This is a follow-up to my previous post, where I explained the modulation and coding of Tianwen-1’s telemetry. In this post I will explain the framing structures and the data contained in the telemetry (though we only understand a few of the telemetry channels). Most of what I’m going to explain here was found first by r00t.cz and is already presented in his Tianwen-1 page. In this post I’ll try to give a bit more detail (especially for those not so familiar with the CCSDS protocols) and some Python code for those interested in digging into the data.
Using, configuring and programming amateur radio related software.
Tianwen-1 telemetry: modulation and coding
As promised in this post, I will now speak about how to demodulate the Tianwen-1 telemetry signal. This post will deal with demodulation and FEC decoding. The structure of the frames will be explained in the next post. In this post I also give a fully working GNU Radio decoder that can store frames in the format used by the orbit state vector extraction Python script.
BY02 telemetry beacon
BY02 (also known as BY70-2) is an Amateur cubesat by the China Aerospace Science and Technology Corporation and Beijing Bayi High School. It was launched on July 3 on a CZ-4B rocket from Taiyuan together with a Gaofen Earth observation satellite. BY02 is intended as a replacement for BY70-1, which was launched on 2016-12-28 and was placed on a short-lived orbit that decayed in a few months because of a launch problem.
Today, Wei Mingchuan BG2BHC announced on Twitter at 09:14 UTC that BY02’s beacon was on and would be left on at least until 12:50 UTC. I believe that this is the first time that the beacon has been on for an extended period of time, since during the early operations the beacon was only active on passes over China.
Since at 11:39 UTC there was a good pass over Spain, I went outside with my handheld Arrow 7 element yagi to do a recording. This post is an in-depth analysis of this recording and includes an explanation of the coding and telemetry format used by BY02.
gr-satellites FSK BER
A few months ago I talked about BER simulations of the gr-satellites demodulators. In there, I showed the BER curves for the BPSK and FSK demodulators that are included in gr-satellites, and gave some explanation about why the current FSK demodulator is far from ideal. Yesterday I was generating again these BER plots to check that I hadn’t broken anything after some small improvements. I was surprised to find that the FSK BER curve I got was much worse than the one in the old post.
On the DSCS-III X-band 1kbaud beacon
Over the last few days, I’ve been looking at some recordings of the DSCS-III A-3 X-band beacon made by Scott Tilley VE7TIL. The beacon has a central carrier, which is BPSK modulated at 800baud and whose details we know pretty well due to this Master’s thesis by James Coppola. It also has two subcarriers modulated with 1kbaud BPSK of which we know very little. In this post I explain what I’ve been able to find about the data in this 1kbaud subcarriers (which isn’t that much, to be honest).
Reverse-engineering the DSCS-III convolutional encoder
One thing I left open in my post yesterday was the convolutional encoder used for FEC in the DSCS-III X-band beacon data. I haven’t seen that the details of the convolutional encoder are described in Coppola’s Master’s thesis, but in a situation such as this one, it is quite easy to use some linear algebra to find the convolutional encoder specification. Here I explain how it is done.
A look at the DSCS-III X-band beacon
The DSCS satellites are a constellation of US military communication satellites. While the constellation is old and it is being replaced by WGS, there are still several active DSCS-III satellites. A few days ago, Scott Tilley VE7TIL tweeted about the DSCS-III-A3 X-band beacon. The satellite DSCS-III-A3, also known as USA-167, is the second most recent DSCS-III satellite, having been launched in 2003. It has an X-band beacon at 7604.6MHz.
Scott’s tweets included a very impressive and interesting find: a Master’s thesis about a DSCS-III beacon decoder made by James Coppola in 1992. The thesis contains a wealth of information about the beacon, as well as the complete C source code for the decoder.
Scott has also been kind enough to share with me some recordings that he made of the beacon, so in this post I’ll be looking at these and how they relate to the information in the thesis.
Ranging through the QO-100 WB transponder
One of the things I’ve always wanted to do since Es’hail 2 was launched is to perform two-way ranging by transmitting a signal through the Amateur transponder and measuring the round trip time. Stefan Biereigel DK3SB first did this about a year ago. His ranging implementation uses a waveform with a chip rate of only 10kHz, as it is thought for Amateur transponders having bandwidths of a few tens of kHz. With this relatively slow chiprate, he achieved a ranging resolution of approximately 1km.
The QO-100 WB transponder allows much wider signals that can be used to achieve a ranging resolution of one metre or less. This weekend I have been doing my first experiments about ranging through the QO-100 WB transponder.
Advances with delta-range and delta-range rate observations in GMAT
A month ago I started modifying the GMAT EstimationPlugin to support delta-range observations. This work is needed in order to perform orbit determination with the VLBI observations that we did with DSLWP-B (Longjiang-2) during its mission. Now I have a version which is able to use both delta-range and delta-range rate observations in simulation and estimation. This is pretty much all that’s needed for the DSLWP-B VLBI observations.
The modified GMAT version and accompanying GMAT scripts for this project can be found in the gmat-dslwp Github repository. This post is an account of the work I’ve made.
Decoding Crew Dragon Demo-2
The launch last Saturday of Crew Dragon Demo-2 undoubtedly was an important event in the history of American space exploration and human spaceflight. This was the first crewed launch from the United States in 9 years and the first crewed launch ever by a commercial provider. Amateur radio operators always follow this kind of events with their hobby, and in the hours and days following the launch, several Amateur operators have posted reception reports of the Crew Dragon C206 “Endeavour” signals.
It seems that the signal received by most people has been the one at 2216 MHz. Among these reports, I can mention the tweets by Scott Tilley VE7TIL (and this one), USA Satcom, Paul Marsh M0EYT. Paul also managed to receive a signal on 2272.5 MHz, which is not in the FCC filing, so this may or may not be from the Crew Dragon.
Scott has also shared with me an IQ recording of one of the passes, and as I showed on Twitter yesterday, I have been able to demodulate the data. This post is my analysis of the signal.