## Soft Viterbi decoder for AO-40 FEC

A year ago, I made a decoder for the AO-40 FEC. While AO-40 has been dead for many years, the same FEC system is used in AO-73 and the rest of the FUNcube satellites. This decoder was later included in gr-satellites and it is currently used in the decoders for AO-73, UKube-1 and Nayif-1.

When I implemented this FEC decoder, for simplicity I used a hard decision Viterbi decoder, since my main concern was to get all the system working. I always intended to replace this by a soft decision Viterbi decoder, but it seems that I forgot completely about it.

Now, while thinking about integrating gr-aausat (my AAUSAT-4 decoder OOT module) into gr-satellites and adding a soft Viterbi decoder for AAUSAT-4, I have remembered about this. While the decoder for AAUSAT-4 will have to wait, since I have found a bug in the GNU Radio Viterbi decoder that makes it segfault, I have already added a soft Viterbi AO-40 FEC decoder to the FUNcube decoders in gr-satellites.

## Decoder for PolyITAN-2-SAU

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.

## BCH decoder for S-NET

In my previous post, I talked about the coding used by the S-NET cubesats and the implementation of my decoder included in gr-satellites. The decoder was still missing a BCH decoder. I have now implemented a BCH decoder and included it in gr-satellites. Here I describe the details of this decoder.

## Decoding S-NET

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.

## Improved signal processing for LilacSat-2 VLBI

Last week I published my results about the LilacSat-2 VLBI experiment. There, I mentioned that there were some things I still wanted to do, such as studying the biases in the calculations or trying to improve the signal processing. Since then, I have continued working on this and I have tried out some ideas I had. These have given good results. For instance, I have been able to reduce the delta-range measurement noise from around 700m to 300m. Here I present the improvements I have made. Reading the previous post before this one is highly recommended. The calculations of this post were performed in this Jupyter notebook.

## TY-2 decoded

In January, I took a look at TY-2 telemetry. This is a Chinese cubesat that transmits 9k6 FSK telemetry in the 70cm Amateur band. In my previous post I tried to reverse engineer the packets from TY-2 and got as far as recognizing the syncword, and noting that the syncword is the same as the one sent by the GOMspace NanoCom AX100 transceiver. However, in all AX100 transceivers I had seen, the syncword was sent scrambled with a G3RUH scrambler, and TY-2 sent it unscrambled. This left me a bit puzzled. The payload seemed to be scrambled and I was unable to descramble it, preventing any further progress.

Since then, I have tried to get in contact with the satellite team to see if they could give me any additional information about TY-2 and its companion TY-6 (which uses the same format). Finally, the satellite team have answered me, giving me some details and confirming me that they use the AX100. This has allowed me to finish the decoder. An updated decoder is now available in gr-satellites. Thanks to BI1AEM for his help. Here I look at the specific details of the format used by TY-2.

## Amateur VLBI experiment with LilacSat-2

On 23 February, Wei Mingchuan BG2BHC published on Twitter the first Amateur VLBI experiment. This consisted of a GPS-synchronized recording of signals from LilacSat-2 using USRPs in groundstations at Harbin and Chongqing, which are about 2500km apart. Wei has made a Github repository containing the recording (in MATLAB file format) and some signal processing in MATLAB. I have done some signal processing of my own with the recording and published my results in a Jupyter notebook. Here I describe some general aspects about VLBI and its use in Amateur radio, and some specific details of the signal processing I have done.