Thanks for the suggestion – in this case they were all sitting on the roof of my vehicle in an open parking lot, with 6-8” separation between radios. I guess there could be minimal shadowing for satellites at low grazing angles, but I’m skeptical of that as a full explanation.
I have a hypothesis that the default 5Hz update rate is problematic on these devices. The serial connection between the GPS receiver the Zynq PS runs at 38400 baud. With standard 8N1 framing, that only allows for 768 bytes of sentence data per 200ms cycle. If I capture the raw GPS serial output (by directly watching /dev/ttyPS1, not the scrubbed data filtered through gpsd), it’s quickly obvious that many sentences get truncated and/or dropped. For example, there are very frequent “time skips” happening in the time-related sentences, as well as random sentence fragments. Some cycles would be expected to have a larger data volume, such as when multiple GPGSV sentences list all satellites in view, and I think that’s mangling the serial stream. This explains discrepancies in what ‘gpsmon’ sees, as well as discrepancies I’ve sometimes seen on E320s trying to sync common GPS time with PPS assertion (sometimes radios are wrong by 200ms). This should not impact the “gps_locked” sensor, which gets its state via an I/O signal from the GPS receiver and not by parsing sentences. However, I am currently using information from sentences to determine lock status because “gps_locked” doesn’t seem to work as expected in UHD 4.4 on the E320 (looks like that might’ve been fixed in UHD 4.5 though). So long story short – I think 5Hz update rate is problematic. It can be changed to 1Hz by removing a resistor, and as far as I can tell, neither UHD nor the radio filesystem would care about that change. I may try this on one radio and see if it helps improve consistency.. -David From: Marcus D. Leech <patchvonbr...@gmail.com> Sent: Wednesday, June 5, 2024 8:56 AM To: usrp-users@lists.ettus.com Subject: [USRP-users] Re: GPS fix behavior on USRP E320 On 05/06/2024 08:43, David Raeman via USRP-users wrote: Hello, I'm having a difficult time getting consistent GPS fix behavior from a set of USRP E320 radios. They are all using UHD 4.4 with the same active GPS antenna (Siretta Tango 21, which has a 28dB LNA and short ~6" coax run). When outside with a view of the sky and 6 radios sitting together, 10-15 minutes after power-on, some of the radios will have a lock and others will not. For radios that get a lock, sometimes they will briefly glitch into "unlocked" state briefly every 20-30 seconds before reporting as locked again. If I let it sit another 10-15 minutes, nothing really changes. Looking at the output of 'gpsmon' on the radio, the radios which never locked will see fewer satellites, and the ones in common will have far different SNR levels. I'm trying to find a solution for more consistent behavior, especially since these are outside with a view of the sky. I confirmed the radio's GPS ANT port has the +3.3V bias so I assume the antennas receive power as expected. Searching the mailing list, over the years this topic has come up a couple times specifically with E320 radios. I know the same Jackson Labs LTE-Lite SOM is also used in the newer X410 radios, though it's configured a bit differently via strapping pins. I think: * The X410 sets the module in 1Hz mode instead of 5Hz. * The X410 uses it in "mobile" mode instead of auto-surveying “stationary” mode. * Curiously, the E320 seems to connect pin 1 (EFC) to pin 2 (NC), though this doesn't make any sense based on the LTE-Lite public tech manual. The X410 leaves them NC. Does anybody know whether any of the changes (or others) represent "lessons learned" that would improve GPS TTFF or disciplining behavior? I don’t mind changing resistor populations if there is a reason to. Or any other suggestions around this topic? Thank you, David Raeman _______________________________________________ USRP-users mailing list -- usrp-users@lists.ettus.com<mailto:usrp-users@lists.ettus.com> To unsubscribe send an email to usrp-users-le...@lists.ettus.com<mailto:usrp-users-le...@lists.ettus.com> IF you move the antennas further apart, what happens? If they are all tightly packed together, there's an opportunity for shadowing (small, but, maybe?).
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