Synthetic airspeed is fused for copters when dead-reckoning using wind estimates

[pavloblindnology]

Bug Report

Synthetic airspeed is fused for copters when dead-reckoning using wind estimates, which leads to very fast explode of EKF3 in moderate wind conitions.

Issue details

AP 4.6 introduced new feature to use last estimated airspeed when dead-reckoning for planes.
This feature also takes action with copters, here

ardupilot/libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp

Line 884 in 12a6efe

if (lastAspdEstIsValid && imuDataDelayed.time_ms - prevTasStep_ms > 200) {

.
This synthetic airspeed is then fused here

ardupilot/libraries/AP_NavEKF3/AP_NavEKF3_AirDataFusion.cpp

Line 211 in 12a6efe

FuseAirspeed();

This results in EKF3 attitude exploding after ~25sec of dead-reckoning using drag forces with moderate wind.
In 4.5.7 copter would steadily drift with attitude gradually increasing till reaching max roll of 30deg in ~2min, but EKF3 would never explode.
Changed parameters:

• EK3_DRAG_BCOEF_X/Y = 9.5
• EK3_DRAG_MCOEF = 0.082
• FS_DR_ENABLE = 0
• FS_DR_TIMEOUT = 300
• SIM_WIND_DIR = 0
• SIM_WIND_SPD = 5

To reproduce, take off to 50m altitude, turn GPS off with long 218 65 2 and wait.

The issue is fixed by extending condition

ardupilot/libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp

Line 884 in 12a6efe

if (lastAspdEstIsValid && imuDataDelayed.time_ms - prevTasStep_ms > 200) {

to

if (lastAspdEstIsValid && imuDataDelayed.time_ms - prevTasStep_ms > 200 && assume_zero_sideslip()) {

Version
4.6.3

Platform
[ ] All
[ ] AntennaTracker
[*] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Airframe type
Copter

Hardware type
SITL

Logs
BIN log

[rmackay9]

@pavloblindnology, thank you very much for this. I have been a bit suspicious that our Copter dead reckoning was no longer as good as what I showed off at the australia dev conference (see video here).

[rmackay9]

Here's the PR that likely broke the feature: #24899 and here's the fix: #33607

Thanks very much @pavloblindnology for finding this!

[pavloblindnology]

@rmackay9 You're welcome.