- 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).
- BPSK radar received through Meridian 8
Ever since SETI Insitute published the news of a possible signal received from Proxima Centauri in some of the Parkes telescope recordings at 982 MHz, Scott Tilley VE7TIL has taken up the interest to search and catalogue the satellites that transmit on this band (specially old, forgotten and zombie satellites). His idea is to try to see if this candidate signal can be explained as interference from some satellite.
This has led him to discover some signals coming from satellites on a Molniya orbit. After examination with the Allen Telescope Array of these signals, we confirmed that they came from wideband transponders (centre frequency around 995 MHz, 13 MHz width) on some of the Meridian Russian communications satellites (in particular Meridian 4 and 8, but also others).
These transponders show all sorts of terrestrial signals that are relayed as unintended traffic through the transponder. By measuring Doppler we know that the uplink is somewhere around 700 or 800 MHz. We have found some OFDM-like signals that seem to be NB-IoT. Unfortunately we haven’t been able to do anything useful with them, maybe because there are several signals overlapping on the same frequency. We also found a wideband FM signal containing music and announcements in Turkmen, which later turned out to be the audio subcarrier of a SECAM analogue TV channel from Turkmenistan.
A few days ago, Scott detected a pulsed strong signal through the transponder of the Meridians at a downlink frequency of 994.2 MHz. He did an IQ recording of this signal on the downlink of Meridian 8. It turns out that this signal is a BPSK pulse radar. In this post I do a detailed analysis of the radar waveform using this recording.
- 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.
- Chang’e 5 interferometric astrometry
Bill Gray, from Project Pluto is doing a great job trying to estimate the orbit of Chang’e 5 as it travels to somewhere around the Sun-Earth L1 Lagrange point (see my previous post). He is using RF pointing data from Amateur observers and the Allen Telescope Array, since the low elongation and the distance of the spacecraft have made it impossible to observe it optically.
For this task, the pointing data I am obtaining with my observations on Allen Telescope Array as part of the activities of the GNU Radio community there is quite valuable, since the 6.1 metre dishes give more accurate pointing measurements than the smaller dishes of Amateurs. The pointing data from ATA should be accurate to within 0.1 or 0.2 degrees.
To try to get more accurate data for Bill, last weekend I decided to do a recording with two dishes from the array, with the goal of using interferometry to obtain a much more precise pointing solution that what can be achieved with a single dish. This post is a report of the processing of the interferometric data.
- Chang’e 5 observations from ATA on December 19 and 20
As we’ve been doing lately, last weekend we observed the Chang’e 5 orbiter at Allen Telescope Array as part of the GNU Radio community activities in the telescope. This post contains a large overview of these observations, including the efforts to determine the spacecraft orbit, the study of the signal polarization, and the data obtained by decoding the telemetry.
I am still transferring the IQ data from the telescope, but I will publish the recordings in Zenodo in a few days and update this post.
Edit 2021-01-02: the recordings are now published and can be found in the following datasets.
- Chang’e 5 RF recording at 8471.2 MHz with Allen Telescope Array on 2020-12-19 (X polarization)
- Chang’e 5 RF recording at 8471.2 MHz with Allen Telescope Array on 2020-12-19 (Y polarization)
- Chang’e 5 RF recording at 8486.2 MHz with Allen Telescope Array on 2020-12-19 (X polarization)
- Chang’e 5 RF recording at 8486.2 MHz with Allen Telescope Array on 2020-12-19 (Y polarization)
- Chang’e 5 RF recording at 8471.2 MHz with Allen Telescope Array on 2020-12-20
- Chang’e 5 RF recording at 8486.2 MHz with Allen Telescope Array on 2020-12-20
- Amateur radio
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