[EMAIL PROTECTED] skrev: > Hi Karl, > > commercial receivers are limited to 1000 knots by regulation. > > Considering that your space based vehicle will probably zomm about pretty > quickly (not geo-stationary) it sounds like you need a special "unlocked" > receiver, or a P-code military one.
This is why the GEC Plessey/Zarlink based receivers with adapted software have been popular for research shots and satellites. The original development kit contained the receiver software in source form, so it could be adapted for all kinds of purposes, including this. You will find several papers online showing such work. US export conditions for commercial receivers includes speed and heigth limits which makes them unsuitable for sat work. However, the actual receiver chips is not that limited, but some can be dopper difference limited. If the sat you are going to do GPS for is a low-flying bird (eg LEO) then a standard GPS solution is possible. To answer the questions: > a) Do all satellites in the constellation broadcast a signal into > space (as opposed to, say, bouncing off some ionospheric boundary > layer) Yes. They fly at 26600 km in fairly circular orbits with a orbit time of 11 hours and 58 min, which makes their orbit pattern stable. They emit signal directed towards the earth, reaching about half the earth. The antenna rig makes the signal strengts straight down weaker than towards the edge of the earth, to compensate for the longer distance and thus increased free space propagation loss. > b) is there some website/technical paper describing expected signal > strength for these space-radiated signals? The free space propagation loss is fairly simple. It's basically 20*log(d). The good news is that there is no tropospheric error and less ionospheric error, but that can be handled. I'd pick up "Understanding GPS Principles and Applications", Second Edition by Kaplan and Hegarty ISBN 1-58053-894-0 and "Global Positioning System; Signals, Measurements and Performance", Second Edition, by Misra and Enge. ISBN 0-9709544-1-7 to read up on GPS. There are further books to pick up, including a pair on GPS software receivers. For a sat I think regular receiver chips or a frontend followed by a FPGA for digital correlator frontend and then use some suitable processor for loop processing. The benefit of a FPGA would be that the receiver can be upgraded in flight. Cheers, Magnus _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
