Im in northern Sweden.. So its snow and ice everywhere.. Any directions on how i should calibrate compass? Can i do it on flat ground with snow/ice?
Compass Calibration, A Complete Primer
- Thread starter ianwood
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You hold the bird in your hands when calibrating the compass. Not sure what snow/ice/flat ground has to do with it.
That should be done on a completely level surface, inside, without being disturbed until the calibration has completed.
Find a cool, level place free from too many vibrations. I use a spot in my basement. Let it sit undisturbed for 20-30 min to stabilize the temperature. Got to imu cal as quickly as possible so that the system doesn't warm up. Not a bad idea to redo the gimbal cal afterward too.
Actually, you SHOULD do that outside! Its COLD if there is snow and ice! You want to do your IMU cal when the bird is as cold as you plan to fly or colder. I did mine at -15f (or somewhere there abouts). By doing it when the bird is COLD, you get absolutely no warmup delay. So do your IMU now and do it outside (in an open garage is fine). I did mine in the garage so it was isolated from any wind. It complained about compass error but thats not a problem since you should be doing your compass cal after IMU anyway.
So, set your bird outside in your garage (if you have one). Let it chill for an hour or so. Keep the battery inside where its warm. Then haul the battery out to the bird, slap it in and do the IMU cal. Once done in the cold you are good to go the rest of the year.
This isn't correct. The field strength variations detected are used to determine on board distortions, and then build a model that can be used to compensate for those distortions. They aren't used to determine the field structure (i.e., the "magnetic fingerprint") of the surrounding area.
To simplify the discussion assume that there are no on board distortions and that any field strength variations on the circumference of the compass dance are caused by external distortions. These field strength variations can't be used to determine the geomagnetic field structure beyond the calibration circumference. A calibration based on these variations would be valid only for flight within the calibration circumference.
To see this consider a simplified example where two calibrations are done next to each other. Further, there is an interference causing field strength variations. Flight with calibration 1 would expect to see maximum field strength in the northeast quadrant whereas using calibration 2 will have the maximum field strength in the northwest quadrant.
Someone may claim that the above example was cleverly constructed so that the two calibrations are different. That by moving the calibrations away from the interference the field will be the same for each calibration. That's true, but, then there won't be any field strength variations on either of the calibrations. Likewise for reducing the intensity of the interference. The key point here is that if the field is distorted enough that the strength varies within the calibration circumference then a different calibration done at a different location will give different results.
N017RW
Premium Pilot
+1
In the past I always used the term 'compensation' rather than calibration to better describe the results.
In the past I always used the term 'compensation' rather than calibration to better describe the results.
I would agree my description is not accurate. More of a weak attempt to try explain it as simply as possible. However, I think the basic concept is still conveyed.
If the basic concept is that the calibration determines the field structure external to the AC then that's not correct.
Interesting. Take a look at this article. Among other things it addresses hard iron and soft iron.
Compensating for Tilt, Hard-Iron, and Soft-Iron Effects | Sensors Magazine
There are two sentences that are particularly applicable to our discussion
"It is also important to recognize that effective compensation of hard- and soft-iron distortions is dependent upon the distorting material(s) rotating/moving with the sensor." and
"In contrast, it is much more difficult—if not impossible—to compensate for distorting effects exhibited by material external to the aircraft/sensor platform."
Localized distortions that vary from one region to the next can be measured and factored. That is part of what calibration does.
GadgetGuy
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All the more reason not to recalibrate the compass after a good compass calibration, unless you are more than 150 miles from the last good compass calibration! Recalibrating before every flight is far more likely to introduce compass errors than to fix what isn't broken in the first place! 
N017RW
Premium Pilot
There's no fixed distance based deciding factor. Sooo many compass based misunderstandings still circulated here.
Can you be more specific? I can't tell if you're referring to points already covered.
If you're referring to geoMagnetic declination then that is not determined by the calibration either. In fact, declination can't be determined because there isn't enough data. To determine declination the calibration procedure would necessarily have a step where the AC were placed in a known, precise, true heading and then the magnetometer heading for that true heading is recorded. In essence, the calibration has to be told what the declination is before it can determine the declination.
Another way to look at this is that if you know X + Y = 10 and you're asked what is the value of X. It's impossible to know this without first being told the value of Y.
Declination is determined by the AC shortly after batteryOn using GPS coords. There is at least one algorithm that computes declination from GPS coords; DatCon uses this algorithm. Here is an example of a P3 at a location where the declination is 12.72 degrees. MagYaw is the heading computed from the magnetometers and Yaw is the Flight Controllers heading value. Within a second or two after batteryOn the magnetometers are producing data and show a heading of 153 degrees. the FC sets Yaw to be the same value as magYaw. Then, when gpsHealth and numSats show the GPS has valid coords the Yaw value gets changed to 165 degrees (i.e. magYaw + the declination).
N017RW
Premium Pilot
This makes sense. The 'original' Naza FCs (and others???) required the compass/GPS puck to be rotated to adjust for this manually after/during flight tests.
Not sure about present offerings as I don't research them but explains the past.
Not sure about present offerings as I don't research them but explains the past.
Would the alignment of the compass/GPS puck be considered part of the compass calibration? At least by some? The reason I ask is that it could be a partial explanation of why there is such a wide spread belief that declination is determined during the calibration.
N017RW
Premium Pilot
Well I'd say more set-up. As I recall the instructions gave you a rough angle then you'd fly and evaluate the TBE and adjust til none or minimum.
So that would be a stretch to call it calibration.
So that would be a stretch to call it calibration.
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