Re: [time-nuts] What position is measured?
On Wed, Sep 8, 2010 at 02:16, jimlux jim...@earthlink.net wrote: Mark J. Blair wrote: On Sep 7, 2010, at 6:30 AM, jimlux wrote: Yes.. except that the cable's physical and electrical length *do* vary with temperature, so if you're looking at the gnat's eyelash sort of thing, you need to take that into account. Maybe 10 ppm/degree, so a 20 meter run will change a bit less than a millimeter. That's down in the fractional picoseconds time-wise. It's an issue if you're doing things like interferometry at higher frequencies.. Would be possible for the receiver to take into account automatically the delay of the antenna cable, by measuring the delay of an echo of a signal it sends towards the antenna? Do such receivers exists? ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Hi Setups like that do exist and are fairly common. I have never seen the technique integrated into a GPS receiver. It's normally done with a secondary setup. My observation is that something like 99% of the GPSDO's out there never get their antenna delay set to the proper number. It's commonly done for surveying work, but not so much for timing. The cell phone guys can get away without that fine an adjustment, so they ignore it. Bob -- From: Pierpaolo Bernardi olopie...@gmail.com Sent: Wednesday, September 08, 2010 6:35 AM To: Discussion of precise time and frequency measurement time-nuts@febo.com Subject: Re: [time-nuts] What position is measured? On Wed, Sep 8, 2010 at 02:16, jimlux jim...@earthlink.net wrote: Mark J. Blair wrote: On Sep 7, 2010, at 6:30 AM, jimlux wrote: Yes.. except that the cable's physical and electrical length *do* vary with temperature, so if you're looking at the gnat's eyelash sort of thing, you need to take that into account. Maybe 10 ppm/degree, so a 20 meter run will change a bit less than a millimeter. That's down in the fractional picoseconds time-wise. It's an issue if you're doing things like interferometry at higher frequencies.. Would be possible for the receiver to take into account automatically the delay of the antenna cable, by measuring the delay of an echo of a signal it sends towards the antenna? Do such receivers exists? ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
It's usually a manual setting of antenna delay on receivers I've used, and based on assumed delay in the particular cable connectors. You can tweak things closer if you have a good 1PPS to compare with. Rob K -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Pierpaolo Bernardi Sent: 08 September 2010 11:36 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] What position is measured? On Wed, Sep 8, 2010 at 02:16, jimlux jim...@earthlink.net wrote: Mark J. Blair wrote: On Sep 7, 2010, at 6:30 AM, jimlux wrote: Yes.. except that the cable's physical and electrical length *do* vary with temperature, so if you're looking at the gnat's eyelash sort of thing, you need to take that into account. Maybe 10 ppm/degree, so a 20 meter run will change a bit less than a millimeter. That's down in the fractional picoseconds time-wise. It's an issue if you're doing things like interferometry at higher frequencies.. Would be possible for the receiver to take into account automatically the delay of the antenna cable, by measuring the delay of an echo of a signal it sends towards the antenna? Do such receivers exists? ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Tom Holmes wrote: One other delay contributor would appear to be processing delay in the receiver, which thus begs the question of how the PPS signal is actually synchronized to the GPS system. The GPS nav messages is synchronized to the seconds, so it's a matter of making sure the output pulse is synced to the appropriate time in the GPS signal. The delay in the receiver is (reasonably) constant, so the mfr essentially calibrates it out. It's not done precisely like this, but conceptually, you have a 1pps on the spacecraft driven by a Cs clock, you receive the signal in your receiver (some time later than the actual change of second) and subtract out the light time delay from satellite to you. (or, more accurately, delay the signal from the receiver to the next second). It's controlling for that light time delay that's the tricky part, since it varies depending on the degree of ionization of the ionosphere. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Pierpaolo Bernardi wrote: On Wed, Sep 8, 2010 at 02:16, jimlux jim...@earthlink.net wrote: Mark J. Blair wrote: On Sep 7, 2010, at 6:30 AM, jimlux wrote: Yes.. except that the cable's physical and electrical length *do* vary with temperature, so if you're looking at the gnat's eyelash sort of thing, you need to take that into account. Maybe 10 ppm/degree, so a 20 meter run will change a bit less than a millimeter. That's down in the fractional picoseconds time-wise. It's an issue if you're doing things like interferometry at higher frequencies.. Would be possible for the receiver to take into account automatically the delay of the antenna cable, by measuring the delay of an echo of a signal it sends towards the antenna? Do such receivers exists? Not for GPS, to my knowledge, but in other time distribution systems, certainly. It's also used in antenna ranges when you need phase information (as in a near field range). It's also been done over the air in radio telescope arrays (e.g. VLA). At JPL, we navigate spacecraft in deep space by very accurately measuring the time delay of a round trip to the spacecraft from earth and back. These days, position uncertainties are in the cm range and velocity in the mm/s, implying measurements of picoseconds in a round trip time of 10,000 seconds. All of this implies that the entire measurement chain (including the cables carrying the maser reference signal) are carefully characterized and controlled. