CompuServe Messages

* Secrets*: Can't give you a proper reply in time for this trip, but on the "fixed height" thing: Generally, 4 satellites are required to give a 3D fix, because you have 4 unknowns: Lat, Long, Height and User Clock Offset from GPS system time…ie , how far away from GPS time your receiver clock is: they can't be fully synchronised since receivers use "cheap" crystal clocks which are worse timekeepers than the caesium and rubidium satellite clocks by several orders of magnitude. You can see, without me going into the maths, I hope, that if you had some knowledge of your height, you could constrain it to a known value and would then only have Lat, Long and UCO as unknowns, allowing a fix with 3 satellites. On one of your other questions, I reiterate that the satellites are not in geostationary orbits, but they're still high ( I'll give you the exact figure tomorrow when I log in from work, since I don't have a textbook handy and I don't want to give you US info). I stated earlier that the Block 2 satellites are in orbits inclined at 55 degrees. I also noted that there are a few functional Block 1 ( test satellites) still around. These are in orbits of 63 degrees. I'll try and figure out a way of representing orbital plots here, but failing that I may try to capture a screen from the Trimble survey planning software suite and upload it as a GIF. Rob