satellites – Page 6 – Daniel Estévez

Trying to observe the Vega-C MEO cubesats

On July 13, the Vega-C maiden flight delivered the LARES-2 passive laser reflector satellite and the following six cubesats to a 5900 km MEO orbit: AstroBio Cubesat, Greencube, ALPHA, Trisat-R, MTCube-2, and CELESTA. This is the first time that cubesats have been put in a MEO orbit (see slide 8 in this presentation). The six cubesats are very similar to those launched in LEO orbits, and use the 435 MHz amateur satellite band for their telemetry downlink (although ALPHA and Trisat-R have been declined IARU coordination, since IARU considers that these missions do not meet the definition of the amateur satellite service).

Communications from this MEO orbit are challenging for small satellites because the slant range compared to a 500 km LEO orbit is about 10 times larger at the closest point of the orbit and 4 times larger near the horizon, giving path losses which are 20 to 12 dB higher than in LEO.

I wanted to try to observe these satellites with my small station: a 7 element UHF yagi from Arrow antennas in a noisy urban location. The nice thing about this MEO orbit is that the passes last some 50 minutes, instead of the 10 to 12 minutes of a LEO pass. This means that I could set the antenna on a tripod and move it infrequently.

As part of the observation, I wanted to perform an absolute power calibration of my SDR (a USRP B205mini) in order to be able to measure the noise power at my location and also the power of the satellite signals power, if I was able to detect them.

Real time Doppler correction with GNU Radio

Satellite RF signals are shifted in frequency proportionally to the line-of-sight velocity between the satellite and groundstation, due to the Doppler effect. The Doppler frequency depends on time, on the location of the groundstation, and on the orbit of the satellite, as well as on the carrier frequency. In satellite communications, it is common to correct for the Doppler present in the downlink signals before processing them. It is also common to correct for the uplink Doppler before transmitting an uplink signal, so that the satellite receiver sees a constant frequency.

For Earth satellites, these kinds of corrections can be done in GNU Radio using the gr-gpredict-doppler out-of-tree module and Gpredict (see this old post). In this method, Gpredict calculates the current Doppler frequency and sends it to gr-gpredict-doppler, which updates a variable in the GNU Radio flowgraph that controls the Doppler correction (for instance by changing the frequency of a Frequency Xlating FIR Filter or Signal Source).

I’m more interested in non Earth orbiting satellites, for which Gpredict, which uses TLEs, doesn’t work. I want to perform Doppler correction using data from NASA HORIZONS or computed with GMAT. To do this, I have added a new Doppler Correction C++ block to gr-satellites. This block reads a text file that lists Doppler frequency versus time, and uses that to perform the Doppler correction. In this post, I describe how the block works.

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.

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.