If you are interested in how to build a low-cost radar altimeter for a lunar lander, then keep an eye on the work being done by Lee Begg from Lunar Numbat, our open design partner down under.
Lee's work is an open design, published for all to see on the Lunar Numbat Twiki site. He also started some radar design calculations on the github site. The calculations are in the form of python scripts and Lee plans to implement these in a breadboard demo of the radar altimeter, probably based on a Blackfin processor. Below is a block diagram of the concept.
Lee Begg lives in Wellington, New Zealand, and in his day job works as a software developer for Harmonic. He and has a Masters degree from the University of Canterbury, Christchurch in Computer Science and Software Engineering.