Who is this feature for?
The majority of setups do not need to use an external compass to get acceptable performance from their HoverflyGPS. Most multi-rotors will not need to take any of the precautions outlined below. You should start by moving your ESCs, batteries, battery wires, Motor wires at least 6" from any edge of the HoverflyGPS. Be sure to remove all magnetic materials (steel, or anything that sticks to a magnet) from your multi-rotor. If after trying the above you are still having problems with circling or flying away when engaging GPS hold, read on.
In some muti-rotors, the magnetic fields produced by electronic components on your multi-rotor interfere with the compass' ability to get a correct magnetic heading. This causes circling or fly-aways when position hold is turned on. The ideal solution is to move these components (ESCs, batteries, wires, etc) further away from your HoverflyGPS. When this is not possible, an external compass may be used and mounted in a remote location away from the other electronic components of your multi-rotor.
How do I know if I need to do this?
The easiest way to tell if you are experiencing magnetic interference is to fly with magnetic hold enabled and with autoleveling turned on. If when varying the throttle (with the rudder stick centered), you see a noticeable change in heading it is very likely you will need an external compass.
What You Need
2x 10k pullup resistors (Any 10k resistor should work)
1x Soldering iron and solder
v4.5 or higher of the HoverflyPro and HoverflyGPS firmware
A HoverflyGPS and HoverflyPro flight controller
What to do
1. Solder the Black wire to GND and the Red wire to VCC on the compass using one of your servo leads
2. Using the same servo lead, solder the Yellow (signal) wire to SDA
3. On the second servo lead, solder the Yellow (signal) wire to SCL.
4. Install the pullup resistors. Each resistor must go from SDA to VCC and SCL to VCC
5. Connect the assembly to your HoverflyGPS' EXT BUS
6. Mount the new compass on your multi-rotor away from any other components
7. Orient the front of the compass with the front of your HoverflyPro
7. Re-calibrate your compass using the compass calibration procedure
How you know its working
When plugging in the battery, your HoverflyGPS' LED will flash Green for 2 seconds before flashing purple. This indicates that the external compass is connected correctly.