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Telemetry receiving

This page describes how acquire telemetry from Vaisala weather radio sonde RS92. It comprises demodulation, descrambling, and conversion of bite stream to bytes and frames. Detailed description of packet structure is available in Telemetry decoding.

Demodulation

  • radiosode type: RS92-SGPD
  • modulation: GFSK - 4800 baud

GFSK block from GNURadio can be used to demodulate this signal. Custom block are used for data decoding.

Decoding

conversion of demodulated signal into bite stream

  • demodulated data stream contains symbols
  • 2 adjescent symbols represent one bite of data
  • symbols to bites mapping:
    • 10 → 0
    • 01 → 1
    • 00, 11 → invalid (error in reception or stream is shifted by one symbol)
Data stream atructure (symbols)
             2 start symbols
             ||
...sSsSsSsS0110sSsSsSsSsSsSsSsS0110sSsSsSsSsSsSsSsS0110sSsSsSsS...
                  16 data      ||
                  symbols      2 stop symbols

conversion of bite stream into byte stream

  • bite stream have structure identical to 8bit asynchronous serial protocol
    • 1 start bite (0), 8 data bites (LSB first), 1 stop bite (1)
Data stream atructure (bites)
        Start bite
        |
...xxxx10xxxxxxxx10xxxxxxxx10xxxx...
          8 data |
          bits   Stop bite
          LSB first
  • removing synchronization bits allow us create stream of bytes

conversion of byte stream into frames (packets)

  • byte stream is dividen into frames 240B long
  • each frame starts with predefined byte sequence

Frame start pattern (8B)

|
SSSSSSSSDDD....DDD
  • frame have attached Reed-solomon correction code, RS(255,231)
  • frame is divided into 4 subblocks
    • each subblock have different, but fixed length

Frame structure 240B

offset (B) lenght (B) content description
0 8 0x2A 0x2A 0x2A 0x2A 0x2A 0x10 0x65 0x10 synchronization pattern
8 34 subframe Calibration data block 1
42 2 0x69 0x0C synchronization pattern
44 26 subframe Measurement data block 2
70 2 0x67 0x3D synchronization pattern
72 124 subframe GPS data block 3
196 2 0x68 0x05 synchronization pattern
198 12 subframe ??? data block 4
210 6 0xff 0x02 0x02 0x00 0x02 0x00 synchronization pattern
216 24 FEC - RS(255, 231) Reed-Solomon correction code

Reed-Solomon

Datasheet for RS92 sonde states that used correction code si RS(255, 231) .

related links

 
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