Hi,

Bo need to put your head under the sand ...

a location device will be necessary : moving at a constant speed doesn't produce any accelaration and compasses changes are too weak to be seen.

so:

1) a string fixed to a reference point ( the simplest ... )
2) a surveyor dog
3) a wheel on the ground
4) the RSSI indicator of a Rx chip ( need a Tx somewhere in a reference point ...)
5) a stupid GPS device ???

you already have locating devices on the market ( for dogs, old persons, cars ....) so, try to see how they work ...

Alain