Re: [ntp:questions] Accuracy of NTP - Advice Needed
Terje Mathisen wrote: If you have ethernet cards that support hw time sync (I don't remember the spec number), 1588 ? -- E-Mail Sent to this address blackl...@anitech-systems.com will be added to the BlackLists. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Paul Sobey wrote:
Some of these sites vary in their willingness to allow
GPS antenas on roofs as well, joy.
If timing is more important than money, you could always get
a rubidium / cesium, frequency / time reference {e.g. Symmetricom},
if the office politics and/or the physical layout of the building,
won't allow for them using GPS.
--
E-Mail Sent to this address blackl...@anitech-systems.com
will be added to the BlackLists.
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Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2012-01-04, E-Mail Sent to this address will be added to the BlackLists
Null@BlackList.Anitech-Systems.invalid wrote:
Paul Sobey wrote:
Some of these sites vary in their willingness to allow
GPS antenas on roofs as well, joy.
roof? roof is not necessarily necessary. -- Try a window facing the sky.
If timing is more important than money, you could always get
a rubidium / cesium, frequency / time reference {e.g. Symmetricom},
if the office politics and/or the physical layout of the building,
won't allow for them using GPS.
___
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Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/29/2011 8:38 PM, Dennis Ferguson wrote: On 29 Dec, 2011, at 23:26 , Terje Mathisen wrote: Danny Mayer wrote: No, they use synchronized Cesium atomic clocks for time accuracy. GPS is only used to get a fix on the location and I'm not sure that 10's of centimeters is good enough for what they are trying to prove. I'd have to look closely at the methods used and the data to even have a clue as to what is needed and I have touched that stuff in years. Danny, how do you think they keep those atomic clocks synchronized? How do they _verify_ that they actually stay in sync (to a single-digit ns level) over the entire length of the experiment(many months)? Even Hydrogen Masers won't give you that performance over a year or so, you have to have some way to sync them either to each other or to UTC. Yes, they use GPS to compare the clocks to each other. One of the articles I read even identified the GPS receiver they use. I think it was a Septentrio PolaRx3eTR PRO (or maybe the older model which that one replaced). Those receivers take a 10 MHz and 1 PPS reference in from the atomic clock so that they can produce GPS carrier phase measurements with respect to the local clock's time. Making these measurements simultaneously at both locations gives you data you can post-process to determine the time difference between the two clocks, independent of the GPS system time. The GPS signals are used only as markers that can be measured at both locations. They used Septentrio PolaRx2e GPS receivers in both places along with a Symmetricom Cs4000 Cs atomic clock. All of this raises additional questions for which I'd have to dig into the references for answers. For example, both ends are underground and they are likely to use heavy shielding around the sites of the source and target so how are they even getting a GPS signal through in the first place? Are they getting signal or did they set up an external antenna in which case they would have to also figure out the distance of the antenna from the receiver (which part of the antenna?). This is not an easy physics experiment and the errors involved can easily overwhelm the result. It used to be that detecting neutrinos was very hard never mind generating them in a reliable way. Now if only we could send NTP packets via neutrino... Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Danny Mayer ma...@ntp.org wrote: On 12/29/2011 8:38 PM, Dennis Ferguson wrote: On 29 Dec, 2011, at 23:26 , Terje Mathisen wrote: Danny Mayer wrote: No, they use synchronized Cesium atomic clocks for time accuracy. GPS is only used to get a fix on the location and I'm not sure that 10's of centimeters is good enough for what they are trying to prove. I'd have to look closely at the methods used and the data to even have a clue as to what is needed and I have touched that stuff in years. Danny, how do you think they keep those atomic clocks synchronized? How do they _verify_ that they actually stay in sync (to a single-digit ns level) over the entire length of the experiment(many months)? Even Hydrogen Masers won't give you that performance over a year or so, you have to have some way to sync them either to each other or to UTC. Yes, they use GPS to compare the clocks to each other. One of the articles I read even identified the GPS receiver they use. I think it was a Septentrio PolaRx3eTR PRO (or maybe the older model which that one replaced). Those receivers take a 10 MHz and 1 PPS reference in from the atomic clock so that they can produce GPS carrier phase measurements with respect to the local clock's time. Making these measurements simultaneously at both locations gives you data you can post-process to determine the time difference between the two clocks, independent of the GPS system time. The GPS signals are used only as markers that can be measured at both locations. They used Septentrio PolaRx2e GPS receivers in both places along with a Symmetricom Cs4000 Cs atomic clock. All of this raises additional questions for which I'd have to dig into the references for answers. For example, both ends are underground and they are likely to use heavy shielding around the sites of the source and target so how are they even getting a GPS signal through in the first place? Are they getting signal or did they set up an external antenna in which case they would have to also figure out the distance of the antenna from the receiver (which part of the antenna?). This is not an easy physics experiment and the errors involved can easily overwhelm the result. Given the size of a GPS antenna, which part they measure from is down in the noise level. And yes, if you actually were to read all the documents available, you would find the antennas are outside and they did measure the distance to the antenna. How else would they be able to know were the expirement was in relation to a known point, i.e. the antenna, and the time delay down the coax if they didn't make such measurements. -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Danny Mayer wrote: No, they use synchronized Cesium atomic clocks for time accuracy. GPS is only used to get a fix on the location and I'm not sure that 10's of centimeters is good enough for what they are trying to prove. I'd have to look closely at the methods used and the data to even have a clue as to what is needed and I have touched that stuff in years. Danny, how do you think they keep those atomic clocks synchronized? How do they _verify_ that they actually stay in sync (to a single-digit ns level) over the entire length of the experiment(many months)? Even Hydrogen Masers won't give you that performance over a year or so, you have to have some way to sync them either to each other or to UTC. Terje -- - Terje.Mathisen at tmsw.no almost all programming can be viewed as an exercise in caching ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
People really do need to read the paper rather then guess. Yes, As some have said, normally GPS is not accurate enough for this level of work but they are not using GPS in the normal way. What they do is agree on ONE specific GPS satellite that happens to be visible at both locations. Each site measures the difference between its own Cesium atomic and the clock aboard the GPS sat.They need to account for Doppler shift, and path delay through the atmosphere. They can double check they got the path delay right by watching a second GPS that is in common view. The method works well because several sources are self canceling. Someone said that were simply using better and more expensive GPS receivers. No, that is NOT the case. (BTW very good GPS receivers are not expensive, cheaper than consumer car navigation GPSs because they don't need a graphic display, map data or a pretty box. (For example, these are only $60 each if you call and ask: http://www.synergy-gps.com/images/stories/pdf/m12mt_brochure.pdf ) This short introduction describes the method used to sync two clocks using GPS common view http://tf.nist.gov/time/commonviewgps.htm. It is short and easy to read. On Thu, Dec 29, 2011 at 7:26 AM, Terje Mathisen terje.mathisen at tmsw.no@ntp.org wrote: Danny Mayer wrote: No, they use synchronized Cesium atomic clocks for time accuracy. GPS is only used to get a fix on the location and I'm not sure that 10's of centimeters is good enough for what they are trying to prove. I'd have to look closely at the methods used and the data to even have a clue as to what is needed and I have touched that stuff in years. Danny, how do you think they keep those atomic clocks synchronized? How do they _verify_ that they actually stay in sync (to a single-digit ns level) over the entire length of the experiment(many months)? Even Hydrogen Masers won't give you that performance over a year or so, you have to have some way to sync them either to each other or to UTC. Terje -- - Terje.Mathisen at tmsw.no almost all programming can be viewed as an exercise in caching ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions -- Chris Albertson Redondo Beach, California ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 29 Dec, 2011, at 23:26 , Terje Mathisen wrote: Danny Mayer wrote: No, they use synchronized Cesium atomic clocks for time accuracy. GPS is only used to get a fix on the location and I'm not sure that 10's of centimeters is good enough for what they are trying to prove. I'd have to look closely at the methods used and the data to even have a clue as to what is needed and I have touched that stuff in years. Danny, how do you think they keep those atomic clocks synchronized? How do they _verify_ that they actually stay in sync (to a single-digit ns level) over the entire length of the experiment(many months)? Even Hydrogen Masers won't give you that performance over a year or so, you have to have some way to sync them either to each other or to UTC. Yes, they use GPS to compare the clocks to each other. One of the articles I read even identified the GPS receiver they use. I think it was a Septentrio PolaRx3eTR PRO (or maybe the older model which that one replaced). Those receivers take a 10 MHz and 1 PPS reference in from the atomic clock so that they can produce GPS carrier phase measurements with respect to the local clock's time. Making these measurements simultaneously at both locations gives you data you can post-process to determine the time difference between the two clocks, independent of the GPS system time. The GPS signals are used only as markers that can be measured at both locations. BIPM Circular T lists GPSPPP (that's two-frequency, all-in-view carrier phase measurements) as being accurate to 0.3 ns. The bigger error is the equipment calibration, which they estimate as 5 ns. The traveling atomic clock would have been used for the equipment calibration. Dennis Ferguson ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/27/2011 11:45 PM, Greg Hennessy wrote: On 2011-12-28, Danny Mayer ma...@ntp.org wrote: On 12/27/2011 9:08 PM, John Hasler wrote: Danny writes: GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. The requirement is for synchronization. They use common view GPS. That's not good enough for experiments like this. In what way is it not good enough? The neutrinos are apparently arriving about 60 nanoseconds early, the distance is known, through GPS to 10's of centimeters, and the time is synchronized, again through GPS (although a second method is used as a double check) to about 1 nanosecond. In what fashion is it 'not good enough'? No, they use synchronized Cesium atomic clocks for time accuracy. GPS is only used to get a fix on the location and I'm not sure that 10's of centimeters is good enough for what they are trying to prove. I'd have to look closely at the methods used and the data to even have a clue as to what is needed and I have touched that stuff in years. Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/28/2011 12:09 AM, j...@specsol.spam.sux.com wrote: Danny Mayer ma...@ntp.org wrote: On 12/27/2011 8:48 PM, j...@specsol.spam.sux.com wrote: Danny Mayer ma...@ntp.org wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. Danny How do you measure distance with an atomic clock? That's a complex question. GPS (even the military version) is not accurate enough. Danny No, it is not complex; you can't measure distance with an atomic clock. An atomic clock is used to measure time intervals. As for GPS, it is pretty trivial these days to determine an absolute location to parts of a centimeter for a fixed location. There's no such thing as an absolute location. See Einstein. Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/28/2011 12:17 AM, unruh wrote: On 2011-12-28, Danny Mayer ma...@ntp.org wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. No they do not. They use GPS. As has been discussed here gps can be made accurate to a few ns. GPS is used by radio astronomers to synchronize very long baseline arrays. (Yes, I also thought that gps was not accurate enough. I was wrong) As a fellow astrophysicist you know that you don't just use GPS for this like you would finding your way around the streets of Vancouver. This is way beyond those kind of calculations. Of course in astrophysics even 1 km is below the noise level... Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. No they do not. They use GPS. The experiment between Cern and San Grasso for superluminal neutrinos uses atomic clocks which are synchrononized with GPS. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Danny Mayer ma...@ntp.org wrote: On 12/27/2011 11:45 PM, Greg Hennessy wrote: On 2011-12-28, Danny Mayer ma...@ntp.org wrote: On 12/27/2011 9:08 PM, John Hasler wrote: Danny writes: GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. The requirement is for synchronization. They use common view GPS. That's not good enough for experiments like this. In what way is it not good enough? The neutrinos are apparently arriving about 60 nanoseconds early, the distance is known, through GPS to 10's of centimeters, and the time is synchronized, again through GPS (although a second method is used as a double check) to about 1 nanosecond. In what fashion is it 'not good enough'? No, they use synchronized Cesium atomic clocks for time accuracy. GPS is only used to get a fix on the location and I'm not sure that 10's of centimeters is good enough for what they are trying to prove. I'd have to look closely at the methods used and the data to even have a clue as to what is needed and I have touched that stuff in years. Danny Why don't you read some of the available literature before you make an even bigger fool of yourself with your arm-waving guesses and conjecture? -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Danny Mayer ma...@ntp.org wrote: On 12/28/2011 12:09 AM, j...@specsol.spam.sux.com wrote: Danny Mayer ma...@ntp.org wrote: On 12/27/2011 8:48 PM, j...@specsol.spam.sux.com wrote: Danny Mayer ma...@ntp.org wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. Danny How do you measure distance with an atomic clock? That's a complex question. GPS (even the military version) is not accurate enough. Danny No, it is not complex; you can't measure distance with an atomic clock. An atomic clock is used to measure time intervals. As for GPS, it is pretty trivial these days to determine an absolute location to parts of a centimeter for a fixed location. There's no such thing as an absolute location. See Einstein. Danny Absolute within the frame of reference of GPS, which in case you didn't know, is the Earth. See Spot run. -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Danny Mayer ma...@ntp.org wrote: On 12/28/2011 12:17 AM, unruh wrote: On 2011-12-28, Danny Mayer ma...@ntp.org wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. No they do not. They use GPS. As has been discussed here gps can be made accurate to a few ns. GPS is used by radio astronomers to synchronize very long baseline arrays. (Yes, I also thought that gps was not accurate enough. I was wrong) As a fellow astrophysicist you know that you don't just use GPS for this like you would finding your way around the streets of Vancouver. This is way beyond those kind of calculations. Of course in astrophysics even 1 km is below the noise level... Danny Well, you got a small clue. Do you think the GPS equipment used came from Best Buy or that perhaps it is a bit more sophisticated, costly, and accurate than consumer equipment? -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Why did they not do these computations in Mathematica. Once one defines a framework (program) in Mathematica to do long and complex calculations, and you test it with known good data to ensure it is correct, you are virtually guaranteed to find the correct answer with real data. Many of the real gains in astrophysics and in developments of the atomic bomb after the Second World War were done on Macsyma, a forerunner of today's computer algebra systems. Mathematica really is a Godsend if you must have the correct answers to long and difficult problems. Charles Elliott -Original Message- From: questions-bounces+elliott.ch=verizon@lists.ntp.org [mailto:questions-bounces+elliott.ch=verizon@lists.ntp.org] On Behalf Of Danny Mayer Sent: Tuesday, December 27, 2011 11:36 PM To: Greg Hennessy Cc: questions@lists.ntp.org Subject: Re: [ntp:questions] Accuracy of NTP - Advice Needed On 12/27/2011 10:39 PM, Greg Hennessy wrote: The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. GPS is indeed used for the measurement of the time of flight in the CERN and Fermilab experiments. You should read the papers. They use GPS to get time to the order of nanosecond accuracy. You can read some of this here: http://www.wired.com/wiredscience/2011/09/scientists-question- neutrinos/ It's not too technical but describes the basic setup. Yes they did use GPS to get accurate locations of the equipment but it's a rather complex and hard to get right. They then used Cesium atomic clocks for timing the events. The calculations you have to do for all this is mind-boggling and there is a lot of work that has to go into ensuring that they are accurate and nothing got missed. That's the principle reason that it's hard to be sure that an FTL result was obtained. There are lots of scientists pouring over calculations (there were something like 150 authors listed on the paper published in arXiv. Hords of other scientists are also analyzing the data. Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-28, Danny Mayer ma...@ntp.org wrote: On 12/27/2011 11:45 PM, Greg Hennessy wrote: On 2011-12-28, Danny Mayer ma...@ntp.org wrote: On 12/27/2011 9:08 PM, John Hasler wrote: Danny writes: GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. The requirement is for synchronization. They use common view GPS. That's not good enough for experiments like this. In what way is it not good enough? The neutrinos are apparently arriving about 60 nanoseconds early, the distance is known, through GPS to 10's of centimeters, and the time is synchronized, again through GPS (although a second method is used as a double check) to about 1 nanosecond. In what fashion is it 'not good enough'? No, they use synchronized Cesium atomic clocks for time accuracy. GPS is And the clocks are synchronized by GPS. They also have some burbling about using a truck with a portable atomic clock to check the synchronization. only used to get a fix on the location and I'm not sure that 10's of centimeters is good enough for what they are trying to prove. I'd have 10s of cm is good enough. The error in the speed of light is 20 m. to look closely at the methods used and the data to even have a clue as to what is needed and I have touched that stuff in years. Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-28, Danny Mayer ma...@ntp.org wrote: On 12/28/2011 12:17 AM, unruh wrote: On 2011-12-28, Danny Mayer ma...@ntp.org wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. No they do not. They use GPS. As has been discussed here gps can be made accurate to a few ns. GPS is used by radio astronomers to synchronize very long baseline arrays. (Yes, I also thought that gps was not accurate enough. I was wrong) As a fellow astrophysicist you know that you don't just use GPS for this like you would finding your way around the streets of Vancouver. This is way beyond those kind of calculations. Of course in astrophysics even 1 km is below the noise level... No idea what you mean. The gps I might use to find my way around the streets of Vancouver does not have a time function at all. I would use a gps with a timing output (PPS) with Sawtooth corrections to get me down to something like 5-10ns precision, making sure I used a GPS that did not have an internal bias. Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-28, Danny Mayer ma...@ntp.org wrote: On 12/28/2011 12:09 AM, j...@specsol.spam.sux.com wrote: Danny Mayer ma...@ntp.org wrote: On 12/27/2011 8:48 PM, j...@specsol.spam.sux.com wrote: Danny Mayer ma...@ntp.org wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. Danny How do you measure distance with an atomic clock? That's a complex question. GPS (even the military version) is not accurate enough. Danny No, it is not complex; you can't measure distance with an atomic clock. An atomic clock is used to measure time intervals. As for GPS, it is pretty trivial these days to determine an absolute location to parts of a centimeter for a fixed location. There's no such thing as an absolute location. See Einstein. Yes, under GPS there is, by definition on the surface of the earth. The center of mass of the earthdefines an origin, the distance from the center a radial distance, Greenwich a zero longitude, and the poles a 90 degrees latitude. On GPS I am not sure if the Long and lat of a point are defined by the local vertical (perpendicular to the goid or parallel to local g) or absolute (equal angles of the radius vector from the center.) Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Danny Mayer wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. Danny, they sync the atomic clocks using common-view GPS. This is exactly the same procedure that is used to compare all the clocks that together define what UTC is. Terje -- - Terje.Mathisen at tmsw.no almost all programming can be viewed as an exercise in caching ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/24/2011 1:11 PM, unruh wrote: On 2011-12-24, John Hasler jhas...@newsguy.com wrote: I wrote: An upcoming experiment at Fermilab will observe neutrinos at both ends (the far end will be in Minnesota). unruh writes: Well, no. At best the electrons or muons at one end. At best the electrical pulse produced by a photomultiplier when struck by a photon generated when a muon or electron emitted as a result of a neutrino collision interacts with the detector medium (there are a variety of detector designs but photomultipliers are almost always involved). However, the use of similar or identical neutrino detectors at both ends means that systemic errors in delay estimation will tend to cancel. I assume that they will try to match up the timing equipment at both ends as well. Just saying, it is not the same neutrino that is being detected at both ends. The detection probability is just too small. Thus again there is the same inference that the timing at one end measures the same class of things as teh timing at the other. Yes, the timing equipment is a worry. They require ns accuracy in the timing and m accuracy in the distance. And the timing is not simply gps ( although they could have gotten that wrong) but then that timing has to be brought down into the mine a km or so below ground and horizontally and that also has to be surveyed for the distance. You need a very good atomic clock at both ends that are synchronized to each other. Chances are very good that they have a number of them at each end. Nothing less than an atomic clock will do. Now what has this to do with the original question? Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Danny writes: GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. The requirement is for synchronization. They use common view GPS. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Danny Mayer ma...@ntp.org wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. Danny How do you measure distance with an atomic clock? -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/27/2011 8:48 PM, j...@specsol.spam.sux.com wrote: Danny Mayer ma...@ntp.org wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. Danny How do you measure distance with an atomic clock? That's a complex question. GPS (even the military version) is not accurate enough. Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/27/2011 9:08 PM, John Hasler wrote: Danny writes: GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. The requirement is for synchronization. They use common view GPS. That's not good enough for experiments like this. Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. GPS is indeed used for the measurement of the time of flight in the CERN and Fermilab experiments. You should read the papers. They use GPS to get time to the order of nanosecond accuracy. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
