CompuServe Messages

I didn't see the original qustion re the " 2 week delay thing" but I presume that what is being discussed here is the availability of Precise Ephemeris data for enhanced processing. The delay there is about 2 weeks. What you get in real time is a position derived from satellite orbit parameters which the satellites themselves are broadcasting as part of the navigation message. This data is based on the last orbital parameter correction from the ground control segment, and is therefore based on extrapolated predictions from orbital measurements made up to the upload time. The tracking stations are continuously measuring the *actual * orbits of the satellites as they pass over. This data is logged and refined to produce a set of parameters which describe where the satellites *were* at various times, as opposed to where they * thought* they were at those times. You're quite right in saying that us surveyors can often wait a couple of weeks for refined ephemeris parameters. However, in real time using differential techniques plus 16,000 dollar multi-channel geodetic GPS receivers we routinely achieve better than the 30 feet ( at the 2 sigma level) which you guys seem to crave. There's nothing to stop you setting up a differential base station on a geodetic point to cover a local area where you might be doing a lot of work. All you need is another Scout and a telemetry system! I'll revert on the errors soon…BTW, I hope you guys appreciate this: I normally charge 600 dollars per day for teaching basic surveying and GPS techniques to geologists and geophysicists in the seismic industry <bg>. Rob