In my previous post I spoke about the recording I made of the X-band telemetry signal of Hera with the Allen Telescope Array shortly after it was launched. Despite the lack of publicly available accurate ephemerides at the time of launch, I managed to track the spacecraft by hand and decode a good amount of telemetry frames. In this post I will do an in-depth analysis of the telemetry.
Tag: ATA
Decoding Hera
Hera is an ESA mission to the Didymos binary asteroid system. It will arrive there in December 2026 to study the asteroids and the effects of the impact of DART on Dimorphos. It was launched on October 7 from Cape Canaveral, exactly one week before Europa Clipper. In the same way as for Europa Clipper, Hera’s launch trajectory allowed me to track it with the Allen Telescope Array, starting approximately 90 minutes after launch.
However, the ephemerides publicly available when the launch happened turned out to be completely wrong, as I will explain below in more detail. I needed to find the spacecraft’s signal by moving the antenna in the blind, and continue tracking it by hand by tweaking the pointing every few minutes. For this reason, the quality of the recordings I have done is not so good. The signal drops down frequently as the spacecraft moves away from where I was pointing or when I made mistakes in my pointing adjustments.
For this reason, I have prioritized decoding the Europa Clipper recordings, since I expected that decoding these low quality recordings of Hera would take more work. Nevertheless I have managed to decode a good amount of telemetry.
I have published the IQ recordings made with the ATA in the following two Zenodo datasets:
Europa Clipper telemetry
In my previous post I spoke about the recording I did of the Europa Clipper X-band telemetry shortly after launch with one of the Allen Telescope Array antennas. In that post I analysed the recording waterfall and the signal modulation and coding, and decoded the telemetry frames with GNU Radio. In this post I analyse the contents of the telemetry. As we will see, there are several similarities with the telemetry of Psyche. This makes sense, because both are NASA missions that have been launched only one year apart.
Decoding Europa Clipper
Europa Clipper is a NASA mission that will study Europa, Jupiter’s icy moon, to investigate if it can support life, perhaps in hydrothermal vents in a global ocean under the ice crust. The mission launched on Monday from Cape Canaveral, after some days of delay due to Hurricane Milton. As happened with Psyche one year ago, the launch trajectory was such that the first pass over the Allen Telescope Array, in northern California, started only about 1.5 hours after launch. To put this in perspective, launch was at 2024-10-14T16:06 UTC, spacecraft separation at T+1:02:39, and my recording began at 17:33:24 UTC, with signal acquisition a couple minutes later as the spacecraft raised above the 16.8 deg elevation mask of the ATA antennas.
I used one of the ATA antennas to record the X-band telemetry signal for about 2 hours and 50 minutes, until the spacecraft set again due to Earth rotation. In this post I overview the recording and decode the telemetry with GNU Radio.
I recorded at 6.144 Msps IQ, but since the telemetry symbol rate was only 12 kbaud throughout all the recording, I have made files decimated to 96 ksps and published them in the dataset “Recording of Europa Clipper X-band telemetry with the Allen Telescope Array shortly after launch” in Zenodo. This decimation discards the sequential ranging tones, which were present during most of the observation, but it greatly reduces the file size.
JUICE Earth flyby
At the beginning of this week, JUICE, an ESA spacecraft that was launched in April 2023 and that will explore Jupiter’s icy moon, completed its lunar-Earth flyby. This is the first time that such a manoeuvre has been done. I’m far from an expert in orbital dynamics, but I hear that by doing a combined lunar-Earth flyby instead of the more traditional Earth flyby there can be delta-V savings for the mission. The IAC paper Improved interplanetary transfers with Lunar-Earth gravity assists seems to be the first that presented this idea, but I haven’t found the paper itself. The JUICE mission analysis report (CReMA) contains some more information, but it is not too detailed and somewhat outdated. Perhaps a good source to learn about this is a master’s thesis called Automatic extension of Earth and Earth-Moon resonant flybys, although I’ve only had time to read a small part of it.
