(2 - 2003) Doppler was not studied, using a shift in frequency to detect a presence on the moon. A radar beam is sent off and reflects back at double the speed it came from. It can be used for range finding as well as velocity measurement. The signal travels through space until it hits an object which then sends it back towards Earth where it is picked up by radio telescope (just physical) which are stationed on the moon, it acted like a mirror to send signals and data from there. It was possible to find out if anything moved with a shift in frequency of the returned signal. It's not hard for you to understand why they would want this kind of information.