Last weekend, AMSAT-DL started some test transmissions of a high-speed multimedia beacon through the QO-100 NB transponder. The beacon uses the high-speed modem by Kurt Moraw DJ0ABR. It is called “high-speed” because the idea is to fit several kbps of data within the typical 2.7 kHz bandwidth of an SSB channel. The modem waveform is 2.4 kbaud 8APSK with Reed-Solomon (255, 223) frames. The net data rate (taking into account FEC and syncword overhead) is about 6.2 kbps.
I had never worked with this modem before, even though it served me as motivation for my 32APSK modem (still a work in progress). With a 24/7 continuous transmission on QO-100, now it was the perfect time to play with the modem, so I quickly put something together in GNU Radio. In this post I explain how my prototype decoder works and what remains to be improved.
The James Webb Space Telescope probably needs no introduction, since it is perhaps the most important and well-known mission of the last years. It was launched on Christmas day from Kourou, French Guiana, into a direct transfer orbit to the Sun-Earth L2 Lagrange point. JWST uses S-band at 2270.5 MHz to transmit telemetry. The science data will be transmitted in K-band at 25.9 GHz, with a rate of up to 28 Mbps.
After launch, the first groundstation to pick the S-band signal from JWST was the 10 m antenna from the Italian Space Agency in Malindi, Kenya. This groundstation commanded the telemetry rate to increase from 1 kbps to 4 kbps. After this, the spacecraft’s footprint continued moving to the east, and it was tracked for a few hours by the DSN in Canberra. One of the things that Canberra did was to increase the telemetry rate to 40 kbps, which apparently is the maximum to be used in the mission.
As JWST moved away from Earth, its footprint started moving west. After Canberra, the spacecraft was tracked by Madrid. Edgar Kaiser DF2MZ, Iban Cardona EB3FRN and other amateur observers in Europe received the S-band telemetry signal. When Iban started receiving the signal, it was again using 4 kbps, but some time after, Madrid switched it to 40 kbps.
At 00:50 UTC on December 26, the spacecraft made its first correction burn, which lasted an impressive 65 minutes. Edgar caught this manoeuvre in the Doppler track.
Later on, between 7:30 and 11:30 UTC, I have been receiving the signal with one of the 6.1 metre dishes at Allen Telescope Array. The telemetry rate was 40 kbps and the spacecraft was presumably in lock with Goldstone, though it didn’t appear in DSN now. I will publish the recording in Zenodo as usual, but since the files are rather large I will probably reduce the sample rate, so publishing the files will take some time.
In the rest of this post I give a description of the telemetry of JWST and do a first look at the telemetry data.
Back in September, I showed how to decode the telemetry signal from Voyager 1 using a recording made with the Green Bank Telescope in 2015 by the Breakthrough Listen project. The recording was only 22.57 seconds long, so it didn’t even contain a complete telemetry frame. To study the contents of the telemetry, more data would be needed. Often we can learn things about the structure of the telemetry frames by comparing several consecutive frames. Fields whose contents don’t change, counters, and other features become apparent.
Some time after writing that post, Steve Croft, from BSRC, pointed me to another set of recordings of Voyager 1 from 16 July 2020 (MJD 59046.8). They were also made by Breakthrough Listen with the Green Bank Telescope, but they are longer. This post is an analysis of this set of recordings.
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.
DART, the Double Asteroid Redirection Test, is a NASA mission that launched last Wednesday from Vandenberg. The goal of this mission is to crash the spacecraft into the small asteroid Dimorphos, allowing us to measure the small change in the orbit of the asteroid caused by the impact.
From the communications perspective, this spacecraft is the first to use a Spiral Radial Line Slot Array (RLSA) as high-gain antenna. Details about the antenna design can be seen in this paper. The paper shows that antenna polarization is LHCP. Most DSN communications use RHCP, although there are a few notable exceptions (for instance Emirates Mars Mission), and the DSN stations are equipped to handle both polarizations. I’m not sure if DART is indeed using LHCP or if this is just a matter of the convention in the definition of the polarization used in the paper (there are actually two opposite conventions to define the sense of circular polarization).
A few hours after launch, as the spacecraft passed over Europe, Miguel CT1BYM and Iban EB3FRN recorded the X-band telemetry signal from DART at 8421.79 MHz. This post is a first analysis of the signal.
Lucy is a spacecraft that will study the Trojan asteroids, during a twelve year mission. It was launched last Saturday at 9:34 UTC from Cape Canaveral on an Atlas V rocket. Its telemetry downlink is on X-band, at a frequency of 8445.768 MHz.
Iban Cardona EB3FRN made a 30 minute recording of the telemetry downlink at 19:00 UTC on Saturday, as the spacecraft first appeared over Europe after launch. r00t.cz did a brief analysis of this recording overnight, and then published some more details about the telemetry data. On Sunday, at 8:52 UTC, I did a long recording with one of the dishes in the Allen Telescope Array. This recording lasts 3 hours 26 minutes, and ends when the spacecraft set below the 16 degree elevation mask of the ATA. In this post I give a first analysis of the telemetry data in both recordings.
The recording done at ATA can be downloaded from the following datasets in Zenodo:
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.
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.
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.
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.