[time-nuts] Re: What time difference to expect from two clocks using internal GPS receivers?
Hi, Have you looked at the delay of the GPS signal passing through the filter to the one unit vs C on the other? I've seen a similar situation where a bias T was the culprit. I don't recall the numbers off hand but it was significant enough to notice, and the difference went away once the T removed (or duplicated on the other device). Dan On 5/1/2022 3:30 AM, time-nuts-requ...@lists.febo.com wrote: Some more info The two GPS do keep their phase stable vs a Rb within ±10 ns. But the absolute time difference of their PPS pulses was, after a cold start, stable within ± 20ns but the average value could be up to 100ns and differed after every cold start. The two GPS antenna cables had a length difference of 1 meter, but that should cater for only 5 ns (?) One module is connected to the antenna with only a C, the other has a 1 GHz CLC high pass filter between antenna and module Erik ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
[time-nuts] Re: What time difference to expect from two clocks using internal GPS receivers?
In late 2019 we successfully employed 4 SRS FS740 (with Rb option; GNSS receiver: Trimble RES SMT 360) to synchronize spatially distributed SDR receivers over distances of 1~2.5 km for the purpose of locating RF emitters via time difference of arrival. As a side product of that measurement campaign we managed to assess the accuracy of the time synchronization of the FS740, which was about 5 ns over the course of 4 hours with a little post-processing relying on log data from the FS740. We circumvented the issues mentioned by Bob via a reference measurement with known emitter location and line of sight to all receivers. If you're interested in details, I'd refer you to a paper we published on the measurement [1]. Best regards, Carsten [1] https://ieeexplore.ieee.org/document/9128562 (behind IEEE paywall; I can send you a personal copy of the paper upon request) On 01.05.22 02:50, Bob kb8tq wrote: Hi If you are looking at time ( = the absolute offset from GPS’s version of UTC) then there are a number of issues. The antenna you use will have a delay and it may well vary more than a bit. The cable to the antenna is in the same category. If both your modules run off a power splitter then those will not show up in an A-B comparison. The modules themselves likely have SAW filters in them. These have group delay just like any other bandpass filter. The tolerance on this is likely in the 10’s of ns module to module. There are other bit and pieces that can contribute at the “nanoseconds” level. A typical set of modules from a good supplier should come in to a +/- 20 ns sort of window for 2/3 of the parts ( = 1 sigma). Geometry errors are fairly simple. A meter is 3 ns in free space. Each meter you are off from “correct” will add 3 ns of “wobble” in the results. Just how this shows up is very dependent on the direction of the error an what sort of satellite view you happen to have. Ionosphere can ( in high sunspot years) contribute 50 ns or more to timing errors. Tropospheric issues can also get into the mix at a bit lower level. You might think these would wash out on co-located units. Unfortunately they are not going to start /stop using this or that sat at exactly the same time. Lots of fun stuff to look for …. Bob On Apr 30, 2022, at 6:41 AM, Erik Kaashoek wrote: Some more info The two GPS do keep their phase stable vs a Rb within +/-10 ns. But the absolute time difference of their PPS pulses was, after a cold start, stable within +/- 20ns but the average value could be up to 100ns and differed after every cold start. The two GPS antenna cables had a length difference of 1 meter, but that should cater for only 5 ns (?) One module is connected to the antenna with only a C, the other has a 1 GHz CLC high pass filter between antenna and module Erik Op za 30 apr. 2022 om 12:32 schreef Erik Kaashoek : The PPS jitter of a cheap Chinese GPS module was measured at about +/- 10 ns. But the phase of the PPS compared to a Rb varied substantial more. To verify if this was possibly due to ionospheric or atmospheric conditions the time difference between the PPS of two identical modules using two identical rooftop antenna was measured. Both only used the GPS constellation. This showed difference of up to 100 ns. Switching to GPS+GLN did not make a visible difference. It was tried to set both GPS modules into fixed position mode but the reported position still kept moving a bit (within 3 m) and the fixed mode did not have a visible impact on the time difference variations. Is a time difference of up to 100 ns to be expected when using two GPS receivers or is this difference possibly due to bad application or performance of the cheap Chinese GPS modules Erik. ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there. -- M.Sc. Carsten Andrich Technische Universität Ilmenau Fachgebiet Elektronische Messtechnik und Signalverarbeitung (EMS) Helmholtzplatz 2 98693 Ilmenau T +49 3677 69-4269 ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
[time-nuts] Re: What time difference to expect from two clocks using internal GPS receivers?
