CompuServe Messages

There are 0nly 24 block 2 satellites in orbit. There are still a couple of old satellites left over from the block 1 ( concept testing) launches which still work. There are no satellites which sit around the equator: they are all in orbits inclined at 55 degrees to the equatorial plane. To reduce the effects of Selective Availability ( dithering) all you need is one or more stationary receivers located on a known point. Corrections to the satellite pseudoranges are produced at this static receiver on a continuous basis and transmitted to the mobiles ( at anything up to 1000 km away). This mode is called Differential mode ( DGPS). The cost comes at the correction transmission end. For short ranges MF, VHF or UHF frequencies can be used. To cover larger areas, the Inmarsat ( Maritime communications) satellites broadcast corrections from several sets of differential reference stations located around the world. All you need is a portable Inmarsat ground station!! I've been using GPS "in anger" for 10 years now. In 1986 I helped to re-triangulate northern Brazil ( finding in the process a 56 metre error in the original trig). At that time, using the block 1 satellites, we were obtaining baseline accuracies of 5 centimetres plus 2-3 parts per million of the baseline distance up tp 200 km, at the 95 % confidence level. Survey units do even better these days. Rob