By clock bias I am referring to the difference between true GPS time and the USRP time.
For example, in my application I am running 2 Separate B210 USRPs each side by side with a split signal coming from a single S band antenna. However, they are both disciplined using separate GPSDOs with separate antennas to simulate being 2 independent devices. They are being synchronized as described in method 2 in this article (https://files.ettus.com/manual/page_sync.html) and then the data collection is being commanded to start at the same GPS time for both devices. My goal is to try to create as phase coherent of an operation as possible between the 2 USRPs. When I cross correlate the 2 split S band signals to calculate difference in phase between the 2 signals, there is a significant drift over time. This phase difference converted is about 300 nanoseconds over the course of 1 second (300 PPB).Because of this, I wished to see if I could use the calculated clock bias that I described above to compensate for this drift in phase. However, now that you have mentioned that I should in fact be getting 1 PPB accuracy when synchronized to GPS time, I am questioning if I am doing the time synchronization process wrong altogether Since I seem to be getting drift around 300 PB after following the instructions in the link above. Sidd >> On Jun 4, 2020, at 1:24 PM, Marcus D Leech <[email protected]> wrote: > What exactly do you mean by clock bias? compared to what? > > Once the unit is locked to GPS and you use the GPSDO as your system clock, > the timing will be under 1PPB. What exactly are you measuring? > > If you’re trying fo derive bit timing from a modulated digital signal, the > usual way is to use a PLL in your DSP algorithms. > > Sent from my iPhone > >>> On Jun 4, 2020, at 11:52 AM, Sidd Subramanyam via USRP-users >>> <[email protected]> wrote: >> >> Hi all, >> >> I was wondering if there was a way to extract or compute a precise Clock >> Bias of the USRP time vs GPS time from the GPSDO. I am using a B210 USRP >> with the board mounted recommended TCXO GPSDO. I have previously used the >> GNSS-SDR (https://gnss-sdr.org/) software to compute pseudorange and clock >> bias, however this software is run by using the front end RX channels and >> not the GPSDO input. >> >> In my use case, I have a need to compute this from my built in GPSDO since I >> am intending to simultaneously use the front end on S-band signals, and the >> B series does not allow 2 separate center frequencies across its 2 RX >> channels. >> >> I am aware of the NMEA messages from which I can extract the position lock >> via serial but it does not seem like there is a time output that is more >> granular than one second which I can use to compute a clock bias. >> >> Any help is appreciated. >> >> Thank you, >> Sidd >> >> >> _______________________________________________ >> USRP-users mailing list >> [email protected] >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
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