The Laser Retroreflector Array, or LRA, is a set of 192 quartz corner-cubes mounted in two concentric rings around the altimeter antenna. The array is a completely passive unit which reflects laser beams originating from one of about thirty ground-based laser tracking stations positioned around the earth. By measuring the length of time the laser beam takes to travel to the spacecraft and back, scientists can calculate TOPEX/Poseidon's orbital radial position to within 3 centimeters.
