The way that the distance-to-ground detectors on many military aircraft works is that at the same time a pulse is shot at the ground, and ramp generator is initiated.

When the signal returns, the ramps voltage is sampled and held.

The held voltage is converted to digital, and an appropriate multiplier is used.

To change the range, all that's needed is to change the speed of the ramp.

Modulating the outgoing pusle so that it can be distinguished from ambient light completes the package. (Same thing as is done with IR remote controls.)