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.
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:
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 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:
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.
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.
Chang’e 5 is a Chinese lunar sample return mission. It was launched a few days ago on 2020-11-23 from Wenchang and is estimated to perform lunar orbit injection on Saturday. Since then, a number of Amateurs such as USA Satcom, Paul Marsh M0EYT, Scott Tilley VE7TIL, Fer IW1DTU and others have been receiving the X-band signals from the spacecraft and posting reports over on Twitter. Meanwhile, r00t.cz has been working in decoding the frames, which has led him to the amazing achievement of being able to retrieve a short video from the signal.
In this post I will look at some of the frames demodulated by USA Satcom and Paul during the first couple of days of the mission. The frame structure has many similarities with Tianwen-1, which I have described in several posts, such as here and here. However, there are some interesting differences.
Following my polarimetry experiments at Allen Telescope Array, on October 31 I did a polarimetric observation of the quasar 3C286 with two dishes from the array to use as a test-bed for polarimetric calibration. 3C286 is a bright, compact, polarized source, with a fractional polarization intensity of around 10% and a polarization angle of 33º over a wide range of frequencies, so it makes an ideal source for polarization calibration. It is the primary polarization calibrator for VLA. The observation duration was slightly more than 2 hours, and it was done around the transit of the source, so the parallactic angle coverage is large (around 90º).
My initial idea was to use this observation to perform a “single dish” polarization calibration of each of the dishes by separate (since the math is somewhat simpler) and then perform an interferometric polarization calibration. However, after initial examination of the data, the SNR doesn’t seem large enough to do a “single dish” calibration. The polarized signal from 3C286 is rather weak and is swamped by noise from other sources in the field and from the receiver, and also by gain variations in the receive chain.
On the contrary, the interferometric calibration has worked well, since correlating the signals from the antennas allows us to discard the uncorrelated receiver noise and to phase on the target and discard other signals from the field, by means of Earth rotation aperture synthesis.
In this post I give my analysis and results of the observation. I have done an ad hoc calibration in Python to determine the polarization leakage and measure the polarization degree and angle of the source, and also a full polarimetric calibration in CASA to compare my calibration with one obtained with professional software.
A few days ago I posted about TCM3, the fourth trajectory correction made by Tianwen-1 so far. After some days, the Chinese DSN has performed precise orbit determination and updated the on-board ephemerides, so that we are now seeing the final trajectory in the telemetry state vectors.
The figure below shows how the state vectors have been updated a couple of times following the TCM, as the DSN computes and uploads an improved trajectory solution. I have plotted this graph in the following way: I have taken the first state vector received after TCM3, on the UTC afternoon of 2020-10-28, and used it to propagate a trajectory in GMAT. The plot shows the difference between the state vectors and the GMAT trajectory.
TCM3 happened on 2020-10-28 at 14:00 UTC, so the reference trajectory computed in GMAT corresponds to the trajectory of the state vectors immediately following the TCM. These are based on a prediction of the burn performance, rather than on the actual results. The graph above shows clearly two changes in the trajectory, one on 2020-10-29, and another one on 2020-11-01.
Since we have already seen the same trajectory for three days without updates, I am confident that this trajectory is now final. The latest state vector we have today is
As always, this gives the UTC timestamp and the ICRF heliocentric position and velocity coordinates in km and km/s respectively.
I have re-run the calculations in the previous post by back-propagating a state vector from the UTC evening of 2020-11-01, which already belongs to the final trajectory. The change in delta-V in comparison to what I should in the previous post is small. The new delta-V is 2.13 m/s rather than 2.09 m/s, and the components have changed around 5%. The detailed calculations and data can be found in the updated Jupyter notebook.
The feeds in the ATA dishes are dual polarization linear feeds, giving two orthogonal linear polarizations that are called X and Y and (corresponding to the horizontal and vertical polarizations). In the setup we currently have, the two RF signals from a single dish are downconverted to an IF around 512 MHz using common LOs and then sampled by the two channels of a USRP N32x. Since we have two USRPs, we are able to receive dual polarization signals from two dishes simultaneously.
The two USRPs are synchronized with the 10MHz and PPS signals from the observatory, but even in these conditions there will be random phase offsets between the different channels. These offsets are caused by fractional-N PLL states and other factors, and change with every device reset. To solve this problem, it is possible to distribute the LO from the first channel of a USRP N321 into its second channel and both channels of a second USRP N320. In fact, it is possible to daisy chain several USRPs to achieve a massive MIMO configuration. By sharing the LO between all the channels, we achieve repeatable phase offsets in every run.
During the first weekends of experiments at ATA we didn’t use LO sharing, and we finally set it up and tested it last weekend. After verifying that phase offsets were in fact repeatable between all the channels, I did some polarimetric observations of GNSS satellites to calibrate the phase offsets. The results are summarised in this post. The data has been published in Zenodo as “Allen Telescope Array polarimetric observation of GNSS satellites“.
