Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/27/2011 11:45 PM, Greg Hennessy wrote: > On 2011-12-28, Danny Mayer 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 wrote: >> On 12/27/2011 8:48 PM, j...@specsol.spam.sux.com wrote: >>> Danny Mayer wrote: On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: > John Hasler 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 wrote: >> On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: >>> John Hasler 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
On 2011-12-28, 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. Your comment is false. It is not try that GPS is used *only* to get a location fix, GPS is also used to synchronise the atomic clocks. As you have admitted in email, you have not read the preprint. I have read the preprint. You are now trying to tell me that I am wrong when I speak to something mentioned in the preprint. I've not seen a more perfect example of Dunning-Kruger in some time. ___ 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 12/27/2011 11:45 PM, Greg Hennessy wrote: >> On 2011-12-28, Danny Mayer 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 wrote: > On 12/28/2011 12:09 AM, j...@specsol.spam.sux.com wrote: >> Danny Mayer wrote: >>> On 12/27/2011 8:48 PM, j...@specsol.spam.sux.com wrote: Danny Mayer wrote: > On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: >> John Hasler 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 wrote: > On 12/28/2011 12:17 AM, unruh wrote: >> On 2011-12-28, Danny Mayer wrote: >>> On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler 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] Windows and Wi-Fi - starts well, frequency steps?
In article , Dave Hart wrote: >On Wed, Dec 28, 2011 at 00:51, Danny Mayer wrote: >> No you don't want to do DNS over TCP if you can avoid it. It would be a >> major hit on the resolver servers and with the kind of high volume that >> you get as mobile devices make increasing use of such networks. You want >> WiFi to drop UDP packets if they are lost rather than attempting to >> retransmit them. > >I do indeed, but UDP is treated as guaranteed by WiFi, and I expect >the reason is DNS over UDP otherwise becomes a user experience killer >due to extra seconds of wait for each loss. Is this defined in an RFC or some other standards document? -- -- Rod -- rodd(at)polylogics(dot)com ___ 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 wrote: > On 12/27/2011 11:45 PM, Greg Hennessy wrote: >> On 2011-12-28, Danny Mayer 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 wrote: > On 12/28/2011 12:17 AM, unruh wrote: >> On 2011-12-28, Danny Mayer wrote: >>> On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: John Hasler 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 wrote: > On 12/28/2011 12:09 AM, j...@specsol.spam.sux.com wrote: >> Danny Mayer wrote: >>> On 12/27/2011 8:48 PM, j...@specsol.spam.sux.com wrote: Danny Mayer wrote: > On 12/24/2011 8:10 PM, j...@specsol.spam.sux.com wrote: >> John Hasler 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
[ntp:questions] Off topic: using delay in routing protocols
Hi, Sorry for the offtopic post, but I really don't see another place to ask this question. I hear that the Fuzzball routing protocol used packet delay as a routing metric. Does anyone recall if that's right? Was it the RTT, or was it attempting to perform an estimate of one-way delay? More generally, I'll be grateful for any pointers to papers on the subject of using delay in routing protocols. Thanks for your help, -- Juliusz Chroboczek ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Windows and Wi-Fi - starts well, frequency steps?
On Wed, Dec 28, 2011 at 18:58, Rod Dorman wrote: > Is this defined in an RFC or some other standards document? http://standards.ieee.org/about/get/802/802.11.html Cheers, Dave Hart ___ 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 -- - "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] Windows and Wi-Fi - starts well, frequency steps?
Danny Mayer writes:
>No you don't want to do DNS over TCP if you can avoid it. It would be a
>major hit on the resolver servers and with the kind of high volume that
>you get as mobile devices make increasing use of such networks. You want
>WiFi to drop UDP packets if they are lost rather than attempting to
>retransmit them.
\begin{ramble}
All unicast MAC layer traffic is retransmitted on WiFi if not
recieved correctly, probably for the same reason that Ethernet also
retransmits packets that collide. If NTP can handle 10base2 Ethernet,
then it should be able to handle WiFi.
The maximum number of retries depends somewhat on the hardware
(usually between 7 and 11 tries), and is subject to backoff, in a
similar way to traditional Ethernet. Determining if a packet was
successful depends on on a MAC-layer ACK, because you can't easily
do collision detection in the same way as Ethernet. The reason that
only unicast packets are retransmitted is because it's tricky to
figure out who sends the ACK if it is multicast or broadcast. I
suspect that if the 802.11 guys could figure it out, multi-/broadcast
traffic would also be retransmitted as needed.
Modern hardware that supports 802.11e (or 802.11n, which requires
much of the QoS part of 11e) can control things like the number of
retries, and you could hack the driver to inspect the packets and
if it is NTP to reduce the number of retires. An alternative would
be to use NTP's broadcast mode on a LAN, which would eliminate
retries. However, I suspect that bufferbloat and asymetric delays
on DSL is probably a much bigger problem for NTP than 802.11 retries.
\end{ramble}
David.
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Re: [ntp:questions] Off topic: using delay in routing protocols
> Sorry for the offtopic post, And sorry for bothering everyone uselessly -- all of my questions are answered in RFC 891. -- Juliusz ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Windows and Wi-Fi - starts well, frequency steps?
On Wed, Dec 28, 2011 at 21:54, David Malone wrote: > Modern hardware that supports 802.11e (or 802.11n, which requires > much of the QoS part of 11e) can control things like the number of > retries, and you could hack the driver to inspect the packets and > if it is NTP to reduce the number of retires. I recognize I'm suggesting a layer violation in wishing 802.11 devices treated UDP differently from TCP, or even worse in terms of layer violation, UDP 123 differently from UDP 53. It's not pretty, but it would make a positive difference. The ideal number of retries for NTP may be zero, assuming the radio layer loss rates are less than 87.5% in practice. > An alternative would > be to use NTP's broadcast mode on a LAN, which would eliminate > retries. Right. > However, I suspect that bufferbloat and asymetric delays > on DSL is probably a much bigger problem for NTP than 802.11 retries. Absolutely. Nearly every consumer broadband connection has bufferbloat issues. Local loop asymmetry is extreme with ADSL. And most NTP clients in the wild are using NTP servers reached over an asymmetric WAN connection. Reverse and forward delay will frequently be slightly different, introducing apparent offset error. Cheers, Dave Hart ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
