Laser Transponders for High-Accuracy Interplanetary Laser Ranging and Time Transfer

Satellite laser ranging (SLR) and lunar laser ranging (LLR) to passive reflectors have been carried out successfully since 1964 and 1969, respectively. The single-ended SLR ranging technique, although capable of providing millimeter precision range data to satellites, is not practical over interplanetary ranges. Double-ended laser transponders for decimeter or better accuracy interplanetary ranging and subnanosecond time transfer are well within the state-of-the-art, however, as was recently demonstrated in two successful transponder experiments carried out by the NASA Goddard Space Flight Center to laser altimeters onboard the Messenger spacecraft (currently enroute to Mercury) and the Mars Global Surveyor spacecraft (presently in Mars Orbit). A high-accuracy interplanetary ranging capability would support a number of new scientific investigations (e.g., solar system and planetary physics, general relativity, etc.) and enhance deep-space mission operations and reliability through vastly improved navigation accuracy and time synchronization with Earth mission control centers. The performance of future lunar or interplanetary laser transponder and laser communications instruments can be simulated and tested at distances to Pluto and beyond using existing passive SLR and LLR targets already in space.