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.
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.
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.
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.
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.
The data used in this post has been published in Zenodo as the dataset “Allen Telescope Array polarimetric observation of 3C286“.
This post belongs to a series about the activities of the GNU Radio community at Allen Telescope Array. For more information about these activities, see my first post.
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“.
A few days ago I read the paper about the Breakthrough Listen experiment. This experiment consists in doing many wideband recordings of different stars using the Green Bank Telescope, and (in the future) Parkes Observatory and then trying to find signals from extraterrestrial intelligent life in the recordings. The Breakthrough Listen project has a nice Github repository with some documentation and an analysis of a recording they did of Voyager 1 to test their setup.
I have also been thinking about how to study the polarization of signals in a dual polarization recording (two coherent channels with orthogonal polarizations). My main goal for this is to study the polarization of the signals of Amateur satellites in low Earth orbit. It seems that there are many myths regarding polarization and the rotation of cubesats, and these myths eventually pop up whenever anyone tries to discuss whether linearly polarized or circularly polarized Yagis are any good for receiving cubesats.
Through the Breakthrough Listen paper I’ve learned of the Stokes parameters. These are a set of parameters to describe polarization which are very popular in optics, since they are easy to measure physically. I have immediately noticed that they are also easy to compute from a dual polarization recording. In comparison with Jones vectors, Stokes parameters disregard all the information about phase, but instead they are computed from the averaged power in different polarizations. This makes their computation less affected by noise and other factors.
As I also wanted to get my hands on the Breakthrough Listen raw recordings, I have been computing the Stokes parameters of the Voyager 1 signal in their recording. Since the Voyager 1 signal is left hand circularly polarized, the results are not particularly interesting. It would be better to use a signal with changing polarization or some form of elliptical polarization.
I have started to use Jupyter notebook. This is something I had been wanting to try since a while ago, and I’ve realised that a Jupyter notebook serves better to document my experiments in Python than a Python script in a gist, which is what I was doing before. I have started a Github repo for my experiments using Jupyter notebooks. The experiment about polarization in the Voyager 1 signal is the first of them. Incidentally, this experiment has been done near Voyager 1’s 40th anniversary.