Studies Of Laser Ranging To The TOPEX Satellite

In the early 1990s NASA, with collaboration from the French national center for space studies (CNES) will launch the ocean topography experiment (TOPEX) satellite. The principal function of TOPEX is to use two onboard altimeters to precisely measure the altitude of the satellite's circular orbit from the ocean surface. The altimeters will be capable of measuring the satellite's altitude with a relative precision of 2-3 cm, and an absolute accuracy of 14 cm. This capability dictates the necessity of calibrating the altimeters to the same level of accuracy and precision. Several means are being considered for accomplishing this calibration. The Navy's TRANET system is the prime candidate under consideration, though questions have been raised as to whether it will have sufficient availability in the early 1990s time frame. Another radio-frequency system is the Global Positioning System (GPS). Though having the requisite accuracy, the availability of the GPS is also in doubt because of the Space Shuttle launch schedule. Finally, there exists a sparse, but world-wide laser tracking network. Satellite laser ranging is a LIDAR technique that works by retroreflecting a laser pulse off of a set of cube corner retroreflectors which are precisely located relative to the satellite center of mass. Principally, there are the Nd:YAG mobile laser (MOBLAS) stations operated by the Goddard Space Flight Center (GSFC). These laser systems have the necessary ranging accuracy and will be available at well surveyed sites throughout the 1990s. Based on previous work done at GSFC, we have developed a computer program to model the result of using different ground-based LIDAR systems in conjunction with various retroreflector array designs on the TOPEX satellite. This PC-based software model is designed to accommodate a wide range of satellite viewing angles, altitudes, retroreflector arrays and cube corner parameters. The program is to be used as a design tool to help understand how the LIDAR return signal strength varies with satellite elevation angle, number and geometry of cube corners, laser strength, transmitter size, etc., and how these parameters are traded off against each other.