Hi Erik, On 2022-04-30 12:32, Erik Kaashoek wrote: The PPS jitter of a cheap Chinese GPS module was measured at about +/- 10 ns. But the phase of the PPS compared to a Rb varied substantial more. To verify if this was possibly due to ionospheric or atmospheric conditions the time difference between the PPS of two identical modules using two identical rooftop antenna was measured. Both only used the GPS constellation. This showed difference of up to 100 ns. Switching to GPS+GLN did not make a visible difference. It was tried to set both GPS modules into fixed position mode but the reported position still kept moving a bit (within 3 m) and the fixed mode did not have a visible impact on the time difference variations. Is a time difference of up to 100 ns to be expected when using two GPS receivers or is this difference possibly due to bad application or performance of the cheap Chinese GPS modules Well, there are many sources of bias both in hardware and firmware. As mentioned already, delays of antennas and cables remains uncompensated. Seeing a difference of 100 ns is equivalent of 20 m of cable. If you know you have about the same length of cable, then that is not your culprit. There is a peculiar effect in that the experienced delay in receiver becomes different depending on the PN code used, so per satellite. This should be lower. At the same time, considering that a single chip of the PN code is just shorter than 1 us, so maybe there is a narrow-band effect there. Still, that should not give such huge difference, so it is really currious. There could be some peculiar issue on how state is set up as it locks up. Try restarting one of them a couple of times and see if the offset varries or is consistent. Try swapping antenna cables to see if the offset follows the receiver or antenna/coax. Try using another receiver in parallel. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
[time-nuts] Re: What time difference to expect from two clocks using internal GPS receivers?
Hi If you are looking at time ( = the absolute offset from GPS’s version of UTC) then there are a number of issues. The antenna you use will have a delay and it may well vary more than a bit. The cable to the antenna is in the same category. If both your modules run off a power splitter then those will not show up in an A-B comparison. The modules themselves likely have SAW filters in them. These have group delay just like any other bandpass filter. The tolerance on this is likely in the 10’s of ns module to module. There are other bit and pieces that can contribute at the “nanoseconds” level. A typical set of modules from a good supplier should come in to a +/- 20 ns sort of window for 2/3 of the parts ( = 1 sigma). Geometry errors are fairly simple. A meter is 3 ns in free space. Each meter you are off from “correct” will add 3 ns of “wobble” in the results. Just how this shows up is very dependent on the direction of the error an what sort of satellite view you happen to have. Ionosphere can ( in high sunspot years) contribute 50 ns or more to timing errors. Tropospheric issues can also get into the mix at a bit lower level. You might think these would wash out on co-located units. Unfortunately they are not going to start /stop using this or that sat at exactly the same time. Lots of fun stuff to look for …. Bob > On Apr 30, 2022, at 6:41 AM, Erik Kaashoek wrote: > > Some more info > The two GPS do keep their phase stable vs a Rb within +/-10 ns. But the > absolute time difference of their PPS pulses was, after a cold start, > stable within +/- 20ns but the average value could be up to 100ns and > differed after every cold start. > The two GPS antenna cables had a length difference of 1 meter, but that > should cater for only 5 ns (?) One module is connected to the antenna with > only a C, the other has a 1 GHz CLC high pass filter between antenna and > module > Erik > > > > Op za 30 apr. 2022 om 12:32 schreef Erik Kaashoek : > >> The PPS jitter of a cheap Chinese GPS module was measured at about +/- >> 10 ns. >> But the phase of the PPS compared to a Rb varied substantial more. >> To verify if this was possibly due to ionospheric or atmospheric >> conditions the time difference between the PPS of two identical modules >> using two identical rooftop antenna was measured. Both only used the GPS >> constellation. >> This showed difference of up to 100 ns. Switching to GPS+GLN did not >> make a visible difference. >> It was tried to set both GPS modules into fixed position mode but the >> reported position still kept moving a bit (within 3 m) and the fixed >> mode did not have a visible impact on the time difference variations. >> Is a time difference of up to 100 ns to be expected when using two GPS >> receivers or is this difference possibly due to bad application or >> performance of the cheap Chinese GPS modules >> Erik. >> > ___ > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an > email to time-nuts-le...@lists.febo.com > To unsubscribe, go to and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
[time-nuts] Re: What time difference to expect from two clocks using internal GPS receivers?
