ALSE

Radar waves with wavelengths between 2 and 60 meters (frequencies of 5, 15, and 150 MHz) were transmitted through a series of antennas near the back of the Apollo Service Module.

(A radar experiment on the Space Shuttle has been similarly used to map ancient river valleys beneath the Sahara Desert.)

However, in Mare Crisium the Lunar Sounder Experiment results were combined with other observations to estimate a total basalt thickness of between 2.4 and 3.4 kilometers.

The two halves of the dipole antenna were retractable, on the two sides of the service module itself, while the Yagi used for VHF was stowed close to the main engine and then deployed into position after launch.

Being the primary objective of the experiment the mapping of subsurface layers, the most critical trade-off in the design was that of penetration depth vs resolution: lower frequencies penetrates more, but allowed a smaller signal bandwidth and, therefore, a worst resolution which, in turn, affected the capability to discriminate subsurface echoes close to the surface.

An automatic gain control (AGC) feature was included in all the channels to optimize the signal allocation within the receiver dynamic range.

Additionally, on this channel, the receiver gain was increased 13 μs after the arrival of the main surface echo to best exploit the dynamic range on weak subsurface returns.

During the developments phase, a modified ALSE prototype was installed on board a KC-135 aircraft to perform sounding tests over the southeast US and over Greenland, demonstrating the capabilities of the system.