The following image that I made in GMAT with the mission SPICE kernels illustrates the geometry of the flybys. The lunar flyby happened on 2024-08-19 21:15 UTC, and the Earth flyby happened on 2024-08-20 21:56 UTC.
The Earth flyby happened 6840 km above the north Pacific ocean, so the spacecraft was visible from the Allen Telescope Array, in northern California, starting some minutes after the flyby. I thus decided to observe JUICE with one of the 6.1 m dishes of the array and record its X-band telemetry signal roughly between 22:00 and 23:00 UTC. That would be a fun way of “saying hello” again to the spacecraft (see the post where I recorded and decoded its telemetry when it launched in April last year).
I prepared a tracking file for antenna pointing in the same way that I did for the OSIRIS-REx flyby during its sample capsule return. I used the SPICE kernels to compute a time series of azimuth and elevations for the antenna. For the spacecraft orbit I used the juice_orbc_000071_230414_310721_v01.bsp SPICE kernel, which was the latest available at the time. This kernel was created on 2024-08-15. The following plot shows the antenna azimuth and elevation and the distance to the spacecraft.
When I was about to begin observing, I saw a tweet from ESA Operations. Strangely the tweet has now been deleted. I hate it when information I once saw suddenly vanishes out of existence. I have the link to the tweet, but not a copy of it. It roughly said something along the lines of: “since JUICE is prepared for the cold thermal environment of Jupiter, to prevent it from overheating during the Earth flyby, data will not be transmitted to Earth until 2024-08-21 04:30 UTC”.
A problem with outreach communications is that they’re often vague with the hopes of simplifying to reach a wider audience. In this case, the context for the tweet was that ESA had run a livestream during the lunar flyby where they transmitted and showed images of the Moon in near-realtime. Postprocessed images with better quality were then released the next morning (see also this and related toots from Mark McCaughrean, who is one of the two persons responsible for the images). I think there was some word in social media that shortly after the Earth flyby some images would be shown, so the tweet served as a heads up that any images would need to wait until the next morning.
What wasn’t clear about the tweet is what this really meant about the spacecraft telemetry signal. Anyone familiar with spacecraft tracking will imagine that during the flyby there will be no groundstation coverage, because deep space stations can’t track that fast and probably won’t have the spacecraft in view. Therefore it’s clear that no images can be downloaded for some time around the flyby. But what about the telemetry signal? Does the tweet mean that there will be no housekeeping telemetry (which typically is sent all the time) either? Even no RF modulation at all? This might well be the case, especially since the tweet mentioned thermal reasons. If there isn’t going to be groundstation coverage for a while, why run the radio transmitter and generate more heat? But the way that the tweet was worded still made me doubt (I think the literal words were “it won’t transmit any more data”, but I don’t have the full sentence).
So after seeing that I wasn’t receiving any signal with the Allen Telescope Array, I wasn’t too surprised. I tweeted about this, as a quote tweet (repost) of the ESA Operations tweet.
I finished recording data at 22:30 UTC, since by then I was convinced that the transmitter was off and there was no point in continuing to record. I ran some additional tests to rule out a problem with the receiver. Unfortunately there were no good deep space X-band satellites in view at that time, since Mars was just setting. I tried with Parker Solar Probe, which was being tracked by Goldstone, but I didn’t know its frequency and didn’t have an estimate for how strong the signal should be. I didn’t see the signal. I also tried with ACE, which transmits at S-band and was also tracked by Goldstone. I could receive it just fine, so I don’t have much reason to suspect about the receiver.
Another potential reason why no signal could be detected is if there was an error in the ephemerides that caused a large enough antenna pointing error. I don’t think this was the case, even though the ephemerides were not updated after the lunar flyby. To make sure, I will rerun the pointing calculations with the next SPICE kernel that gets published and compare them. Pointing error is also the reason that I kept recording until 22:30 UTC. The effect of ephemerides error in pointing error would become smaller as the distance to the spacecraft increased with time.
