RADAR MAPPING OF PLANETARY SURFACES
Pulsed signals sent and received by radar have given us relief maps of Mercury, Venus, and Mars. This is important and somewhat remarkable for Venus since a thick cloudy atmosphere prevents visual observations of its surface. The signals are reflected from the planet's surface and return to the earth (or an orbiting satellite) as an echo modified by the surface terrain. The returning wave train from different portions of the surface is analyzed by a computer and then can be used to construct a topographic map of the planet's surface.
Analyzing the Doppler shift in the reflected radar signal caused by the rotation of the planet has given us the period of rotation for Mercury and Venus; it could not have been found for Venus by optical methods because of cloud cover. The radio wave reflected from the part of the surface that is approaching the earth has its wavelength slightly decreased because of the Doppler effect. The echo from the side of the planet that is receding from us is slightly increased in wavelength.
