Originally Posted by rickair7777

Police radar actually uses dopler shift of the emitted freq, since it's looking for relative velocity, not distance. Not the same as time differential, which is what longer-range radars use.

In the case of RADALT, I'm guessing the distance is probably just too small to reliably and precisely measure with time differential. The required precision is very, very high (probably 1-2 feet I'd guess) and at close range that would require some VERY, VERY precise timing... at the speed of light. Normal radar measures distance in miles with precision in thousands or maybe hundreds of feet. Could it be done? Maybe. It might even be easy with the latest technology but RADALTS have been around for a while, and nobody wants to retrofit the whole fleet.