[time-nuts] Re: Question about GPS 1PPS ADEV
On Donnerstag, 21. April 2022 13:48:42 CEST Markus Kleinhenz via time-nuts wrote: > Hello Matthias, > > Could you explain further how enabled GLONASS messed things up for you? Noticeably more transients in the phase error. Phase jumps of 15ns or more. Since then I only enable GPS and Galileo, to mitigate my incomplete sky view a bit. I only have view west to east over south and I filter all SVs below 20° elevation. > > Regards, > > Markus > ___ > 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: Question about GPS 1PPS ADEV
Private email from the folks at NRCan indicated that having both constellations in the data would potentially result in (more) clock offset jumps in the PPP results. Remember that GLONASS and GPS are synced to different master clocks which are some small (and potentially variable) number of nanoseconds apart. But part of it may also be that their modeling of GLONASS clocks and orbitals isn't as robust as for GPS due to having less experience with it. If you're purely interested in location, multiple constellations can be helpful, but the precision timing application is a bit different. Also, note that this is probably a non-issue for simple non-post-processed GPS PPS, as the other error sources would swamp any timescale differences. John On Apr 21, 2022, 12:09 PM, at 12:09 PM, Markus Kleinhenz via time-nuts wrote: >Hello Matthias, > >> Of course curiosity got the better of me. I switched the GPSDO to >hold-over >> and recorded about 4000 seconds of data. Result is attached. BTW I >terminated >> the experiment after the temperature compensation kicked the DAC one >LSB down. >> I am quite pleased with the performance of the OCXO, though. I was >not >> expecting that, it is really nothing special. Not a surplus Chinesium >OCXO, >> but something you can buy for quite reasonable money from Digikey. >> >> So, the bulge is getting a bit more prominent, but it is in no way as > >> prominent as the figure 26 in John Ackermann's paper. >> >> John, what constellations did you have enabled during the test? Just >GPS, or >> also others? I usually run with GPS and Galileo enabled (and I avoid >GLONASS, >> it messes everything up). Can this make a difference? >> >> >Could you explain further how enabled GLONASS messed things up for you? > >Regards, > >Markus >___ >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: Question about GPS 1PPS ADEV
I have yet to find a reliable source for this, but something I have heard in the past is that since the different constellations' satellites may be controlled and disciplined differently (and to different standards), it is not wise to use more than one constellation on receiver that is used to discipline a time server. Like I said though, I have not found a reliable source for this, and I haven't even been able to confirm it in my own testing. I would love to know more on the subject. Regards, John Miller > On Apr 21, 2022, at 7:48 AM, Markus Kleinhenz via time-nuts > wrote: > > Hello Matthias, > >> Of course curiosity got the better of me. I switched the GPSDO to hold-over >> and recorded about 4000 seconds of data. Result is attached. BTW I >> terminated >> the experiment after the temperature compensation kicked the DAC one LSB >> down. >> I am quite pleased with the performance of the OCXO, though. I was not >> expecting that, it is really nothing special. Not a surplus Chinesium OCXO, >> but something you can buy for quite reasonable money from Digikey. >> >> So, the bulge is getting a bit more prominent, but it is in no way as >> prominent as the figure 26 in John Ackermann's paper. >> >> John, what constellations did you have enabled during the test? Just GPS, or >> also others? I usually run with GPS and Galileo enabled (and I avoid >> GLONASS, >> it messes everything up). Can this make a difference? >> >> > Could you explain further how enabled GLONASS messed things up for you? > > Regards, > > Markus > ___ > 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: Question about GPS 1PPS ADEV
Hello Matthias, > Of course curiosity got the better of me. I switched the GPSDO to hold-over > and recorded about 4000 seconds of data. Result is attached. BTW I terminated > the experiment after the temperature compensation kicked the DAC one LSB > down. > I am quite pleased with the performance of the OCXO, though. I was not > expecting that, it is really nothing special. Not a surplus Chinesium OCXO, > but something you can buy for quite reasonable money from Digikey. > > So, the bulge is getting a bit more prominent, but it is in no way as > prominent as the figure 26 in John Ackermann's paper. > > John, what constellations did you have enabled during the test? Just GPS, or > also others? I usually run with GPS and Galileo enabled (and I avoid GLONASS, > it messes everything up). Can this make a difference? > > Could you explain further how enabled GLONASS messed things up for you? Regards, Markus ___ 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: Question about GPS 1PPS ADEV
