Sentence:  This method involves the use of a radar transponder sited (at any rate for the purpose of this illustration) at a known distance <WSD>upwing</WSD> from the landing threshold — say 4,000 yards — and complementary airborne equipment capable of measuring to an accuracy of 1/10 n. ml. 
Meaning of the word marked with <WSD> tags:  Toward the side of the plane where the wing is higher than the plane.