Back in '81, there was a Nimrod doing hot weather trials at NAS Corpus Christi where I was in training. I got to sit in on the brief and debrief, and to go on board for a surveillance sortie over the Gulf of Mexico. A few of the winged aviators, instructors, were invited up on to the flight deck with the plane commander, but we students were not.
One of the navs showed those of us in the back a digital radar display that I think was a precursor to ISAR, or in the same family. (Based on the ISAR I worked with ten years later in Seahawks). The actual purposes of the test as I remember it was was to exercise the environmental control systems, since Corpus Christi is notorious for being hot and humid in July and August. As it turns out, that particular month was not as hot and humid as they had hoped for their tests.
My memory may be shot, but I recall that the Nimrod was operated by the RAF. If it was the RN, we'll chalk that up to my memory going bad.