Software – Page 6 – Daniel Estévez

Measuring Tianwen-1’s modulation

This is a post I had announced since I first described Tianwen-1’s modulation. Since we have very high SNR recordings of the Tianwen-1 low rate rate telemetry signal made with the 20m dish in Bochum observatory, it is interesting to make detailed measurements of the modulation parameters. In fact, there is something curious about the way the modulation is implemented in the spacecraft’s transmitter. This analysis will show it clearly, but I will reserve the details for later in the post.

Here I will be using a recording that already appeared in a previous post. It was made on 2020-07-26 07:47:20 UTC in Bochum shortly after the switch to the high gain antenna, so the SNR is fantastic. The recording was done at 2.5Msps, and the spectrum can be seen below. The asymmetry (especially around +1MHz) might be due to the receive chain.

The signal is residual carrier phase modulation, with 16348 baud BPSK data on a 65536Hz square wave subcarrier. There is also a 500kHz ranging tone.

Decoding Mars 2020

Mars 2020, NASA’s latest mission to Mars, was launched a couple weeks ago. However, with all the Tianwen-1 work down the pipeline, until now I haven’t had time to dedicate an appropriate post to this mission (though I showed some sneak peek on Twitter). This mission consists of a rover and helicopter (a real novelty in space exploration). Both were launched with the cruise stage and the entry, descent and landing system on July 30 from Cape Canaveral, an are currently on their transfer orbit to Mars, as Tianwen-1 and Emirates Mars Mission.

In this post I will be working with some recordings made by AMSAT-DL using the 20m radio telescope at Bochum’s observatory. These feature the low rate safe mode telemetry, which was very strong and caused some anecdotes as it saturated some NASA DSN receivers, and the nominal 10kbps telemetry signal that was switched on later. Here I will describe the modulation and coding, giving GNU Radio decoders, and also take a look at the data. r00t.cz has also written a post where he shows similar information.

Tianwen-1 high speed data signal

In a previous post I talked about how the high data rate signal of Tianwen-1 can be used to replay recorded telemetry. I did an analysis of the telemetry transmitted over the high speed data signal on 2020-07-30 and showed how to interpret the ADCS data, but left the detailed description of the modulation and coding for a future post.

Here I will talk about the modulation and coding, and how the signal switches from the ordinary low rate telemetry to the high speed signal. I also give GNU Radio decoder flowgraphs, tianwen1_hsd.grc, which works with the 8192 bit frames, and tianwen1_hsd_shortframes.grc, which works with the 2048 bit short frames.

Tianwen-1 telemetry: framing and data

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.

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.