ASYNCHRONOUS LASER TRANSPONDERS:: A NEW TOOL FOR IMPROVED FUNDAMENTAL PHYSICS EXPERIMENTS.

Since 1964, the NASA Goddard Space Flight Center (GSFC) has been using short pulse lasers to range to artificial satellites equipped with passive retroreflectors. Today, a global network of 40 satellite laser ranging (SLR) stations, under the auspices of the International Laser Ranging Service (ILRS), routinely tracks two dozen international space missions with few-millimeter precision using picosecond pulse lasers in support of Earth science. Lunar laser ranging (LLR) began in 1969, shortly after NASA's Apollo 11 mission placed the first of five retroreflector packages on the Moon. An important LLR data product has been the verification of Einstein's equivalence principle and other tests of general relativity. In 1975, the University of Maryland used a laser ranging system to continuously transfer time between two sets of atomic clocks — one set on the ground and the other in an aircraft — to observe the predicted relativistic effects of gravity and velocity on the clock rates. Two-way asynchronous laser transponders promise to extend these precise ranging and time transfer capabilities beyond the Moon to the planets, as evidenced by two successful experiments carried out in 2005 at distances of 24 and 80 million km respectively. [ABSTRACT FROM AUTHOR]