What is the purpose and function of the COMPASS_DEC parameter?

jstroup1986 · November 7, 2024, 1:00pm

In working on navigation without GPS, I was introduced to the COMPASS_DEC parameter.

My understanding is the EKF3 (and maybe EKF2 also) has a look up table to correct for compass variance. (declination)

This parameter defaults to “0”.

Does this mean that the value of this parameter applied to look-up table value? To “fine tune” the look-up table value, so to speak?

Or is it applied based on some specific conditions for some other purpose?

Also - it’s odd that the units for COMPASS_DEC is in radians. I’ve never seen radians used in navigation tasks.

I’d appreciate any help in understanding this parameter. Thanks!

Yuri_Rage · November 7, 2024, 7:30pm

Release notes explain fairly clearly:

Compass declination can be overriden with COMPASS_DEC param (and EKF stops using world magnetic tables)

github.com

ArduPilot/ardupilot/blob/b17f7a557630319fc8a2406988884f2065dfed99/ArduCopter/ReleaseNotes.txt#L2061


      
              a) I/O CPU fix so safety remains off after inflight reboot (Critical fix)
              b) Acro mode yaw expo supports values under 0.5 (see ACRO_Y_EXPO param)
              c) Auto mode Loiter-Turn commands points towards center
              d) Change-Speed commands applied smoothly
              e) Compass scaling factor threshhold increased
              f) EKF compass variance reporting to GCS made consistent with onboard logs
              g) Gimbal control using RC input ignores RCx_TRIM param
              h) Holybro Durandal buzzer fix
              i) Parameter reset fix caused by Eeprom race condition
              j) Read-only parameter write failure msg only sent once to GCS
              k) Compass declination can be overriden with COMPASS_DEC param (and EKF stops using world magnetic tables)
              l) Terrain database (SRTM) file fix (will cause all terrain to be reloaded after upgrade)
          2) Bootloader update to reduce chance of param resets during firmware load
          3) Compass ordering and prioritisation improvements
          4) Flight controller support:
              a) CUAV-Nora
              b) CUAV-X7
              c) MatekSys H743
              e) mRo Nexus
              d) R9Pilot
          5) GPS moving baseline (aka Yaw from GPS) for UBlox F9 GPSs

jstroup1986 · November 8, 2024, 2:39pm

Thanks Yuri -

Yes - that’s what the parameter list says.

I guess the question is when this parameter “will” override EKF’s world magnetic tables.

As I noted, the default value is “0” - so it would not appear that this parameter is used unless something else triggers it. Otherwise by default, EKF would be navigating by true north.

Yuri_Rage · November 8, 2024, 2:55pm

A non-zero value is the “trigger.”

jstroup1986 · November 8, 2024, 4:01pm

Interesting.

I guess that means it’s not possible to set declination to 0.