At 10:40 PM 12/27/2011, Danny Mayer wrote... On 12/27/2011 9:08 PM, John Hasler wrote: The requirement is for synchronization. They use common view GPS. That's not good enough for experiments like this. You say that as if it's a fact. You're on the wrong list to just make such an unsupported statement, there are people here who know better. Support your argument with authoritative references, such as these: http://www.boulder.nist.gov/timefreq/general/pdf/192.pdf http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=278607 http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA508388 http://tf.nist.gov/general/pdf/1379.pdf Here's a representative quote: When averaged past one day, the time deviation of the multi-channel common-views remains between 1 to 2 ns, while the noise of the single-channel GPS common-view drops below 1 ns for averaging longer than 2 days. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/27/2011 10:39 PM, Greg Hennessy wrote: The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. GPS is indeed used for the measurement of the time of flight in the CERN and Fermilab experiments. You should read the papers. They use GPS to get time to the order of nanosecond accuracy. You can read some of this here: http://www.wired.com/wiredscience/2011/09/scientists-question-neutrinos/ It's not too technical but describes the basic setup. Yes they did use GPS to get accurate locations of the equipment but it's a rather complex and hard to get right. They then used Cesium atomic clocks for timing the events. The calculations you have to do for all this is mind-boggling and there is a lot of work that has to go into ensuring that they are accurate and nothing got missed. That's the principle reason that it's hard to be sure that an FTL result was obtained. There are lots of scientists pouring over calculations (there were something like 150 authors listed on the paper published in arXiv. Hords of other scientists are also analyzing the data. Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Greg Hennessy greg.henne...@cox.net wrote: The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. GPS is indeed used for the measurement of the time of flight in the CERN and Fermilab experiments. You should read the papers. They use GPS to get time to the order of nanosecond accuracy. What a concept; someone that actually read the papers instead of just pulling crap out their ass and arm waving. Prepare to be inundated with drivel for actually knowing something. -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-28, Danny Mayer ma...@ntp.org wrote: On 12/27/2011 9:08 PM, John Hasler wrote: Danny writes: GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. The requirement is for synchronization. They use common view GPS. That's not good enough for experiments like this. In what way is it not good enough? The neutrinos are apparently arriving about 60 nanoseconds early, the distance is known, through GPS to 10's of centimeters, and the time is synchronized, again through GPS (although a second method is used as a double check) to about 1 nanosecond. In what fashion is it 'not good enough'? ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-28, Danny Mayer ma...@ntp.org wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. No they do not. They use GPS. As has been discussed here gps can be made accurate to a few ns. GPS is used by radio astronomers to synchronize very long baseline arrays. (Yes, I also thought that gps was not accurate enough. I was wrong) Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Danny Mayer ma...@ntp.org wrote: On 12/27/2011 8:48 PM, j...@specsol.spam.sux.com wrote: Danny Mayer ma...@ntp.org wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. GPS is not used for this kind of thing, they are too inaccurate, so it doesn't matter. They use atomic clocks. Danny How do you measure distance with an atomic clock? That's a complex question. GPS (even the military version) is not accurate enough. Danny No, it is not complex; you can't measure distance with an atomic clock. An atomic clock is used to measure time intervals. As for GPS, it is pretty trivial these days to determine an absolute location to parts of a centimeter for a fixed location. -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
j...@specsol.spam.sux.com j...@specsol.spam.sux.com wrote: Again, were do you see the word few in what I wrote? That makes the statement so meaningless. Every distance can be measured in feet. Of couse, nobody at Cern would even think of doing that, but that is another matter. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/25/2011 5:49 AM, Rob wrote: j...@specsol.spam.sux.comj...@specsol.spam.sux.com wrote: Again, were do you see the word few in what I wrote? That makes the statement so meaningless. Every distance can be measured in feet. Of couse, nobody at Cern would even think of doing that, but that is another matter. How many feet in a light year?? ;-) That should keep you out of trouble for a little while! ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Rob nom...@example.com wrote: j...@specsol.spam.sux.com j...@specsol.spam.sux.com wrote: Again, were do you see the word few in what I wrote? That makes the statement so meaningless. Every distance can be measured in feet. If I had written exactly the same thing with the exception of using the word meters instead of the word feet, would you then get the point or would it still go whooshing over your head? -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
I wrote: An upcoming experiment at Fermilab will observe neutrinos at both ends (the far end will be in Minnesota). unruh writes: Well, no. At best the electrons or muons at one end. At best the electrical pulse produced by a photomultiplier when struck by a photon generated when a muon or electron emitted as a result of a neutrino collision interacts with the detector medium (there are a variety of detector designs but photomultipliers are almost always involved). However, the use of similar or identical neutrino detectors at both ends means that systemic errors in delay estimation will tend to cancel. I assume that they will try to match up the timing equipment at both ends as well. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-24, John Hasler jhas...@newsguy.com wrote: I wrote: An upcoming experiment at Fermilab will observe neutrinos at both ends (the far end will be in Minnesota). unruh writes: Well, no. At best the electrons or muons at one end. At best the electrical pulse produced by a photomultiplier when struck by a photon generated when a muon or electron emitted as a result of a neutrino collision interacts with the detector medium (there are a variety of detector designs but photomultipliers are almost always involved). However, the use of similar or identical neutrino detectors at both ends means that systemic errors in delay estimation will tend to cancel. I assume that they will try to match up the timing equipment at both ends as well. Just saying, it is not the same neutrino that is being detected at both ends. The detection probability is just too small. Thus again there is the same inference that the timing at one end measures the same class of things as teh timing at the other. Yes, the timing equipment is a worry. They require ns accuracy in the timing and m accuracy in the distance. And the timing is not simply gps ( although they could have gotten that wrong) but then that timing has to be brought down into the mine a km or so below ground and horizontally and that also has to be surveyed for the distance. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
unruh writes: They require ns accuracy in the timing and m accuracy in the distance. And the timing is not simply gps ( although they could have gotten that wrong) but then that timing has to be brought down into the mine a km or so below ground and horizontally and that also has to be surveyed for the distance. The NOvA detector is not in a mine so it should be possible to site the GPS receiver directly above it and drop a cable straight down. The same should be possible at the Fermi end. You could set up both timing chains at Fermilab (using indentical components including cable lengths if you want to be fanatical), calibrate them against each other for delay from antenna to output, and then pack one up and ship it up north (of course there may be good reasons not to do it this way). The surveying should be easier than in Europe: there's no mountain range in the way. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
John Hasler jhas...@newsguy.com wrote: unruh writes: They require ns accuracy in the timing and m accuracy in the distance. And the timing is not simply gps ( although they could have gotten that wrong) but then that timing has to be brought down into the mine a km or so below ground and horizontally and that also has to be surveyed for the distance. The NOvA detector is not in a mine so it should be possible to site the GPS receiver directly above it and drop a cable straight down. The same should be possible at the Fermi end. You could set up both timing chains at Fermilab (using indentical components including cable lengths if you want to be fanatical), calibrate them against each other for delay from antenna to output, and then pack one up and ship it up north (of course there may be good reasons not to do it this way). The surveying should be easier than in Europe: there's no mountain range in the way. That's the common misconception of the geology. Basically the lab is in a tunnel in the side of a mountain and is no more a km underground than is the lobby of a 20 story hotel 20 stories underground. -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-24, j...@specsol.spam.sux.com j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: unruh writes: They require ns accuracy in the timing and m accuracy in the distance. And the timing is not simply gps ( although they could have gotten that wrong) but then that timing has to be brought down into the mine a km or so below ground and horizontally and that also has to be surveyed for the distance. The NOvA detector is not in a mine so it should be possible to site the GPS receiver directly above it and drop a cable straight down. The same should be possible at the Fermi end. You could set up both timing chains at Fermilab (using indentical components including cable lengths if you want to be fanatical), calibrate them against each other for delay from antenna to output, and then pack one up and ship it up north (of course there may be good reasons not to do it this way). The surveying should be easier than in Europe: there's no mountain range in the way. That's the common misconception of the geology. Basically the lab is in a tunnel in the side of a mountain and is no more a km underground than is the lobby of a 20 story hotel 20 stories underground. But it is a few km inside the mountain. Is a mine in Denver not underground just because Denver is 1600 m above sea level? ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-24, John Hasler jhas...@newsguy.com wrote: unruh writes: They require ns accuracy in the timing and m accuracy in the distance. And the timing is not simply gps ( although they could have gotten that wrong) but then that timing has to be brought down into the mine a km or so below ground and horizontally and that also has to be surveyed for the distance. The NOvA detector is not in a mine so it should be possible to site the GPS receiver directly above it and drop a cable straight down. The same should be possible at the Fermi end. You could set up both timing chains at Fermilab (using indentical components including cable lengths if you want to be fanatical), calibrate them against each other for delay from antenna to output, and then pack one up and ship it up north (of course there may be good reasons not to do it this way). The surveying should be easier than in Europe: there's no mountain range in the way. Surveying is done by GPS, as is timing so mountain ranges do not really matter. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
