Use a handheld GPS in one of the problem areas; if it can show 4+ satellites in state 5 (code + carrier track), and your Tesla GPS is still flipping out, then it’s absolutely not a GPS constellation problem, external GPS interference issue, bad satellite geometry, etc. If you know an exact time and place this happened, you can probably find an online GPS constellation status page that can tell you what the satellite geometry was at that time and location, if any of the satellites were having issues, etc.
Other things to ask them (although I would be surprised if anyone at the service center even knows what any of these things are):
- Did they verify that the +5VDC to the antenna is there? Essentially all GPS antennas are active and get powered through the GPS antenna RF cable’s center connector; if that DC power isn’t there, the antenna won’t work properly.
- Did they force a cold start of the receiver by clearing out the almanac and ephemeris from the GPS receiver memory? This test can take up to 20 minutes, but it forces the receiver to download a new satellite almanac/ephemeris from the satellites; there’s a chance the almanac and/or ephemeris have become corrupt, which could easily cause tracking problems.
- Was the receiver sending any built-in-test failure codes? I have no idea how many BIT flags Tesla pays attention to in the GPS receiver, but I’ll bet they log them somewhere.
- Did they set your car up in a known good location for GPS reception, with another Tesla or a GPS test set as a reference, and verify that it is tracking (in state 5) the same number of satellites as the reference, the C/No (code-to-noise ratio) for each satellite is comparable between your car and the reference, the GDOP and HDOP were both 1, and no error codes were being logged?
As I said, I’d be surprised if the service center knows what any of the above means, much less can actually run the tests, but the above is what a knowledgeable GPS troubleshooting shop would do to diagnose GPS issues. To me, the symptoms sound like an issue in the RF side of the system: a bad antenna, issue with the antenna power supply, issue with the antenna cable or fittings, the RF demodulation, etc. Could also be issues with the correlators, that would make code tracking difficult and/or intermittent. But if you can show that a separate receiver works when the Tesla one doesn’t, and there were no constellation issues at that place and time, then that will show Tesla the issue has to be with the car. Whether they’ll listen…