- Tianwen-1 attitude in Mars orbit
It has been a while since the last time I wrote an update about Tianwen-1’s attitude. In that post I showed that Tianwen-1 had changed to a sun angle of 0 degrees with respect to the spacecraft’s X axis on 2020-10-22. Since then, nothing changed, even after the spacecraft arrived to Mars orbit on February 2021, so I have been sloppy and ignored the ADCS telemetry for quite a while.
As you may have heard, the 20 metre antenna at Bochum observatory had a failure at the beginning of June due to a thunderstorm. It has been under repair until the end of August, when it became operational again. As AMSAT-DL has now returned to receive telemetry from Tianwen-1 at Bochum, I am now reviewing the new data to see if anything interesting might have happened in the last months.
I have updated my attitude study Jupyter notebook, since the previous version assumed that the spacecraft’s state vectors were heliocentric coordinates, so it didn’t work in Mars orbit, due to the change to Mars-centric coordinates.
When plotting the new data I have learned that Tianwen-1 changed attitude during the release of the lander on May 14, and has maintained this new attitude until the present day.
- Decoding Voyager 1
Today is the 44th anniversary of the launch of Voyager 1, so I want to celebrate by showing how to decode the Voyager 1 telemetry signal using GNU Radio and some Python. I will use a recording that was done back in 30 December 2015 with the Green Bank Telescope in the context of the Breakthrough Listen project. Most of the data from this project is open data and can be accessed through this portal.
In contrast to other posts about deep space probes in this blog, which are of a very specialized nature, I will try to keep this post accessible to a wider audience by giving more details about the basics. Those interested in learning further can refer to the workshop “Decoding Interplanetary Spacecraft” that I gave in GRCon 2020, and also take a look at other posts in this blog.
- An update about Chang’e 5 whereabouts
It’s been a while since the last time I posted about Chang’e 5, back at the end of January. Since then we have been occasionally keeping an eye on it with the Allen Telescope Array, and Scott Tilley VE7TIL has also been monitoring with its home station. Until now, the spacecraft has spent its time slowly drifting around the Sun-Earth L1 point. However, it appears that now it is on the move.
The observations I’ve done last weekend from ATA, as well as reports from Scott and other observers show that it has deviated significantly from the ephemerides made by Bill Gray using our radio observations. This is not so surprising, because we haven’t been maintaining those ephemerides with new observations through part of July and all August. What is more surprising is that now the spacecraft’s right ascension and declination is changing rather fast.
The following are the observations I took with the ATA this weekend, using the Chang’e 5 telemetry signal at 8486.3 MHz.
Timestamp (UTC) RA (h) DEC (deg) 2021-08-28 21:00 7.931 14.73 2021-08-29 16:00 8.117 14.05 2021-08-29 21:30 8.150 13.80 This represents an angular movement of 3.25 deg per day, which is a lot if we compare it with the ephemerides around the end of July, which had an angular movement of 0.43 deg per day.
Still we don’t have enough data to fit a good trajectory. It is uncertain whether Chang’e 5 has manoeuvred or is simply being ejected out of the L1 point (orbits around the Langrage points are typically unstable and require station-keeping). We will keep gathering more data points to try to fit a better trajectory.
Yesterday I observed that the signal strength was really good. The figure below shows the spectrum of the signal. The two smaller spikes to the sides of the carrier are the data sidebands, which have an SNR of approximately 5 dB. The spacecraft is still transmitting at 512 baud.
Chang’e 5 signal on 2021-08-29 In comparison, this is the signal on Saturday, which is a few dB weaker.
Chang’e 5 signal on 2021-08-28 And here is the signal on 2021-07-31, which is the previous observation I made.
Chang’e 5 signal on 2021-07-31 These last two spectra are more typical of the signal strengths we’ve been seeing with the ATA (using a single 6 metre dish) since Chang’e 5 arrived the neighbourhood of the L1 point.
Since the signal was so good, I left the ATA recording IQ data in order to decode the telemetry later. Unfortunately the spacecraft movement was fast enough that it exited the beam of the dish in approximately 3 hours (I was tracking a constant right ascension and declination). Still, during these 3 hours we got very few lost frames, as shown in these figures.
The telemetry data in the different APIDs in virtual channel 1 (which is the only virtual channel in use) looks very similar to the data decoded on 2021-01-24. The only noteworthy aspect is that APID 1029 is missing in the data from 2021-08-29. As during the end of January, the data looks rather static, with some periodic changes in channels that could correspond to temperature cycles.
The full plots of the decoded data can be seen in this Jupyter notebook, and the decoded data can be found here.
- Imaging Cygnus A at 8.45 GHz with ATA
Earlier this year, I published a post showing our results of the interferometric imaging of Cassiopeia A and Cygnus A at 4.9 GHz with the Allen Telescope Array. Near the end of July, I decided to perform more interferometric observations of Cygnus A at a higher frequency, in order to obtain better resolution. I chose a frequency of 8.45 GHz because it is usually a band clean of interference (since it is allocated to deep space communications), it is used by other radio observatories, so flux densities can be compared directly with previous results, and because going higher up in frequency the sensitivity of the old feeds at ATA starts to decrease.
This post is a summary of the observations and results. The code and data is included at the end of the post.
- 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.
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