Hi Erik, Since each Chinese GPS receiver module has its own Xtal oscillator driving a very low power 32-bit processor usually at around 48MHz, and we can expect these oscillators to have a slightly different frequency, let's say the difference is 500ppm, then I would imagine there would be a correspondingly significant phase difference in the PPS pulses of these two Chinese GPS receivers. I would also imagine if you took two Neo-M9N modules and adjusted their PPS with the reported quantization error, the PPS phase difference would be much, much smaller. As usual, you get what you pay for. But then, depending on the use case, the Chinese GPS receiver modules may be perfectly adequate, it all depends on your specific needs. On Sat, Apr 30, 2022 at 9:32 PM Erik Kaashoek wrote: > Some more info > The two GPS do keep their phase stable vs a Rb within +/-10 ns. But the > absolute time difference of their PPS pulses was, after a cold start, > stable within +/- 20ns but the average value could be up to 100ns and > differed after every cold start. > The two GPS antenna cables had a length difference of 1 meter, but that > should cater for only 5 ns (?) One module is connected to the antenna with > only a C, the other has a 1 GHz CLC high pass filter between antenna and > module > Erik > > > > Op za 30 apr. 2022 om 12:32 schreef Erik Kaashoek : > > > The PPS jitter of a cheap Chinese GPS module was measured at about +/- > > 10 ns. > > But the phase of the PPS compared to a Rb varied substantial more. > > To verify if this was possibly due to ionospheric or atmospheric > > conditions the time difference between the PPS of two identical modules > > using two identical rooftop antenna was measured. Both only used the GPS > > constellation. > > This showed difference of up to 100 ns. Switching to GPS+GLN did not > > make a visible difference. > > It was tried to set both GPS modules into fixed position mode but the > > reported position still kept moving a bit (within 3 m) and the fixed > > mode did not have a visible impact on the time difference variations. > > Is a time difference of up to 100 ns to be expected when using two GPS > > receivers or is this difference possibly due to bad application or > > performance of the cheap Chinese GPS modules > > Erik. > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send > an email to time-nuts-le...@lists.febo.com > To unsubscribe, go to and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
[time-nuts] Re: What time difference to expect from two clocks using internal GPS receivers?
Some more info The two GPS do keep their phase stable vs a Rb within +/-10 ns. But the absolute time difference of their PPS pulses was, after a cold start, stable within +/- 20ns but the average value could be up to 100ns and differed after every cold start. The two GPS antenna cables had a length difference of 1 meter, but that should cater for only 5 ns (?) One module is connected to the antenna with only a C, the other has a 1 GHz CLC high pass filter between antenna and module Erik Op za 30 apr. 2022 om 12:32 schreef Erik Kaashoek : > The PPS jitter of a cheap Chinese GPS module was measured at about +/- > 10 ns. > But the phase of the PPS compared to a Rb varied substantial more. > To verify if this was possibly due to ionospheric or atmospheric > conditions the time difference between the PPS of two identical modules > using two identical rooftop antenna was measured. Both only used the GPS > constellation. > This showed difference of up to 100 ns. Switching to GPS+GLN did not > make a visible difference. > It was tried to set both GPS modules into fixed position mode but the > reported position still kept moving a bit (within 3 m) and the fixed > mode did not have a visible impact on the time difference variations. > Is a time difference of up to 100 ns to be expected when using two GPS > receivers or is this difference possibly due to bad application or > performance of the cheap Chinese GPS modules > Erik. > ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
