Well, actually not quite. Both Radar and Radio-Altimeters measure distances
by measuring the time required to transmit a signal, then receive its
reflection from a target. Radar generally uses pulses, while Radio Altimeters
use either pulses or a modulated continuous-wave transmission. In the case of
the latter, highly accurate distance measurement can be made. As an example,
the original Bendix ALA-52 Radio Altimeter was accurate to 1/8 foot at 2500
feet altitude.

Note, however that this is a different method of measuring than the poster
originally asked about. The problem with gaining accurate measurements between
a transmitter and a seperate receiver is that you need a highly accurate
time base which starts at the receiver at the exact instant the transmitter
triggers. This cannot be wire connected, since radio waves will actually
travel faster in free-space (air, in this case) than wire (the difference
is called the velocity factor of the cable). So you need to resort to a
common timebase that is automatically corrected for distance, etc. Something
like a PLL connected to a GPS receiver should do the trick, triggering both
the transmitter and receiver simultaneously. Sound expensive? Not too bad,
but plan on spending a few bucks in both equipment and effort.

Why not go to a different method? Surveyors use a laser-light system where again
the reflection time is measured. Why not try this? (Sounds like something a P.E.
should know about anyway ;-).


This is actually highly inaccurate, since the power output of a transmitter
varies from unit to unit, there are variances in the antenna and transmission
line, and the receiver may also vary, both from unit to unit, and the same unit
over time. You would need to continuously calibrate the entire system. With
the radio altimeter this is also done, but since everything is located at
one place, it is much easier to do. Note especially that the time base for
the R.A. receiver and transmitter is one unit also...


Nope. FM capture effect says that the strongest signal wins. That is, unless
the two interfering signals are seperated by more than 3 db in signal strength.
This is the one problem that makes altimeters inaccurate at very low altitudes.
Signals bouncing off runways tend to be VERY strong...


As high as possible to eliminate outside influence, and also to enhance
attenuation of multipath signals. Radio Altimeters typically use frequencies
around 4 GHz.

Hope this helps...
