Today, I want to build a satellite that receives ADS-B signals from airplanes over the open ocean, far away from land. With a decent receiver and a simple antenna, it should be possible on a gravity-stabilized cubesat. I know about terrestrial receivers picking signals 200..300 km out, surely with care one can do better. But I highly doubt that it's possible to finance such a toy — unless someone has already done that. I know that people somehow manage to finance AIS receivers, which are basically the same thing, only for ships. How do they do that?
UPDATE: Reportedly, hosted payloads by Aireon on Iridium NEXT satellites do ADS-B. The working altitude of the previous generation of Iridium was 780 km, the NEXT is probably the same.