@Wiggy, provided the GPS is connected to an external antenna then it will work accurately, down to a few feet or less of the location of the antenna. In the air it will be receiving data from perhaps a dozen or so satellites at any one time which gives great accuracy. On the ground much less so as not only will it receive fewer satellites most of the time but it may also receive reflected signals from buildings which could cause spurious readings. Try driving along a motorway with one at a constant speed and and watch the fluctuations as you go under bridges, or the huge spikes if you go through tunnels. Imagine the latter effect on a fixed antenna on a rotating body where it is seeing a different set of satellites as it rotates. Mine will often tell me I reached 600 mph as I exited the tunnel (I had it set to record every second), with no signal in the tunnel it estimates position until you exit and it receives again, if there was a turn in the tunnel you will not be where it expected so it will correct it the next second, similarly if you change speed. This should go some way to explain what happened after the event in respect of GPS readings.