GPS spectrometry at Allen Telescope Array

Over the last few weeks I have been helping the Allen Telescope Array by calibrating the pointing of some of the recently upgraded antennas using the GNU Radio backend, which consists of two USRP N32x devices that are connected to the IF output of the RFCB downconverter. For this calibration, GPS satellites are used, since they are very bright, cover most of the sky, and have precise ephemerides.

The calibration procedure is described in this memo. Essentially, it involves pointing at a few points that describe a cross in elevation and cross-elevation coordinates and which is centred at the position of the GPS satellite. Power measurements are taken at each of these points and a Gaussian is fitted to compute the pointing error.

The script I am using is based on this script for the CASPER SNAP boards, with a few modifications to use my GNU Radio polarimetric correlator, which uses the USRPs and a software FX correlator that computes the crosscorrelations and autocorrelations of the two polarizations of two antennas. For the pointing calibration, only the autocorrelations are used to measure Stokes I, but all the correlations are saved to disk, which allows later analysis.

In this post I analyse the single-dish polarimetric spectra of the GPS satellites we have observed during some of these calibrations.

Interferometric imaging with Allen Telescope Array

In the weekend experiments that we are doing with the GNU Radio community at Allen Telescope Array we usually have access to some three antennas from the array, since the rest are usually busy doing science (perhaps hunting FRBs). This is more than enough for most of the experiments we do. In fact, we only have two N32x USRPs, so typically we can only use two antennas simultaneously.

However, for doing interferometry, and specially for imaging, the more antennas the better, since the number of baselines scales with the square of the number of antennas. To allow us to do some interferometric imaging experiments that are not possible with the few antennas we normally use, we arranged with the telescope staff to have a day where we could access a larger number of antennas.

After preparing the observations and our software so that everything would run as smoothly as possible, on 2021-02-21 we had a 18 hour slot where we had access to 12 antennas. The sources we observed where Cassiopeia A and Cygnus A, as well as several compact calibrators. After some calibration and imaging work in CASA, we have produced good images of these two sources.

Many thanks to all the telescope staff, specially to Wael Farah, for their help in planning together with us this experiment and getting everything ready. Also thanks to the GNU Radio team at ATA, specially Paul Boven, with whom I’ve worked side by side for this project.

This post is a long report of the experiment set up, the software stack, and the results. All the data and software is linked below.

Voyager-1 single dish detection at Allen Telescope Array

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:

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 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 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.