Here is a DIY radio altimeter designed by Matjaž Vidmar. It is a good start for anyone looking at a non-GPS method of finding altitude while landing.
There are no schematics or source code available on the site, but it is a good design reference and it does give the PCB layout of the microwave circuit. That alone helps
immensely as that is the most difficult part of an RF design.
::Update::
Found another vertical radar project on his website with much more documentation.
Comments
Prabably not a big thing, but I think borometric altimeters have been the prefered measure for measuring altitude before landings for quite some time. They are usually more accurate then gps'.