PolyITAN-2-SAU 9600bps BPSK









(C)NTUU


[Complement]

Decoder for PolyITAN-2-SAU by EA4GPZ

PolyITAN-2-SAU, or QB50 UA01, is a cubesat from National Technical University of Ukraine,
that was launched on May 2017 as part of the QB50 project. When it was launched,
I made a recording of several QB50 satellites, including PolyITAN-2-SAU. Presumably,
the modulation and coding used by this satellite is 9k6 BPSK AX.25, with G3RUH scrambling.
Back then, I commented that although the signal was strong and I could get a clean
constellation plot, I was unable to get valid AX.25 packets.

I had completely forgotten this satellite, but the other day I saw that Andy UZ7HO had
added support for PolyITAN-2-SAU to his SoundModem. I asked Andy for some help, since
I suspected that the coding wasn't exactly standard G3RUH AX.25.

The trick is that this satellite uses two "layers" of NRZI encoding. The relevant part
of the decoder is shown below. The BPSK symbols come in from the left of the figure and
the AX.25 packets exit by the right. A standard G3RUH AX.25 decoder wouldn't have the
extra NRZI decoder on the right.


PolyITAN-2-SAU decoder


Note that NRZI decoding and G3RUH descrambling commute, since the G3RUH polynomial has
an odd number of terms. Therefore, the decoder can also be organized in a different way,
with both NRZI decoders at one side (either the input or output) of the descrambler.

Having two NRZI decoders in chain is a really funny concept, so it almost seems as
some kind of mistake from the satellite team (most QB50 satellites use standard BPSK
or FSK AX.25 packet radio for compatibility). In fact, if we write an NRZI decoder as

  yn=1+xn+xn-1, where xn is the input sequence, yn is the output sequence

and the operations are performed on the finite field GF(2), then the effect of two
NRZI decoders in chain can be written as

  zn=1+yn+yn-1=1+xn+xn-2,

which is a rather strange form of differential decoder.

Thanks to Andy for giving me the clue about the extra NRZI decoder, as I would have had
a hard time in finding it by myself (although, in retrospective, it is not that difficult
to guess it by looking at the descrambled stream and seeing how HDLC 0x7e flags can be
obtained from it). I have now added a decoder for PolyITAN-2-SAU to gr-satellites.


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