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Thanks, Jim. I assume that neither the satellite nor the receiver knows what the variation in the light time delay is, so it must be small enough to allow the claimed nanosecond accuracy of the PPS edge. Although one sat is sufficient for time work, would using more improve the PPS accuracy? Seems like having more inputs would help with the light delay and other corrections, but it probably is no different than having multiple Rb's in the lab (the guy with two is never quite sure and all that). Mostly just curious, as my Z3801 is quite good enough for my needs. Tom Holmes, N8ZM Tipp City, OH EM79 -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of jimlux Sent: Wednesday, September 08, 2010 9:42 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] What position is measured? Tom Holmes wrote: One other delay contributor would appear to be processing delay in the receiver, which thus begs the question of how the PPS signal is actually synchronized to the GPS system. The GPS nav messages is synchronized to the seconds, so it's a matter of making sure the output pulse is synced to the appropriate time in the GPS signal. The delay in the receiver is (reasonably) constant, so the mfr essentially calibrates it out. It's not done precisely like this, but conceptually, you have a 1pps on the spacecraft driven by a Cs clock, you receive the signal in your receiver (some time later than the actual change of second) and subtract out the light time delay from satellite to you. (or, more accurately, delay the signal from the receiver to the next second). It's controlling for that light time delay that's the tricky part, since it varies depending on the degree of ionization of the ionosphere. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Tom Holmes wrote: Thanks, Jim. I assume that neither the satellite nor the receiver knows what the variation in the light time delay is, so it must be small enough to allow the claimed nanosecond accuracy of the PPS edge. Well.. that's the difference between a L1 only and a L1/L2 receiver. If you measure the same signal at two different frequencies, you can use that to estimate the total electron content (TEC) of the path, which in turn can be turned into a delay correction. The uncertainties are on the order of meters/few ns, so keeping the 1pps to within 10ns is doable with a L1 receiver. Although one sat is sufficient for time work, would using more improve the PPS accuracy? Seems like having more inputs would help with the light delay and other corrections, but it probably is no different than having multiple Rb's in the lab (the guy with two is never quite sure and all that). One sat works *if* you know where it and you are. In practice, though, you look at multiple satellites and solve for position and time offset simultaneously. The secret sauce in GPS receivers that distinguishes one from another is: 1) acquisition (how long does it take to find the signal and start tracking) 2) how do you best form the estimate of position and clock offset. Typically it's done with some form of Extended Kalman Filter (EKF) so you also wind up with estimates of the covariance matrix. Whether or not that gets shoved out to the user is another matter. the timing receivers separate the where am I and the what time is it questions.. you do a survey mode to get a precise position, then lock that down, and go to timing only mode, essentially averaging the time info from multiple satellites (not true averaging, almost always a weighted average) ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Hi Actually they do know a bit about the light delay. They include that data in the information the stat's broadcast. The data is fairly coarse grained. I posted some links a week or so back that go into all the grubby details. Bob -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Tom Holmes Sent: Wednesday, September 08, 2010 10:19 AM To: 'Discussion of precise time and frequency measurement' Subject: Re: [time-nuts] What position is measured? Thanks, Jim. I assume that neither the satellite nor the receiver knows what the variation in the light time delay is, so it must be small enough to allow the claimed nanosecond accuracy of the PPS edge. Although one sat is sufficient for time work, would using more improve the PPS accuracy? Seems like having more inputs would help with the light delay and other corrections, but it probably is no different than having multiple Rb's in the lab (the guy with two is never quite sure and all that). Mostly just curious, as my Z3801 is quite good enough for my needs. Tom Holmes, N8ZM Tipp City, OH EM79 -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of jimlux Sent: Wednesday, September 08, 2010 9:42 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] What position is measured? Tom Holmes wrote: One other delay contributor would appear to be processing delay in the receiver, which thus begs the question of how the PPS signal is actually synchronized to the GPS system. The GPS nav messages is synchronized to the seconds, so it's a matter of making sure the output pulse is synced to the appropriate time in the GPS signal. The delay in the receiver is (reasonably) constant, so the mfr essentially calibrates it out. It's not done precisely like this, but conceptually, you have a 1pps on the spacecraft driven by a Cs clock, you receive the signal in your receiver (some time later than the actual change of second) and subtract out the light time delay from satellite to you. (or, more accurately, delay the signal from the receiver to the next second). It's controlling for that light time delay that's the tricky part, since it varies depending on the degree of ionization of the ionosphere. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
