[time-nuts] Using GPS for space-based instrument

Magnus Danielson magnus at rubidium.dyndns.org
Tue Nov 11 00:13:02 UTC 2008


SAIDJACK at aol.com 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




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