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.

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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.

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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.

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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.

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Decoding BepiColombo

BepiColombo is a joint mission between ESA and JAXA to send two scientific spacecraft to Mercury. The two spacecraft, the Mercury Planetary Orbiter, built by ESA, and the Mercury Magnetospheric Orbiter, built by JAXA, travel together, joined by the Mercury Transfer Module, which provides propulsion and support during cruise, and will separate upon arrival to Mercury. The mission was launched on October 2018 and will arrive to an orbit around Mercury on December 2025. The long cruise consists of one Earth flyby, two Venus flybys, and six Mercury flybys.

The Earth flyby will happen in a few days, on 2020-04-10, so currently BepiColombo is quickly approaching Earth at a speed of 4km/s. Yesterday, on 2020-04-04, the spacecraft was 2 million km away from Earth, which is close enough so that Amateur DSN stations can receive the data modulation sidebands. Paul Marsh M0EYT, Jean-Luc Milette and others have been posting their reception reports on Twitter.

Paul sent me a short recording he made on 2020-04-04 at 15:16 UTC at a frequency of 8420.535MHz, so that I could see if it was possible to decode the signal. I’ve successfully decoded the frames, with very few errors. This post is a summary of my decoding.

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Decoding images from AMICal Sat

AMICal Sat is a 2U cubesat developed by the Space Centre of the Grenoble University, France, and the Skobeltsyn Institute of Nuclear Physics in the Lomonosov Moscow State University. Its scientific mission consists in taking images of auroras from low Earth orbit. The satellite bus was built by SatRevolution. Currently, the satellite is in Grenoble waiting to be launched on a future date (which is uncertain due to the COVID-19 situation).

A few weeks ago I was working with Julien Nicolas F4HVX to try to decode some of the images transmitted by AMICal Sat. Julien is an Amateur radio operator and he is helping the satellite team at Grenoble with the communications of the satellite.

This post is an account of our progress so far.

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Decoding ESA Solar Orbiter

Solar Orbiter is an ESA Sun observation satellite that was launched on February 10 from Cape Canaveral, USA. It will perform detailed measurements of the heliosphere from close distances reaching down to around 60 solar radii.

As usual, Amateur observers have been interested in tracking this mission since launch, but apparently ESA refused to publish state vectors to aid them locate the spacecraft. However, 18 hours after launch, Solar Orbiter was found by Amateurs, first visually, and then by radio. Since then, it has been actively tracked by several Amateur DSN stations, which are publishing reception reports on Twitter and other media.

On February 13, the spacecraft deployed its high gain antenna. Since it is not so far from Earth yet, even stations with relatively small dishes are able to receive the data modulation on the X band downlink signal. Spectrum plots showing the sidelobes of this signal have been published in Twitter by Paul Marsh M0EYT, Ferruccio IW1DTU, and others.

I have used an IQ recording made by Paul on 2020-02-13 16:43:25 UTC at 8427.070MHz to decode the data transmitted by Solar Orbiter. In this post, I show the details.

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Extracting AX.25 satellites from SatNOGS DB

In my last post about gr-satellites 3, I announced that gr-satellites would start to support all the AX.25 satellites transmitting in Amateur bands. Historically, gr-satellites didn’t support packet radio (AFSK and FSK AX.25) satellites since there were too many of them and there were already other good decoders such as Direwolf. At one point Rocco Valenzano W2RTV convinced me to add “generic” packet radio decoders to gr-satellites and since then these have been seeing quite some use.

In gr-satellites 3 it is very easy to add new satellites, since this is done with a SatYAML file, which is a brief YAML file describing basic information about the satellite and its transmitters. Therefore, I decided to make a script to get this data from SatNOGS DB and write the SatYAMLs automatically for all the AFSK and FSK AX.25 satellites.

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Third alpha for gr-satellites 3

gr-satellites v3 is a large refactor of the gr-satellites codebase that I introduced in September. Since then, I have been working and releasing alphas to showcase the new features and get feedback from the community. Today I have released the third alpha in the series: v3-alpha2.

Each of the alphas has focused on a different topic or feature, and v3-alpha2 focuses on extending the number of satellites supported and bringing back most of the satellites supported in gr-satellites v2. Whereas previous alphas supported only a few different satellites, this alpha supports a large number. Therefore, I think that this is the first gr-satellites v3 release that is really useful. I expect that interested people will be able to use v3-alpha2 as a replacement of gr-satellites v2 in their usual activities.

In this post, I explain the main features that this alpha brings. For the basic usage of gr-satellites v3, please refer to the post about the second alpha.

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First tests of a narrowband data modem for QO-100

Since a while ago, I have had the idea to design a data modem for the NB transponder of QO-100 (Es’hail 2). The main design criteria of this modem is that it should fit in a bandwidth of 2.7kHz and be able to work at a signal power equal to that of the transponder BPSK beacon, since these are the bandwidth and power constraints when using the NB transponder.

Currently, the following modes are used for medium speed data (understood as a few kbps) on the NB transponder. First, there are the FreeDV modes, whose use has been covered in this Lime microsystems community post. Most of these modes use OFDM or multi-carrier modems and are designed having HF fading channels in mind. These don’t give good performance over the QO-100 transponder, since the frequency instabilities of the transmitters and receivers give problems with OFDM modems. A single carrier modem is much better. David Rowe VK5DGR has made some modifications to the FreeDV 2020 modem to improve performance over QO-100, and it certainly works quite well, but better results can be obtained with a single carrier modem.

There are some people using DRM for DSSTV. This is also an OFDM modem intended for HF, and the symbol time is quite long, so the frequency instabilities can give problems. Finally, there is KG-STV, which was relatively unpopular before QO-100 but it is seeing a lot of use due to its good performance. It uses a single carrier MSK modem. This is probably the most popular medium speed mode on the NB transponder, but it is only 1200bps.

One important characteristic of the NB transponder is that there is a lot of SNR available. The rule is that no signal should be stronger than the beacons, but the BPSK beacon has a CN0 of around 54dB as received in my station. It is also not difficult (in terms of uplink EIRP) to achieve the same power as the beacon. Therefore, it is a reasonable assumption that stations interested in using a medium speed data modem will adjust their uplink power to be as strong as the BPSK beacon. I already hinted at what is possible with such a strong signal in this post.

I have decided to do some preliminary tests to check the performance of a 2kbaud 8PSK signal over the NB transponder. This post summarizes my results. The material for the post can be found in the qo100-modem Github repository.

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