CompuServe Messages

>>>I _think_ (but am not at all sure) that they mean if you start at point A and head for point B 96km away, and navigate only by a GPS system, you will wind up at point B with a maximum error of 1mm plus or minus.<<< I suspect what they are actually saying is that if you make simultaneous measurements from A and B against the same satellites, you can determine the distance from A to B very precisely. There's a subtle difference between saying that and saying the you can navigate to the same precision. Simultaneous measurements against three or more satellites will allow you to measure distances between base stations knowing just the distances from the satellites to each other. To navigate over time, you need to know the satellite position very accurately at different times. While the arithmetic to predict satellite motion is routine rocket science, actually predicting satellite motion with very great accuracy isn't all that easy.