Calibrating the Phantom 3 Compass
Why Calibrate?
Compass calibration is important to safe, controlled flight. It compensates for changing background magnetic "noise", a.k.a. magnetic inclination and deviation (not to be confused with declination). Inclination and deviation that isn't corrected through compass calibration will cause inconsistencies between GPS and compass that can result in "toilet bowl effect", a swirling motion that can cause the Phantom to fly out of control.
What is Magnetic Inclination and Deviation?
Magnetic deviation is a horizontal variation that comes from the Phantom itself and the equipment you have installed on it as well as the magnetic makeup of the area you are flying in (again not to be confused with declination). Sometimes the deviation will be insignificant, but other times it can be big enough to cause you to lose control. Inclination is a vertical magnetic variation that shifts depending on where you are.
Warning Signs
The Phantom can detect when the compass is providing extremely poor (implausible) data. This typically occurs if you place it near a strong magnetic field or do not calibrate it properly. It will flash red and yellow lights and will not start the motors when this happens. Unfortunately, it can only detect this in extreme conditions and you can still fly with really bad compass data if you're not careful.
Another important safeguard is the compass mod value. This is the total magnetic field as measured by the sensor. You can check this with the Phantom Assistant software. According to DJI, it should be above 750 and below 2,250 but ideally it should be between 1,000 and 1,700. Between 1,200 to 1,500 is very good. Check it away from magnetic influences. If it reads very high or very low, check it again in a different location. If it is still off, it could be magnetised and need degaussing or it could be damaged.
What Does Calibration Actually Do?
Calibration measures the magnetic fingerprint of the surrounding area. By turning the compass 360 degrees, the Phantom can see where the compass reading doesn't smoothly increase or decrease. It uses this information to build an adaption table so that when the Phantom turns during flight, the reading is smooth and linear.
When Should I Calibrate?
You do not need to calibrate before every flight and in some cases you definitely should not calibrate. That doesn't mean you shouldn't ever bother doing it. It only takes one time for it to go very wrong. The most important aspect of compass calibration is making sure the magnetic "noise" around your Phantom is consistent between calibration and during flight.
DO Calibrate
If you go to a new location that is a good distance (i.e. >100 miles) from the last place you calibrated the compass.
If the terrain has changed significantly i.e. going from prairie to mountainous.
If you change any equipment on your Phantom.
If you just installed new firmware.
If you just degaussed your compass (BTW, don't degauss unless you are absolutely positively sure you need to).
If you have taken all the precautions to make sure there are no localized magnetic fields near you.
DO NOT Calibrate
If you're in an urban area surrounded by concrete, buildings, and hidden or overhead power lines / pipes / etc.
If you're on the beach or on a boat.
If you're in immediate proximity to metallic objects or anything magnetic.
