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Yet another DIY GPSDO - yes, another one

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--- Quote from: Johnny B Good on July 28, 2022, 06:17:05 pm --- Today, I decided to disable the SBAS option which initially had shown a reduction of the ionospheric errors that normally account for most of the 7 to 8ns phase wander in the output from my G3RUH based GPSDO since I suspected this was the 'low hanging fruit' most likely to suffer the attentions of the Russian military's electronic countermeasures division.

 After allowing ten minutes for things to settle down, I did see a significant improvement over the 5ns pk-pk phase wander in between the randomly spaced 'transient events' over the past 3 weeks or so. IMO, the 1mHz frequency accuracy you mentioned can easily be attained with a modernised version of the G3RUH design (in peace time at least!) but bearing in mind that a 1mHz offset requires a sustained 100ps per second phase drift rate (one full cycle slip at 10MHz over a 1000 second interval), you can be the judge of that by examining the 10 screenshots taken over a 20 minute time period started just after the GPSDO had recovered from its loss of SBAS assistance (or hindrance!) that I've attached to this reply.

--- End quote ---

I've experimented with using SBAS, too, reasoning that it would help timing accuracy as much as it does for positioning, but I found, like you, that it doesn't help a lot. On the contrary, when I logged the TIC output from my GPSDO over a day or so, I would frequently see phase jumps of the GPS/GNSS time signal against the OCXO in the 15-20 ns range that went away when SBAS was disabled. Now I only enable it during survey-in for faster convergence of the true position. I generally found it to be a bad idea to have too many GNSS constellations enabled in a receiver and I stay clear from GLONASS in particular. GPS and Galileo coexist nicely, though, and help with having good coverage even if you antenna position is compromised.

Johnny B Good:

 I'm in total agreement with all of your observations. I had some doubts over the SBAS issue until I disabled it just over 12 hours ago  but not any more from what I'm now seeing in the 'scope traces.  :palm:

 I'd read about similar doubts over SBAS about two years ago (possibly one of your postings) but I was still making do with fake M8N modules back then when I tried testing the use of SBAS before deciding that on balance, it wasn't doing me any favours. The only reason as to why I decided to enable it again was on account I'd acquired a set of three genuine M8T modules for cheap from an Amazon seller (late last year afaicr) and had upgraded the MK II gpsdo with one and three weeks ago thought it might be worth trying the effect of SBAS once more with a timing module.

 The test initially suggested this was providing a benefit during the first week but since I was still fine tuning the temperature controlling algorithm, I was probably not able to see these wobbles through those induced by my own testing until more than a week had gone by when I actually witnessed one such event taking place. I'd forgotten about the (seemingly non-) issue of SBAS hindrance by then and started hypothesising about the possibility of electronic countermeasures being used in the Ukraine as a more palatable  alternative to my LPRO starting to 'go bad'.

 At the time, overlooking the possibility that using the SBAS option was the cause as now seems to be the case, the only way I could prove whether it was the LPRO or some weirdness with the GPS, was to buy another rubidium oscillator, hence my purchasing, literally, the last SA22C in the shop.  https://tinyurl.com/258vuuhj

 It was quite a shock to discover that the supply of salvaged LPROs and the slightly less desirable second best FEI rubidium oscillators had completely dried up, leaving only the overpriced Temex units with their failing electrolytic caps which I wouldn't give tuppence for, let alone the 500 dollar and up prices being asked. I guess I've only just discovered the reason for the apparent waning of interest in DIYing a rubidium oscillator based home lab frequency standard. :( :palm:

 Anyway, here's another sequence of screen grabs taken overnight which seem free of the SBAS nonsense. And, btw, just in case there is any confusion as to what the scope traces are displaying (I didn't explicitly describe them afaicr), the CH1 (yel) is the gpsdo output triggering the timebase (hence it remaining fixed in place despite the 4 to 5ns wobbles it's imposing on the LPRO trace shown in blue on CH3.

