Radiometry for DELFI-PQ, EASAT-2 and HADES

On January 13, the SpaceX Transporter-3 mission launched many small satellites into a 540 km sun-synchronous orbit. Among these satellites were DELFI-PQ, a 3U PocketQube from TU Delft (Netherlands), which will serve for education and research, and EASAT-2 and HADES, two 1.5U PocketQubes from AMSAT-EA (Spain), which have FM repeaters for amateur radio. The three satellites were deployed close together with an Albapod deployer from Alba orbital.

While DELFI-PQ worked well, neither AMSAT-EA nor other amateur operators were able to receive signals from EASAT-2 or HADES during the first days after launch. Because of this, I decided to help AMSAT-EA and use some antennas from the Allen Telescope Array over the weekend to observe these satellites and try to find more information about their health status. I conducted an observation on Saturday 15 and another on Sunday 16, both during daytime passes. Fortunately, I was able to detect EASAT-2 and HADES in both observations. AMSAT-EA could decode some telemetry from EASAT-2 using the recordings of these observations, although the signals from HADES were too weak to be decoded. After my ATA observations, some amateur operators having sensitive stations have reported receiving weak signals from EASAT-2.

AMSAT-EA suspects that the antennas of their satellites haven’t been able to deploy, and this is what causes the signals to be much weaker than expected. However, it is not trivial to see what is exactly the status of the antennas and whether this is the only failure that has happened to the RF transmitter.

Readers are probably familiar with the concept of telemetry, which involves sensing several parameters on board the spacecraft and sending this data with a digital RF signal. A related concept is radiometry, where the physical properties of the RF signal, such as its power, frequency (including Doppler) and polarization, are directly used to measure parameters of the spacecraft. Here I will perform a radiometric analysis of the recordings I did with the ATA.

List of RF recordings

Happy New Year! To celebrate, I have put together a list of RF recordings. Over the last few years, and specially since the start of the collaboration between GNU Radio and SETI Institute, I have been publishing a number of RF recordings in Zenodo. The search function of Zenodo is not very good, and I thought that readers of this blog would find useful to have a list of all the recordings I have published. The list of recordings can be accessed here and in the website menu, under “Publications“.

I have also published an excerpt of the recording of James Webb Space Telescope that I did on December 26. This is just the first 25 minutes of the recording, so that both polarizations fit into maximum 50 GB of a Zenodo dataset. The sample rate is still 3.84 Msps, so the sequential ranging tones are present in these files. The dataset is called “James Webb Space Telescope S-band recording with Allen Telescope Array (wideband excerpt)“. In some days I will also publish a decimated version (containing the telemetry but not the ranging tones) of the full recording.

Update 2022-01-03: I have now published the full recording decimated to 320 ksps. This is available in the dataset “James Webb Space Telescope S-band recording with Allen Telescope Array (320 kHz bandwidth)“.

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Waterfalls from the December 2021 eclipse frequency measurement

The HamSci Ham Radio Scienze Citizen Investigation community organized earlier this month the December 2021 Eclipse Festival of Frequency Measurement. The goal of this activity was to measure the frequency of HF time signals such as WWV and RWM over the course of ten days. The experiment lasted from December 1 to December 10, so it included the total eclipse over Antarctica of December 4, which happened between 5:29 and 9:37 UTC.

I participated in this activity with my HF station, which consists of a Hermes-Lite 2 beta2 DDC/DUC SDR transceiver and an end-fed random wire antenna about 17 metres long. I used a 10 MHz reference from a GPSDO as described in this post to lock the Hermes-Lite 2 sampling clock. Instead of measuring frequency in real time, I recorded IQ data at 200 sps for the WWV carrier at 5000, 10000 and 15000 kHz and for the RWM carrier at 4996, 9996 and 14996 kHz, so that the data could be post processed later with any kind of algorithms. I have published my recordings in the “December 2021 Eclipse Festival of Frequency Measurment IQ recording by station EA4GPZ” dataset in Zenodo.

In this post I process the IQ recordings to produce waterfalls that give us an overview of the data. The frequency measurement will be done in a later post.

Hermes-Lite 2 external 10 MHz reference

Interested by the forthcoming HamSci December 2021 eclipse festival of frequency measurement, I have decided to enable and test the external 10 MHz input of my Hermes-Lite 2 DDC/DUC HF transceiver. This will allow me to use a GPSDO (the Vectron MD-011 which has appeared in other posts) to reference the Hermes-Lite 2 in order to measure frequency accurately.

GNU Radio 3.9 in Buildroot

Recently I’ve had to cross-compile GNU Radio for an ARM embedded system. I have decided to use Buildroot to build GNU Radio and its dependencies, since I’m fairly familiar with using Buildroot to generate embedded Linux images. Earlier this year, Jean-Michel Friedt and
Gwenhaël Goavec-Merou
presented in FOSDEM their work about adding a GNU Radio package in buildroot. They gave a talk called “Never compile on the target!“.

