Repeatable GPS phase drift - Page 1
Forum section: electronics / general

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: April 25, 2021, 11:13:46 am
content: I have been testing two almost identical GPSDOs agains each other. Their antennas are different types and are about 1m away from each other. In both cases GPSDOs were running for several days.By accident I have pulled up two traces separated by 4 days but started within 14 minutes of each in relation to midnight.They look too correlated to me. Overall drift is different because Ublox navigation settings were not identical on these runs. Low level curve structure is different too - loop bandwidth settings have been changed.Not sure what to make of this, I will make more tests to see if this is repeatable.Comments? Speculations? Explanations?Leo

author: edpalmer42
author_rank: Super Contributor
timestamp: April 25, 2021, 05:44:53 pm
content: GPS satellites have an orbital period of 12 sidereal hours so I guess that you should see a phase pattern repeat at that rate. The other GNSS systems have different orbital periods.

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2021, 03:02:47 am
content: There is also a 24 hour cycle from changes in the ionosphere, which is what dual frequency receivers compensate for.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 26, 2021, 03:09:03 am
content: If you had a multi-day phase record, you could load it into Plotter and do an Autocorrelation on it. It's slow, but automated, and would show any daily or semi-daily peaks in the data.

author: MIS42N
author_rank: Frequent Contributor
timestamp: April 26, 2021, 05:22:40 am
content: A sidereal day is 23 h 56 min 4.0905. 4 days difference from UTC = 944s about, 15 minutes and 44 seconds. If satellite position was the cause of repeatability, the two recordings would be offset by this amount and the later (in days) recording would need to be starting earlier (in the day) by this amount. That's the case with your recordings, but because the recordings were not 15m44s apart but 14m, there should be an observable offset of about 100 s between them. Hard to see from the graph but the magenta line does seem to lag slightly. I'd say that's sufficient to say you are looking at changes due to the changing satellite positions.You could verify this by looking at two traces separated by 11h58m and see if they correlate.I conclude from my imprecise testing of antenna that once the signal is "good enough" there's little or no improvement. I tested a GPS with patch antenna on the back (I don't think these have active gain), and another with a 3 meter lead and active gain in the same position. The NMEA messages showed about a 7 (dB?) increase for the active gain antenna, but the GPSDO response was pretty much the same. These were in what I'd call 'reasonable' viewing areas. For development the antenna is in the best position I could find and seems able to get a few (dB?) extra. HDOP is < 1 for more than 97% of the time, and nothing over 2.

author: nctnico
author_rank: Super Contributor
timestamp: April 26, 2021, 04:59:30 pm
content: In Europe there are various time transmission services (like DCF77 in Germany) for which the carrier frequency is driven by an NMI. Short term these carriers can have quite some fluctuation but with an OCXO added to the mix you should be able to create a stable reference frequency which can be used to test GPS receivers against where it comes to frequency stability.