Month: June 2025

Z-Sat VHF transmissions

Z-Sat is a microsatellite by Mitsubishi Heavy Industries that was launched in 2021. It is a demonstrator for multi-wavelength infrared Earth observation technologies. It carries an amateur radio payload that was coordinated by IARU and which consists of a BBS (bulletin board system) with a 145.875 MHz downlink and 435.480 MHz uplink. I have not been able to find more information about the amateur radio payload on this satellite.

Recently, Daniel Ekman SA2KNG asked me to analyze some transmissions by this satellite. Apparently it has recently begun to transmit a digital modulation, as shown in this SatNOGS observation, while it typically had transmitted CW telemetry in the past. The point where this started appears to be on 2025-06-20, as there is a SatNOGS observation of CW telemetry on that day followed by an observation of the digital modulation. In this post I analyze this digital modulation and explain what it is.

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5G NR PDSCH

In my previous post in the 5G NR RAN series, I showed how to decode the PDCCH (physical downlink control channel), which is used to send control information from the gNB (base station) to the UEs (cellphones). In this series I am using as an example a short recording of the downlink of an srsRAN gNB. The PDCCH transmission that I decoded in the previous post was a DCI (downlink control information) containing the scheduling of the SIB1 PDSCH transmission. The PDSCH is the physical downlink shared channel, which is the channel where the gNB transmits data. The SIB1 is the system information block 1. It contains basic information about the cell, and it is decoded by the UE after decoding the MIB in the PBCH, as part of the cell attach procedure. In this post I will show how to decode this PDSCH SIB1 transmission.

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