Not really frequency so much, but radio, and especially, radar, can be
used to find the distance between a transmitter and a receiver.
If a pulse of radio waves is emitted by a transmitter, and the
pulse (possibly reflected from a target) is received by a receiver
(possibly very close to the emitter), then precise knowledge of the
travel time gives a precise measure of the total distance the radio
wave traveled. The conversion factor is the speed of light, which is
known to high accuracy.
Radar is very commonly used to determine the range (and direction)
from the radar tower to airplanes in the air, and also to get the
ranges on thunderstorms (especially here in the Midwest).
Frequency shifts of radio waves reflected from moving objects are
used to measure their speeds. This is called Doppler radar, and is how
police often measure the speeds of cars on the road. It's also used by
meteorologists to detect rain.
(republished on 07/28/06)