Decoding Peregrine Mission One

Peregrine Mission One is a lunar lander built by Astrobotic Technology. It is the first mission to be launched under the NASA Commercial Lunar Payload Services program, and Astrobotic’s first mission. It was launched in January 8 from Cape Canaveral in the maiden flight of ULA‘s Vulcan Centaur. Shortly after the launch, the team detected an issue with a propellant leak that prevented the spacecraft from achieving a soft landing on the Moon. Since then, the team has continued operating the spacecraft to the best of their capacity and collecting as much engineering and science data as they can for the next mission. Astrobotic has been doing a superb work of communicating the progress of the mission with regular updates in the Twitter account, which should specially be praised because of the difficulties they’ve faced. Congratulations for all they have achieved so far, and best luck in the upcoming missions.

In this post I won’t speak about propulsion anomalies, but rather about low-level technical details of the communications system, as I usually do. Peregrine Mission One, or APM1, which is NASA DSN‘s code for the mission, uses the DSN groundstations for communications, as many other lunar missions have done. However, it is not technically a deep space mission. In CCSDS terms, it is a Category A mission rather than a Category B mission (see Section 1.5 in this CCSDS book), since it operates within 2 million km of the Earth. Communications recommendations and usual practices are somewhat different between deep space and non-deep space missions, but APM1 is specially interesting in this sense because it differs in several aspects of what typical deep space missions and other lunar missions look like.

For this post I have used some IQ recordings done by the AMSAT-DL team with the 20 metre antenna at Bochum Observatory. To my knowledge, these recordings are not publicly available.

Decoding MOVE-II

MOVE-II is a cubesat from Technical University of Munich that was launched in December 2018. It transmits telemetry in the 145 MHz amateur satellite band using a protocol that uses CCSDS LDPC codewords. Back in the day, there was a GNU Radio out-of-tree module developed by the satellite team to decode this satellite. Given the additional effort required to support LDPC decoding for just this satellite and since there was already a GNU Radio decoder available, I never added a decoder for MOVE-II to gr-satellites.

Fast forward 5 years, and MOVE-II is still active, but apparently its GNU Radio out-of-tree module has bit rotten. The Gitlab repository where this was hosted (I believe it was a self-hosted Gitlab) has disappeared, and while it was originally developed for GNU Radio 3.7, it was never ported to newer GNU Radio versions. Some days ago, some amateurs including Scott Chapman K4KDR and Bob Mattaliano N6RFM started doing some experiments to try to get a decoder for MOVE-II working.

Seeing this, I decided to revisit the situation and try to add a decoder for MOVE-II to gr-satellites. Since this satellite was launched, I have been dealing with CCSDS LDPC for the Artemis Orion, made my own LDPC decoder, and participated in fixing the GNU Radio in-tree LDPC decoder. Therefore, most of the heavy lifting seemed to be already done.

I have now added an example decoder flowgraph for MOVE-II to gr-satellites. Here I describe the details of this example, and why it is only an example instead of a fully supported decoder as the ones that exist for other satellites.

ldpc-toolbox gets LDPC decoding

Recently I have implemented an FPGA LDPC decoder for a commercial project. The belief propagation LDPC decoder algorithm admits many different approximations in the arithmetic, and other tricks that can be used to trade off between decoding sensitivity (BER versus Eb/N0 performance) and computational complexity. To help me benchmark the different belief propagation algorithms, I have extended my ldpc-toolbox project to implement many different LDPC decoding algorithms and perform BER simulations.

ldpc-toolbox is a Rust library and command line tool for the design of LDPC codes. I initially created this project when I was trying to design a suitable LDPC code for a narrowband 32APSK modem to be used over the QO-100 amateur GEO transponder. The tool so far supported some classical pseudorandom constructions of LDPC codes, computed Tanner graph girths, and could construct the alists for all the DVB-S2 and CCSDS LDPC codes. Extending this tool to support LDPC encoding, decoding and BER simulation is a natural step.

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.