I was working on a GPS/magnetometer mounting bracket and wanted to insure that the screws holding the assembly to the (printed) frame of one of my quads didn't come loose. Most of the GPS units you buy for mutlitirotors also include a magnetometer as part of the assembly.
Magnetometers (electronic compasses) are, of course, sensitive to magnetic fields. Whenever a wire carries current, it develops a magnetic field around it. For that reason, it is vitally important that you keep magnetometers as far away from current-carrying wires as possible. Since multirotors have lots of current going to the motors, it is difficult to keep the magnetometers far away from those wires.If the magnetometers are too close to the wiring, the multirotor will turn (YAW) during periods of high-throttle. The problem can get severe enough that the mutirotor will go "nuts" in autonomous mode, since the GPS says the craft is going in one direction, and the compass says it is going in another. The GPS is not affected by the magnetic field.
In order to improve the situation, I often use a piece of magnetic steel or else "mu-metal" under the magnetometer. Thin steel is an effective magnetic shield, mu metal is even better, but is more expensive and harder to find. I also support the platform on standoffs or long screws to place the assembly well above the rest of the mutirotor frame. In this particular case, I used long #6 screws for that support.
I normally use nylok nuts on the screws, but I had no #6 nylok nuts, so I decided to use Lock-tite (blue).
LOCK-TITE DESTROYS ABS!!!!