Geodetic and Geophysical Applications of Laser Satellite Ranging

The Smithsonian Astrophysical Observatory (SAO) uses computed satellite orbits in its geodesy program and for studies of the gravitational field and other parameters of the earth. As the accuracy of these orbits increases, so does their field of application. The measurement of temporal changes, such as continental drift or other crustal motion, is in prospect. Laser systems are providing satellite-range measurements from which orbits are computed with significantly increased accuracy. SAO uses three pulsed ruby-laser systems routinely in its worldwide network of satellite observing stations and cooperates on an international basis with other agencies who have their own systems. The 500-MW laser at Smithsonian's Mt. Hopkins Observatory in Arizona is described. This system uses currently available orbits to position the laser transmitter and receiving telescope in azimuth and altitude. The laser is pulsed at the proper epoch without the satellite's being acquired visually. The accuracy of this method of positioning, its advantages for the present application, and its future potential will be discussed.