unruh un...@invalid.ca wrote: On 2011-12-24, j...@specsol.spam.sux.com j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: unruh writes: They require ns accuracy in the timing and m accuracy in the distance. And the timing is not simply gps ( although they could have gotten that wrong) but then that timing has to be brought down into the mine a km or so below ground and horizontally and that also has to be surveyed for the distance. The NOvA detector is not in a mine so it should be possible to site the GPS receiver directly above it and drop a cable straight down. The same should be possible at the Fermi end. You could set up both timing chains at Fermilab (using indentical components including cable lengths if you want to be fanatical), calibrate them against each other for delay from antenna to output, and then pack one up and ship it up north (of course there may be good reasons not to do it this way). The surveying should be easier than in Europe: there's no mountain range in the way. That's the common misconception of the geology. Basically the lab is in a tunnel in the side of a mountain and is no more a km underground than is the lobby of a 20 story hotel 20 stories underground. But it is a few km inside the mountain. Is a mine in Denver not underground just because Denver is 1600 m above sea level? The issue is that most people don't seem to be able to understand how to get an accurate position of a location that is vertically under a km or so of dirt, yet horizontally feet from wide open sky and GPS signals. -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
unruh writes: Surveying is done by GPS, as is timing so mountain ranges do not really matter. The OPERA team had to survey a traverse through the Gran Sasso highway tunnel to get to suitable benchmarks. You're right though: they did not survey the entire distance. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Jim Pennino writes: The issue is that most people don't seem to be able to understand how to get an accurate position of a location that is vertically under a km or so of dirt, yet horizontally feet from wide open sky and GPS signals. The open sky nearest the OPERA detector is straight up through 1400m of rock. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
John Hasler jhas...@newsguy.com wrote: Jim Pennino writes: The issue is that most people don't seem to be able to understand how to get an accurate position of a location that is vertically under a km or so of dirt, yet horizontally feet from wide open sky and GPS signals. The open sky nearest the OPERA detector is straight up through 1400m of rock. And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Everything else is bloviation. -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
unruh un...@invalid.ca wrote: On 2011-12-24, j...@specsol.spam.sux.com j...@specsol.spam.sux.com wrote: unruh un...@invalid.ca wrote: On 2011-12-24, j...@specsol.spam.sux.com j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: unruh writes: They require ns accuracy in the timing and m accuracy in the distance. And the timing is not simply gps ( although they could have gotten that wrong) but then that timing has to be brought down into the mine a km or so below ground and horizontally and that also has to be surveyed for the distance. The NOvA detector is not in a mine so it should be possible to site the GPS receiver directly above it and drop a cable straight down. The same should be possible at the Fermi end. You could set up both timing chains at Fermilab (using indentical components including cable lengths if you want to be fanatical), calibrate them against each other for delay from antenna to output, and then pack one up and ship it up north (of course there may be good reasons not to do it this way). The surveying should be easier than in Europe: there's no mountain range in the way. That's the common misconception of the geology. Basically the lab is in a tunnel in the side of a mountain and is no more a km underground than is the lobby of a 20 story hotel 20 stories underground. But it is a few km inside the mountain. Is a mine in Denver not underground just because Denver is 1600 m above sea level? The issue is that most people don't seem to be able to understand how to get an accurate position of a location that is vertically under a km or so of dirt, yet horizontally feet from wide open sky and GPS signals. A few feet? I assume that was a misprint for a few km. Where do you see the words few feet in what I wrote? The bottom line is that the only thing that is relevant is the path to the GPS antenna with a clear view of the sky. Everything else is bloviation. -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-25, j...@specsol.spam.sux.com j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Yes. 5km away horizontally or 1.5km away vertically. Everything else is bloviation. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
unruh un...@invalid.ca wrote: On 2011-12-25, j...@specsol.spam.sux.com j...@specsol.spam.sux.com wrote: John Hasler jhas...@newsguy.com wrote: The open sky nearest the OPERA detector is straight up through 1400m of rock. Jim Pennino writes: And the easiest open sky to get to is horizontally down the tunnel to the entrance which is next to a freeway. Yes, the entrance is next to a freeway. The entrance to the LNGS facility where the OPERA detector is located is near the middle of the 10 km long Gran Sasso highway tunnel. The bottom line is that the only thing that is relevant is how easy it is to get to a GPS antenna with an open view of the sky. Yes. 5km away horizontally or 1.5km away vertically. Distance is not automatically a metric of ease. But bloviate away. -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Richard B. Gilbert wrote: On 12/22/2011 9:17 PM, Chris Adams wrote: The securities traders (especially HFT) want it. I suspect the OP is in that group. That level of timekeeping has been discussed here before. I think that the radio astronomers are some of the most demanding. They need far better than just PPS accuracy, they normally use common-view GPS in the same mode as is used for comparing UTC master clocks, i.e. with eventual accuracy in the low ns range. This is sufficient to phase-lock multiple radio observatories synthesizing a _very_ long baseline receiver. Terje -- - Terje.Mathisen at tmsw.no almost all programming can be viewed as an exercise in caching ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Paul Sobey wrote: Our internal testing to this point is that a stock ntpd pointed against a stratum 1 clock on a low contention gigabit ethernet (stratum 1 source and client less than 1ms apart) reports its own accuracy at approx 200 microseconds. Further tuning the ntp config by adding the minpoll 4, maxpoll 6 and burst keywords result in ntpd reported accuracy dropping to within 10-20 microseconds (as reported by ntpq -p and borne out by loopstats). Further improvements can be made running ntpd in the RT priority class. Good, you've done your homework! :-) My questions to you all, if you've read through the above waffle are: - what is a sensible expected accuracy of ntpd if pointed at several stratum 1 time sources across a low jitter gigabit network (we'd probably spread them over several UK and US sites for resiliency but all paths are low jitter and highly deterministic latency) Gbit and low jitter is not quite compatible: 100 Mbit switches were using cut-through, while (afaik) all Gbit and up switches use store forward, leading to higher latency and jitter. - are there any obvious tunables to improve accuracy other than minpoll/burst and process scheduling class, and how agressive can the polling cycles be sensible made? - can ntpd's own reported offset (ntpq -p or loopstats) be trusted (assuming high priority means it gets scheduled as desired)? I've quoted our apparent numbers at several people and the response is always 'pfft you can't trust ntpd to know its own offset' - but nobody can ever tell me why You can use ntpd's internal numbers to verify the maximum possible offset (half the round trip time), you should be able to use statistics to show that the jitter is quite low as well. I appreciate these may appear to be silly questions with obvious answers - I am grateful in advance for your patience, and any research sources you may direct me to. The best (and probably only possible) solution that does give you single-digit us is to route a PPS signal to each and every server, then use the network for approximate (~100 us) timing, with the PPS doing the last two orders of magnitude. Terje -- - Terje.Mathisen at tmsw.no almost all programming can be viewed as an exercise in caching ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Paul Sobey wrote: Our internal testing to this point is that a stock ntpd pointed against a stratum 1 clock on a low contention gigabit ethernet (stratum 1 source and client less than 1ms apart) reports its own accuracy at approx 200 microseconds. Further tuning the ntp config by adding the minpoll 4, maxpoll 6 and burst keywords result in ntpd reported accuracy dropping to within 10-20 microseconds (as reported by ntpq -p and borne out by loopstats). Further improvements can be made running ntpd in the RT priority class. Good, you've done your homework! :-) I've been trying! It's a challenging subject to get to grips with! A long way to go I think... My questions to you all, if you've read through the above waffle are: - what is a sensible expected accuracy of ntpd if pointed at several stratum 1 time sources across a low jitter gigabit network (we'd probably spread them over several UK and US sites for resiliency but all paths are low jitter and highly deterministic latency) Gbit and low jitter is not quite compatible: 100 Mbit switches were using cut-through, while (afaik) all Gbit and up switches use store forward, leading to higher latency and jitter. There are several varieties of cut-through gig/10GB switch available now - but noting that store and forward adds latency is a good point. Presumably layer 3 switching (routing) is always store and forward since we're effectively writing a new packet. It's all done in hardware though - negligible jitter. - are there any obvious tunables to improve accuracy other than minpoll/burst and process scheduling class, and how agressive can the polling cycles be sensible made? - can ntpd's own reported offset (ntpq -p or loopstats) be trusted (assuming high priority means it gets scheduled as desired)? I've quoted our apparent numbers at several people and the response is always 'pfft you can't trust ntpd to know its own offset' - but nobody can ever tell me why You can use ntpd's internal numbers to verify the maximum possible offset (half the round trip time), you should be able to use statistics to show that the jitter is quite low as well. At the risk of pushing my luck, can you expand on this? I appreciate these may appear to be silly questions with obvious answers - I am grateful in advance for your patience, and any research sources you may direct me to. The best (and probably only possible) solution that does give you single-digit us is to route a PPS signal to each and every server, then use the network for approximate (~100 us) timing, with the PPS doing the last two orders of magnitude. Our problem will be that running coax around many sites to lots of machines, many of which don't have serial ports (think blades), is both highly time consuming and maintenance intensive. If we have to do it then we will but I'd like a clear idea as to the whys before I start down that particular path. In particular at this stage I'm trying to understand more about the theoretical accuracies obtainable under ideal conditions, and most important, how to independently verify the results of any tweaks we might apply. Say I have coalesence turned on a nic and I disable it - I'd like to be able to determine the effect, if any of that change. Is it possible for ntpd (or ptpd) to accurately determine its own accuracy, if that makes sense? If not what techniques might I use to independently measure? On a related note, I'm aware that there are various methods for querying the system time, some of which involve a context switch, and some of which can be done cheaply in userspace. I'm not sure whether the same is true for setting the time. Is anyone aware of how much ntpd operation involves context switches, which obviously would place quite a high ceiling on accuracy since we're at the mercy of the OS scheduler? Cheers, Paul ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