On 09/08/2010 04:51 PM, jimlux wrote: Tom Holmes wrote: Thanks, Jim. I assume that neither the satellite nor the receiver knows what the variation in the light time delay is, so it must be small enough to allow the claimed nanosecond accuracy of the PPS edge. Well.. that's the difference between a L1 only and a L1/L2 receiver. If you measure the same signal at two different frequencies, you can use that to estimate the total electron content (TEC) of the path, which in turn can be turned into a delay correction. The uncertainties are on the order of meters/few ns, so keeping the 1pps to within 10ns is doable with a L1 receiver. Although one sat is sufficient for time work, would using more improve the PPS accuracy? Seems like having more inputs would help with the light delay and other corrections, but it probably is no different than having multiple Rb's in the lab (the guy with two is never quite sure and all that). One sat works *if* you know where it and you are. In practice, though, you look at multiple satellites and solve for position and time offset simultaneously. The secret sauce in GPS receivers that distinguishes one from another is: 1) acquisition (how long does it take to find the signal and start tracking) 2) how do you best form the estimate of position and clock offset. Typically it's done with some form of Extended Kalman Filter (EKF) so you also wind up with estimates of the covariance matrix. Whether or not that gets shoved out to the user is another matter. the timing receivers separate the where am I and the what time is it questions.. you do a survey mode to get a precise position, then lock that down, and go to timing only mode, essentially averaging the time info from multiple satellites (not true averaging, almost always a weighted average) Missing is the RAIM and in timing context the T-RAIM. RAIM helps to drop false-tickers from the solution and this process is done dynamically for every solution. It is fairly straightforward. The remaining sources forms an average after the RAIM. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
On 09/08/2010 05:34 PM, Bob Camp wrote: Hi Actually they do know a bit about the light delay. They include that data in the information the stat's broadcast. The data is fairly coarse grained. I posted some links a week or so back that go into all the grubby details. Coarse grain is certainly a good way of saying it. The ionosphere corrections are based on a simplification and then fitting data to the model curve. The way to improve on this is to use DGPS sources such as WAAS/EGNOS, which should be good enough for most timing purposes of the hobbyist. Estimating ionospheric delay using dual frequency beats that. Still leaves tropospheric delays thought. Reference network helps for that. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] What position is measured?
Hi nuts, have got some interesting question from friends. They asked What coordinates are measured and registrated by a GPS. Is it the position of the antenna, it could be 100m (300ft) away from me or is it the position of the GPS RX as this is the position of the µC where signals are processed? I answered, its allways the antenna position as there is the point where wave fronts coming in from the sats becomes electrical signals (note we are not talking about PPS but coordinates)... What do you think? Peter, DG4EK ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Yes, it is the position of the antenna. Once the signals enter the downlead, their relative positions in time are fixed. It is the relative positions in time that are used to determine position. John WA4WDL -- From: Peter Krengel krengelda...@gmx.de Sent: Tuesday, September 07, 2010 1:57 AM To: time-nuts@febo.com Subject: [time-nuts] What position is measured? Hi nuts, have got some interesting question from friends. They asked What coordinates are measured and registrated by a GPS. Is it the position of the antenna, it could be 100m (300ft) away from me or is it the position of the GPS RX as this is the position of the µC where signals are processed? I answered, its allways the antenna position as there is the point where wave fronts coming in from the sats becomes electrical signals (note we are not talking about PPS but coordinates)... What do you think? Peter, DG4EK ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Peter Krengel wrote: Hi nuts, have got some interesting question from friends. They asked What coordinates are measured and registrated by a GPS. Is it the position of the antenna, it could be 100m (300ft) away from me or is it the position of the GPS RX as this is the position of the µC where signals are processed? Where the antenna is.. I answered, its allways the antenna position as there is the point where wave fronts coming in from the sats becomes electrical signals (note we are not talking about PPS but coordinates)... Pretty good.. Another analogy is that if you had a machine that recorded all the signals, mounted right at the antenna, and then carried the recording half way around the world, and then ran the recording into a receiver, it would give you the position of the antenna, not the receiver. The cable is just a time delay. What do you think? Peter, DG4EK ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
