ce5 – Daniel Estévez

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.

In comparison, this is the signal on Saturday, which is a few dB weaker.

And here is the signal on 2021-07-31, which is the previous observation I made.

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.

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.

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:

Chang’e 5 polarization in the ATA observations

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.

Chang’e 5 LOI-2 observed with Allen Telescope Array

If you follow me on Twitter you’ll probably have seem that lately I’m quite busy with the Chang’e 5 mission, doing observations with Allen Telescope Array as part of the GNU Radio activities there and also following what other people such as Scott Tilley VE7TIL, Paul Marsh M0EYT, r00t.cz, Edgar Kaiser DF2MZ, USA Satcom, and even AMSAT-DL at Bochum are doing with their own observations. I have now a considerable backlog of posts to write, recordings to share and data to process. Hopefully I’ll be able to keep a steady stream of information coming out.

In this post I study the observation I did with Allen Telescope Array last Sunday 2019-11-29. During the observation, I was tweeting live the most interesting events. The observation is approximately 3 hours long and contains the LOI-2 (lunar orbit injection) manoeuvre near its end. LOI-2 was a burn that circularized the elliptical lunar orbit into an orbit with a height of approximately 207km over the lunar surface.