Polarimetric observation of 3C286 with Allen Telescope Array

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“.

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ATA polarimetry test with GNSS satellites

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.

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GNSS interferometry at Allen Telescope Array

Since the beginning of October, together with a group of people from the GNU Radio community, we are doing some experiments and tests remotely at Allen Telescope Array (ATA). This amazing opportunity forms part of the recent collaboration agreement between SETI Institute and GNU Radio. We are taking advantage of the fact that the ATA hardware is relatively unused on weekends, and putting it to good use for our experiments. One of the goal of these activities is to put in contact GNU Radio people and radio astronomy people, to learn from each other and discover what features of GNU Radio could benefit radio astronomy and SETI, particularly at the ATA.

I’m very grateful to Wael Farah, Alex Pollak, Steve Croft and Ellie White from ATA and SETI Institute for their support of this project and the very interesting conversations we’ve had, to Derek Kozel, who is Principal Investigator for GNU Radio at SETI, for organizing and supporting all this, and to the rest of my GNU Radio teammates for what’s being an excellent collaboration of ideas and sharing of resources.

From the work I’ve been doing at ATA, I already have several recordings and data, and also some studies and material that I’ll be publishing in the near future. Hopefully this post will be the first in a series of many.

Here I will speak about one of the first experiments I did at ATA, which is a recording of one Galileo GNSS satellite using two of the dishes from the array. This kind of recording can be used to perform interferometry. GNSS satellites are good test targets because they have strong wideband signals and their location is known precisely. The IQ recording described in this post is published as the dataset “Allen Telescope Array Galileo E31 RF recording with 2 antennas and 2 polarizations” in Zenodo.

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Measuring Tianwen-1’s modulation

This is a post I had announced since I first described Tianwen-1’s modulation. Since we have very high SNR recordings of the Tianwen-1 low rate rate telemetry signal made with the 20m dish in Bochum observatory, it is interesting to make detailed measurements of the modulation parameters. In fact, there is something curious about the way the modulation is implemented in the spacecraft’s transmitter. This analysis will show it clearly, but I will reserve the details for later in the post.

Here I will be using a recording that already appeared in a previous post. It was made on 2020-07-26 07:47:20 UTC in Bochum shortly after the switch to the high gain antenna, so the SNR is fantastic. The recording was done at 2.5Msps, and the spectrum can be seen below. The asymmetry (especially around +1MHz) might be due to the receive chain.

The signal is residual carrier phase modulation, with 16348 baud BPSK data on a 65536Hz square wave subcarrier. There is also a 500kHz ranging tone.

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BY02 telemetry beacon

BY02 (also known as BY70-2) is an Amateur cubesat by the China Aerospace Science and Technology Corporation and Beijing Bayi High School. It was launched on July 3 on a CZ-4B rocket from Taiyuan together with a Gaofen Earth observation satellite. BY02 is intended as a replacement for BY70-1, which was launched on 2016-12-28 and was placed on a short-lived orbit that decayed in a few months because of a launch problem.

Today, Wei Mingchuan BG2BHC announced on Twitter at 09:14 UTC that BY02’s beacon was on and would be left on at least until 12:50 UTC. I believe that this is the first time that the beacon has been on for an extended period of time, since during the early operations the beacon was only active on passes over China.

Since at 11:39 UTC there was a good pass over Spain, I went outside with my handheld Arrow 7 element yagi to do a recording. This post is an in-depth analysis of this recording and includes an explanation of the coding and telemetry format used by BY02.

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Receiving Arecibo in HF

The well known Arecibo observatory, besides being used as a radiotelescope and planetary radar, has a powerful HF transmitter that is used to artificially excite the ionosphere, in order to study ionospheric effects using 430MHz incoherent scatter radar. More information about this can be found in the HF proposals page of the observatory web, and in this poster that details the characteristics of the HF facilities.

The HF transmitter has a power of up to 600kW and can use the frequencies 5.1MHz and 8.175MHz. At those frequencies, the dish has a gain of 22dB (13º beamwidth) and 25.5dB (8.5deg) respectively, so the power that is beamed up to the ionosphere is huge. The 430MHz incoherent scatter radar is even more powerful, with up to 2MW. For an introduction to ionospheric incoherent scatter radar, see this lecture by Juha Vierinen, which explains why such huge powers are needed, due to the very weak radar return of ionospheric plasma.

