- 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.
- 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.
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