[email protected] 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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