The receiver had to compensate for the Doppler shift in frequency of the reflected signal due to the Moon 's orbital motion relative to the Earth 's surface so this motion had to be carefully calculated for each trial .
The Earth 's surface was different each day .
The receiver had to compensate for the Doppler shift in frequency of the reflected signal due to the Moon 's orbital motion relative to the Earth 's surface , which was different each day , so this motion had to be carefully calculated for each trial .