Organisms on earth have a wide variety of ways to detect the presence of other objects. Echolocation, electrolocation, vision, etc. One kind they're missing is detection based around bouncing microwaves off things, i.e. radar.
On my planet I have a taxon of flying creatures that live their lives completely airborne. They mostly live at high altitudes except to feed on fish and other marine life that lives close to the surface. I'd like for them to be able to locate prey from high altitudes using microwaves or radio waves; it seems like a fairly logical adaptation and it's also just pretty cool. Echolocation is out due to its range being too short; my atmosphere is thicker than earth's but not that much thicker. (Echolocation in bats seems to be capped in the tens of meters; my creatures prefer to fly thousands of meters above the water.) In addition to locating prey they could also use their radar for communicating over long distances. (These creatures are fairly smart; not anywhere near human intelligence or probably even toothed whales, but around as smart as an elephant or racoon. They have some need to communicate, if only to find each other to mate, since they're often very far apart and unlike e.g. albatrosses they don't return to a specific area of land to mate.)
I'm wondering how feasible this is. Quite a lot of things on earth are bioluminescent but none, as far as I know, produce wavelengths beyond the infrared spectrum. My flyers don't necessarily have to do this chemically; producing electricity and using some kind of internal antenna to produce their radar would be fine too.
I realize radar does not penetrate water very well, but I think this isn't too big of a problem since they are only looking for things on or just below the surface.
In short: What sort of mechanism could my flyers use to generate their radar? How could they receive it? (antennas, special eyes?) What other sorts of problems might they encounter or what other uses could it have?