On Sep 7, 2010, at 6:30 AM, jimlux wrote: Another analogy is that if you had a machine that recorded all the signals, mounted right at the antenna, and then carried the recording half way around the world, and then ran the recording into a receiver, it would give you the position of the antenna, not the receiver. The cable is just a time delay. Does this mean that while the antenna feedline cable length does not influence the measured position (at the phase center of the antenna), and it does not influence the accuracy of a disciplined frequency reference output, it does introduce an error into the absolute time output (i.e., adding a delay to the PPS output)? In other words, do I correctly assume that I may safely ignore the length of my TBolt's antenna feedline if I am only interested in its 10 MHz OCXO output, but I may want to compensate for it if I ever find a need to use its PPS output as an absolute time marker? -- Mark J. Blair, NF6X n...@nf6x.net Web page: http://www.nf6x.net/ GnuPG public key available from my web page. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Exactly, well put. John WA4WDL -- From: Mark J. Blair n...@nf6x.net Sent: Tuesday, September 07, 2010 12:24 PM To: Discussion of precise time and frequency measurement time-nuts@febo.com Subject: Re: [time-nuts] What position is measured? On Sep 7, 2010, at 6:30 AM, jimlux wrote: Another analogy is that if you had a machine that recorded all the signals, mounted right at the antenna, and then carried the recording half way around the world, and then ran the recording into a receiver, it would give you the position of the antenna, not the receiver. The cable is just a time delay. Does this mean that while the antenna feedline cable length does not influence the measured position (at the phase center of the antenna), and it does not influence the accuracy of a disciplined frequency reference output, it does introduce an error into the absolute time output (i.e., adding a delay to the PPS output)? In other words, do I correctly assume that I may safely ignore the length of my TBolt's antenna feedline if I am only interested in its 10 MHz OCXO output, but I may want to compensate for it if I ever find a need to use its PPS output as an absolute time marker? -- Mark J. Blair, NF6X n...@nf6x.net Web page: http://www.nf6x.net/ GnuPG public key available from my web page. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Hi If you are only interested in frequency, antenna feed line delay will not matter to you. Bob -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Mark J. Blair Sent: Tuesday, September 07, 2010 12:25 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] What position is measured? On Sep 7, 2010, at 6:30 AM, jimlux wrote: Another analogy is that if you had a machine that recorded all the signals, mounted right at the antenna, and then carried the recording half way around the world, and then ran the recording into a receiver, it would give you the position of the antenna, not the receiver. The cable is just a time delay. Does this mean that while the antenna feedline cable length does not influence the measured position (at the phase center of the antenna), and it does not influence the accuracy of a disciplined frequency reference output, it does introduce an error into the absolute time output (i.e., adding a delay to the PPS output)? In other words, do I correctly assume that I may safely ignore the length of my TBolt's antenna feedline if I am only interested in its 10 MHz OCXO output, but I may want to compensate for it if I ever find a need to use its PPS output as an absolute time marker? -- Mark J. Blair, NF6X n...@nf6x.net Web page: http://www.nf6x.net/ GnuPG public key available from my web page. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Hi Mark: Exactly. Have Fun, Brooke Clarke http://www.PRC68.com Mark J. Blair wrote: On Sep 7, 2010, at 6:30 AM, jimlux wrote: Another analogy is that if you had a machine that recorded all the signals, mounted right at the antenna, and then carried the recording half way around the world, and then ran the recording into a receiver, it would give you the position of the antenna, not the receiver. The cable is just a time delay. Does this mean that while the antenna feedline cable length does not influence the measured position (at the phase center of the antenna), and it does not influence the accuracy of a disciplined frequency reference output, it does introduce an error into the absolute time output (i.e., adding a delay to the PPS output)? In other words, do I correctly assume that I may safely ignore the length of my TBolt's antenna feedline if I am only interested in its 10 MHz OCXO output, but I may want to compensate for it if I ever find a need to use its PPS output as an absolute time marker? -- Have Fun, Brooke Clarke http://www.PRC68.com ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Yep :) Sent from my BlackBerry Wireless thingy while I do other things... -Original Message- From: Mark J. Blair n...@nf6x.net Sender: time-nuts-boun...@febo.com Date: Tue, 7 Sep 2010 09:24:53 To: Discussion of precise time and frequency measurementtime-nuts@febo.com Reply-To: Discussion of precise time and frequency measurement time-nuts@febo.com Subject: Re: [time-nuts] What position is measured? On Sep 7, 2010, at 6:30 AM, jimlux wrote: Another analogy is that if you had a machine that recorded all the signals, mounted right at the antenna, and then carried the recording half way around the world, and then ran the recording into a receiver, it would give you the position of the antenna, not the receiver. The cable is just a time delay. Does this mean that while the antenna feedline cable length does not influence the measured position (at the phase center of the antenna), and it does not influence the accuracy of a disciplined frequency reference output, it does introduce an error into the absolute time output (i.e., adding a delay to the PPS output)? In other words, do I correctly assume that I may safely ignore the length of my TBolt's antenna feedline if I am only interested in its 10 MHz OCXO output, but I may want to compensate for it if I ever find a need to use its PPS output as an absolute time marker? -- Mark J. Blair, NF6X n...