I hear from many vendors and industry colleagues that 'ntp just isn't suitable for high precision work and anything less than 1-2ms precision requires ptp or direct connection to gps clock'. I find these numbers For microsecond accuracy, I would say that NTP needs direct (PPS) connection to a GPS clock. On a very well managed LAN, with very good temperature control and a constant power dissipation on the machines, you might achieve it over LANs. On realistic internet connections, I don't think any technology will achieve it. (I guess you could use the internet to coordinate atomic clocks.) Thanks for this reponse. Assuming I give NTP a direct connection to a PPS clock, can you advise how I might determine what my accuracy actually is? This is the piece I'm very unsure of - I had hoped I could simply use the offset estimations from loopstats, now it seems I was mistaken in that assumption, but I'm still unclear as to why, and what numbers I can actually use to gain a measure. You need to consider more than this; for example, ethernet switches can be a major source of degradation. Is this true even with the kind of architecture I've described? I'm aware that switches and networks come in varying flavours, but I think I'm being fair when I describe ours as low latency and low jitter. Most paths have very little contention on. - are there any obvious tunables to improve accuracy other than minpoll/burst and process scheduling class, and how agressive can the polling cycles be sensible made? Very good temperature control, and maintaining a constant load on the machine. It is also essential that you calibrate the system. This either means using GPS or a portable atomic clock. What does calibrate the system mean? Is this 'use a GPS clock as an ntp source' or some other technique I haven't heard of? - can ntpd's own reported offset (ntpq -p or loopstats) be trusted (assuming high priority means it gets scheduled as desired)? I've quoted our apparent numbers at several people and the response is always 'pfft you can't trust ntpd to know its own offset' - but nobody can ever tell me why It can be trusted as what it actually measures. It is not a measurement of the error from true time. If it were, and it could be trusted, ntpd would be remiss not to use it to correct the time to a point where the remaining offset was no longer a good measure. The offset on a locked up system should be several times larger than the RMS error in the actual system time. Understood, at least in part. I have a nice Christmas reading list of man pages and white papers! Cheers, Paul ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
What OS are your hosts running? If it's Windows, millisecond, not microsecond accuracy will be what you can get at best when syncing over the network. A mixture of linux flavours and solaris. For the linux hosts I have a little more control over which version of ntpd I deploy. For the solaris ones, the stock version is ancient, not sure I have much control over that one but it could be raised if necessary. If you really need microsecond, I suspect you will be looking at a GPS receiver or two at each site, and distributing the PPS (pulse per second) signal to each host, and praying that the hosts have a serial port connection! Well that's the rub - some of them don't :) If nothing else it might inform new hardware purchases though. Some of these sites vary in their willingness to allow GPS antenas on roofs as well, joy. Cheers, Paul ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
What OS are your hosts running? If it's Windows, millisecond, not microsecond accuracy will be what you can get at best when syncing over the network. A mixture of linux flavours and solaris. For the linux hosts I have a little more control over which version of ntpd I deploy. For the solaris ones, the stock version is ancient, not sure I have much control over that one but it could be raised if necessary. I read that folk who want the best accuracy will run FreeBSD rather than Linux, but also that recent versions of Linux may not be as bad. How much the NTP version will affect the accuracy on such systems I don't know, nor how well Solaris performs. If you really need microsecond, I suspect you will be looking at a GPS receiver or two at each site, and distributing the PPS (pulse per second) signal to each host, and praying that the hosts have a serial port connection! Well that's the rub - some of them don't :) If nothing else it might inform new hardware purchases though. Some of these sites vary in their willingness to allow GPS antenas on roofs as well, joy. Cheers, Paul My gut feeling is that without GPS, microsecond accuracy is out of reach, and likely without a direct PPS connection to the box as well. It may not apply here, but is the requirement at all realistic? Was it dreamt up off the top of someone's head? What happens if you say that (for example) 100 microseconds is the best you can get. It's not high-energy physics or radio astronomy, is it? Not sure I can help any further, except to point you to my best PC, which is a dual-core Intel Atom box, doing nothing other than time serving, connected to a roof-mounted GPS, running FreeBSD. It's in a domestic, non-temperature controlled, environment. http://www.satsignal.eu/mrtg/pixie_ntp.html The zero line of the graph is at +20 microseconds, as MRTG doesn't plot negative numbers. The long term drift on the yearly graph is an MRTG issue. The spike of several microseconds you see on Wednesday is just the heating coming on here. Windows systems do not do as well: http://www.satsignal.eu/mrtg/performance_ntp.php You were planning on temperature controlled rooms for these systems, I suppose? Cheers, David ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-23, Paul Sobey bud...@the-annexe.net wrote: I hear from many vendors and industry colleagues that 'ntp just isn't suitable for high precision work and anything less than 1-2ms precision requires ptp or direct connection to gps clock'. I find these numbers For microsecond accuracy, I would say that NTP needs direct (PPS) connection to a GPS clock. On a very well managed LAN, with very good temperature control and a constant power dissipation on the machines, you might achieve it over LANs. On realistic internet connections, I don't think any technology will achieve it. (I guess you could use the internet to coordinate atomic clocks.) Thanks for this reponse. Assuming I give NTP a direct connection to a PPS clock, can you advise how I might determine what my accuracy actually is? This is the piece I'm very unsure of - I had hoped I could simply use the offset estimations from loopstats, now it seems I was mistaken in that assumption, but I'm still unclear as to why, and what numbers I can actually use to gain a measure. With a good GPS receiver, AND a system which directly time tags the interrupt (no driver in between like a serial driver) you can assume that the time stamps on the gps signal are a reasonable estimate of the true offset (main problem is that interrupt latency). You could have the system put out a pulse the instant it receives the interrupt pulse (eg toggle a line on a parallel port) and attack an oscilliscope on the two lines (gps PPS line and the output port line ) to see what the time lag is. You need to consider more than this; for example, ethernet switches can be a major source of degradation. Is this true even with the kind of architecture I've described? I'm aware that switches and networks come in varying flavours, but I think I'm being fair when I describe ours as low latency and low jitter. Most paths have very little contention on. Not the problem. The routers have delays. Teh NICs have delays (some purposely wait to collect a reasonable amount of data before sending it out on the line). I used to have all 100MbS NICS and switches. I now have mixed 100Mbs and 1Gbs system and I see a really terrible degredation of the ntp network performance. While it used to be a very reliable 120us roundtrip on the ntp packets, it is now bimodal ( some about half 120-140, half 300 and a few up to 10ms delays). This change occured when the routers were replaced by Gbrouters. And there is very little contention on this network. - are there any obvious tunables to improve accuracy other than minpoll/burst and process scheduling class, and how agressive can the polling cycles be sensible made? Very good temperature control, and maintaining a constant load on the machine. It is also essential that you calibrate the system. This either means using GPS or a portable atomic clock. What does calibrate the system mean? Is this 'use a GPS clock as an ntp source' or some other technique I haven't heard of? It means using a pps source to measure the offsets of your system. - can ntpd's own reported offset (ntpq -p or loopstats) be trusted (assuming high priority means it gets scheduled as desired)? I've quoted It can be trusted to tell you what the offsets are. It cannot be trusted to tell you what the difference between your system time and true time is. our apparent numbers at several people and the response is always 'pfft you can't trust ntpd to know its own offset' - but nobody can ever tell me why It can be trusted as what it actually measures. It is not a measurement of the error from true time. If it were, and it could be trusted, ntpd would be remiss not to use it to correct the time to a point where the remaining offset was no longer a good measure. The offset on a locked up system should be several times larger than the RMS error in the actual system time. Understood, at least in part. I have a nice Christmas reading list of man pages and white papers! Cheers, Paul ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Paul Sobey wrote: Gbit and low jitter is not quite compatible: 100 Mbit switches were using cut-through, while (afaik) all Gbit and up switches use store forward, leading to higher latency and jitter. There are several varieties of cut-through gig/10GB switch available now - but noting that store and forward adds latency is a good point. Presumably layer 3 switching (routing) is always store and forward since we're effectively writing a new packet. It's all done in hardware though - negligible jitter. Negligible only in a statistical way, when running on a lightly loaded network (close to zero collisions/arbitration for access to the same port) You can use ntpd's internal numbers to verify the maximum possible offset (half the round trip time), you should be able to use statistics to show that the jitter is quite low as well. At the risk of pushing my luck, can you expand on this? NTPD statistics will easily give you an order of magnitude better accuracy than that defined by half the RTT, so if you have ping times well below 1 ms, you should get actual offsets in the dual-digit us range. The best (and probably only possible) solution that does give you single-digit us is to route a PPS signal to each and every server, then use the network for approximate (~100 us) timing, with the PPS doing the last two orders of magnitude. Our problem will be that running coax around many sites to lots of machines, many of which don't have serial ports (think blades), is both highly time consuming and maintenance intensive. If we have to do it then we will but I'd like a clear idea as to the whys before I start down that particular path. In particular at this stage I'm trying to understand more about the theoretical accuracies obtainable under ideal conditions, and most important, how to independently verify the results of any tweaks we might apply. Say I have coalesence turned on a nic and I disable it - That is just one of the things you _have_ to do in order to get rid of jitter. If you have ethernet cards that support hw time sync (I don't remember the spec number), then you can indeed get low us over network links, but that isn't ntp any longer, and you'll still have to replace the blade infrastructure. I'd like to be able to determine the effect, if any of that change. Is it possible for ntpd (or ptpd) to accurately determine its own accuracy, if that makes sense? If not what techniques might I use to independently measure? On a related note, I'm aware that there are various methods for querying the system time, some of which involve a context switch, and some of which can be done cheaply in userspace. I'm not sure whether the same is A fast system time query will load the time as of the last hw clock tick, along with the corresponding RDTSC (or similar, constant-rate highres clock source), load the current RDTSC value, then re-read the OS tick. If the OS tick has been updated in the meantime, restart the process. Finally, scale the RDTSC delta by the (OS-provided) frequency and add to the tick time: The result is the current system time in ~100 clock cycles, with sub-us precision. true for setting the time. Is anyone aware of how much ntpd operation involves context switches, which obviously would place quite a high ceiling on accuracy since we're at the mercy of the OS scheduler? NTP does not set the time, it just tells the OS how much the tick rate should be tuned by, i.e. effectively how short/long each tick really is. Terje -- - Terje.Mathisen at tmsw.no almost all programming can be viewed as an exercise in caching ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On Fri, Dec 23, 2011 at 17:02, Terje Mathisen terje.mathisen at tmsw.no@ntp.org wrote: A fast system time query will load the time as of the last hw clock tick, along with the corresponding RDTSC (or similar, constant-rate highres clock source), load the current RDTSC value, then re-read the OS tick. If the OS tick has been updated in the meantime, restart the process. Finally, scale the RDTSC delta by the (OS-provided) frequency and add to the tick time: The result is the current system time in ~100 clock cycles, with sub-us precision. That's a good description of most OS clocks. There are exceptions where no high-resolution counter is used, such as often seen with ntpd on Windows Vista and later. On such systems, reading the clock can be as fast as reading 64 bits from shared memory, possibly twice if partial updates are visible. Cheers, Dave Hart ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
