A laser altimeter uses focused light pulses to do the same. MOLA's solid state laser fires short pulses of infrared light 10 times per second at the surface of Mars and measures the time for the reflections to return. Upon return to the satellite, a telescope focuses the light scattered by the terrain and possibly clouds, onto a series of detectors. By knowing the speed of light, the satellite's position and its height above the planet, one can map the surface based on the return time of the pulse. The heights of mountains and the depths of valley's will be revealed to a precision of 40 centimeters.
Laser altimeters have a small footprint on the planet's surface.
MOLA's beam is only 130 meters wide. Each track along the planet's surface measures elevation continuously. In 1976 two space craft were sent to map Mars and then land to take surface pictures and conduct tests. Prior to these missions the Mariner 9 craft was sent to the planet on a mapping mission. Mariner images where used to select Viking landing sites.