[speed150mph]

you run into some big issues there too.

1: size. The sonar sets in a buoy are very compact. sound carries further in water so you don't need a large receiver or transmitter to do the job. Buoys also have a relatively short range as it is. Compare that to the size of even a portable air search set. Without some breakthrough, I doubt you'd be able to fit an air search radar into the same space.

2: power. Radars use a lot more power to transmit which means you need a lot of batteries (which adds to the size) or you have a very short battery life (possibly not enough to fire your missile.)

3: detection. A buoy mounted ASR would have a very limited range, which means if the helo is close enough to be a threat, you'd be launching the buoy right above yourself which, sitting on the surface and radiating, gives your position away just as bad as if your raised a mast.

4: cost. Can you make a radar that would get the job done that would be cheap enough that it would be disposable?

In the end, even that is not fiesable.