Compass calibration - errors and solution found

[n144sh]

I have the Skyview ADAHRS mounted in the fuselage of a steel tube Bearhawk. There really isn't any place in that plane that is ideal and meets the requirements for the ADAHRS and is far away enough from ferrous material that can interfere with the compass - hence the Dynon intro of the SV-236 remote magnetometer. That said, I thought I would give it a try. I did the ground calibration and then in-air calibrations and could get quality factors of 125 or better. However, even with the good calibration numbers, I had a consistent 30 degree or more error. Assuming that it was due to the steel frame fuselage interfering, I purchased and installed the SV-236 remote magnetometer out in the wingtip, well away from any interference sources. Again did the ground calibration and then air calibration. Calibration quality of 120 or better - but I still had a 20 degree error! Better, but really not very good.

The short story version is that the ground calibration is very important. I had assumed wrongly so that if the air calibration showed a good quality factor it would correct for errors in the ground calibration and report a bad quality factor if they couldn't be resolved. I had been using my iPhone compass app for the ground calibration. Even though I did it carefully, the iPhone was set to "use true north" instead of magnetic. I redid the calibration using a regular old aircraft compass and everything is now good.

I have not gone back to see if the ADAHRS in the fuselage can be calibrated sufficiently but suspect it would actually be pretty close given the very similar behavior.

 

[Dynon]

Thanks for the note.

To recap of what N144SH discovered, if you are using a GPS-derived compass, app, etc, as a reference source, make sure that it is set up to display magnetic direction, not true direction. In the calibration process for the ADAHRS you are trying to calibrate it to magnetic north, just a like a compass rose on an airport or a magnetic whiskey compass.

 

[llacy]

I can't get the remote compass to work. Do you have to do the ground calibration first before the calibration in the air. Actually I got better magnetic readings before I used the remote compass. I checked the location under the right rear seat with a hand held compass with no noticeable interference.
Lou Lacy

 

[Dynon]

After you installed the remote compass and connected it, did you perform a SkyView Network configuration so that it would be seen by the system? Does it show up under SETUP MENU > SYSTEM SETUP > SKYVIEW NETWORK SETUP > NETWORK STATUS?

Yes, you must perform the ground calibration. If you choose to do the in-air calibration (which is optional), you should perform it after the on-ground calibration. Know that performing the in-air calibration can only improve things.

 

[llacy]

When trying to do the in air calibration I get an indication that it is still in ground mode.

 

[Dynon]

I'm not sure I'm following exactly what you're seeing. Can you send pictures (you can grab screenshots by pressing buttons 2+7 with a USB stick connected, and then they'll be in the screenshots sub-folder) to us at support at dynonavionics dot com?

 

[mmarien]

Out of curiosity, how good does the ground calibration need to be? If the aircraft is roughly pointed it in the approximate compass cardinal directions for the ground calibration, will a good (100+) in air calibration remove the errors?

Also I got a few more points by doing lazy eights after the initial 360's. Hey, they were fun! The last few points were increasingly difficult to get and I suspect there is a limit. Is there a perfect score other than more than 100?

 

[jakej]

You just have to do better FWIW my initial air cal was about 125. Maybe it's the coriolis force up there :-*

Do you need an excuse to go fly ?

Jake J

 

[mmarien]

Curse you Red Baron. You did beat me :-[

Next time I'll add a few chandells or Immelmanns to see if that improves things. The lazy eight inevitably turn into wing overs after the first couple but the score stopped at 123. Still the question of will the air calibration fix a shoddy on ground calibration.

 

[Dynon]

An in-air calibration can improve what you obtain on the ground, but if it's truly bad, it's not going to do magic. The in-air calibration is primarily tuning the 3D aspect of the sensors - so that heading is as accurate as possible in all flight attitudes.