[zo1]

>"The secondary surveillance radar, which is now the main system used by ATC, would be completely blind to them, as it relies on active equipment being installed in the aircraft. Requiring the installation of transponders in drones might be tempting, but they are large and expensive compared to a quadcopter, so it's impractical."

Interesting factoid: The new regulations put into place last year by the South African civil aviation authority mandates transponders for drones entering restricted/controlled airspace.

Apparently, there are commercial transponders in place that comply with the specs required, but are prohibitively expensive. At least, compared to most consumer drones.