I see a common misconception here. Most of the concepts about NTP can be explained using a common wristwatch. One is that more frequent checking to a standard keeps the time closer to the standard. That would be true only if you set your watch to match the standard each time. NTP does not do that. It plays with the fast and slow lever and adjusts the rate. To set a rate the longer you wait between checks the better. It takes time to see it you are running fast or slow. Frequent jumping the clock to match a standard while ignoring the rate adjustment produces a sawtooth shape error function. Making small adjustments to the rate is better and is what NTP does. NTP can't really know it's own absolute error from true UTC time. It if did it would be simple enough to make it zero. All it can do is report statistics about how in the recent past it differed along with an error bar on that.To really know how NTP is doing you need a second local clock that is about at least an order of magnitude better. Practically speaking for most of us that means a timing grade GPS receiver. But how do you know you have that set up right and you have accounted for the cable and internal delays. You get a second GPS, hopefully a different type. The good news is that these GPSes are dirt cheap. Under $20 if you hunt around. But it is absolutely true that unless you have access to true UTC and also have a way to double/tripple check then you can never know if your NTP server is accurate. You will need a way to measure phase differences in the PPS signals too.None of this stuff is very expensive and there seem to be endless supply from China. But it takes time to build up your ability to measure. So if you want an NTP server that works at the uS level, first assemble a timing lab that can measure nS. New modern timing grade receivers cost under $100 and are good for single digit nanoseconds. Finally, you are ready to start. Setting up an NTP server that does better then 1uS requires a very stable internal clock in side the PC. Most PCs are unsuitable and their internal clock frequency will track the building's air conditioning/heating cycles. If these cycles are faster than NTP's control loop there is not much the software can do. People have resorted to hardware mods like temperature control and upgrading the crystal clock on the computer main board. I found an Atom powered main board that does not need a heat sync, very stable. The bottom line is that getting NTP to sub uS is entering the realm of rocket science. You can do it but not by tweaking software parameters and reading NTPs own status print outs. That method is fine for mS but not for uS BTW there're some guys in a lab at work (not me) who are measuring femtoseconds. I'm impressed. -- Chris Albertson Redondo Beach, California ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On Fri, Dec 23, 2011 at 2:09 AM, Terje Mathisen terje.mathisen at tmsw.no@ntp.org wrote: The best (and probably only possible) solution that does give you single-digit us is to route a PPS signal to each and every server, then use the network for approximate (~100 us) timing, with the PPS doing the last two orders of magnitude. I think the above is true. Even then getting NTP to track the PPS at the uS level is not plug and play. But there is no hop of doing uS level over the gigabit either net. Distributinng PPS is no simple task either. I'm experiment here with that. Im using the the extra wire pairs in the install cat-5 wire that s already in the walls. Of course PPS can't go through a router so you have to physically connect a twisted pair from the source (In my case this is a GPS) to every computer. This is very hard to do in a big campus, impossible really. So PPS only works within a building and only over about a few hundred meters of cable. I'm using a simple RS232 driver chip from Max as the transmuter. These are driven by a simple hex inverter TTL chip. The Max driver converts the signal to -9 and +9 volts. There is delay in this. Some in the driver chips and some in the cable. Cable is roughly about nanosecond per foot. You need to measure and remove this and it's different for every cable run. But most people with more than a few computers have wire closets and plenty of unused cat-5 wire pairs. This is a very cheap and good way to distribute PPS in a building. If you have two buildings buy a GPS for each building. Chris Albertson Redondo Beach, California ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/22/2011 11:35 PM, unruh wrote: On 2011-12-23, Richard B. Gilbertrgilber...@comcast.net wrote: On 12/22/2011 2:11 PM, Paul Sobey wrote: Dear All, I work for a firm which requires clocks to be synchronised to quite a high degree of accuracy. We have an existing ntp-based infrastructure but want to improve on it to the point where the bulk of our hosts are synchronised to single digit microseconds of each other if possible. We have about 400 hosts in production, spread across about 15 sites. I hear from many vendors and industry colleagues that 'ntp just isn't suitable for high precision work and anything less than 1-2ms precision requires ptp or direct connection to gps clock'. I find these numbers somewhat suspect, and wanted to ask the advice of you experts. In particular I've read several threads on this list and other sites which suggest that highly accurate synchronisations are possible, assuming OS and network jitter can be minimised. Our internal testing to this point is that a stock ntpd pointed against a stratum 1 clock on a low contention gigabit ethernet (stratum 1 source and client less than 1ms apart) reports its own accuracy at approx 200 microseconds. Further tuning the ntp config by adding the minpoll 4, maxpoll 6 and burst keywords result in ntpd reported accuracy dropping to within 10-20 microseconds (as reported by ntpq -p and borne out by loopstats). Further improvements can be made running ntpd in the RT priority class. My questions to you all, if you've read through the above waffle are: - what is a sensible expected accuracy of ntpd if pointed at several stratum 1 time sources across a low jitter gigabit network (we'd probably spread them over several UK and US sites for resiliency but all paths are low jitter and highly deterministic latency) - are there any obvious tunables to improve accuracy other than minpoll/burst and process scheduling class, and how agressive can the polling cycles be sensible made? - can ntpd's own reported offset (ntpq -p or loopstats) be trusted (assuming high priority means it gets scheduled as desired)? I've quoted our apparent numbers at several people and the response is always 'pfft you can't trust ntpd to know its own offset' - but nobody can ever tell me why I appreciate these may appear to be silly questions with obvious answers - I am grateful in advance for your patience, and any research sources you may direct me to. Many thanks, Paul If you can possibly site a GPS antenna and receiver at your location, you can get microsecond accuracy or better. The receiver will output a tick each second. One edge of the tick signal will be within about 50 nanoseconds of the start of a second. The receivers cost anywhere from $100 and up. Some people need, or just want this level of accuracy. You do need to be able to site an antenna with a clear view of the sky. The last time I heard, there were twenty-seven GPS satellites in service. There are usually anywhere from three to five or six above the horizon at any given time. Given at least three satellites in line-of-sight your GPS receiver can figure out the latitude, longitude, and elevation of your antenna. Once it has done this it only needs to see a single GPS satellite to get the time. This kind of accuracy is far more than most people really need. It's there if you need it even if you only need it for bragging rights! Or timing how long it takes neutrinos to get from Cern to Grand Sasso. How do you tag a neutrino so that you can say with assurance that the the neutrino that left Cern is the same neutrino that arrives at Sasso? ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Hi Paul, This is probably going further up the track for you, however, with the HP c class blades you can use a PCI expansion blade with a serial IO card in it. The downside is that the expansion blade takes up one bay (to the left of a half-height blade, or bottom left slot next to a full height blade) If your enclosures are already quite full this maybe a problem for you? Another thought is the SUV Dongle that goes on the front, breaks out com1 to the DB9 on the Cable. However, loss of PPS due someone hijacking the cable for service reasons is probably quite high... I wouldn't recommend epoxying the cable in unless you are going to use the cheapest available blade and intend to chuck it if it fails. Another thought is to use the HP SL type blades, some of which have expansion slots natively. These are not so well known, however video rendering farms seem to love them.. I wish you luck on your exciting project, feel free to drop me a line if you have any HP blade or ISS technical related questions? Mark -Original Message- From: Paul Sobey [mailto:bud...@the-annexe.net] Sent: Saturday, 24 December 2011 1:47 AM To: questions@lists.ntp.org Subject: Re: [ntp:questions] Accuracy of NTP - Advice Needed Our problem will be that running coax around many sites to lots of machines, many of which don't have serial ports (think blades), is both highly time consuming and maintenance intensive. If we have to do it then we will but I'd like a clear idea as to the whys before I start down that particular path. In particular at this stage I'm trying to understand more about the theoretical accuracies obtainable under ideal conditions, and most important, how to independently verify the results of any tweaks we might apply. Say I have coalesence turned on a nic and I disable it - I'd like to be able to determine the effect, if any of that change. Is it possible for ntpd (or ptpd) to accurately determine its own accuracy, if that makes sense? If not what techniques might I use to independently measure? On a related note, I'm aware that there are various methods for querying the system time, some of which involve a context switch, and some of which can be done cheaply in userspace. I'm not sure whether the same is true for setting the time. Is anyone aware of how much ntpd operation involves context switches, which obviously would place quite a high ceiling on accuracy since we're at the mercy of the OS scheduler? Cheers, Paul ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Richard B. Gilbert wrote: How do you tag a neutrino so that you can say with assurance that the the neutrino that left Cern is the same neutrino that arrives at Sasso? Jim Pennino writes: By sending them in a pulse of a known width. It should be noted, however, that you cannot observe the same neutrino twice. In fact, no neutrinos at all are observed at the Cern end: just the protons that produce the pions that produce the neutrinos. An upcoming experiment at Fermilab will observe neutrinos at both ends (the far end will be in Minnesota). -- John Hasler jhas...@newsguy.com Dancing Horse Hill Elmwood, WI USA ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
John Hasler jhas...@newsguy.com wrote: Richard B. Gilbert wrote: How do you tag a neutrino so that you can say with assurance that the the neutrino that left Cern is the same neutrino that arrives at Sasso? Jim Pennino writes: By sending them in a pulse of a known width. It should be noted, however, that you cannot observe the same neutrino twice. In fact, no neutrinos at all are observed at the Cern end: just the protons that produce the pions that produce the neutrinos. An upcoming experiment at Fermilab will observe neutrinos at both ends (the far end will be in Minnesota). And your point would be? -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
