In my previous post, I looked at the Doppler of the SLIM S-band telemetry signal during its landing on the Moon. I showed some waterfall plots of the signal around the residual carrier. In these, a reflection on the lunar surface was visible. The following figure shows a waterfall of the signal around the residual carrier, after performing Doppler correction and using a PLL to lock to the residual carrier. I was intrigued by the patterns made by these reflections, specially by some bands that look like a ‘1’ shape (the most prominent happens at 14:58).
In this post I study the geometry of the lunar reflection and find what causes these bands.
SLIM, JAXA’s Smart Lander for Investigating Moon, landed near Shioli crater on January 19. Shortly after the landing, the spacecraft was powered down to conserve power, since the probe had landed in an unexpected attitude which shaded its solar panels. After a few days of trying to contact SLIM, JAXA succeeded to reestablish communication with it on January 29. By then the Sun had moved west in the sky at SLIM’s location and had started illuminating the solar panels.
AMSAT-DL recorded the S-band signal from SLIM during the landing with the 20-meter antenna in Bochum Observatory. In this post I will analyse a recording done between 14:51:51 and 15:21:54 UTC (the touchdown was at 15:20 UTC). I will study the Doppler of the residual carrier and other radiometric quantities rather than the telemetry.
LEV-1 is a small lunar hopper that was carried by the SLIM lunar lander. It was released a few metres above the surface on January 19, as part of the lunar landing of SLIM. LEV-1 transmits telemetry in the 435 MHz amateur satellite band (it has an IARU satellite coordination approval), and also in S-band. Shortly after the landing, CAMRASreceived the 437.410 MHz signal from LEV-1 using the 25 m radiotelescope at Dwingeloo. They have published a couple of IQ recordings in their directory of miscellaneous recordings (see the filenames starting by slim_).
The information about the telemetry signal of LEV-1 is scarce. Its website just says
Telemetry format of LEV-1 stands on CCSDS. The contents of telemetry are under developing.
The IARU coordination sheet contains other clues, such as the mention of PCM/PSK/PM, CW, and bitrates of 31, 31.25 and 32 bps, but not much else. Regardless of the mention of CCSDS, I have found that the signal from LEV-1 is quite peculiar. This post is an account of my attempt to decode the data.
Galileo OSNMA (Open Service Navigation Message Authentication) is a service in the Galileo GNSS that allows receivers to authenticate cryptographically the navigation data transmitted in the Open Service signal. This is one of the mechanisms to avoid spoofing that are being deployed in Galileo. Currently, OSNMA is in its Public Observation Test Phase. Two years ago I presented a Rust library called galileo-osnma that implements OSNMA and includes some demo software for a small microcontroller, and also a PC CLI application. Since then, some breaking changes have happened in the format of the OSNMA signal-in-space, which have required updates in galileo-osnma. I have also implemented some new features. This post is an update about the current status of my galileo-osnma library and the OSNMA test phase.
Peregrine Mission One is a lunar lander built by Astrobotic Technology. It is the first mission to be launched under the NASA Commercial Lunar Payload Services program, and Astrobotic’s first mission. It was launched in January 8 from Cape Canaveral in the maiden flight of ULA‘s Vulcan Centaur. Shortly after the launch, the team detected an issue with a propellant leak that prevented the spacecraft from achieving a soft landing on the Moon. Since then, the team has continued operating the spacecraft to the best of their capacity and collecting as much engineering and science data as they can for the next mission. Astrobotic has been doing a superb work of communicating the progress of the mission with regular updates in the Twitter account, which should specially be praised because of the difficulties they’ve faced. Congratulations for all they have achieved so far, and best luck in the upcoming missions.
In this post I won’t speak about propulsion anomalies, but rather about low-level technical details of the communications system, as I usually do. Peregrine Mission One, or APM1, which is NASA DSN‘s code for the mission, uses the DSN groundstations for communications, as many other lunar missions have done. However, it is not technically a deep space mission. In CCSDS terms, it is a Category A mission rather than a Category B mission (see Section 1.5 in this CCSDS book), since it operates within 2 million km of the Earth. Communications recommendations and usual practices are somewhat different between deep space and non-deep space missions, but APM1 is specially interesting in this sense because it differs in several aspects of what typical deep space missions and other lunar missions look like.
MOVE-II is a cubesat from Technical University of Munich that was launched in December 2018. It transmits telemetry in the 145 MHz amateur satellite band using a protocol that uses CCSDS LDPC codewords. Back in the day, there was a GNU Radio out-of-tree module developed by the satellite team to decode this satellite. Given the additional effort required to support LDPC decoding for just this satellite and since there was already a GNU Radio decoder available, I never added a decoder for MOVE-II to gr-satellites.
Fast forward 5 years, and MOVE-II is still active, but apparently its GNU Radio out-of-tree module has bit rotten. The Gitlab repository where this was hosted (I believe it was a self-hosted Gitlab) has disappeared, and while it was originally developed for GNU Radio 3.7, it was never ported to newer GNU Radio versions. Some days ago, some amateurs including Scott Chapman K4KDR and Bob Mattaliano N6RFM started doing some experiments to try to get a decoder for MOVE-II working.
I have now added an example decoder flowgraph for MOVE-II to gr-satellites. Here I describe the details of this example, and why it is only an example instead of a fully supported decoder as the ones that exist for other satellites.
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.
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:
On September 24, the OSIRIX-RExsample 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).
Trajectory of OSIRIX-REx and sample return capsule
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.
For this post I have only used final solutions for the CODE precise clock biases. I have replaced the rapid solutions that I used in the last post by their final versions. The data under analysis now spans from day of year 225 (2023-08-13) to day of year 266 (2023-09-23).
The plot of the difference between the broadcast clock biases and the CODE precise clock biases now has the following aspect. Comparing with the previous post, we see that the difference has stayed around -15 ns until 2023-09-08, and then it has started increasing towards zero, but it has overshoot and it is at around 20 ns by 2023-09-23.
The plot of the system time differences in the broadcast messages is also quite interesting. For this I am using the IGS BRDC data until day of year 275 (2023-10-03). Recall that it appeared that the GST-UTC drift had the wrong sing, because it was clearly increasing but the drift had negative sign. Now the situation looks more complicated. Though it appears that the drift has the wrong sign around 2023-08-28 and 2023-09-22, there is also a segment around 2023-09-08 where the sign looks correct. Additionally, around 2023-09-01 the sign should be close to zero but is not.
For comparison, here is the same plot with the sign of the GST-UTC drift flipped. Arguably, the drift gives a better match most of the time, but certainly not around 2023-09-08. Therefore, the problem with the modelling of the GST-UTC drift in the Galileo broadcast message looks more complicated than just the sign bit being wrong.
The updated Jupyter notebook and data is in this repository.