Just in case there was a faint signal, I have processed the recording to compute a waterfall with a resolution of 93.75 Hz and 10 seconds. The following two plots definitely show that there is no signal.
So with this, I finish the post as a failed observation of JUICE. I’ve decided do this write up because I think that, in science, negative results can be as important as positive results. In this case, the observation provides good evidence (especially now that the ESA Operations tweet is misteriously gone) that JUICE had the RF transmitter turned off during the Earth flyby. This is nothing transcendental, but rather an interesting tidbit of spacecraft operations trivia. The lack to detect any signals also reminds me of many astronomical observations, particularly those done as a followup to some transient, where if no signal is detected, the conclusion is often stated as “we establish upper bounds for the flux radiated by this source”.
To close with something more positive, I’ll mention that Alan Antoine F4LAU had a much more successful time tracking the lunar flyby with the AMSAT-DL 20 m antenna at Bochum Observatory, and he even found and decoded many images transmitted in the telemetry.
The materials used in this post can be found in this repository.
Psyche telemetry
In my previous post I spoke about the recording of the telemetry signal from the Psyche spacecraft that I made just a few hours after launch with the Allen Telescope Array. In that post I analysed the physical aspects of the signal and the modulation and coding, but left the analysis of the telemetry frames for another post. That is the topic of this post. It will be a rather long and in-depth look at the telemetry, since I have managed to make sense of much of the structure of the data and there are several protocol layers to cover.
As a reminder from the previous post, the recording was around four hours long. During most of the first three hours, the spacecraft was slowly rotating around one of its axes, so the signal was only visible when the low-gain antenna pointed towards Earth. It was transmitting a low-rate 2 kbps signal. At some point it stopped rotating and switched to a higher rate 61.1 kbps signal. We will see important changes in the telemetry when this switch happens. Even though the high-rate signal represents only one quarter of the recording by duration, due to its 30x higher data rate, it represents most of the received telemetry by size.
Receiving the Psyche launch
On Friday, the Psyche mission launched on a Falcon Heavy from Cape Canaveral. This mission will study the metal-rich asteroid of the same name, 16 Psyche. For more details about this mission you can refer to the talk that Lindy Elkins-Tanton, the mission principal investigator, gave a month ago at GRCon23.
The launch trajectory was such that the spacecraft could be observed from the Allen Telescope Array shortly after launch. The launch was at 14:19 UTC. Spacecraft separation was at 15:21 UTC. The spacecraft then rose above the ATA 16.8 degree elevation mask in the western sky at 15:53 UTC. However, the signal was so strong that it could be received even when the spacecraft was a couple degrees below the elevation mask, so I confirmed the presence of the signal and started recording a couple minutes earlier. At this moment, the spacecraft was at a distance of 18450 km. The spacecraft continued to rise in the sky, achieving a maximum elevation of 32.9 degrees at 16:53 UTC, and setting below the elevation mask on the west at 19:22 UTC. At this moment the spacecraft was 103800 km away. The signal could still be received for a few minutes afterwards, but eventually became very weak and I stopped recording.
Since the recording started only 30 minutes after spacecraft separation, we get to see some of the events that happen very early on in the mission. Most of the observations of deep space launches that I have done with the ATA have started several hours after launch. This Twitter thread by Lindy Elkins-Tanton gives some insight about the first steps following spacecraft separation, and I will be referring to it to explain what we see in the recording.
I intend to publish the recordings in Zenodo as usual, but the platform has been upgraded recently and is showing the following message “Oct 14 12:03 UTC: We are working to resolve issues reported by users.” So far I have been unable to upload large files, but I will keep retrying and update this post when I manage.