At 01:58 PM 12/23/2011, Chris Albertson wrote... But there is no hop of doing uS level over the gigabit either net. IEEE 1588. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-23, John Hasler jhas...@newsguy.com wrote: An upcoming experiment at Fermilab will observe neutrinos at both ends (the far end will be in Minnesota). But not the same neutrino, since you can only detect the neutrino after it has collided with something else. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On Fri, Dec 23, 2011 at 5:51 PM, Mike S mi...@flatsurface.com wrote: At 01:58 PM 12/23/2011, Chris Albertson wrote... But there is no hop of doing uS level over the gigabit either net. IEEE 1588. Seem 1588 can work at the sub uS level but actual real-work software on Linux works at the tens of uS level. The limiting factor is the hardware's ability to time stamp eithernet packets. If you have special hardware you can get to sub-uS but not using PTPd on Linux or BSD. Linux's and BSD's time stamper for the PPS interface works better (less jitter) then its time stamper for Ethernet. In a standard Cat-5 cable there are 8 wires in 4 pair. Ethernet only uses 2 pair. I simply used a spare pair for a pulse. The parts required cost under $5 assuming you have tools to build cables already or old cables you can hack. -- Chris Albertson Redondo Beach, California ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-23, John Hasler jhas...@newsguy.com wrote: Richard B. Gilbert wrote: How do you tag a neutrino so that you can say with assurance that the the neutrino that left Cern is the same neutrino that arrives at Sasso? Jim Pennino writes: By sending them in a pulse of a known width. It should be noted, however, that you cannot observe the same neutrino twice. In fact, no neutrinos at all are observed at the Cern end: just the protons that produce the pions that produce the neutrinos. An upcoming experiment at Fermilab will observe neutrinos at both ends (the far end will be in Minnesota). Well, no. At best the electrons or muons at one end. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-23, Richard B. Gilbert rgilber...@comcast.net wrote: On 12/22/2011 11:35 PM, unruh wrote: On 2011-12-23, Richard B. Gilbertrgilber...@comcast.net wrote: On 12/22/2011 2:11 PM, Paul Sobey wrote: Dear All, I work for a firm which requires clocks to be synchronised to quite a high degree of accuracy. We have an existing ntp-based infrastructure but want to improve on it to the point where the bulk of our hosts are synchronised to single digit microseconds of each other if possible. We have about 400 hosts in production, spread across about 15 sites. I hear from many vendors and industry colleagues that 'ntp just isn't suitable for high precision work and anything less than 1-2ms precision requires ptp or direct connection to gps clock'. I find these numbers somewhat suspect, and wanted to ask the advice of you experts. In particular I've read several threads on this list and other sites which suggest that highly accurate synchronisations are possible, assuming OS and network jitter can be minimised. Our internal testing to this point is that a stock ntpd pointed against a stratum 1 clock on a low contention gigabit ethernet (stratum 1 source and client less than 1ms apart) reports its own accuracy at approx 200 microseconds. Further tuning the ntp config by adding the minpoll 4, maxpoll 6 and burst keywords result in ntpd reported accuracy dropping to within 10-20 microseconds (as reported by ntpq -p and borne out by loopstats). Further improvements can be made running ntpd in the RT priority class. My questions to you all, if you've read through the above waffle are: - what is a sensible expected accuracy of ntpd if pointed at several stratum 1 time sources across a low jitter gigabit network (we'd probably spread them over several UK and US sites for resiliency but all paths are low jitter and highly deterministic latency) - are there any obvious tunables to improve accuracy other than minpoll/burst and process scheduling class, and how agressive can the polling cycles be sensible made? - can ntpd's own reported offset (ntpq -p or loopstats) be trusted (assuming high priority means it gets scheduled as desired)? I've quoted our apparent numbers at several people and the response is always 'pfft you can't trust ntpd to know its own offset' - but nobody can ever tell me why I appreciate these may appear to be silly questions with obvious answers - I am grateful in advance for your patience, and any research sources you may direct me to. Many thanks, Paul If you can possibly site a GPS antenna and receiver at your location, you can get microsecond accuracy or better. The receiver will output a tick each second. One edge of the tick signal will be within about 50 nanoseconds of the start of a second. The receivers cost anywhere from $100 and up. Some people need, or just want this level of accuracy. You do need to be able to site an antenna with a clear view of the sky. The last time I heard, there were twenty-seven GPS satellites in service. There are usually anywhere from three to five or six above the horizon at any given time. Given at least three satellites in line-of-sight your GPS receiver can figure out the latitude, longitude, and elevation of your antenna. Once it has done this it only needs to see a single GPS satellite to get the time. This kind of accuracy is far more than most people really need. It's there if you need it even if you only need it for bragging rights! Or timing how long it takes neutrinos to get from Cern to Grand Sasso. How do you tag a neutrino so that you can say with assurance that the the neutrino that left Cern is the same neutrino that arrives at Sasso? The neutrinos are created by collisions of protons which produce pions which decal amonst other things to neutrinos. The protons come in bunches, so knowing when the protons raced by (at 99.9%c) the neutrinos are created in the same line and thus you know what the timing is between when the proton went by and the neutrino was absorbed. So you tag it by time. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 23 Dec, 2011, at 22:47 , Paul Sobey wrote: I appreciate these may appear to be silly questions with obvious answers - I am grateful in advance for your patience, and any research sources you may direct me to. The best (and probably only possible) solution that does give you single-digit us is to route a PPS signal to each and every server, then use the network for approximate (~100 us) timing, with the PPS doing the last two orders of magnitude. Our problem will be that running coax around many sites to lots of machines, many of which don't have serial ports (think blades), is both highly time consuming and maintenance intensive. If we have to do it then we will but I'd like a clear idea as to the whys before I start down that particular path. In particular at this stage I'm trying to understand more about the theoretical accuracies obtainable under ideal conditions, and most important, how to independently verify the results of any tweaks we might apply. Say I have coalesence turned on a nic and I disable it - I'd like to be able to determine the effect, if any of that change. Is it possible for ntpd (or ptpd) to accurately determine its own accuracy, if that makes sense? If not what techniques might I use to independently measure? If you really want to do this, with either NTP (the protocol, maybe not ntpd the implementation) or PTP, then I think the place you need to start is, unfortunately, with your operating system kernels. I have a board which implements a clock which can be synchronized to the 10 MHz and 1 PPS outputs from a GPS receiver. The board's clock resolution is about 3 ns (i.e. a 320 MHz internal clock) and the PIO interface to the board is designed so that it should be possible to transfer time from the board clock to the computer's clock with no more than +/- 10 ns or so of ambiguity (call it +/- 20 ns to be safe). When I first used this with a stock NetBSD kernel (whose clock code I think was copied from FreeBSD at some point) I was a little bit surprised to find that, despite the low tens of nanoseconds of accuracy the hardware was capable of, sampling the card against system timestamps gave me a result which jittered by on the order of several microseconds. After looking at why this was, I found that the jitter was in fact coming from the system clock itself and was caused by the way clock adjustments are applied at clock interrupt time (I believe some of the complaints about interrupt latency of the serial PPS driver are in fact seeing this system clock jitter and blaming it on something else; my very brief measurement of that driver found that while the fixed interrupt latency is 100's of nanoseconds it is also relatively constant, with outliers which are fairly easy to filter). Needless to say, if you can't get your system clock stable to better than microseconds you are unlikely to be able to synchronize it to a network source at that level. I fixed this by replacing the clock code, instead computing the time as a linear function of the value of the underlying counter, and getting rid of the clock interrupt discrete adjustments altogether (except when the NTP adjustment interface is in use, though that's a whole other story), so now my system clock doesn't jitter. The second operating system issue that's useful to address, whether the data is coming from NTP or PTP, is the clock adjustment system call interface. In particular, there are huge advantages to be gained by having a system call interface which allows you to make both clock frequency (i.e. rate of clock advance) and time offset adjustments, and which makes the adjustments you tell it to with great precision (or at least, tells you precisely what it did). The reason this is advantageous would require a long explanation, but the summary is that it allows you to treat the clock control process as solely a measurement process, rather than a feedback control process, and this makes it possible to begin to look at a broader variety of filtering procedures for incoming data to try to maximize the signal while minimizing the noise, without the additional burden of having to consider the stability (in the control system sense) of the adjustment process. The adjustments can be done open-loop. I believe that the operating system work described above, plus maybe some work on your ethernet card drivers, is necessary to achieve what you want with either with NTP or PTP. With my own implementation of the NTP daemon I can generally keep a client machine within 10 us of a server (measured with one of cards mentioned above in each machine) separated by (I think) 4 gigabit ethernet switches, I think with one 10 Gbps circuit in there, carrying company network traffic, with a 16 second polling interval. Note that I haven't tested this with ntpd yet, mostly because I don't like the way I had to jam support for the NTP system call interface into an otherwise very clean
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-24, Chris Albertson albertson.ch...@gmail.com wrote: On Fri, Dec 23, 2011 at 5:51 PM, Mike S mi...@flatsurface.com wrote: At 01:58 PM 12/23/2011, Chris Albertson wrote... But there is no hop of doing uS level over the gigabit either net. IEEE 1588. Seem 1588 can work at the sub uS level but actual real-work software on Linux works at the tens of uS level. The limiting factor is the hardware's ability to time stamp eithernet packets. If you have special hardware you can get to sub-uS but not using PTPd on Linux or BSD. If you really go to stamping the interrupt directly as it comes in on the kernel level, rather than waiting for a driver (eg the serial driver) to report that the itnerrupt has occured to userland, you can get it down to 1-2us. Linux's and BSD's time stamper for the PPS interface works better (less jitter) then its time stamper for Ethernet. In a standard Cat-5 cable there are 8 wires in 4 pair. Ethernet only uses 2 pair. I simply used a spare pair for a pulse. The parts required cost under $5 assuming you have tools to build cables already or old cables you can hack. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On Fri, Dec 23, 2011 at 10:06 PM, unruh un...