Author: destevez

Recently I have added a telemetry parser to the S-NET decoder in gr-satellites. Recall that I have talked about S-NET and its decoder in a previous post. To implement this telemetry parser I have used the information in Mike Rupprecht DK3WN’s web, some additional information shared by the S-NET team, as well as some recordings done by Mike. Many thanks to Mike and the S-NET team for all their help. Here I describe a few details about the telemetry.

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 proyect. 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.

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 \(y_n = 1 + x_n + x_{n-1}\), where \(x_n\) is the input sequence, \(y_n\) 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\[z_n = 1 + y_n + y_{n-1} = 1 + x_n + x_{n-2},\] 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.

S-NET is a swarm of 4 cubesats from TU-Berlin. Their mission is to test SLink, an S-band transceiver for inter-satellite communications. They were launched on February 1 this year and they use use Amateur frequencies for their telemetry downlink on the 70cm band. Several weeks ago, Mike Rupprecht DK3WN raised my attention towards these satellites. Since they use a rather particular coding, custom software would be needed to decode the telemetry. Then, I set to add support for S-NET to gr-satellites

After some really helpful communication with the S-NET team, in particular with Walter Frese, and some exchanges of ideas with Andrei Kopanchuk UZ7HO, who was also working to add an S-NET decoder to his soundmodem, I have finally added a basic S-NET decoder to gr-satellites.