Space – Page 7 – Daniel Estévez

JWST sequential ranging

In my last post I spoke about the James Webb Space Telescope telemetry, and I decoded a recording I made with the Allen Telescope Array. I used an IQ sample rate of 3.84 Msps when doing this recording because I wanted to see if there were any ranging signals. Usually, ranging signals have a bandwidth of 1.5 MHz or less in baseband, so after phase modulation, approximately 3 MHz are used. Thus, 3.84 Msps gives enough bandwidth to record the typical ranging signals.

After looking at the waterfall of the recording carefully, I saw that there are sequential ranging signals present almost all the time. This is expected. Since the recording was done 7 hours after the first correction manoeuvre, the DSN would be doing ranging to compute accurate ephemerides. Often, ranging signals are not used every time that a spacecraft is tracked, but only when the ephemerides need to be refined, such as when planning a manoeuvre or shortly after executing one.

In this post I analyse these sequential ranging signals. I still haven’t had time to publish the recordings in Zenodo. After seeing that the wideband recording is of interest, due to the presence of these signals, I’m planning to publish a shorter segment of the wideband recording (the full recording is 241 GB per polarization) and publish a decimated version of the full recording where only around 100 kHz of spectrum are present (which is enough for the telemetry signal).

Decoding James Webb Space Telescope

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.

One month of Tianwen-1 remote sensing orbit

In a previous post, I described the remote sensing orbit into which Tianwen-1 had moved on November 8. Now it has been in this orbit for more than one month, and AMSAT-DL has been collecting telemetry almost daily with the 20 metre antenna at Bochum obseratory. Therefore, it is a good moment to review the state vector data and look at how the orbit has evolved with time.

Voyager 1 and Reed-Solomon

In one of my previous posts about Voyager 1, I stated that the Voyager probes used as forward error correction only the k=7, r=1/2 CCSDS convolutional code, and that Reed-Solomon wasn’t used. However, some days ago, Brett Gottula asked about this, citing several sources that stated that the Voyager probes used Reed-Solomon coding after their encounter with Saturn.

My source for stating that Reed-Solomon wasn’t used was some private communication with DSN operators. Since the XML files describing the configuration of the DSN receivers for Voyager 1 didn’t mention Reed-Solomon either, I had no reason to question this. However, the DSN only processes the spacecraft data up to some point (which usually includes all FEC decoding), and then passes the spacecraft frames to the mission project team without really looking at their contents. Therefore, it might be the case that it’s the project team the one who handles the Reed-Solomon code for the Voyagers. This would make sense specially if the code was something custom, rather than the CCSDS code (recall that Voyager predates the CCSDS standards). If this were true, the DSN wouldn’t really care if there is Reed-Solomon or not, and they might have just forgotten about it.

After looking at the frames I had decoded from Voyager 1 in more detail, I remarked that Brett might be right. Doing some more analysis, I have managed to check that in fact the Voyager 1 frames used Reed-Solomon as described in the references that Brett mentioned. In this post I give a detailed look at the Reed-Solomon code used by the Voyager probes, compare it with the CCSDS code, and show how to perform Reed-Solomon decoding in the frames I decoded in the last post. The middle section of this post is rather math heavy, so readers might want to skip it and go directly to the section where Reed-Solomon codewords in the Voyager 1 frames are decoded.

Reed-Solomon bug in Queqiao

Queqiao is the communications relay satellite for the Chang’e 4 Chinese lunar lander mission to the far side of the Moon. It is in a halo orbit around the Earth-Moon Largrange L2 point and provides communications to the lander in Von Kármán crater.

Queqiao transmits telemetry in S-band, using the frequency 2234.5 MHz. The modulation and coding is similar to other recent Chinese probes, such as Chang’e 5 and Tianwen-1. Here I report an interesting bug that I found in the Reed-Solomon encoding performed by Queqiao.

More data from Voyager 1

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.

Decoding DART

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.

Tianwen-1 remote sensing orbit

On November 8, the Tianwen-1 orbiter made a manoeuvre to move itself to the remote sensing orbit, as reported by Chinese media. This orbit is the final orbit in the mission, as depicted in this figure from Wikipedia. The main goal of this orbit is to study the geophysical properties of Mars with all the orbiter instruments (see this paper) and to continue acting as a communications relay for the rover Zhurong.

As usual, AMSAT-DL has been collecting telemetry from Tianwen-1 with the 20 metre antenna at Bochum observatory, including spacecraft state vectors. This allows us to study the orbit change manoeuvre and the properties of the remote sensing orbit. This post is a first look at the state vector data.

An update about Lucy’s telemetry

In my previous post I presented a description of the X-band telemetry of Lucy, including a GNU Radio decoder, some recordings from the Allen Telecope Array, and an analysis of some of the telemetry received in the two days following the launch. This is a short post with some updates about the telemetry analysis of the Lucy mission.

Decoding Lucy

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: