CRSF: GPS Heading, Speed, Alt, Sats malfunction

[r1000ru]

Hello. On Tandem X20, Ethos 1.4.6, CRSF(Elrs), next sensor is malfunction:

• GPS Speed - has limit 0.0 - 20.0 km/h, decimals 10. Correct value = uint16_t / 100 = 0-655.35 km/h, decimals 100.
• GPS Heading - has limits 0.000 - 0.360 deg, decimals 1000. Correct value = uint16_t / 100 = 0.00 - 359.99 (655.35), decimals 100.
• GPS Altitude - has limits -100 - 1000m, decimals 0. Correct value = uint16_t - 1000m = -1000 - 64535m, decimals 0.
• GPS Sats - has limits 0-360, decimals 0, Correct value = uint8_t = 0-255, decimals 0

[bsongis-frsky]

Ethos has 2 kind of limits:

• sensor limits
• user modifiable limits

It seems you have mixed those 2 kind of limits in your request, here are my remarks:

• GPS Speed: range agreed, but both OpenTX and EdgeTX have only 1 decimal (protocol).
• GPS Heading: range agreed, but both OpenTX and EdgeTX have 3 decimals (protocol), which is very strange (2 bytes are not enough)
• GPS Altitude: this is a user limit, you may change it.
• GPS Sats: agreed

[r1000ru]

@bsongis-frsky

1. UBLOX protocol return struct, then ground_speed in sm/s and heading value scale to deg/1e-5. For sample take speed 12m/s and course 90 degrees. In ublox struct it will be 12100 = 1200 (sm/h) and 90/1e-5 = 90/ 0.00001 = 90 100 000 = 9 000 000
2. When parsing , INAV makes transformations:

   gpsSol.groundSpeed = _buffer.velned.speed_2d;    // cm/s
   gpsSol.groundCourse = (uint16_t) (_buffer.velned.heading_2d / 10000);     // Heading 2D deg * 100000 rescaled to deg * 10

3. Then INAV make CRSF GPS frame use next scales:
   • speed cm/m to km/h 10 = speed(sm/h) 3600(s in h) / 100000 (sm in km) 10 = speed 36 / 100;
   • heading deg 10 to deg 100 = heading * 10;

     #define DECIDEGREES_TO_CENTIDEGREES(angle) ((angle) * 10)
     crsfSerialize16(dst, (gpsSol.groundSpeed * 36 + 50) / 100); // gpsSol.groundSpeed is in cm/s
     crsfSerialize16(dst, DECIDEGREES_TO_CENTIDEGREES(gpsSol.groundCourse)); // gpsSol.groundCourse is 0.1 degrees, need 0.01 deg

     These transformations correspond to a document that TBS forbids publishing, called crsf rev07.pdf

[bsongis-frsky]

Thanks for the reference to the documentation.

Changes made for 1.4.7: 1) GPS Speed: 2 decimals, max = 650.00km/h 2) GPS Heading: 2 decimals, max = 360.00° 3) GPS Alt: max = 65000m 4) GPS Sats: max = 255 satellites

[bsongis-frsky]

I have found this exact document in my computer, it is a little bit old. I wonder if it has changed since, as OpenTX uses a different precision (so EdgeTX the same) and it seems there is no issue with the GPS fields.