Laser Ranging toGPS Satellites with Centimeter Accuracy

In 1960, Theodore H. Maiman, of the Hughes Aircraft Company, successfully operated the first device to generate an intense beam of highly coherent monochromatic radiation. He called his device a laser for light amplification by the stimulated emission of radiation. The laser has become ubiquitous, with literally hundreds of uses ranging from optical surgery to precision machining. Lecturers use laser pointers; surveyors use laser distancemeasuring devices; police officers use laser radar units to catch speeders. Most of us unwittingly use a laser each time we listen to our CD playersthe light reflected from the microscopic pits on the CD is generated by a precisely positioned laser. One application of the laser that is not so well known is satellite laser ranging. In this month's column, John Degnan and Erricos Pavlis, from the Laboratory for Terrestrial Physics ( LTP) at NASA's Goddard Space Flight Center (GSFC), in Greenbelt, Maryland, introduce us to satellite laser ranging and describe the efforts to track two of the Navstar GPS satellites using this technique. Dr. Degnan is the head of LTP's Space Geodesy and Altimetry Projects Office. He has been employed at GSFC since 1964 when, as a coop student from Drexel University, he participated in the first laserranging experiments to the Beacon Explorer B satellite. Dr. Pavlis is a senior geodesist in the LTP and is affiliated with the Department of Astronomy at the University of Maryland. His research interests include satellite orbital dynamics and the analysis of space geodetic data. "Innovation" is a regular column in GPS World,featuring discussions on recent