We use the radio telescope at the Arecibo Observatory, Puerto Rico, to transmit a radar signal toward the Moon, and receive the reflected echoes at the Green Bank Telescope in West Virginia. The Arecibo radar can transmit signals at a wavelength of either 12.6 cm or 70 cm. These signals penetrate into the lunar soil, and it is this significant depth of probing that makes the new observations so interesting. Radar has been used to locate ancient river channels beneath the Sahara, and to see below the layers of dust that cover much of Mars.
The radar echoes do not form a picture in the same way as a camera. Instead, we measure the strength of the reflected signal as a function of time and frequency. Because the Moon is a sphere, the radar return from the surface at a particular time corresponds to a circular region about the nearest point to us. The Moon's spin causes echoes to be shifted in frequency: increasing frequency on the approaching hemisphere and decreasing frequency on the receding hemisphere. Mathematical models for the motion of the Earth and Moon make it possible to convert the raw observations to maps of the lunar surface.