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I wrote a feature for ArduPlane that allows the home position to be somewhere else than where the APM was started. This is useful for flight under "flying line" rules or if you want to ready your plane in one place and fly it from another.

The feature was requested and discussed in this thread and here. My modified code is here.

The original APM behavior: Home position is set to one of the first positions the GPS reports after powerup. Even if you desired another position as home. The position is written into waypoint 0 too, so you will see the home position by (re)loading the mission into MP.

Modified behavior: There is a new parameter STICKYHOME_RAD, in meters. If the GPS position is nearer than this from waypoint 0, home will be se to waypoint 0. Otherwise it will remain unchanged and the controls will wiggle 3 times. This allows one to set an alternative home position without risking a flyaway of more than STICKYHOME_RAD meters. In this version, I have removed the servo wiggling at normal startup because I find it unnecessary and it makes my planes rock'n'roll at gyro calibration. Wiggling = something was not as expected.

Waypoint 0 is only overwritten if it was previously undefined. Reading the mission, the waypoint 0 read is the one last written. Any reason to change that?

There are a few console messages too. It appears that not all get though because they come in a too rapid succession. See init_home() in commands.pde.

The home altitude may be quite inaccurate too, because GPS takes some time to get altitude right. If you fly with pure baro altimeter (ALT_MIX=1) and you use relative-altitude waypoints, you will not be affected. Funny enough, this is default. I fly in hilly terrain and need to check my missions carefully against Google Earth etc. and therefore need absolute altitude. I often get home altitudes that are 100s of feet off, potentially disastrous if not rejected.

To try solve that problem, the altitude of waypoint 0 is also used. If the horizontal distance to waypoint 0 was OK, the GPS altitude is compared to the waypoint 0 ditto. If off by less than STICKYHOME_MAD (Max Altitude Deviation), the altitude of waypoint 0 is used for home. Else, the controls will wiggle 5 times.

So all in all

No wiggles: Waypoint 0 was undefined at startup, or both position and altitude were within bounds of waypoint 0
3 wiggles: Position was not within bounds of waypoint 0. GPS position and altitude were used for home3 wiggles:
5 wiggles: Position was within bounds of waypoint 0 but not altitude. GPS altitude was used for home. Recommended action if using absolute altitude: Wait a few minutes, try reset APM.

For consideration: When reading the mission, should the EEPROM stored waypoint 0 be returned as waypoint 0, or should the actually used home position (which is the same if within bounds)?

If postion is OK but altitude is off, should the APM go into a wait loop until altitude is OK?