Airsim: yaw problem

[KeyneZzz]

in airsim my drone is heading exactly at north, but the hitl output GPS says that the yaw is around 10 degrees. i think it may related to ekf2 estimator. however, since it's in hitl, is there any way to ignore the deviation?
thx

[madratman]

@clovett do you think this is fixed with the recent commits?

[clovett]

Unfortunately no. I call this "crabbing" - the drone has a small yaw delta that stops it from pointing directly forward in whatever path it is flying when you use YawMode(False,0). It should be pointing directly forwards along the path it is flying, but it is not, usually rotated about 10 degrees anti clockwise. It has done this for a long time, and still does this - but the behavior is random, sometimes it doesn't do this at all and I have not figured out the cause. I suspect it might have something to do with differences in the calculation of magnetic declination between what AirSim is sending to the PX4 and what the ekf2 is calculating based on given GPS locations. In the picture below it should be pointing down the road on the red arrow, but instead it is pointing to the orange arrow.

Interestingly the drone is placed initially on the map completely aligned with the direction of the road:

But as soon as it takes off the 0 degrees position points north, so north is not aligned with the road:

and I'm fine with that, but once I start flying down the street with YawMode(False,0) it should point the drone in the direction of the flight, but it is not doing that, instead it seems to continue with the initial delta throughout the entire moveOnPathAsync.

[clovett]

Ah, ha, it is declination. When I set start point to somewhere on the equator the take off is square and this continues throughout the flight using this setting:

    "OriginGeopoint": {
        "Latitude": 0,
        "Longitude": 112.553869,
        "Altitude": 20
    }

and it continues that way throughout the flight:

[sytelus]

Probably relevant issue:
https://github.com/PX4/ecl/issues/30

So looks like PX4 has setting for declination and it’s default is probably valid for equator. If you change this setting in PX4 then you might be able to set correct latitude.

[madratman]

this is the page to look at (I am just doing a ctrl +f "declination")

EKF2_MAG_DECL is defaulted to 0 deg
EKF2_DECL_TYPE is a bitmask setting variable which allows us to lookup the declination from gps from the geo_lookup library if it's set to 0. By default, it's set to 7 though
Chris
So, should we add this to our px4 docs? @clovett, can you confirm what EKF2_DECL_TYPE is on your setup?

[lovettchris]

Turns out this is an AirSim bug, the declination doesn't match what PX4 is computing so when PX4 tries to remove the declination that AirSim added, it is over compensating. Until this is fixed it is recommended you set a geo position on the equator. If you must fly somewhere else, then find out what declination AirSim is generating then edit the PX4 startup config file “build/posix_sitl_ekf2/tmp/posix-configs/SITL/init/ekf2/iris” and set EKF2_MAG_TYPE to 0 and EKF2_MAG_DECL to whatever AirSim is providing, and then they will cancel out nicely and the drone will fly nice and straight.