[time-nuts] Jupiter GPS Question

Magnus Danielson magnus at rubidium.dyndns.org
Wed Dec 2 21:30:27 UTC 2009


bg at lysator.liu.se wrote:
> SBAS (WAAS/EGNOS/etc) use L1 carrier frequency and "compatible" PRN codes,
> but the datamessage goes at a higher rate (200bps (?)) than the normal GPS
> which uses 50 bits per second. This means that most receiver will have
> added dedicated/modified hardware for SBAS signals reception.

SBAS uses a 500 symbol rate signal which uses the standard "JPL" 1/2 
rate Convolution Encoder (actually not using the output inverter of the 
JPL convolution encode) for a 250 b/s stream. This requires a Viterbi 
decoder. However, it requires no extra hardware, but just a normal 
digital frontend, it just needs to support the SBAS C/A codes which many 
if not most frontends support. Dumping each milisecond works well 
enought. No, all it takes is additional software for SBAS symbol 
alignment, symbol-pair-alignment, viterbi-decoding, and the SBAS 
frame-structure decoding etc. Very similar to normal L1 C/A processing. 
Then comes the SBAS processing which hooks into the normal processing.

It's not trivial, but not that complex either.

A viterbi-decoder takes about 100-200 lines of C code. The SBAS 
processing takes up more.

Cheers,
Magnus




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