Testing the sound mirrors that protected Britain

Over on the RAF Starrship channel, I'm talking about the history of radar: https://youtu.be/VDC_RCh0ws8 - but over here, we're testing a 90-year-old piece of technology that was meant to be part of Britain's air defence. The Sound Mirrors, on Romney Marsh, were built in the late 1920s as a way to amplify the sound from aircraft engines over the English Channel. We're flying a bit closer than that, with a drone.