Yay - although current transponders way at least 0.5 kg, and cost over £1500, so there's a fair way to go before 'light' or 'cheap' so we can have another 'no effect on bad guys' action that clutters radar screens. It does, of course, mean we can no close an airport by turning on a transponder that says there is a drone in the wrong place, without the inconvenience of actually putting a drone there.