Update 2023-10-19: Zenodo have now solved their problems and I have been able to upload the recordings. They are published in the following datasets:
Observing OSIRIS-REx during the capsule reentry
On September 24, the OSIRIX-REx sample return capsule landed in the Utah Test and Training Range at 14:52 UTC. The capsule had been released on a reentry trajectory by the spacecraft a few hours earlier, at 10:42 UTC. The spacecraft then performed an evasion manoeuvre at 11:02 and passed by Earth on a hyperbolic orbit with a perigee altitude of 773 km. The spacecraft has now continued to a second mission to study asteroid Apophis, and has been renamed as OSIRIS-APEX.
This simulation I did in GMAT shows the trajectories of the spacecraft (red) and sample return capsule (yellow).
Since the Allen Telescope Array (ATA) is in northern California, its location provided a great opportunity to observe this event. Looking at the trajectories in NASA HORIZONS, I saw that the sample return capsule would pass south of the ATA. It would be above the horizon between 14:34 and 14:43 UTC, but it would be very low in the sky, only reaching a peak elevation of 17 degrees. Apparently the capsule had some kind of UHF locator beacon, but I had no information of whether this would be on during the descent (during the sample return livestream I then learned that the main method of tracking the capsule descent was optically, from airplanes and helicopters). Furthermore, the ATA antennas can only point as low as 16.8 degrees, so it wasn’t really possible to track the capsule. Therefore, I decided to observe the spacecraft X-band beacon instead. The spacecraft would also pass south of the ATA, but would be much higher in the sky, reaching an elevation above 80 degrees. The closest approach would be only 1000 km, which is pretty close for a deep space satellite flyby.
As I will explain below in more detail, I prepared a custom tracking file for the ATA using the SPICE kernels from NAIF and recorded the full X-band deep space band at 61.44 Msps using two antennas. The signal from OSIRIS-REx was extremely strong, so this recording can serve for detailed modulation analysis. To reduce the file size to something manageable, I have decimated the recording to 2.048 Msps centred around 8445.8 MHz, where the X-band downlink of OSIRIS-REx is located, and published these files in the Zenodo dataset “Recording of OSIRIS-REx with the Allen Telescope Array during SRC reentry“.
In the rest of this post, I describe the observation setup, analyse the recording and spacecraft telemetry, and describe some possible further work.
A very quick RFI survey at the Allen Telescope Array
At the beginning of August I visited the Allen Telescope Array (ATA) for the Breakthrough Listen REU program field trip (I have been collaborating with the REU program for the past few years). One of the things I did during my visit was to use an ADALM Pluto running Maia SDR to do a very quick scan of the radio frequency interference (RFI) on-site. This was not intended as a proper study of any sort (for that we already have the Hat Creek Radio Observatory National Dynamic Radio Zone project), but rather as a spoor-of-the-moment proof of concept.
The idea was to use equipment I had in my backpack (the Pluto, a small antenna, my phone and a USB cable), step just outside the main office, scroll quickly through the spectrum from 70 MHz to 6 GHz, stop when I saw any signals (hopefully not too many, since Hat Creek Radio Observatory is supposed to be a relatively quiet radio location), and make a SigMF recording of each of the signals I found. It took me about 15 minutes to do this, and I made 6 recordings in the process. A considerable amount of time was spent downloading the recordings to my phone, which takes about one minute per recording (since recordings are stored on the Pluto DDR, only one recording can be stored on the Pluto at a time, and it must be downloaded to the phone before making a new recording).
This shows that Maia SDR can be a very effective tool to get a quick idea of how the local RF environment looks like, and also to hunt for local RFI in the field, since it is quite easy to carry around a Pluto and a phone. The antenna I used was far from ideal: a short monopole for ~450 MHz, shown in the picture below. This does a fine job at receiving strong signals regardless of frequency, but its sensitivity is probably very poor outside of its intended frequency range.
