Originally Posted by head_wind
Quick discussion of GPS accuracy: with 3 satellites strong and in view 2 dimension
mapping works and a 4th is required for 3D which provides altitude. I believe
that every vendors software will use more satellites but don't actually know that.
Imagine 3 satellites in a straight line at the zenith which are very close together.
Compare that with three satellites near the horizon as far as possible from one
another (120 degrees apart). The time signals and location from the first three
in a row will be very close together while the three in a triangle will be seperated
more. Any computational errors from the three in a line will be a much larger
proportion of their time differences compared to the big triangle satellite configuration.
Therefore the three in a row will have noticably more error.
Actually, four satellites is the minimum number of satellites required to give an unambiguous position. With three, there are two possible locations. Of course, if one of the two positions is outside the altitude limit of a commercial GPS unit, or below the geoid surface, then the other position can be assumed correct.
But, even in the best possible conditions, the average positional error is on the order of 15-20 ft.