Last Thursday 2020-07-23 at 04:41 UTC, Tianwen-1, a Chinese mission to Mars consisting of an orbiter, a lander and a rover, launched from Wenchang. Usually, I would be posting an analysis of a recording of the telemetry signal, made by Paul Marsh M0EYT or another of my Amateur DSN contributors, as I did a few days ago for the Emirates Mars Mission. However, something amazing has happened that has kept me quite busy. Rest assured that the analysis of the signal will come in a future post, but here I’m going to tell a story about Tianwen-1’s orbit.
Tag: satellites
Decoding Emirates Mars Mission Hope
Last Sunday 2020-07-19, the first mission of United Arab Emirates to Mars, known as Emirates Mars Mission “Hope probe” launched from Tanegashima, Japan. This probe is expect to reach Mars in approximately 200 days and study its atmosphere over the course of two years. The scientific instruments onboard the probe are a digital camera, an infrared spectrometer, and an ultraviolet spectrometer.
Shortly after launch, several Amateur radio operators and Amateur spacecraft trackers received signals from the X-band beacon of the Hope probe at 8402.655 MHz and posted reports on Twitter, such as Paul Marsh M0EYT, Ferrucio IW1DTU, Edgar Kaiser DF2MZ, and others. Since the spacecraft was still near Earth, its signal was so strong that a data modulation with a main lobe of approximately 20kHz wide and several sidelobes could easily be seen in the spectrum, which is shown below.
Paul has been quite kind to send me a recording that he made with his station on 2019-07-19 at 23:29 UTC and I have been decoding the data in GNU Radio and looking at the frames. The recording can be downloaded here (193MB). It is an int16 IQ recording at 99998 samples per second. This post is an account of my results.
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.
RTCM clock corrections for Galileo E24
In my previous post I looked at the BGDs and related topics for the Galileo satellites. We saw that satellite E24 has atypically large BGDs, but everything else seems fine and consistent with that satellite. However, Bert Hubert from galmon.eu shows that several RTCM sources broadcast a clock correction of around -5ns for E24. Here we look at the possible causes for that correction, and discuss whether it might be problematic.
About Galileo BGDs
A few days ago, Bert Hubert, from galmon.eu noticed that Galileo satellite E24 was somewhat special because it had unusually large BGDs. This raised a number of questions, such as what is the physical interpretation of BGDs, what they have to do with broadcast clock models, and so on.
In this post I will explain a few basic facts about BGDs and related topics, following an approach that is perhaps different to that of the usual GNSS literature, and also study the current values for the Galileo constellation. People who know all the details about the BGDs or who just want to see a few pretty plots can skip all the first section of the post.
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.