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Chapter VI Satellite Geodesy

Scientific papers advocating the use of satellites for geodetic purposes were published as early as 1956. Geodetic applications were outlined by the Smithsonian Astrophysical Observatory for data obtained from Project Vanguard during the 1958-59 International Geophysical Year. Many techniques and a great deal of knowledge were ultimately derived from this project. With this information, the constant growth of space technology, the development of electronic distance measuring devices, and the perfection of electronic data processing equipment, satellites specifically equipped for geodetic purposes have been developed, launched, observed and the data utilized.

The first real geodetic satellite was ANNA-1B launched in 1962. Project ANNA was a truly cooperative effort involving the Department of Defense (DoD), the National Aeronautics and Space Administration (NASA), and other civil agencies. Several observational systems were developed and improved during ANNA. These systems included geodetic cameras, electronic ranging and Doppler. Knowledge gained from Project ANNA was also useful in the development of Laser ranging systems.

Observational Systems

Two basic systems have been used for obtaining geodetic information from artificial earth satellites – optical and electronic. These systems have made it possible to perform various geodetic measurements to relate known or unknown positions to the earth's center, to relate unknown positions to existing triangulation networks, and to relate the triangulation networks to each other. Important parameters of the earth's gravitational field and values for the earth's flattening have also been obtained.

Historical Systems

Optical tracking with the BC-4, PC-1000, MOTS, or Baker Nunn cameras consisted of photographic observations of a satellite, or flashing light on the satellite, against a background of stars. The stars, whose positions were accurately determined, provided a framework on the photographic plate or film for a determination of precise directions from camera station to satellite. Geodetic positioning work with cameras was usually performed with one camera observing simultaneously with one or more other cameras (Figure 25). Camera systems are weather dependent and that is one major reason why they are little used today. Laser systems discussed later, are also weather dependent but their extreme accuracy justifies their use and development.

The U.S. Army developed the SECOR (Sequential Collation of Range) system and the first SECOR transponder was orbited on ANNA-1B in 1962. The SECOR system continued to be in use through 1970. The system operated on the principle that an electromagnetic wave propagated through space undergoes a phase shift proportional to the distance traveled. A ground station transmitted a phase modulated signal which was received by the satellite-borne transponder and returned to the ground. The phase shift experienced by the signal during the round trip from ground to satellite and back to ground was measured electronically at the ground station which provided as its output a digitized representation of range.