Japanese ship – coastal station marine comms

 

Concept Diagram of Maritime Communications. Image source: soumu.go.jp

The shortwave frequency range is commonly used for two-way radio communications between ships at sea, as well as between ships and nearby coastal stations across the world. Usually, these are low-power, non-directional transmissions of around 1 kW and can only be heard well locally. For example, in London it is often possible to hear sailors in the territorial waters of France, Spain, Portugal and, occasionally, Greece. I was therefore very surprised when I discovered what appeared to be maritime communications in Japanese on 8261 kHz in a spectrum recording I made on October 10, 2022:

The nature of this transmission was kindly confirmed to me by the user @shinyaradio on Twitter:

Hello. This is a communication between a coast station and a ship station.
The content is numbers and ciphers and I don't understand it.https://t.co/DRWyHnsJ7O

— 長谷川 眞也 (@shinyaradio) October 18, 2022

The link in the tweet states that this frequency is indeed allocated for maritime communications in Japan and the power of these transmissions should not exceed 1.2 kW. If this transmission did in fact originate in Japan, the propagation conditions must have been truly excellent that evening!