Author Topic: Incorrect time in Aircrew replay if GPS unlocks.  (Read 1056 times)

buzz53

Incorrect time in Aircrew replay if GPS unlocks.
« on: August 28, 2020, 02:19:13 pm »
In the course of using the excellent Aircrew track analyser quite a lot recently, I came across an oddity whch is fairly obscure and does not affect the normal use of PAW but worth mentioning as it had me very confused until James and I got to the bottom of it. It's not a fault as such with PAW or James's program, but a combination of the two, which is why I mention it here. Probably both ought to be tweaked!

The visible effect was that the time displayed in the tracker at various points in the flight was not correct, and got worse as the flight progressed.

On investigation, it turns out that (I think) James only reads the actual time at the start of ther flight, and then assumes it advances by one second every time a $GPGGA messge is logged in the track file. Now as it it happens, on this flight my GPS was dropping out from time to time, mainly in steep turns, due to poor antenna positioning (I was trying something else out). It seems that when there is no GPS lock, PAW writes the GPS messages to the track file 4 times per second instead of the normal 1. This therefore causes the displayed time in the Aircrew analyser app to run fast by a factor of 4 when the GPS is unlocked.

I have checked the GPS itself and it continues to send messages only once per second at all times as expected. Also, the GPS message counter on the PAW home screen always advances at the same rate. So it is the PAW that for some reason is doing the repeat logging and I can't imagine it is intentional.

Of itself it doesn't seem a big deal but maybe needs a look at the PAW end in case it's a sign of something else not right? Also I imagine a fix at the Aircrew end would not be too hard to do.

The other thing James noticed, was that the time in the GGA messages jumped forward by exactly 3600 seconds for the first message received after GPS lock was restored. This doesn't affect the Aircrew app, or hopefully anything else, but again indicates something not quite right in this department.

Alan