LDPC code design for my QO-100 narrowband modem

A couple months ago I presented my work-in-progress design for a data modem intended to be used through the QO-100 NB transponder. The main design goal for this modem is to give the maximum data rate possible in a 2.7 kHz channel at 50 dB·Hz CN0. For the physical layer I settled on an RRC-filtered single-carrier modulation with 32APSK data symbols and an interleaved BPSK pilot sequence for synchronization. Simulation and over-the-air tests of this modulation showed good performance. The next step was designing an appropriate FEC.

Owing to the properties of the synchronization sequence, a natural size for the FEC codewords of this modem is 7595 bits (transmitted in 1519 data symbols). The modem uses a baudrate of 2570 baud, so at 50 dB·Hz CN0 the Es/N0 is 15.90 dB. In my previous post I considered using an LDPC code with a rate of 8/9 or 9/10 for FEC, taking as a reference the target Es/N0 performance of the DVB-S2 MODCODs. After some performing some simulations, it turns out that 9/10 is a bit too high with 7595 bit codewords (the DVB-S2 normal FECFRAMEs are 64800 bits long, giving a lower LDPC decoding threshold). Therefore, I’ve settled on trying to design a good rate 8/9 FEC. At this rate, the Eb/N0 is 9.42 dB.

32APSK narrowband modem for QO-100

Some time ago I did a few experiments about pushing 2kbaud 8PSK and differential 8PSK through the QO-100 NB transponder. I didn’t develop these experiments further into a complete modem, but in part they served as inspiration to Kurt Moraw DJ0ABR, who has now made a QO-100 Highspeed Multimedia Modem application that uses up to 2.4 kbaud 8PSK to send image, files and digital voice. Motivated by this, I have decided to pick up these experiments again and try to up the game by cramming as much bits per second as possible into a 2.7 kHz SSB channel.

Now I have a definition for the modem waveform, and an implementation in GNU Radio of the modulation, synchronization and demodulation that is working quite well both on simulation and in over-the-air tests on the QO-100 NB transponder. The next step would be to choose or design an appropriate FEC for error-free copy.

In this post I give an overview of the design choices for the modem, and present the GNU Radio implementation, which is available in gr-qo100_modem.

Decoding IDEASSat

One of the interesting Amateur cubesats in yesterday’s SpaceX Transporter-1 launch from Cape Canaveral is IDEASSat, a 3U cubesat from the National Central University of the Republic of China (Taiwan) designed to study ionospheric plasma. Jan van Gils PE0SAT drew my attention to this satellite as he was trying to see if it was possible to decode it with any of the decoders existing in gr-satellites. Mike Rupprecht DK3WN also helped with a good recording, much cleaner than the SatNOGS recording that Jan was using.

Presumably this satellite uses “AX25, 9k6, GMSK”, as listed at the bottom of this page from Taiwan’s National Space Organization, and also in this research paper. However, this is not true. It’s simple to check that the usual 9k6 FSK AX.25 decoders aren’t able to decode this signal, and a look at the FSK symbols shows that there is no scrambler (9k6 AX.25 uses the G3RUH scrambler) and that the symbol sequence doesn’t have much to do with AX.25.

After some reverse-enginnering, yesterday I figured out how the coding used by IDEASSat worked, and today I added a decoder to gr-satellites to help Mike investigate what kind of telemetry the packets contain. The protocol is not very good, so I think it’s interesting to document it in detail, as some sort of lessons learned. In this post I’ll do so. As it turns out, the protocol has some elements that loosely resemble AX.25, so I’m left wondering whether this is some unsuccessful attempt at implementing standard AX.25 (we’ve already seen very weird attempts, such as ESEO).

Decoding the NEXUS π/4-DQPSK signal

NEXUS, also called FO-99, is a Japanese Amateur satellite built by Nihon University and JAMSAT. It was launched on January 2019, and one of its interesting features is a π/4-DQPSK high-speed transmitter for the 435 MHz Amateur satellite band.

I was always interested in implementing a decoder for this satellite, due to its unusual modulation, but the technical information that is publicly available is scarce, so I never set to do it. A few days ago, Andrei Kopanchuk UZ7HO asked me a question about the Reed-Solomon code used in this satellite. He was working on a decoder for this satellite, and had some extra documentation. This renewed my interest in building a decoder for this satellite.

With some help from Andy regarding the symbol mapping for the π/4-DQPSK constellation, I have made a decoder GNU Radio flowgraph that I have added to gr-satellites.

Decoding AMICal Sat in-orbit images

Back in March, I was helping Julien Nicolas F4HVX to test the S-band image transmitter of AMICal Sat before launch. In my post back then, I explained that AMICal Sat uses a Nordic Semiconductor nRF24L01+ 2.4GHz FSK transceiver chip to transmit Shockburst packets at 1Mbaud. I also explained how the Onyx EV76C664 CMOS image sensor works and how to process raw images.

AMICal Sat was finally launched on 2020-09-03, and since them the satellite team has been busy trying to downlink some images, both using the UHF transmitter (which uses the same protocol as Światowid) and the S-band transmitter. This has proven a bit difficult because the ADCS of the satellite is not working, and the downlink protocols are not very robust.

Julien has been sending me recordings done by their groundstation in Russia with the hope that we could be able to decode some of the data. Before several failed attempts where we were hardly able to decode a few packets, we got a particularly good S-band recording done on 2020-10-05. Using that recording, I have been able to decode a full image.

Analysis of Object A’s S-band signal

On 2020-09-04, China launched a “reusable experimental spacecraft” of which very little is publicly known. The most popular hypothesis is that this is a robotic spaceplane similar to the X-37B. The spacecraft spent two days in orbit and landed back at Earth, most likely near Lop Nur nuclear test site. Marco Langbroek has a nice post detailing all we know about the mission.

During the time it spent in orbit, the spacecraft released an object which has been catalogued as 2020-063G and is commonly known as “Object A”. On 2020-09-14, Dmitry Pashkov R4UAB detected an S-band signal coming from Object A at around 2280 MHz. This was verified later by Scott Tilley VE7TIL, who received a strong signal with lots of fading, suggesting that the object is tumbling. Marco also did some optical observations of the object.

Scott has sent me a recording of the S-band signal that he did on 2020-09-15 so that I can analyse it and we can learn more about this mysterious object. This post shows the results of my analysis.

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.

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.

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.

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.