@invalid.ca wrote: If you really go to stamping the interrupt directly as it comes in on the kernel level, rather than waiting for a driver (eg the serial driver) to report that the itnerrupt has occured to userland, you can get it down to 1-2us. That is exactly how the Linux PPS system works. The kernel level driver is all of about a dozen lines of code. It is very simple. When the interrupt happens the driver copies the hardware nanosecond clock to some location in RAM and sets a flag that says data available A user land program waits for the flag. When it sees the flag set it can read the counter value that was saved by the driver and then the flag is re-set. It is unimportance how long it take the program to notice there is data and read it, so long as it does so before the next pulse. Linux does not queue the data. The next pulse over writes the time stamp. As you say 1 or 2 uS is about what it gets. On my system the nanosecond clock jumps in 1000 nS steps But notice this is not just a pulse per second interface. It is a pulse interface and can time stamp random pulses too. I know it works for 60Hz pulses. The interpretation of what to do with the data is a user land thing. NTP uses it to sync the clock. Other people use it for general-purpose pulse timing. Chris Albertson Redondo Beach, California ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/22/2011 2:11 PM, Paul Sobey wrote: Dear All, I work for a firm which requires clocks to be synchronised to quite a high degree of accuracy. We have an existing ntp-based infrastructure but want to improve on it to the point where the bulk of our hosts are synchronised to single digit microseconds of each other if possible. We have about 400 hosts in production, spread across about 15 sites. I hear from many vendors and industry colleagues that 'ntp just isn't suitable for high precision work and anything less than 1-2ms precision requires ptp or direct connection to gps clock'. I find these numbers somewhat suspect, and wanted to ask the advice of you experts. In particular I've read several threads on this list and other sites which suggest that highly accurate synchronisations are possible, assuming OS and network jitter can be minimised. Our internal testing to this point is that a stock ntpd pointed against a stratum 1 clock on a low contention gigabit ethernet (stratum 1 source and client less than 1ms apart) reports its own accuracy at approx 200 microseconds. Further tuning the ntp config by adding the minpoll 4, maxpoll 6 and burst keywords result in ntpd reported accuracy dropping to within 10-20 microseconds (as reported by ntpq -p and borne out by loopstats). Further improvements can be made running ntpd in the RT priority class. My questions to you all, if you've read through the above waffle are: - what is a sensible expected accuracy of ntpd if pointed at several stratum 1 time sources across a low jitter gigabit network (we'd probably spread them over several UK and US sites for resiliency but all paths are low jitter and highly deterministic latency) - are there any obvious tunables to improve accuracy other than minpoll/burst and process scheduling class, and how agressive can the polling cycles be sensible made? - can ntpd's own reported offset (ntpq -p or loopstats) be trusted (assuming high priority means it gets scheduled as desired)? I've quoted our apparent numbers at several people and the response is always 'pfft you can't trust ntpd to know its own offset' - but nobody can ever tell me why I appreciate these may appear to be silly questions with obvious answers - I am grateful in advance for your patience, and any research sources you may direct me to. Many thanks, Paul If you can possibly site a GPS antenna and receiver at your location, you can get microsecond accuracy or better. The receiver will output a tick each second. One edge of the tick signal will be within about 50 nanoseconds of the start of a second. The receivers cost anywhere from $100 and up. Some people need, or just want this level of accuracy. You do need to be able to site an antenna with a clear view of the sky. The last time I heard, there were twenty-seven GPS satellites in service. There are usually anywhere from three to five or six above the horizon at any given time. Given at least three satellites in line-of-sight your GPS receiver can figure out the latitude, longitude, and elevation of your antenna. Once it has done this it only needs to see a single GPS satellite to get the time. This kind of accuracy is far more than most people really need. It's there if you need it even if you only need it for bragging rights! ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Once upon a time, Richard B. Gilbert rgilber...@comcast.net said: The last time I heard, there were twenty-seven GPS satellites in service. There are currently 31 active. There are usually anywhere from three to five or six above the horizon at any given time. It can be up to 8 or 9. This kind of accuracy is far more than most people really need. It's there if you need it even if you only need it for bragging rights! The securities traders (especially HFT) want it. I suspect the OP is in that group. That level of timekeeping has been discussed here before. -- Chris Adams cmad...@hiwaay.net Systems and Network Administrator - HiWAAY Internet Services I don't speak for anybody but myself - that's enough trouble. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/22/2011 9:17 PM, Chris Adams wrote: Once upon a time, Richard B. Gilbertrgilber...@comcast.net said: The last time I heard, there were twenty-seven GPS satellites in service. There are currently 31 active. There are usually anywhere from three to five or six above the horizon at any given time. It can be up to 8 or 9. This kind of accuracy is far more than most people really need. It's there if you need it even if you only need it for bragging rights! The securities traders (especially HFT) want it. I suspect the OP is in that group. That level of timekeeping has been discussed here before. I think that the radio astronomers are some of the most demanding. Joe Average just needs to get to the bus stop or railway station in time to catch his chosen mode of transportation. The securities traders generally need to time-stamp their transactions within two seconds. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On Thu, Dec 22, 2011 at 19:11, Paul Sobey bud...@the-annexe.net wrote: - can ntpd's own reported offset (ntpq -p or loopstats) be trusted (assuming high priority means it gets scheduled as desired)? I've quoted our apparent numbers at several people and the response is always 'pfft you can't trust ntpd to know its own offset' - but nobody can ever tell me why This is an interesting issue. Consider the case where ntpd has just started and is reporting 50 msec offset from its source(s). Using ntpd 4.2.7, within the first five (with drift file) or ten (without) minutes that offset will be reduced to less than a half millisecond, thanks to the new for 4.2.7 initial offset slew, which also avoids perturbing the frequency correction during that initial slew. During such startup, it is reasonable to assume the best estimate of UTC available on that system is the system clock plus the reported offset from ntpq. However, once ntpd has settled down, the best estimate of UTC will be the system clock alone, ignoring any offset reported by ntpq. To help understand why, recognize why ntpd has a damped response rather than working furiously to eliminate any apparent offset as fast as possible: doing so would be respecting noise that gets drowned out by signal through the slower feedback loop. ntpq -c rv 0 rootdisp gives you the estimate of maximum error between the local clock and the ultimate source of time (GPS or other refclock). You won't find small-microseconds numbers there. While the average and instant error is likely to be much less, if you're careful in talking about ntpd accuracy, you'll want to minimize root dispersion. A good way to measure ntpd performance is to use a local PPS marked noselect as well as a nearby NTP server with PPS. With peerstats enabled, you can consider the PPS offsets logged by the noselect refclock a good measurement of the offset of the local clock. Cheers, Dave Hart ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 2011-12-22, Paul Sobey bud...@the-annexe.net wrote: Dear All, I work for a firm which requires clocks to be synchronised to quite a high degree of accuracy. What does quite a high degree of accuracy mean? Nearst hour, minute, second, millisecond, microsecond, nanosecond, picosecond,...? Those could all fall under that description. We have an existing ntp-based infrastructure but want to improve on it to the point where the bulk of our hosts are synchronised to single digit microseconds of each other if possible. We have about 400 hosts in production, spread across about 15 sites. Not possible. Even if you give each a gps it is going to be hard (both temp effects and time it takes for the signal to be timestamped by the system are on the usec level.) And over the net it is absolutely impossible. I hear from many vendors and industry colleagues that 'ntp just isn't suitable for high precision work and anything less than 1-2ms precision requires ptp or direct connection to gps clock'. I find these numbers Well, no. direct network can give you 10s of usec accuracy. But not gigabit networks or switches. That is more like 100s of usec. somewhat suspect, and wanted to ask the advice of you experts. In particular I've read several threads on this list and other sites which suggest that highly accurate synchronisations are possible, assuming OS and network jitter can be minimised. Our internal testing to this point is that a stock ntpd pointed against a stratum 1 clock on a low contention gigabit ethernet (stratum 1 source and client less than 1ms apart) reports its own accuracy at approx 200 microseconds. Further tuning the ntp config by adding the minpoll 4, maxpoll 6 and burst keywords result in ntpd reported accuracy dropping to within 10-20 microseconds (as reported by ntpq -p and borne out by loopstats). Further improvements can be made running ntpd in the RT priority class. No Network delays will dominate. My questions to you all, if you've read through the above waffle are: - what is a sensible expected accuracy of ntpd if pointed at several stratum 1 time sources across a low jitter gigabit network (we'd probably spread them over several UK and US sites for resiliency but all paths are low jitter and highly deterministic latency) ?? If the servers are off site, you will not get that. And gigabit systems have variable latencies due to interrupt consilidation, etc. - are there any obvious tunables to improve accuracy other than minpoll/burst and process scheduling class, and how agressive can the polling cycles be sensible made? Attach a gps to each machine. On a local network you can make them as agressive as you like. It is your network. - can ntpd's own reported offset (ntpq -p or loopstats) be trusted (assuming high priority means it gets scheduled as desired)? I've quoted our apparent numbers at several people and the response is always 'pfft you can't trust ntpd to know its own offset' - but nobody can ever tell me why Because if it did it could discipline the clock better. Attach a gps to the system, write a direct interrupt driven timestamping routine and test it. (That was what I did) I appreciate these may appear to be silly questions with obvious answers - I am grateful in advance for your patience, and any research sources you may direct me to. Many thanks, Paul ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On Friday 23 December 2011 03:25:18 Dave Hart wrote: On Thu, Dec 22, 2011 at 19:11, Paul Sobey bud...@the-annexe.net wrote: - can ntpd's own reported offset (ntpq -p or loopstats) be trusted (assuming high priority means it gets scheduled as desired)? I've quoted our apparent numbers at several people and the response is always 'pfft you can't trust ntpd to know its own offset' - but nobody can ever tell me why This is an interesting issue. Consider the case where ntpd has just started and is reporting 50 msec offset from its source(s). Using ntpd 4.2.7, within the first five (with drift file) or ten (without) minutes that offset will be reduced to less than a half millisecond, thanks to the new for 4.2.7 initial offset slew, which also avoids perturbing the frequency correction during that initial slew. During such startup, it is reasonable to assume the best estimate of UTC available on that system is the system clock plus the reported offset from ntpq. However, once ntpd has settled down, the best estimate of UTC will be the system clock alone, ignoring any offset reported by ntpq. To help understand why, recognize why ntpd has a damped response rather than working furiously to eliminate any apparent offset as fast as possible: doing so would be respecting noise that gets drowned out by signal through the slower feedback loop. ntpq -c rv 0 rootdisp gives you the estimate of maximum error between the local clock and the ultimate source of time (GPS or other refclock). You won't find small-microseconds numbers there. While the average and instant error is likely to be much less, if you're careful in talking about ntpd accuracy, you'll want to minimize root dispersion. A good way to measure ntpd performance is to use a local PPS marked noselect as well as a nearby NTP server with PPS. With peerstats enabled, you can consider the PPS offsets logged by the noselect refclock a good measurement of the offset of the local clock. This is great - appreciate you taking the time to write it. Paul ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