On 4/16/22 15:53, Matthias Welwarsky wrote: John, what constellations did you have enabled during the test? Just GPS, or also others? I usually run with GPS and Galileo enabled (and I avoid GLONASS, it messes everything up). Can this make a difference? I'm afraid I'm not certain. Normally, I do all my work with GPS only, as for timing purposes adding in the other constellations can result in clock-jumping and other issues. But at the beginning of the paper I said that all receivers were set to default configuration except for setting to 0D timing mode when available. So I don't know if that means I left all the default constellations were enabled. My gut feeling, though, is that I probably turned everything off but GPS. John ___ 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: Question about GPS 1PPS ADEV
Hi Any time you look inside a control loop, there will be bumps and the like that are a function of the control loop plus the dynamics of the device being controlled plus the environment. If you have a straight line … something is wrong. Bob > On Apr 16, 2022, at 1:20 PM, Matthias Welwarsky > wrote: > > On Samstag, 16. April 2022 17:55:17 CEST John Ackermann N8UR wrote: >> Finally, it looks like you're comparing the raw PPS with an oscillator >> that is steered by that same PPS. I have to think their correlation >> could lead to possible and unpredictable errors. It would be better to >> have the OCXO remain unsteered during the measurement. > > That's correct. But I'm not trying to characterize the OCXO or the GNSS > receiver. I was just wondering about this perceived discrepancy. > >> >> Best, >> John >> >> >> >>> Best regards, >>> Matthias >>> >>> https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/ >>> N8UR_GPS_Evaluation_August2020.pdf >>> >>> >>> ___ >>> 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 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: Question about GPS 1PPS ADEV
Hi > On Apr 16, 2022, at 11:55 AM, John Ackermann N8UR wrote: > > > > On 4/16/22 09:52, Matthias Welwarsky wrote: >> Dear list members, >> in 2020 John Ackermann published an evaluative survey of current day GPS and >> GNSS receivers (URL below). I have a question about figure 26, which shows, >> among others, the ADEV of a NEO-M8T against a Cesium reference, with >> quantization correction applied. The curve shows a significant "bulge" above >> 1e-10 between about 10s and 100s tau. >> I'm using a LEA-M8T in my DIY GPSDO, which I think is the same chipset in a >> slightly different package. I have attached an image worth about 3000 seconds >> of data, raw 1PPS phase difference against the LO used in the GPSDO. The >> GPSDO >> is locked, not in hold-over mode. >> The bulge between 10s and 100s is not really visible here. There is a slight >> bend, but not as pronounced. My explanation is that this is due to the LO >> being pulled by the GNSS receiver so that it is no longer fully visible. I >> reason that, were the LO more stable, more loosely coupled to the GNSS, I >> should see the bulge from figure 26. Would you agree? > > My theory of the ADEV flat spot when the M8T is qErr corrected is that > multiple things contribute to noise on the PPS output, and the qErr is only > one of them, and it consists of a noise source (clock granularity) that is > unrelated to any external analog process. > > Since Qerr has a fixed limit of ± XX nanoseconds (half the receiver clock > granularity), its contribution to the PPS noise decreases with longer > averaging times. Meanwhile, the other sources of noise such as ionosphere, > etc., are slower and become more pronounced as tau increases. At around 30 > seconds, the external noise factors become larger than the qErr. > > So at short tau cancelling out the qErr gets rid of a major noise source and > improves ADEV but as tau increases beyond ~30 seconds the qErr contribution > is gradually outweighed by the other noise sources and disappears, returning > the ADEV slope to its normal -1. > > Also note that in Fig. 26, at tau below 3 seconds it's possible that both the > M8T and F9T corrected plots are limited by the TICC resolution, which is > around 8e-11 @ 1 second on a good day. That's below these traces, but may > still be high enough to impact the measurement. > > Finally, it looks like you're comparing the raw PPS with an oscillator that > is steered by that same PPS. I have to think their correlation could lead to > possible and unpredictable errors. It would be better to have the OCXO > remain unsteered during the measurement. The only way to make meaningful single comparison measurements of what’s going on is to compare to an external “free running” standard that has a stability adequate to the task. Normally this means an ADEV 5 to 10X better than what you expect to see on the device under test. Bob > > Best, > John > > >> Best regards, >> Matthias >> https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/ >> N8UR_GPS_Evaluation_August2020.pdf >> ___ >> 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 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: Question about GPS 1PPS ADEV
