ft,
it takes 3 for a 2D fix, 4 for a 3D fix.
One might wonder whether the movement caused by summer/winter is large enough to measure without the use of DGPS (as they do in geology). GPS uses a model of the earth's shape anyway (WGS84) which in itself causes small position errors in reference to the earth. As all users in aviation use this same model, there is not really an issue.
Or should we start using different summer and winter charts?
Erik.