 Basically, the LPRO is acting as a referee by which to judge the wobbly behaviour of the gpsdo due to mostly ionospheric disturbances that the correction data provided by the gps SV's are unable to fully cancel out. It has the benefit of not being molested by ionospheric disturbances and all the other smaller imperfections of the GPS system as a time transfer system - only the more mundane issues that plague all free running oscillators (temperature, pressure, psu voltage variations etc plus, of course, calibration error).

 A rubidium oscillator offers the electronic hobbyist the best chance of generating a reference at least stable enough for the hour or so it takes to observe the disciplining action of all (bar lab grade) gpsdos. In my case, with the effects of temperature and pressure variations being almost completely mitigated, it is more than good enough for the task, even if calibration is not absolutely perfect when the calibration reference happens to be the DUT itself ::)

Johnny B Good:
 Well, it seems the funny business with the GPS looks very much like it was a consequence of enabling SBAS (hindrance) so my thanks for reminding me of your experience testing out its worth. Disabling it certainly appears to have restored the status quo as the following seven screen grabs covering the remaining  four hours of the day (and 40 minutes more) seem to indicate.

 I'll resist the urge to twiddle with the calibration knob until at least tomorrow afternoon simply to give the GPS a chance to show its hand.

Since we seem to keep hijacking André's thread for topics only peripherally related to his project, how about opening a separate GNSS timing discussion thread?

--- Quote from: thinkfat on July 29, 2022, 07:17:24 am ---I've experimented with using SBAS, too, reasoning that it would help timing accuracy as much as it does for positioning, but I found, like you, that it doesn't help a lot. On the contrary, when I logged the TIC output from my GPSDO over a day or so, I would frequently see phase jumps of the GPS/GNSS time signal against the OCXO in the 15-20 ns range that went away when SBAS was disabled.

--- End quote ---
For the ZED-F9T I'm tinkering with u-blox explicitly warns that using SBAS can degrade timing performance, which is why it's disabled by default.

--- Quote from: thinkfat on July 29, 2022, 07:17:24 am ---Now I only enable it during survey-in for faster convergence of the true position.
--- End quote ---
As a superior alternative to SBAS, you can use an RTK reference for survey in, if your receiver supports it. The majority of German states provide these free of charge thanks to open data laws.

Johnny B Good:
 Did anyone notice a complete 3 or 4 minutes long black out of GPS a few minutes ago (around 14:15 UTC 2022-08-17)?

 I wasn't monitoring with U-blox's ucentre at the time - just spotted my gpsdo's 4 per second PPS indication of loss of sync and the consequent wiping out of the rubidium trace against that of the gpsdo with the infinite persistence that normally shows a 30ns band over 24 hour and longer periods.

 I connected the usb link and fired up ucentre to check the state of play, by which time service had been resumed with 5 satellites above the 30 deg elevation threshold, suggesting it hadn't been a case of no in service satellites being available to appear above the threshold as has happened in the past when I'd been testing with 45 and 50 degree elevation mask angles.

 It took some 5 or 6 minutes to settle back down after the GPS SV's had come back on line, leaving the earlier phase difference between the ruby and the gpsdo within a ns of their previous state recorded just an hour earlier as can bee seen in the attached screen shots (time stamped in BST).

 Please note that the 3 or 4 ns drift between the last two screen shots are typical of the +/- 3ns disciplining 'wobble' caused largely by the combination of imprecise wide area ionosphere correction data carried in the GPS message packets and in my case, the limited averaging time of a simple G3RUH based design of gpsdo.


 I've double checked the antenna connections and it looks like it may well have been a connection issue with the cheap Banggood sourced Wilkinson splitter (modified with a DC-block) and/or the half meter SMA ended cables I've kept in line ever since I'd tested out a second M8T module a fortnight or so back in readiness to experiment with a long time constant version of the G3RUH setup to discipline the ruby to deal with its long term ageing drift (circa 1E-13 to 1E-14 per day).

 Although antenna connection issues with my setup would appear to be the most likely cause of this loss of GPS signal event, I'd still appreciate any reports that it wasn't due to the GPS system itself dropping out during the time period in question.

 In the meantime, I'll use the time honoured method of retensioning the suspect SMA-F contacts with the aid of a jeweller's loupe and a pin.


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