- Decoding IEEE 802.11ah
Since some time, I’ve been thinking about doing something similar to my posts about LTE and 5G NR, but for WiFi (IEEE 802.11). In these posts, I take a signal recording and write a Jupyter notebook from scratch to analyze the signal and decode the data. I use these posts as a way of learning all the details of how these standards work, and I have seen that some people find them very useful.
Recently I was taking a look at a baby monitor camera system, composed by a camera and a monitor screen, since I was curious about how the camera transmits the video. Using Maia SDR, I located the signal at 866 MHz and realized that both the camera and the monitor screen were transmitting OFDM packets of approximately 2 MHz of bandwidth on this frequency. With some cyclostationary analysis, I found that the subcarrier spacing was 31.25 kHz (which works out to be 2 MHz / 64 FFT points), and that the cyclic prefix was 1/4 of the useful symbol duration. This pointed me straight to IEEE 802.11ah (WiFi HaLow), a variant of WiFi designed for the 800 MHz and 900 MHz license-exempt bands. After comparing the packet formats on the 802.11ah standard with the waterfall of my recording, I was sure that this was indeed 802.11ah. What started as a fun and short signal recording experiment has ended up going through the rabbit hole of implementing 802.11ah decoding from scratch in a Jupyter notebook. In this post I explain my implementation and the analysis of this recording.
- Tianwen-1 second apoapsis raise
Some weeks ago I reported about an apoapsis raise manoeuvre done by Tianwen-1, the Chinese Mars orbiter. This has now happened again. Using state vectors from the telemetry decoded with the 20 m antenna in Bochum observatory by AMSAT-DL, we have detected an apoapsis raise manoeuvre done on 2025-01-08. This new apoapsis raise is much larger than the previous one. I have done the same kind of calculations as in the previous post, and also corrected a bug in my Keplerian elements plots (the periapsis and apoapsis passings were being paired incorrectly, which caused the SMA and eccentricity not to change in the plots I did in the previous post).
- Tianwen-1 apoapsis raise
For a long time, AMSAT-DL has been using the 20 meter antenna in Bochum observatory to receive some telemetry from Tianwen-1, the Chinese Mars orbiter, almost daily. Since the telemetry includes the spacecraft’s state vectors, we can use this to monitor the spacecraft’s orbit. In 8 November 2021, Tianwen-1 entered its remote sensing orbit. This is an elliptical orbit with a period approximately 2/7 Mars sidereal days plus 170 seconds. This causes a ground track that is almost repeating, but drifts slowly to cover all the surface area of the planet.
I have been posting yearly updates about Tianwen-1’s orbit, the last of them this summer. In these updates, we can see that no manoeuvres have happened, and the changes in the Keplerian elements correspond to orbital perturbations caused by external forces. The orbit is in fact designed to cause the latitude of the periapsis to precess. In this way, all the surface of Mars can be scanned from low altitude.
Now we have some news. In the telemetry of the last few days we have detected that Tianwen-1 has raised its apoapsis radius from about 14134 km to 14489 km. All the data we have indicates that a propulsive burn has happened recently. In this post I give the details about this apoapsis raise manoeuvre.
- 5G NR PBCH
This post is a continuation of my series about the 5G NR RAN. In these posts, I’m analyzing a recording of the downlink of an srsRAN gNB in a Jupyter notebook written from scratch. In this post I will show how to decode the PBCH (physical broadcast channel). The PBCH contains the MIB (master information block). It is transmitted in the SSB (synchronization signals / PBCH block). After detecting and measuring the synchronization signals, a UE must decode the PBCH to obtain the MIB, which contains some parameters which are essential to decode other physical downlink channels, including the PDSCH (physical downlink shared channel), which transmits the SIBs (system information blocks).
In my first post in the series, I already demodulated the PBCH. Therefore, in this post I will continue from there and show how to decode the MIB from the PBCH symbols. First I will give a summary of the encoding process. Decoding involves undoing each of these steps. Then I will show in detail how the decoding procedure works.
- Hera telemetry
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.
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