Going back about 40 years so might have recalled incorrectly but it's down to the duration of the pulse. While transmitting it cannot receive so no matter how brief the pulse is there will be a minimum range. Could be crucial at fifty feet. Seem to remember seeing a pulse Radar Altimeter for high altitude applications long ago, perhaps military.
And as to accuracy very small changes due to Doppler effect are detectable.
Last edited by Wodrick; 25th June 2010 at 07:52.
Reason: Addition