While checking for Patriot activity in South Korea I stumbled on the interesting increase of interferences in the seas surrounding Japan. This pushed me to investigate a little further to isolate the potential sources for them.
Hyūga class helicopter destroyers
The vessels (2) which became operational in the late first decade of 2000 saw the introduction of the indigenous ATECS (advanced technology command system) system and it’s main aerial radar, the FCS-3 .
FCS-3 is based on two different set of antennas, the first set operating in the C band used for surveillance and tracking and the second set operating in the X band for fire and control.
The C band antennas signals should be the ones causing the interference lines in the SENTINEL-1 data.
FCS-3 became the main aerial radar system for modern Japanese Maritime Self-Defense Force vessel.
Akizuki class destroyers
The four vessels operational since late 2014 are meant as escort for the Hyuga class helicopter destroyers. All four employ ATECS and the FCS-3 radars.
Asahi class destroyers
Two vessels commissioned between March 2018 and late February 2019. The destroyers employ ATECS and the FCS-3 radars.
Izumo class helicopter destroyer
Two vessels commissioned between March 2015 and March 2017. They employ a version of FCS-3 lacking the X band fire and control antennas, but still have the C band ones.
Conclusions
Given the area where the interferences are detectable, the specs of the FCS-3 radars and it being installed on always more vessels, it is quite plausible that most if not all, interferences detected by SENTINEL-1 in the area are traceable back to vessels employing the modern Japanese radar.
If any reader knows of other C band radars operating in the around Japan please let me know, would love to keep this list updated with additional discoveries.
You can read more about SENTINEL-1 and how and why it can be used to track some military radars here.
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