Unfortunately, the version they used was GNU Radio 3.8, and the package hasn’t been updated to GNU Radio 3.9 yet. I wanted to use GNU Radio 3.9, so I decided to try to update the Buildroot package. After some assorted problems, I have managed to get GNU Radio 3.9 running on my ARM target. The fixes I’ve done are really horrible, so I’ve been quite tempted not to share my changes. I’ve finally decide to share this even though it’s far from perfect, because it might save someone from having to replicate this work, and because if anyone wants to do this properly and update the upstream package, this could be useful as a starting point.

Decoding Voyager 1

Today is the 44th anniversary of the launch of Voyager 1, so I want to celebrate by showing how to decode the Voyager 1 telemetry signal using GNU Radio and some Python. I will use a recording that was done back in 30 December 2015 with the Green Bank Telescope in the context of the Breakthrough Listen project. Most of the data from this project is open data and can be accessed through this portal.

In contrast to other posts about deep space probes in this blog, which are of a very specialized nature, I will try to keep this post accessible to a wider audience by giving more details about the basics. Those interested in learning further can refer to the workshop “Decoding Interplanetary Spacecraft” that I gave in GRCon 2020, and also take a look at other posts in this blog.

Imaging Cygnus A at 8.45 GHz with ATA

Earlier this year, I published a post showing our results of the interferometric imaging of Cassiopeia A and Cygnus A at 4.9 GHz with the Allen Telescope Array. Near the end of July, I decided to perform more interferometric observations of Cygnus A at a higher frequency, in order to obtain better resolution. I chose a frequency of 8.45 GHz because it is usually a band clean of interference (since it is allocated to deep space communications), it is used by other radio observatories, so flux densities can be compared directly with previous results, and because going higher up in frequency the sensitivity of the old feeds at ATA starts to decrease.

This post is a summary of the observations and results. The code and data is included at the end of the post.

LDPC code design for my QO-100 narrowband modem

A couple months ago I presented my work-in-progress design for a data modem intended to be used through the QO-100 NB transponder. The main design goal for this modem is to give the maximum data rate possible in a 2.7 kHz channel at 50 dB·Hz CN0. For the physical layer I settled on an RRC-filtered single-carrier modulation with 32APSK data symbols and an interleaved BPSK pilot sequence for synchronization. Simulation and over-the-air tests of this modulation showed good performance. The next step was designing an appropriate FEC.

Owing to the properties of the synchronization sequence, a natural size for the FEC codewords of this modem is 7595 bits (transmitted in 1519 data symbols). The modem uses a baudrate of 2570 baud, so at 50 dB·Hz CN0 the Es/N0 is 15.90 dB. In my previous post I considered using an LDPC code with a rate of 8/9 or 9/10 for FEC, taking as a reference the target Es/N0 performance of the DVB-S2 MODCODs. After some performing some simulations, it turns out that 9/10 is a bit too high with 7595 bit codewords (the DVB-S2 normal FECFRAMEs are 64800 bits long, giving a lower LDPC decoding threshold). Therefore, I’ve settled on trying to design a good rate 8/9 FEC. At this rate, the Eb/N0 is 9.42 dB.

Solution to the EU GNU Radio Days challenge

For the European GNU Radio Days 2021, Jean-Michel Friedt, as part of the organizing team set up a signal decoding challenge based on GPS signals. The price to the two best solutions was two USRP B205mini‘s kindly provided by NI Ettus, who sponsored the conference.

I managed to solve this challenge shortly after it was published, and sent Jean-Michel a Jupyter notebook explaining my solution. Jean-Michel liked this approach and invited me to present my solution today at the conference. This presentation can be watched in the recording of the conference livestream.

I have now published a repository with all the material of my solution. Thanks to Jean-Michel for putting together this interesting and enjoyable challenge and to NI for providing a prize to make the challenge more attractive.

Rain backscatter on 10 GHz

Yesterday we had a strong storm in Madrid at around 16:30 UTC. The storm was rather short but intense. Seeing the heavy rain, it occurred to me that I might be able to receive the 10 GHz beacon ED4YAE at Alto del León using my QO-100 groundstation (without moving the antenna).

The 10 GHz beacon is 39.4 km away and the direct path to my station is obstructed by some hill in the middle, as shown in the link profile.

Link profile ED4YAE -> EA4GPZ (from HeyWhatsThat.com)

In the countryside just outside town it is possible to receive the beacon, probably because it diffracts on the hills. However, it is impossible for me to receive it directly from home, as there are too many tall buildings in the way.

In fact, when I fired up my receiver as the storm raged, I was able to see the beacon signal, with a huge Doppler spread of some 700 Hz (20 m/s). The CW ID of the beacon was easy to copy.

ED4YAE -> EA4GPZ at 10 GHz via rain backscatter

Then I started recording the signal. As the rain got weaker, it started disappearing, until it faded away completely. This post is a short analysis of the scatter geometry and the recording.