Posts – Page 15 – Daniel Estévez

Chang’e 5 low rate telemetry

Even though I haven’t been posting updates about Chang’e 5 lately, we have continued tracking it with Allen Telescope Array most weekends since my last post. The main goal of these observations has been to give Bill Gray updated pointing data so that he can refine his ephemerides. Additionally, we have been decoding telemetry from the recordings we’ve made.

One of the interesting things that have happened is the change to a lower baudrate in the telemetry signals. Until 2020-12-27 the baudrate was 4096 baud, while starting with the observation on 2021-01-02 we are seeing a new baudrate of 512 baud. This means that at some point around the end of last year the spacecraft was commanded to switch to a lower baudrate, to account for the increase in path loss caused by the increasing distance as the spacecraft travels towards the Sun-Earth L1 point.

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.

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.

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 reception from ATA on its return trajectory: polarization

This post is a follow up to my previous post about the recordings made by the GNU Radio team at Allen Telescope Array on December 12 and 13. In that post I looked at the telemetry decoding in two full pass observations done last weekend, each of them lasting around 4 to 5 hours.

In this post, I will study the signal polarization in those recording, following the same method as in my previous post about the Chang’e 5 polarization. In these recordings, only the signal at 8471.2 MHz from the orbiter was active.

Chang’e 5 reception from ATA on its return trajectory: telemetry

Last weekend, we did two long observations of Chang’e 5 with one of the dishes from Allen Telescope Array as part of the activities of the GNU Radio community in the telescope. The recordings were done during the UTC evenings on Saturday 2020-12-12 and Sunday 2020-12-13, and almost lasted for all the time that the spacecraft was above 16.8 degrees, which is the elevation mask for the telescope. Since the Moon was at a low declination, the observations were not so long, only around 4 to 5 hours.

On Saturday, the spacecraft had already performed its TEI-1 (trans-Earth injection burn) and was on an elliptical lunar orbit. On Sunday, the spacecraft had performed TEI-2 and was already on its transfer orbit to Earth, and several degrees away from the Moon, as shown by the blue cross in the figure below, done with Stellarium.

Position of Chang’e 5 on the sky on Sunday evening

The IQ recordings of the observations will be published in Zenodo in a few days, since I need to transfer them over the slow internet connection of the telescope. This post will be updated when they are ready.

Update 2020-12-19: The recordings are now published in the following datasets:

In this post, I look at the telemetry decoded from these recordings. Future posts will look at other aspects, such as the polarization of the signal.

Chang’e 5 telemetry from the lunar surface received by Bochum

A few days ago, Achim Vollhardt DH2VA shared with me some recordings of the lander+ascender combo of Chang’e 5 with the 20m antenna at Bochum observatory, which is operated by AMSAT-DL. The recordings were made on 2020-12-02, while the lander+ascender combo was still on the lunar surface collecting samples (see this tweet by Scott Tilley VE7TIL for the detailed mission timeline). The successful reception of Chang’e 5 by Bochum was announced by AMSAT-DL in Twitter.

The recordings that Achim made are the following:

Recording of the low data rate telemetry at 8463.7 MHz for some 15 minutes at 6:00 UTC. This frequency was in ground-lock at that time, as shown by the telecommand loopback at +/-8kHz from the main carrier (there are several telecommand packets being transmitted, plus the usual idle telecommand subcarrier)
Five recordings of a high-speed signal at 8495 MHz. The recording was done at 21:10 UTC, has a length of 5 minutes, and is split in five files due to a constraint of 2GB in the size of the recorded files.

In this post I look at the telemetry decoded from these recordings.

Chang’e 5 telemetry from the ATA 2020-11-28 observation

In one of my last posts I’ve analysed a recording I made at Allen Telescope Array of the four low rate telemetry signals of Chang’e 5 during the LOI-2 manoeuvre. The previous day, I did an observation several hours before the spacecraft arrived to the Moon and performed the LOI-1 burn. In this observation I only recorded the signal at 8463.7 MHz (which later we discovered that corresponds to the lander), as it was the strongest of all four. In this post I give the analysis of the telemetry in this recording.

The recording corresponding to this observation will be published in Zenodo, but this will be done in a few days, since I’m still transferring files from the telescope. I’ll update the post when it is published.

Update 2020-12-11: the recording is now published in the following datasets: