On Wed, 19 Jan 2005 20:30:12 GMT, Me wrote:

Nope, not something that a RADAR as discribed could do. These were
analog Reatime video displays, and didn't have any ARPA features
included.

I suspect that it could have been one of the early Xband Transponders
but how it could be triggered at that distance without being at some
altitude, would be an interesting question.
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My last boat had a Raytheon R41xx with an analog display, circa 1983
or so. It was capable of tracking targets via ARPA.