Sentence:  The ratio of the measured antenna temperature change during a drift scan across the moon to the average brightness temperature of the moon over the antenna <WSD>beam</WSD> ( assuming that the brightness temperature of the sky is negligible ) was found , by graphical integration of the antenna directivity diagram , to be 0.85 . 
Meaning of the word marked with <WSD> tags:  a signal transmitted along a narrow path; guides airplane pilots in darkness or bad weather