> The use of primary radar requires a great deal of signal power, because objects further from the antenna will reflect or send back a weak signal. At longer distances from the antenna, radar becomes unreliable as a way to determine aircraft position with only reflected signals.

http://www.wisegeek.com/what-is-a-primary-radar.htm

Radar derives altitude from measurements of distance and elevation angle. The greater the distance, the less sensitive elevation angle is to vertical displacement (simple trigonometry), and hence signal:error decreases.

I think it has to do with electromagnetic ducting or other clutter effects:

http://www.radartutorial.eu/07.waves/wa17.en.html

If the last effect on the page were in effect, you might get a return, know it's relative bearing well, it's range a little less well, and it's altitude not very well at all.