And since a moving object causes a tiny change (known as a Dopplershift) in the frequency of the reflected signal, Silent Sentry can also calculate how fast it is moving.
The recent conference I attended at the Lowell Observatory in Flagstaff to celebrate the 100th anniversary of his first measured Dopplershift, was a great opportunity to learn more about him.
Rather than looking for the tiny Dopplershift of a bright star, in other words, Dr Cameron and his team were looking for the much larger Dopplershift of a much fainter planet.
As is to be expected, ordinary digital transmission is nowhere near robust enough to cope with the problems experienced by a mobile receiver, which can suffer from all sorts of doppler-shift effects and multipath radio interference as it moves around the place.