[mrabcx]

According to Flightradar24, https://twitter.com/flightradar24/status/1742136715253313938, the Japanese coast guard aircraft was not equipped with a ADS-B transponder. Probably because it's not a requirement for military aircraft. That can explain why the ATC and/or JAL was not aware of the exact location of the plane.

[throw0101d]

It has a transponder, and previous flights for it were recorded:

* https://www.flightradar24.com/data/aircraft/ja722a

* Via: https://twitter.com/flightradar24/status/1742136715253313938

* https://www.planespotters.net/airframe/de-havilland-canada-d...

[icehawk]

It has a mode-s transponder. If you look at, say ADSBExchanbe you can see the signal source is MLAT

https://globe.adsbexchange.com/?icao=867bee&lat=36.840&lon=1...

[Dalewyn]

Minor nitpick, but the Japan Coast Guard is not a military (and so the concerned aircraft isn't military).

It serves under the Ministry of Land, Infrastructure, Transport and Tourism; unlike the JSDF which serves under the Ministry of Defense.

[alistairSH]

Curious, what's the rule for USCG and transponders?

USCG is part of DHS and considered a military branch (despite not being in DoD alongside the other military branches and having a primarily law-enforcement mission).

[manquer]

Civilian transponders would give away the position of the craft to smugglers and others trying to evade the coast guard. I would expect they would have the ability to switch off their transponders at minimum for missions.

[rootusrootus]

I'm most curious about why the JAL plane couldn't see it. An airliner has a pretty bright headlight, and the CG plane should have had lighting on it as well that would distinguish it from the runway lighting.

Clearly, as usual, accidents like this require a number of things to all go wrong at the same time.