@nf6x.net Web page: http://www.nf6x.net/ GnuPG public key available from my web page. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Does this mean that while the antenna feedline cable length does not influence the measured position (at the phase center of the antenna), and it does not influence the accuracy of a disciplined frequency reference output, it does introduce an error into the absolute time output (i.e., adding a delay to the PPS output)? In other words, do I correctly assume that I may safely ignore the length of my TBolt's antenna feedline if I am only interested in its 10 MHz OCXO output, but I may want to compensate for it if I ever find a need to use its PPS output as an absolute time marker? Yes. Here is the way I would look at it. Consider the PPS case where you have a setup like this: antenna-cable(RF)-receiver-cable(PPS)-testgear. Use the same type of coax on the PPS signal as you used for the antenna feed. (Or measure the length of the cables in ns rather than meters.) If you measure the offset of the PPS signal at your test gear, you can't tell if the delay comes from the antenna cable or the PPS cable. You could move the receiver back and forth or move chunks of cable from one side to the other and the PPS signal wouldn't move. This also means that you can use the GPS receiver's antenna correction to correct for the delay in your PPS distribution. -- These are my opinions, not necessarily my employer's. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
Mark J. Blair wrote: On Sep 7, 2010, at 6:30 AM, jimlux wrote: Another analogy is that if you had a machine that recorded all the signals, mounted right at the antenna, and then carried the recording half way around the world, and then ran the recording into a receiver, it would give you the position of the antenna, not the receiver. The cable is just a time delay. Does this mean that while the antenna feedline cable length does not influence the measured position (at the phase center of the antenna), and it does not influence the accuracy of a disciplined frequency reference output, it does introduce an error into the absolute time output (i.e., adding a delay to the PPS output)? In other words, do I correctly assume that I may safely ignore the length of my TBolt's antenna feedline if I am only interested in its 10 MHz OCXO output, but I may want to compensate for it if I ever find a need to use its PPS output as an absolute time marker? Yes.. except that the cable's physical and electrical length *do* vary with temperature, so if you're looking at the gnat's eyelash sort of thing, you need to take that into account. Maybe 10 ppm/degree, so a 20 meter run will change a bit less than a millimeter. That's down in the fractional picoseconds time-wise. It's an issue if you're doing things like interferometry at higher frequencies.. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
On Sep 7, 2010, at 5:16 PM, jimlux wrote: Yes.. except that the cable's physical and electrical length *do* vary with temperature, so if you're looking at the gnat's eyelash sort of thing, you need to take that into account. Maybe 10 ppm/degree, so a 20 meter run will change a bit less than a millimeter. That's down in the fractional picoseconds time-wise. It's an issue if you're doing things like interferometry at higher frequencies.. I see a bright future selling oven-controlled speaker cables to audiophiles... ;-) -- Mark J. Blair, NF6X n...@nf6x.net Web page: http://www.nf6x.net/ GnuPG public key available from my web page. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] What position is measured?
One other delay contributor would appear to be processing delay in the receiver, which thus begs the question of how the PPS signal is actually synchronized to the GPS system. Tom Holmes, N8ZM Tipp City, OH EM79 -Original Message- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Hal Murray Sent: Tuesday, September 07, 2010 5:33 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] What position is measured? Does this mean that while the antenna feedline cable length does not influence the measured position (at the phase center of the antenna), and it does not influence the accuracy of a disciplined frequency reference output, it does introduce an error into the absolute time output (i.e., adding a delay to the PPS output)? In other words, do I correctly assume that I may safely ignore the length of my TBolt's antenna feedline if I am only interested in its 10 MHz OCXO output, but I may want to compensate for it if I ever find a need to use its PPS output as an absolute time marker? Yes. Here is the way I would look at it. Consider the PPS case where you have a setup like this: antenna-cable(RF)-receiver-cable(PPS)-testgear. Use the same type of coax on the PPS signal as you used for the antenna feed. (Or measure the length of the cables in ns rather than meters.) If you measure the offset of the PPS signal at your test gear, you can't tell if the delay comes from the antenna cable or the PPS cable. You could move the receiver back and forth or move chunks of cable from one side to the other and the PPS signal wouldn't move. This also means that you can use the GPS receiver's antenna correction to correct for the delay in your PPS distribution. -- These are my opinions, not necessarily my employer's. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
