The Compass and its errors are one of the most discussed and misunderstood items on the DJI machines. The posts on various forums and flight-line gossip that gets repeated perpetuating the mistakes is unbelievable.
First : On first receiving your nice new DJI - please go through all calibrations properly before flying. I know its a pain in the **** but believe me - its worth it.
Calibration - lets remove one error in advice ... The IMU calibration may not directly have compass BUT it has a referencing part - look at the IMU calibration page bottom set of figures ... Therefore I suggest ...
note SUGGEST ... that when doing IMU - do this in same place as you would do your compass calibration ... CLEAR OF ALL INFLUENCES that may interfere with Compass and its referencing in the IMU. Of course thousands will laugh at that and say ... no way Hose ... we've been doing IMU in the garage ... in the house ... on Aunt Fanny's knee ! Fine ... carry on.
OK we move onto compass calibration - why do we do it ?
Its quite simple really but based on some serious geomagnetic stuff that really would take up page after page of data. To give an example VARIATION (sorry I'm International and all the 50 odd years I've been with compasses Variation is the word ... Declination is the Celestial equivalent of Latitude) ... anyway Variation is the magnetic lines of flux of the earth and their offset from Pole. The Magnetic North lies in Alaska west of the North Pole and is slowly moving ever westward. The Geomag lines are distorted on the earth as well giving rise to a non-uniform setup.
The picture gives a good idea of the difficulty faced with it. ie Change of Variation of up to 25 degrees between Florida and California coasts.
OK that's illustrated.
Next we have DEVIATION - the effects of on-board items that deflect compasses ... now herein lies another error made by people. They think that this is just a fixed figure and that's it. Not quite true. The DEVIATION may be fixed if we take the item and place in a totally non-magnetic environment - but sadly we live on the earths surface and orientation of that DEviation causes varying effect.
OK ... now that we have two basic items noted ... Earths Variation and Items Deviation ... how do they affect us ?
This is where vectors come into it .... similar to two forces meeting at varying angles - the sum of which changes with angles.
DJI advise us to calibrate in VERTICAL and HORIZONTAL orientation. This is actually a good way to arrive at the different results of the vector addition of the VARIATION and DEVIATION and so to calibrate ... Remember we are flying a model that acts in 3 dimension. As you rotate the model - the results of the two lines of flux interact and produce the necessary numbers for the calibration. Those numbers alter as you rotate. The amount and direction of change tells the compass enough to correct sufficient for our purposes.
Where does GPS come into it as some suggest ? It doesn't. GPS is only a Position indicating system. It has no way to determine orientation in any form. Any solutions provided by GPS are the result of consecutive positions and the calculations of movement from one to other. Speed, direction, CTS, DTG .. these are all derived from repeated position fixing and averaging. To get our model to start RTH for example - needs first model position and the compass to say what is the heading at that moment. The calculations are then made to turn the model and point it in correct direction based on Compass heading ... with direction confirmed by repeated GPS position information.
It has been suggested that GPS provides VARIATION data to the model and that with DEVIATION corrects the compass ............ Poppycock is all I can say. Go back and read what happens when a body orientates within a magnetic environment ... The Chip used in the model may have Geomag data programmed in ... fine - but the GPS can only tell where you are ... model can pick up relevant data from the chip ... but what to do with it ? Come in the calibration routine we do.
OK ... when to do Compass Calibration ?
a) When its new and you haven't flown it before.
b) Update of FW and / or components of the model.
c) Addition of items such as electronic tracking device etc. that can influence models DEVIATION.
d) Moving significant distance from last calibration location - note my quote of 25 degrees change Florida to California.
When not to do Compass Calibration ?
a) On a boat dock - they have metal in the structure
b) On a boat - they have engines / various magnetic items all over
c) Next to cars / metal structures
d) In your house driveway - usually there are pipes and cables running under
e) Mall Car parks - same as house driveway
f) Each time you go flying !! This is the most common mistake of all ... Basically once you have a good calibration - LEAVE IT ALONE !!
Finally : I know I have trodden on a few sensitive 'toes' with this post and I know that various will not agree with me. That's their choice and I just hope that they do not have any incidents or accidents as a result.
I have posted in all fair and reasonable manner based on many years of Compass knowledge from a professional career. The Laws of Physics apply and cannot be avoided. Even with todays MEMS based Compasses as DJI and many others use.
People will make up their own minds and decide for themselves. Mine was made up by Professors at Plymouth College back in the 1970's and the 17 years navigating ships around the world. I have been active in RC hobby since about 6yrs old ... and seen the changes and development from mechanical gyros to todays sophisticated solid state MEMs devices. Compasses, Flight Controllers ...
I do not know or pretend to know all the answers and am learning new everyday ... but one thing I am sure of - Magnetism and Compasses do not change whether needle, digital, Fluxgate or any other that relies on magnetic influence.
Nigel
