Don't know how/what it is in SH3 terms. Or how it can be used as a sensor. But a 'radio direction finder' is that circular antenna rotating on the bridge. It's a dipole antenna that is sensitive in a certain direction, and by turning you can determine the bearing to the transmitter (or opposite bearing). But the radio operators around here in the forum can explain it in far more detail than me, I'm sure. Here's a wiki page to start-off.
http://en.wikipedia.org/wiki/Radio_direction_finder