This post has been delayed by several months, as some other things (like Chang’e 5) kept getting in the way. As part of the GNU Radio activities in Allen Telescope Array, on 14 November 2020 we tried to detect the X-band signal of Voyager-1, which at that time was at a distance of 151.72 au (22697 millions of km) from Earth. After analysing the recorded IQ data to carefully correct for Doppler and stack up all the signal power, I published in Twitter the news that the signal could clearly be seen in some of the recordings.
Since then, I have been intending to write a post explaining in detail the signal processing and publishing the recorded data. I must add that detecting Voyager-1 with ATA was a significant feat. Since November, we have attempted to detect Voyager-1 again on another occasion, using the same signal processing pipeline, without any luck. Since in the optimal conditions the signal is already very weak, it has to be ensured that all the equipment is working properly. Problems are difficult to debug, because any issue will typically impede successful detection, without giving an indication of what went wrong.
I have published the IQ recordings of this observation in the following datasets in Zenodo:
Today at 9:00 UTC Tianwen-1 made its plane change manoeuvre, as reported by Xinhua. Yesterday I showed my planning for this manoeuvre. Shortly after the spacecraft returned to the high gain antenna after the manoeuvre, the Bochum 20m antenna operated by AMSAT-DL received state vectors with the new trajectory. These state vectors allow us to calculate the timestamp of the burn and the delta-V vector, as I have done in other occasions. It is convenient to remark that the state vectors that we are seeing right now are probably a prediction. In the next few days we will see updates in the trajectory as the Chinese DSN measures the effects of the actual burn and updates the onboard ephemerides.
Today, the Chinese media published a short piece of news stating that tomorrow, 2021-02-15, Tiawen-1 will make make a plane change to a polar orbit. The post is accompanied by an short video, which includes an animation depicting the manoeuvre. A screenshot of the video is shown below. As the spacecraft arrives to apoapsis, it effects a plane change into an ascending polar orbit.
This is a good moment to review the maths behind a plane change manoeuvre and compute what the manoeuvre will look like.
In the case of Tianwen-1 the signal was pretty strong even while the spacecraft was on the low gain antenna, and we could clearly see the change in Doppler rate as the thrusters fired up. However, in the case of Emirates Mars Mission the signal disappeared as soon as the spacecraft switched to the low gain antenna. In fact DSN Now reported a received power of -155 dBm with the 34m DSS55. That was a large drop from the -118 dBm that it was reporting with the high gain antenna. Therefore, nothing could be seen in the livestream waterfall until the spacecraft returned to the high gain antenna, well after the manoeuvre was finished.
Nevertheless, a weak trace of the carrier was still present in the livestream audio, and it could be seen by appropriate FFT processing, for example with inspectrum. I put up a couple of tweets showing this, but at the moment I wasn’t completely sure if what I was seeing was the spacecraft’s signal or some interference. After the livestream ended, I’ve been able to analyse the audio more carefully and realize that not only this weak signal was in fact the Hope probe, but that the start of the burn was recorded in perfect conditions.
In this post I’ll show how to process the livestream audio to clearly show the change in drift rate at the start of the burn and measure the acceleration of the spacecraft.
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).
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.
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.
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.
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.
In my previous post, I talked about an observation of Chang’e 5 made with Allen Telescope Array last Sunday, 2020-11-29. I still need to write the report corresponding to the observation from Saturday 2020-11-28. However, before doing so, I thought it would be interesting to look at the polarization of each of the signals in these recordings. As I already advanced, the polarization is not perfect RHCP, but rather elliptical and time varying.
In fact, it seems likely that most of the antennas of Chang’e 5 are not steerable antennas, but rather, patch-like medium-gain or low-gain antennas. These are circularly-polarized only when seen from the front. They are linearly polarized when seen from a side.
Therefore, by studying the polarization of the Chang’e X-band signals, we can try to learn more about the spacecraft’s attitude and its antennas.