Mounting Pixhawk facing rearward

When mounting the Pixhawk with the arrow facing rearward on the airframe what should I select in Mission Planner for orientation? (manual / yaw180?). And should I leave my compass/GPS module pointing forward (or do I have to mount this the same direction as the Pixhawk (rearward)?

The orientation should be set at yaw180. Every time you change a setting like this or move anything around, you should always re-calibrate the “level” position, accelerator and compass again. I find it useful to open up mission planner and point the vehicle somewhere, like exactly towards the sun. Mission planner should show it pointing directly east or west, if not, you have set the orientation wrong. Another useful test is to bank the aircraft in each direction and make sure the previous test still holds true. (Once I had set it wrong and it worked on the ground, but once I started flying and thus banking, I knew something was wrong)

You should leave the GPS/Compass pointing towards the front of the vehicle. If this isn’t practical, or you want to change the orientation of that also, you can simply change the COMPASS_ORIENT parameter to suit.