Sentence:  Measurements at 3.15 cm were obtained on 11 days spread over the interval May 3 to June 19 , 1956 , using the 50 - <WSD>foot</WSD> reflector at the U.S. Naval Research Laboratory in Washington . 
Meaning of the word marked with <WSD> tags:  a linear unit of length equal to 12 inches or a third of a yard