The following plot shows the impedance of the antenna measured with a NanoVNA V2. The impedance changes noticeably if I put my hand on the NanoVNA, with the resonant peak shifting down in frequency by 50 to 100 MHz. Therefore, this measurement should be taken just as a rough ballpark of how the antenna looks like on the Pluto, which I was holding with my hand.
As the antenna I used for this survey is pretty bad, the scan will only show signals that are actually very strong on the ATA dishes. The log-periodic feeds on the ATA dishes tend to pick up signals that do not bounce off the dish reflectors, and instead arrive to the feed directly from a side. This is different from a waveguide type feed, in which signals need to enter through the waveguide opening. Therefore, besides having the main lobe corresponding to a 6.1 m reflector, the dishes also have relatively strong sidelobes in many directions, with a gain roughly comparable to an omnidirectional antenna. The system noise temperature of the dishes is around 100 to 150 K (including atmospheric noise and spillover), while the noise figure of the Pluto is probably around 5 dB. This all means that the dishes are more sensitive to detect signals from any direction that the set up I was using. In fact, signals from GNSS satellites from all directions can easily be seen with the dishes several dB above the noise floor, but not with this antenna and the Pluto.
Additionally, the scan only shows signals that are present all or most of the time, or that I just happened to come across by chance. This quick survey hasn’t turned up any new RFI signals. All the signals I found are signals we already knew about and have previously encountered with the dishes. Still, it gives a good indication of what are the strongest sources of RFI on-site. Something else to keep in mind is that the frequency range of the Allen Telescope Array is usually taken as 1 – 12 GHz. Although the feeds probably work with some reduced performance somewhat below 1 GHz, observations are done above 1 GHz. Therefore, none of the signals I have detected below 1 GHz are particularly important for the telescope observations, since they are not strong enough to cause out-of-band interference.
I have published the SigMF recordings in the Zenodo dataset Quick RFI Survey at the Allen Telescope Array. All the recordings have a sample rate of 61.44 Msps, since this is what I was using to view the largest possible amount of spectrum at the same time, and a duration of 2.274 seconds, which is what fits in 400 MiB of the Pluto DDR when recording at 61.44 Msps 12 bit IQ (this is the maximum recording size for Maia SDR). The published files are as produced by Maia SDR. At some point it could be interesting to add additional metadata and annotations.
In the following, I do a quick description of each of the 6 recordings I made.
Decoding Euclid
Euclid is an ESA near-infrared space telescope that was launched to the Sun-Earth Lagrange L2 point on July 1, using a Falcon 9 from Cape Canaveral. The spacecraft uses K-band to transmit science data, and X-band with a downlink frequency of 8455 MHz for TT&C. On July 2 at 07:00 UTC, 16 hours after launch and with the spacecraft at a distance of 167000 km from Earth, I recorded the X-band telemetry signal using antennas 1a and 1f from the Allen Telescope Array. The recording lasted approximately 4 hours and 30 minutes, until the spacecraft set.
Even though the telemetry signal fits in about 300 kHz, I recorded at 4.096 Msps, since I wanted to see if there were ranging signals at some point. Since the IQ recordings for two antennas at 4.096 Msps 16-bit are quite large, I have done some data reduction before publishing to Zenodo. I have decided to publish only the data for antenna 1a, since the SNR in a single antenna is already very good, so there is no point in using the data for the second antenna unless someone wants to do interferometry. Since looking in the waterfall I saw no signals outside of the central 2 MHz, I decimated to 2.048 Msps 8-bit. Also, I synthesized the signal polarization from the two linear polarizations. To fit within Zenodo’s constraints for a 50 GB dataset, I split the recording in two parts of 32 GiB each.
The recording is published in these two datasets:
- Recording of Euclid with the Allen Telescope Array on 2023-07-02 (part 1/2)
- Recording of Euclid with the Allen Telescope Array on 2023-07-02 (part 2/2)
In this post I will look at the signal modulation and coding and present GNU Radio decoders. I will look at the contents of the telemetry frames in a future post.