ekf2: Fix description of EKF2_HDG_GATE

[mortenfyhn]

Solved Problem

The description of EKF2_HDG_GATE made me assume this parameter was irrelevant for a vehicle that doesn't use a magnetometer:

I'm working on a multicopter with external odometry input (via mavlink ODOMETRY type VISION). I want the external odometry system's heading to always be trusted, so I had set EKF2_HDG_GATE very high. Later, having forgotten about the original reason for setting EKF2_HDG_GATE, I found that the docs for this parameter say "Gate size for magnetic heading fusion". Since I do not use a magnetometer, this made me think I'd set this parameter by mistake, so I reset it to the default value. When flying, it became clear that it does indeed also affect yaw estimation with only external vision/odometry yaw, but the parameter description is a bit misleading.

Solution

The simplest fix is to just not mention "magnetometer" in the description of EKF2_HDG_GATE.

Changelog Entry

For release notes:

Feature/Bugfix XYZ
New parameter: XYZ_Z
Documentation: Clarified description of EKF2_HDG_GATE

Alternatives

Not sure, maybe separate parameters for magnetometer heading fusion and other types of heading fusion, but that seems overkill.

Test coverage

N/A

Context

I verified that EKF2_HDG_GATE does affect my system (no magnetometer, using external odometry), by

1. Flying, and seeing that with low EKF2_HDG_GATE, PX4's heading can diverge from the odemetry's yaw. This does not happen when EKF2_HDG_GATE is set to a high value.
2. Looking at the call sequence:
   • mavlink_receiver gets mavlink ODOMETRY type VISION
   • published as uorb vehicle_visual_odometry
   • received by the ekf, calls setExtVisionData
   • data pushed into the _ext_vision_buffer
   • Ekf::update runs
   • Ekf::controlFusionModes
   • Ekf::controlExternalVisionFusion
   • Ekf::controlEvYawFusion
   • Ekf::fuseYaw(3 args)
   • Ekf::fuseYaw(4 args)
   • this uses _params.heading_innov_gate which is EKF2_HDG_GATE
   • (it seems to also be used in Ekf::updateGpsYaw)

[bresch]

Thanks for the PR. I double-checked in the code and it's used for the "generic yaw fusion" (used by mag heading and EV yaw) and even in the GNSS yaw fusion.

[mortenfyhn]

Thank you for the very swift feedback!