The biggest problem with GPS is that the American military incorporate a built in "error" for readings from their satellites for positional readings available to the "general public" (different for military forces). This error only allows an accuracy no greater than + or - a 5 metre radius circle (at best), that is why for the accuracy required for surveying when using GPS, an independent "radio base station" with known XYZ coordinates has to set up and used in conjunction with many multiple GPS readings so that the error can be (almost) negated. For general use, particularly in determining speed over the surface of the earth, the results can only be taken as an indication, not fact. For long or medium distances at sea (in particular) this degree of accuracy is not a problem, but for short distances ie 500 metres, it is just not accurate enough for a precise result.