I have drone with Pixhawk and 3DR GPS. I have placed the GPS on a mast, away from the Pixhawk as instructed in Ardupilot. I would like to know the GPS can be placed inside. I do not want it to be sticking out. I understand that it needs visibility but how are the commercial drones doing it? DJI and Parrot have their GPS inside and its not visible. Gives a good look to the drone. Do I need to buy a more powerful GPS? What is the solution to this?
GPS uses a radio receiver which operates at about 1.5ghz. As long as no RF blocking material blocks the signal (coming from satellites overhead), the GPS will receive data. RF blocking material; metal or carbon fiber (i.e., electrically conductive material) will degrade the signal.
In addition, some might argue there are RF harmonics interfering with the GPS. Any RF harmonic emission from another device on the copter is probably in violation of FCC Part 15 rules on EMI.
The compass (magnetometer) is sensitive magnetic fields; generated by motors and/or heavy power cables.
The main reason for mounting the GPS(which probably has a compass chip in it's case) is to keep the compass away from magnetic fields. Typically on a quad it is safe to mount a compass that is equidistant from the four motors.
One of the dumbest ideas that has ever been propagated on quad-copters is mounting a GPS on a stalk. One crash and flip; crunch... I think it was done for the "cool factor". Ranks up there with the idea of heating a gyro to improve stability...just not necessary!
As Thomas said, because the GPS unit has the compass in it it needs to be kept it away from any wires carrying high DC currents and a clear view of the sky. DJI Phantom puts the compass in one or two of the legs, I think.
You can experiment and find how close you can get it before it affects your flight.