Over on the Reddit /r/SpaceXLounge discussion board user /u/Xerbot has made an interesting post showing how u/derekcz was able to receive the telemetry signals from the latest SpaceX Falcon 9 rocket launch using a HackRF and a 1.2m prime focus dish with homebuilt feed designed for the 2232.5 MHz downlink frequency. Then after demodulating the signal with GNU Radio, /u/Xerbot was able to convert that signal into binary data, and then into plain text strings.
Another user /u/Origin_of_Mind then figured out that these strings are debug messages being sent by the software-defined GPS receiver, which amongst other data contains the GPS coordinates of the second stage. The GPS data indicates that the second stage was tracking over the north of Serbia at an altitude of 219 km and velocity of 7483m/s. /u/derekcz was able to then confirm that he was indeed recording the signal when the satellite would have been crossing Serbia, confirming the received telemetry was correct.
The entire thread is an interesting read, with multiple users dissecting the plaintext and finding out information about the launch. /u/Origin_of_Mind's post in particular explains the meaning of each of the data fields, which includes the system time, the XYZ coordinates in the earth-centered earth-fixed (ECEF) coordinate system, the loss of precision due to unfavorable GPS satellite positions and the number of GPS satellites currently received.
Another user /u/softwaresaur even notes that there was an "radiation_fdir_activation_guard" event. FDIR stands for Fault Detection, Isolation and Recovery (FDIR) and this event was triggered due to 0.06 s mission time discrepancy between the rocket and GPS true time.
Of course the image is only grayscale (or in Dmitrii's case he decided to use greenscale), but adding false color and various other image enhancements found in advanced software like WXtoIMG are just standard image processing techniques.
Dmitrii concludes with the following:
Interesting to mention, that there are not so many operational radio communication systems in the world, the signal of which can be decoded using 20 lines of code. The NOAA satellites are about 20 years old, and when they finally will retire, the new ones will most likely be digital and format will be much more complex (the new Russian Meteor-M2 satellite is already transmitting digital data at 137 MHz). So those who want to try something simple to decode can be advised to hurry up.
Over on GitHub stdevPavelmc has released his software called FAASGS (Fully Automatic Amateur Satellite Ground Station). FAASGS is an open source program that allows RTL-SDR users to set up a satellite ground station that tunes, record and generate images for NOAA APT weather satellites, as well as records FM amateur radio satellites. The software runs on a single board computer such as a Raspberry Pi, however in the authors own setup he uses an Orange Pi Prime board. The features include:
Web interface to see the next passes, the recorded ones, and details for it.
Receive any satellite in FM mode (SSB is possible but no there is doppler control yet, so no SSB by now)
Record the satellite pass and keep the audio for later.
APT WX audio is preserved in wav format and 22050 hz of sampling (the format wximage needs to work with)
FM audio satellites is preserved in .mp3 mode but with high quality settings, and other tricks.
The spectrogram of the audio is embedded as album art (see below).
The pass data and receiving station are stored in the mp3 tags.
Automatic decode APT images from WX sats (NOAA 15, 18 and 19)
For the voice FM sats we craft a spectrogram and embedd the metadata of the pass on the image
Back in November 2020 we posted about the release of a decoder for the FengYun line of geostationary weather satellites which provide full disk images of the Earth and are positioned to cover parts of Europe, Africa, the Middle East, Asia, Russia, and Australia. Back then only a few people had attempted decoding this, and it was believed that a 120cm satellite dish or larger would be required.
We do note that u/Harrison_Clark55's image appears to be missing a few lines of data, and they are based in Australia where the elevation of FY-2G could be quite high depending on what side of the continent they are on. So it's possible that receivers in lower elevations may still require a larger dish size to work.
Compared to the SATNOGS design, the SATRAN design appears to be much simpler and easier to build. Although being a smaller unit it's only design to handle small compact antennas such as a 70cm Yagi. SATRAN is also controllable via a web interface and there is an Android App. The design is capable of rotating 360 degrees, and 110 degrees from zenith, which allows a user to cover the entire sky.
Daniel notes that SATRAN kits should be available for sale from Feburary/March 2021. He also notes that it is possible to 3D print most of the parts and to just purchase the electronics for a lower price.
Turns out that the TRANSIT 5B-5 satellite's telemetry still has signs of some of the satellite's systems operating (albeit with a questionable reliability). The satellite represents an amazing legacy for all the people that worked on it in the 1950s and 60s, but due to its age it is also very difficult to find technical documentation about the telemetry (or I should rather say impossible), so to make sense of the data that's being broadcast by the satellite would require many people receiving, decoding, and comparing their results, mainly to identify any patterns in the satellite's behavior and the resulting demodulated data.
Derek and u/Xerbot are asking the SDR community to help collect more sample data, which might help in finding a way to decode some of the telemetry. If you have data to contribute, you can contact @ok9sgc on Twitter, and u/Xerbot on Reddit.
This reminds us of an old post from reader happysat where he demonstrated with an RTL-SDR that many "dead" satellites are actually still transmitting telemetry. Due to suspected chemical breakdown of the onboard batteries, the satellites tend to turn themselves on again when the solar panels receive sunlight.
Over on his YouTube channel TechMinds has uploaded a new video showing how to decode signals from Orbcomm satellites. Orbcomm run a global network of low earth orbit satellites that perform services such as Internet of Things (IoT), Machine 2 Machine (M2M) communications, asset tracking, utilities telemetry, government communications and much more. The signals can be received at around 137 MHz.
In the video he explains how the private client data is encrypted, however it is possible to at least see the encrypted data coming down, and decode some of the data management information such as the transmitted uplink frequencies using a program called Orbcomm Plotter. Ultimately, the data available is quite boring to monitor, however decoding these satellites is still an interesting exercise.
Decoding Orbcomm Satellite Transmissions Using Software Defined Radio
Back in 2017 we posted about Adam 9A4QV's simple V-Dipole antenna design which works very well for receiving NOAA and Meteor weather satellites at 137 MHz. This type of antenna is a lot easier to build compared to a QFH or turnstile, and it results in good performance if built and set up correctly. Over the years he notes that he's received a number of questions asking to clarify the design and so he's uploaded a YouTube video which explains the built and dimensions of the antenna clearly.