[Ovmsdev] GPS location glitch (flatbed theft false alert)

Michael Balzer dexter at expeedo.de
Wed Jun 30 16:05:21 HKT 2021


Am 30.06.21 um 01:44 schrieb Craig Leres:
> On 6/29/21 12:27 AM, Michael Balzer wrote:
>> How about testing latitude & longitude individually for sudden peaks 
>> from their previous average, and discarding a new coordinate 
>> completely while such a peak is present?
> Overnight I had the idea that instead of looking for specific broken 
> values (0.0, 0.05, etc.) maybe the way to go is to suppress values 
> when the only the latitude or the longitude has a big jump. Maybe 
> something like this:
>     lat1, lon1 // previous position
>     lat2, lon2 // new position
>     tlat = (lat1 + lat2) / 2
>     delta_lat = haversine(tlat, lon1, tlat, lon2)
>     tlon = (lon1 + lon2) / 2
>     delta_lon = haversine(lat1, tlon, lat2, tlon)
>     if (delta_lat > simcombugdist && delta_lon < (simcombugdist / 10))
>         # toss
>     if (delta_lon > simcombugdist && delta_lat < (simcombugdist / 10))
>         # toss

…or maybe just testing the value deltas for m_latitude & m_longitude in 
their Updated…() methods. Calculating an actual distance doesn't seem to 
be necessary here.

A sudden jump after having no GPS lock or on init should be considered OK.

> Another (possibly far fetched) idea I had was is it possible we' 
> seeing corruption transferring the gps location from the simcom to the 
> esp32? A race condition where the latitude or the longitude gets 
> clobbered while reading it? I don't have a good idea of what other 
> symptoms that kind of problem would have.

Actually… not far fetched at all, as we continuously see multiplex frame 
errors in the logs -- see my previous posts on this. We don't have flow 
control on the ESP-SIMCOM serial line, and the handler may not catch all 

You can test that theory by enabling the verbose log on "gsm-nmea". Be 
aware that's a lot of log messages.

Better option: I already do some consistency checks in 
GsmNMEA::IncomingLine(), but I didn't implement the NMEA checksum. You 
might give that a try first.


Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
Fon 02333 / 833 5735 * Handy 0176 / 206 989 26

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