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.
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.
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.
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.
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.