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.>> This is not the case ( my previous posts on Differential GPS must have scrolled). In differential mode, you occupy each end of a baseline ( points A and B ) *simultaneously* with 2 GPS receivers. Over baselines of " a few hundred" km, if you are tracking the same satellite constellation from each rx, effects of timing, orbit, ionospheric etc errors will largely be modelled out, allowing you to resolve the length of your baseline to around 1mm , PLUS 1-2 parts per million of baseline length ( at the 2drms confidence level) with a reliability of, say, 80%. (NB: quoting an accuracy number *without* giving a confidence level and reliability is meaningless to a surveyor). Navigating in single receiver mode, you'll do well to locate yourself within 30 m. Rob