Decoding IDEASSat

One of the interesting Amateur cubesats in yesterday’s SpaceX Transporter-1 launch from Cape Canaveral is IDEASSat, a 3U cubesat from the National Central University of the Republic of China (Taiwan) designed to study ionospheric plasma. Jan van Gils PE0SAT drew my attention to this satellite as he was trying to see if it was possible to decode it with any of the decoders existing in gr-satellites. Mike Rupprecht DK3WN also helped with a good recording, much cleaner than the SatNOGS recording that Jan was using.

Presumably this satellite uses “AX25, 9k6, GMSK”, as listed at the bottom of this page from Taiwan’s National Space Organization, and also in this research paper. However, this is not true. It’s simple to check that the usual 9k6 FSK AX.25 decoders aren’t able to decode this signal, and a look at the FSK symbols shows that there is no scrambler (9k6 AX.25 uses the G3RUH scrambler) and that the symbol sequence doesn’t have much to do with AX.25.

After some reverse-enginnering, yesterday I figured out how the coding used by IDEASSat worked, and today I added a decoder to gr-satellites to help Mike investigate what kind of telemetry the packets contain. The protocol is not very good, so I think it’s interesting to document it in detail, as some sort of lessons learned. In this post I’ll do so. As it turns out, the protocol has some elements that loosely resemble AX.25, so I’m left wondering whether this is some unsuccessful attempt at implementing standard AX.25 (we’ve already seen very weird attempts, such as ESEO).

BPSK radar received through Meridian 8

Ever since SETI Insitute published the news of a possible signal received from Proxima Centauri in some of the Parkes telescope recordings at 982 MHz, Scott Tilley VE7TIL has taken up the interest to search and catalogue the satellites that transmit on this band (specially old, forgotten and zombie satellites). His idea is to try to see if this candidate signal can be explained as interference from some satellite.

This has led him to discover some signals coming from satellites on a Molniya orbit. After examination with the Allen Telescope Array of these signals, we confirmed that they came from wideband transponders (centre frequency around 995 MHz, 13 MHz width) on some of the Meridian Russian communications satellites (in particular Meridian 4 and 8, but also others).

These transponders show all sorts of terrestrial signals that are relayed as unintended traffic through the transponder. By measuring Doppler we know that the uplink is somewhere around 700 or 800 MHz. We have found some OFDM-like signals that seem to be NB-IoT. Unfortunately we haven’t been able to do anything useful with them, maybe because there are several signals overlapping on the same frequency. We also found a wideband FM signal containing music and announcements in Turkmen, which later turned out to be the audio subcarrier of a SECAM analogue TV channel from Turkmenistan.

A few days ago, Scott detected a pulsed strong signal through the transponder of the Meridians at a downlink frequency of 994.2 MHz. He did an IQ recording of this signal on the downlink of Meridian 8. It turns out that this signal is a BPSK pulse radar. In this post I do a detailed analysis of the radar waveform using this recording.

Decoding the NEXUS π/4-DQPSK signal

NEXUS, also called FO-99, is a Japanese Amateur satellite built by Nihon University and JAMSAT. It was launched on January 2019, and one of its interesting features is a π/4-DQPSK high-speed transmitter for the 435 MHz Amateur satellite band.

I was always interested in implementing a decoder for this satellite, due to its unusual modulation, but the technical information that is publicly available is scarce, so I never set to do it. A few days ago, Andrei Kopanchuk UZ7HO asked me a question about the Reed-Solomon code used in this satellite. He was working on a decoder for this satellite, and had some extra documentation. This renewed my interest in building a decoder for this satellite.

With some help from Andy regarding the symbol mapping for the π/4-DQPSK constellation, I have made a decoder GNU Radio flowgraph that I have added to gr-satellites.

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“.

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.

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.

Decoding Mars 2020

Mars 2020, NASA’s latest mission to Mars, was launched a couple weeks ago. However, with all the Tianwen-1 work down the pipeline, until now I haven’t had time to dedicate an appropriate post to this mission (though I showed some sneak peek on Twitter). This mission consists of a rover and helicopter (a real novelty in space exploration). Both were launched with the cruise stage and the entry, descent and landing system on July 30 from Cape Canaveral, an are currently on their transfer orbit to Mars, as Tianwen-1 and Emirates Mars Mission.

In this post I will be working with some recordings made by AMSAT-DL using the 20m radio telescope at Bochum’s observatory. These feature the low rate safe mode telemetry, which was very strong and caused some anecdotes as it saturated some NASA DSN receivers, and the nominal 10kbps telemetry signal that was switched on later. Here I will describe the modulation and coding, giving GNU Radio decoders, and also take a look at the data. r00t.cz has also written a post where he shows similar information.

Tianwen-1 high speed data signal

In a previous post I talked about how the high data rate signal of Tianwen-1 can be used to replay recorded telemetry. I did an analysis of the telemetry transmitted over the high speed data signal on 2020-07-30 and showed how to interpret the ADCS data, but left the detailed description of the modulation and coding for a future post.

Here I will talk about the modulation and coding, and how the signal switches from the ordinary low rate telemetry to the high speed signal. I also give GNU Radio decoder flowgraphs, tianwen1_hsd.grc, which works with the 8192 bit frames, and tianwen1_hsd_shortframes.grc, which works with the 2048 bit short frames.

Decoding Emirates Mars Mission Hope

Last Sunday 2020-07-19, the first mission of United Arab Emirates to Mars, known as Emirates Mars Mission “Hope probe” launched from Tanegashima, Japan. This probe is expect to reach Mars in approximately 200 days and study its atmosphere over the course of two years. The scientific instruments onboard the probe are a digital camera, an infrared spectrometer, and an ultraviolet spectrometer.

Shortly after launch, several Amateur radio operators and Amateur spacecraft trackers received signals from the X-band beacon of the Hope probe at 8402.655 MHz and posted reports on Twitter, such as Paul Marsh M0EYT, Ferrucio IW1DTU, Edgar Kaiser DF2MZ, and others. Since the spacecraft was still near Earth, its signal was so strong that a data modulation with a main lobe of approximately 20kHz wide and several sidelobes could easily be seen in the spectrum, which is shown below.

Emirates Mars Mision Hope X-band downlink spectrum

Paul has been quite kind to send me a recording that he made with his station on 2019-07-19 at 23:29 UTC and I have been decoding the data in GNU Radio and looking at the frames. The recording can be downloaded here (193MB). It is an int16 IQ recording at 99998 samples per second. This post is an account of my results.

gr-satellites FSK BER

A few months ago I talked about BER simulations of the gr-satellites demodulators. In there, I showed the BER curves for the BPSK and FSK demodulators that are included in gr-satellites, and gave some explanation about why the current FSK demodulator is far from ideal. Yesterday I was generating again these BER plots to check that I hadn’t broken anything after some small improvements. I was surprised to find that the FSK BER curve I got was much worse than the one in the old post.