Never be lost again.
No attempt at profundity here — just a quick note about an idea for a system to find desert-island-castaways. You know, folks whose plane or boat goes off course, off radar, and out of radio contact, then wrecks on or near a deserted island. Castaways seem much more common in the TV/film realm than in real life (which is interesting in itself), but still, it would be nice to know that if you were unlucky enough to be shipwrecked on some uninhabited isle, you would be found quickly.
How? Spy satellites. Even in Gilligan’s day we had them.
Optical surveillance spy satellites (as far as I know — I have no special expertise here) normally use very narrow image fields. Their primary function is not to search, but to inspect a given location. So you might think that they would be useless for wide-area search applications. But they must have the ability to “look” sideways across a given territory as they pass it, to some extent. And they almost certainly also have the ability to “zoom out” to a wider angle view if needed. So it should be no problem for them (if power’s not an issue and they’re otherwise not doing anything) to, say, scan in raster fashion, side to side, across a relatively wide swath of ocean, as they glide over it from north pole to equator to south pole and head north again (they usually are placed in such polar orbits). All that would be needed then — and there are cheap “apps” for this if the NRO hasn’t paid some contractor a jillion dollars for one already — is simple word-recognition software that can sift through that sea of visual data (skipping inhabited areas, of course) and find a target word or phrase, such as “Help” or “S.O.S.”
Thus, all a castaway would have to do is write one of these appeals on a beach, or field, or sail, or whatever, at a scale large enough for satellites in wide-field scanning mode to pick up. Maybe two meters high? Again, I’m no expert, but with the huge CCDs they must have on those satellites, I’d think that would be enough.