1) Sometimes, the GPS could only obtain less than four pseudo-range/satellite location data, and the pseudo-range equations are unsolvable. But the GPS still would produce positioning data;

2) The positioning solution of the pseudo-range equations could fluctuate greatly from time to time, while the variation of the positioning data the GPS gave me appear to be much smoother.

Does anyone know what algorithm a GPS usually use? Do they use some form of Kalman filtering or maybe other data that I have not considered?

Thanks.