Satellite Geodesy

the branch of geodesy that studies the theory and methods of solving practical and theoretical geodetic problems on the basis of observations of artificial earth satellites and other objects in space. Through observations of satellites—in particular, by photographing the satellite against the background of stars with special cameras or by measuring the satellite’s distance and radial velocity with radio and laser instruments—both geometrical and dynamical problems can be dealt with. The geometrical problems involve the determination of the coordinates of points and the directions of chords of the earth’s surface. As for the dynamical problems, observations of satellites permit the parameters characterizing the earth’s gravitational field to be determined more precisely. Observations of satellites can also be used to determine the relative positions of islands and continents, to investigate the motion of the earth’s poles, and to study changes in the earth’s geodetic parameters over time. The application of the laser to distance measurement has revived interest in the moon as an object of observation for the solution of problems in satellite geodesy. In solving geometrical problems, the satellite is regarded as a point fixed in space at a certain moment in time. Synchronous, or simultaneous, observations of the satellite from a number of reference points and from a point whose coordinates are unknown allow the point’s position to be determined in a single system of coordinates of reference points. The observation of several satellites makes it possible to construct a satellite triangulation network or to lay out a vector path (seeSPACE GEODESY).

To solve dynamical problems, the laws of the motion of the satellite in its orbit must be known (seeCELESTIAL MECHANICS). In what is sometimes called the orbital method, knowledge of the satellite’s laws of motion permits the coordinates of the observation point to be determined from observations of the satellite. The problem of determining more precisely the parameters of the earth’s gravitational field presents a special difficulty: the solution of the problem is complicated by the large number of parameters being determined and by the need to take into account the influence of factors that perturb the satellite’s motion. The best solution to the problem is achieved when data from ground gravimetric surveys are used together with observations or data on the motion of satellites whose orbits are of different height and inclination. To investigate or eliminate perturbations, such as the perturbation due to atmospheric drag, geodetic satellites are used whose orbits are specially selected for this purpose. At the present time, radio and laser methods of observing the motion of satellites and remote space objects are playing an increasing role in solving the dynamical problems of satellite geodesy.