Firstly I'd recommend avoiding anything that needs certifying, otherwise you'll never be able to test it (unless you have use of a permit to fly aeroplane, but I'll discount that for the moment). So that eliminates sensors attached to most parts of the aircraft. Have you considered:-
- Transponder return detector (otherwise known as TPAS) - not original but you'll be on your own because nobody's going to tell you how the commercial units work.
- Any kind of passive air data recorder - this could cover a very wide range of designs and functions.
- Anything at
http://www.bmaa.org/019_2.pdf
- Any kind of ground device for, say, measuring wind, visibility, light levels. Many American airfields have such devices with voice synthesisers that transmit something similar to realtime ATIS information - how cheaply could that me done?
- DGPS?
G