NASA TND-6849 (1.4Mb PDF) NASA Technical Note, "Apollo Experience Report - Lunar Module Landing Radar and Rendezvoud Radar" by Patrick Rozas and Allen R. Cunningham, June 1972. Scan downloaded from the NASA Technical Reports Server.
Journal Contributor Phil Karn writes:
"I studied the operation of the landing radar recently. The best reference is NASA TN D-6849, Apollo Experience Report Lunar Module Landing Radar and Rendezvous Radar."
"The velocity (Doppler) radar works on exactly the same principle as police radars and the microwave door openers at grocery stores. The reflected signal is beat against the transmitted signal, producing an audio tone whose frequency is proportional to velocity. There are three separate beams, two to the sides and one behind. Quadrature mixers are used to determine the sign of the velocity readings, i.e., whether the ground is coming or going."
"The altitude radar seems pretty clever though it may be common; I don't really know radars. The beam points down. The transmitted signal is swept rapidly in frequency (chirped) with a 130 Hz triangular waveform so frequency changes at a constant rate. Above 2500 feet, the frequency shift is +/- 20 MHz. Below 2500 feet, it's +/- 4 MHz. (The radar problem on Apollo 14 occurred because the radar had incorrectly switched to the low altitude mode.) When the reflected, delayed signal arrives back at the radar, the transmitter frequency has changed by an amount proportional to the distance. Once again the received signal is beat against the transmitted signal and this time the resulting audio tone frequency is proportional to altitude."
"The altitude return signal will also have a velocity-dependent Doppler shift. The Apollo Experience Report says that this shift was taken out by reference to the Doppler radars, but I think it could be done in the altitude radar itself. The beat frequency due to altitude should not change with the chirp direction while that due to velocity would, and this gives a way to distinguish the two."