Radar to the Moon

One of Apollo's major contributions to planetary science was to help to push the development of radar on an orbiting vehicle as a tool to probe the surface and subsurface of a planet or a moon. From the simple experiments conducted on Apollo, such technology has developed profound capabilities that allow the shape or topography of a surface to be accurately profiled. It can 'see' buried surfaces up to a depth of 1 kilometre, depending on the nature of the planet's soil. It can also gather its imagery in the absence of light, allowing unlit and cloud-covered terrain to be viewed.

Beginning with Apollo 14, radar tests became a normal part of the CMP's solo tasks when the spacecraft's S-band (around 2,200 MHz) and VHF transmissions (around 260 MHz) were aimed at the Moon to be received by large dish antennae on Earth. These bistatic tests were so called because, unlike most radar setups where the transmitting and receiving antenna is one and the same, here the antennae were separated by a distance similar to the target distance. As a result, these tests required no additional equipment on the CSM and were a happy result of using what was already available. Researchers could determine the electrical properties of the surface by seeing how the radio wave's reflected strength varied with its incident angle.

Moreover, the interplay between the spacecraft's orbital motion and the resultant Doppler effect on the signal's frequency allowed discrete lunar features to be 'seen' in the signal's received spectrum.

For Apollo 17's lunar sounder, researchers took the technology to the next level, mounting specialised antennae on the SM to send pulses of radio energy towards the Moon and receiving the reflection, including any modification that resulted from its interaction with the surface. Results from the radar were recorded optically on film for later analysis on Earth.

The lunar sounder was the prototype for later radar systems that successfully imaged the cloud-covered landscapes of Venus and Titan, searched for underground geology and ice deposits on Mars, and mapped the surface topography of most of Earth.

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