On Samstag, 16. April 2022 17:55:17 CEST John Ackermann N8UR wrote: > Finally, it looks like you're comparing the raw PPS with an oscillator > that is steered by that same PPS. I have to think their correlation > could lead to possible and unpredictable errors. It would be better to > have the OCXO remain unsteered during the measurement. That's correct. But I'm not trying to characterize the OCXO or the GNSS receiver. I was just wondering about this perceived discrepancy. > > Best, > John > > > > > Best regards, > > Matthias > > > > https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/ > > N8UR_GPS_Evaluation_August2020.pdf > > > > > > ___ > > 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 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: Question about GPS 1PPS ADEV
On 4/16/22 09:52, Matthias Welwarsky wrote: Dear list members, in 2020 John Ackermann published an evaluative survey of current day GPS and GNSS receivers (URL below). I have a question about figure 26, which shows, among others, the ADEV of a NEO-M8T against a Cesium reference, with quantization correction applied. The curve shows a significant "bulge" above 1e-10 between about 10s and 100s tau. I'm using a LEA-M8T in my DIY GPSDO, which I think is the same chipset in a slightly different package. I have attached an image worth about 3000 seconds of data, raw 1PPS phase difference against the LO used in the GPSDO. The GPSDO is locked, not in hold-over mode. The bulge between 10s and 100s is not really visible here. There is a slight bend, but not as pronounced. My explanation is that this is due to the LO being pulled by the GNSS receiver so that it is no longer fully visible. I reason that, were the LO more stable, more loosely coupled to the GNSS, I should see the bulge from figure 26. Would you agree? My theory of the ADEV flat spot when the M8T is qErr corrected is that multiple things contribute to noise on the PPS output, and the qErr is only one of them, and it consists of a noise source (clock granularity) that is unrelated to any external analog process. Since Qerr has a fixed limit of ± XX nanoseconds (half the receiver clock granularity), its contribution to the PPS noise decreases with longer averaging times. Meanwhile, the other sources of noise such as ionosphere, etc., are slower and become more pronounced as tau increases. At around 30 seconds, the external noise factors become larger than the qErr. So at short tau cancelling out the qErr gets rid of a major noise source and improves ADEV but as tau increases beyond ~30 seconds the qErr contribution is gradually outweighed by the other noise sources and disappears, returning the ADEV slope to its normal -1. Also note that in Fig. 26, at tau below 3 seconds it's possible that both the M8T and F9T corrected plots are limited by the TICC resolution, which is around 8e-11 @ 1 second on a good day. That's below these traces, but may still be high enough to impact the measurement. Finally, it looks like you're comparing the raw PPS with an oscillator that is steered by that same PPS. I have to think their correlation could lead to possible and unpredictable errors. It would be better to have the OCXO remain unsteered during the measurement. Best, John Best regards, Matthias https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/ N8UR_GPS_Evaluation_August2020.pdf ___ 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: Question about GPS 1PPS ADEV
Hello Matthias, provided your OCXO is sufficiently stable, i.e., better stability than the 1PPS, I agree that you should see said "bulge". Judging from the DAC plot, the EFC voltage is being adjusted within the tau range in which you expect the NEO-M8T bulge (10~100 s), so I'd expect that to have an effect on the TIC ADEV. Could you measure in holdover mode with fixed EFC voltage? Best regards, Carsten On 16.04.22 15:52, Matthias Welwarsky wrote: I reason that, were the LO more stable, more loosely coupled to the GNSS, I should see the bulge from figure 26. Would you agree? ___ 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.
