For instance if distance is about 100 miles, then this is the scenario:
measure the delay: 1 millisec, or .001 sec.
speed of light 186,000 miles per sec.
186,000 miles/sec * .001 sec = 186 miles.
Since this is round trip, divide by 2, and your distance is 93 miles.
You may need an oscilloscope for this measurement.
In effect this would be active radar, light an airplane transponder.
To make the electronics simple, just use a crossband repeating HT, and leave
it on. periodically send a tone. When the tone comes back, you can measure the
distance on an oscilloscope.
Just my theory.
Of course I have never done such a thing, so don't take my word for it.
John Marcus, MD.