A few days ago, on Wednesday 24, Chris Fallen tweeted that the Arecibo transmitter was active at 5.1MHz. According to the telescope schedule, which can be seen in the figure below (click on the image to view it in full size), there was an experiment that involved the HF transmitter on 2020-06-24 from 18:00 to 22:00 UTC, on 2020-06-25 from 17:00 to 21:00 UTC, and on 2020-06-26 from 17:00 to 21:00 UTC.

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Ranging through the QO-100 WB transponder

One of the things I’ve always wanted to do since Es’hail 2 was launched is to perform two-way ranging by transmitting a signal through the Amateur transponder and measuring the round trip time. Stefan Biereigel DK3SB first did this about a year ago. His ranging implementation uses a waveform with a chip rate of only 10kHz, as it is thought for Amateur transponders having bandwidths of a few tens of kHz. With this relatively slow chiprate, he achieved a ranging resolution of approximately 1km.

The QO-100 WB transponder allows much wider signals that can be used to achieve a ranging resolution of one metre or less. This weekend I have been doing my first experiments about ranging through the QO-100 WB transponder.

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Decoding Crew Dragon Demo-2

The launch last Saturday of Crew Dragon Demo-2 undoubtedly was an important event in the history of American space exploration and human spaceflight. This was the first crewed launch from the United States in 9 years and the first crewed launch ever by a commercial provider. Amateur radio operators always follow this kind of events with their hobby, and in the hours and days following the launch, several Amateur operators have posted reception reports of the Crew Dragon C206 “Endeavour” signals.

It seems that the signal received by most people has been the one at 2216 MHz. Among these reports, I can mention the tweets by Scott Tilley VE7TIL (and this one), USA Satcom, Paul Marsh M0EYT. Paul also managed to receive a signal on 2272.5 MHz, which is not in the FCC filing, so this may or may not be from the Crew Dragon.

Scott has also shared with me an IQ recording of one of the passes, and as I showed on Twitter yesterday, I have been able to demodulate the data. This post is my analysis of the signal.

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LES-5 telemetry Grafana dashboard

Scott Tilley VE7TIL is making a serious effort and a great job of recording and processing LES-5 telemetry. He is recording all the passes over his home in western Canada (which last several days, due to the sub-synchronous orbit), and sharing the data on a Github repository, together with Jupyter notebooks that analyse the data and plot some of the telemetry variables, such as the values recorded by the RFI experiment.

I am storing this data in InfluxDB 2.0 and using Grafana to plot it and explore it. The Grafana server has been running for quite some time now, but I never announced it publicly, so very few people have used it. I guess that now is a good time to share it with a wider audience. The server is at eala.destevez.net:3000 and the LES-5 dashboards can be accessed by using user “les5” and password “les5”.

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Update on the QO-100 local oscillator wiggles

This post is a follow up to my study of the “wiggles” observed in the local oscillator of the QO-100 NB transponder. After writing that post, I have continued measuring the frequency of the BPSK beacon with my station almost without interruptions. Now I have some 44 days of measurements, spanning from April 9 to May 23. This data can be interesting to look at, so I’m doing this short post to share the data and look at it briefly.

The Jupyter notebook with all the data can be found here. The data is also linked in my jupyter_notebooks Github repository, which now uses git-annex to store the data in my home server. See the README for instructions on how to download some or all of the data files in the repository.

The whole time series can be seen in the figure below. We note that the typical Doppler sinusoidal curve varies slowly due to orbit perturbations and sometimes suddenly as a consequence of a station-keeping manoeuvre. I tweeted about one of the manoeuvres a while back.

There are now too many days in order to see things clearly when the frequency curves for each day are overlaid, but hopefully the figure below gives a good idea. We can see that the wiggles still happen approximately between 21:00 and 06:00 UTC, and between 11:00 and 17:00 UTC.

If we add an artificial offset of -15 Hz per day to the curves to prevent them from overlapping, we obtain the figure below. We see that the pattern of the wiggles keeps changing slightly, but also remains quite similar.

In my last post about this topic I said that it seemed that the wiggles repeated with a period of a sidereal day. Now it is clear that it is not the case. The wiggles seem to repeat roughly with a period of a solar day (24 hours). In fact, in 44 days sidereal time “advances” 2.88 hours with respect to solar time. However, it is clear that the wiggles haven’t shifted that much in time.