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TIANHE-1 / Chinese Space Station (CSS)
S-band downlink signal spectrum
S-band downlink parameters
| Frequency: | 2280 MHz |
| Modulation: | 3069000Bd BPSK spread spectrum (1023 chips) |
| Despread mod.: | 40000Bd BPSK |
| FEC: | CCSDS r=1/2 k=7 |
| Framing: | 8192 bit CCSDS AOS frames |
| ECC: | 4x interleaved RS(255,223), duobinary |
| Scrambling: | standard CCSDS scrambler |
Spread spectrum parameters
Spreading sequence is generated by LFSR x^20+x^19+x^18+x^16+x^11+x^8+x^5+x^2+1 (0x1D0925):
0101001001110100111011110101111110100111001100000001001001110111010001111110100100010010100100001000 0011000001011001011111110010000001001101101110011110100000100110100001100101101100010011010111110000 1010110111001001110100110111000010110110011110001100011000101011000100011001111011100101101011000001 0011110100111000000111100111000010000001000110101110110010011100010111100101001010100010101000100001 0111010010010100100101110001001111111001010111110000100101000101111000001100001000111110110111101000 0101011111101001111011010111011011100010110010000011010111101001110100111111100001010001111010000100 1110000001111101111011000100101110001000100010111000010011010111101100011001100010011001111010110111 0101111110001100000010011101010101011001011001110101010111101111110000111011111010100011100100101111 1101001110000111001001110001000101010100101100011110110110011010111101100000110101001111011010001000 0000001101011101000110110110110001011010110111000111100010001101101100110111010000001010100011000110 11000100010011101100000
Despread signal spectrum
Note that data bits are not aligned to spreading sequence, 1023 chips contain 76.725 data bits.
Decoded frames
After despreading, 1/2 FEC is used, then usual CCSDS frames of length 8192 bits, 4x interleaved RS code and CCSDS scrambler:
Frame header looks like this:
1A CF FC 1D 4C 01 2E 0B 03 00 33 00 00 (10 random bytes/IV) 00 (26x 0xFF) (more random/encrypted bytes) 1A CF FC 1D 4C 01 2E 0B 04 00 33 00 00 ... 1A CF FC 1D 4C 01 2E 0B 07 00 33 00 00 ... 1A CF FC 1D 4C 01 2E 0B 08 00 33 00 00 ... 1A CF FC 1D 4C 01 2E 0B 0B 00 33 00 00 ... 1A CF FC 1D 4C 01 2E 0B 0C 00 33 00 00 ... 1A CF FC 1D AA AA AA AA AA AA AA AA AA ... (idle frame)
Downlink is clearly encrypted, the 10 changing bytes in the header is probably encryption IV followed by encrypted data.
Odditities
The downlink format is not really CCSDS compatible and there some deviations from standard or design mistakes:
- Last frame byte is always 0x55, this means one of the RS ECC block codes will always have 1 error
- Idle frame doesn't even have valid TM transfer frame header like it should, all bytes are just filled with 0xAA
- Spacecraft ID is 0x30 and this ID is NOT SANA registered for this mission. Actually this ID is already assigned and in use by JAXA Hisaki SPRINT-A.
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