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Link layer (TM)

From symbols to bits

The demodulated soft symbols of the BPSK subcarrier were passed to ccsds-tool in guess mode, with a sync SNR of 15 dB (this is a tool-specific value used to define a threshold for detection of ASM sequences). The tool reported a CCSDS turbocode at rate 1/2 in the Q channel, with a frame length of 8920 bits:

$ ccsds-tool -g channel-capture-PM-52639-baud-0001.raw -S 15 
Looking for syncwords for r = 1/2
Looking for syncwords for r = 1/3
Looking for syncwords for r = 1/4
Looking for syncwords for r = 1/6
Candidate CCSDS turbocode found
    Code rate:    1/2
    Channel Q:    6590 frames
    Frame length: 8920 bits

After calling ccsds-tool again with the 1/2 code-rate, we managed to obtain a frames.bin file, containing all the successfully decoded turbo-code blocks. The validity of the decodification is verified against the block's CRC.

$  ccsds-tool -r 1/2 -f channel-capture-PM-52639-baud-0001.raw -S 15 > frames.bin
CCSDS tool v0.1 for the Amateur DSN by EA1IYR
(c) 2021 Gonzalo J. Carracedo - https://actinid.org
  Code rate:       1/2
  Block length:    8920 bits
  Turbocode iters: 1
  Channel:         Q
  Sync SNR:        15 dB
  Input file: channel-capture-PM-52639-baud-0001.raw
  Output file: stdout
Decode rate: 1.61 Mbpsps
ccsds-tool: 7348965 bytes decoded

Data link format

We know now that the frames are 8920 in length. We then used the chopper tool to take the bits from frames.bin and produce a bitmap image with 8920 columns, so that similar bits in the same position in each decoded turbo block shows up in the same place, forming patterns:

$ gcc chopper.c -o chopper                                    
$ ./chopper frames.bin > image.pbm
Total: 6591 blocks

The resulting image looked like follows:

Further analysis on the repeating patterns showed that they were actually CCSDS TM Data Link frames with a 6-byte primary header, no secondary header, 6-byte trailer with a CRC. Two virtual channel IDs were in use along this capture: Virtual Channel 0 (containing encapsulated data of the upper layer) and Virtual Channel 7 (idle data produced from a featureless PRNG noise, with a polynomial that is not what the specification recommends).