Space Geodesy

the branch of geodesy that studies methods of determining the relative position of points on the earth’s surface, the dimensions and figure of the earth, and the parameters of the earth’s gravitational field on the basis of observations of solar eclipses; observations of occultations of stars by the moon; photographing (against a background of stars) the moon, balloons with light sources, raised to heights of 20–30 km, and artificial earth satellites; and measurement of the distances to such satellites.

The first works relating to space geodesy were published in the second half of the 18th century. By the mid-20th century “lunar” methods of space geodesy were the most highly developed. However, beginning in the 1960’s space geodesy work has relied exclusively on positional and distance-measuring observations of artificial earth satellites (this branch of space geodesy is usually called satellite geodesy) and of balloons. The methods of photographic astrometry are used extensively for resolving space geodesy problems by observations of artificial and natural space objects and celestial phenomena.

One of the basic methods of resolving the geometric problems of space geodesy consists of simultaneous (synchronized) observations of a space object (moon, artificial earth satellite) from several points on the earth’s surface. If the positions of two or more such points are known in a certain system of coordinates relative to the earth, by mathematically solving the spatial triangles with one of the apexes at the position of the space object it is possible to compute the positions of the other points from which observations were made. This method of establishing the geodetic relationship between points on the earth’s surface is called space (satellite) triangulation. With simultaneous positional and distance-measuring observations of an artificial earth satellite (by means of radio equipment or satellite laser range finders), geodetic relations can be determined even for one point with a known position using the geodetic vector path method.

In the space geodesy methods that have been described the space object signifies only a point fixed in space at a certain moment in time. The orbital methods of space geodesy include methods of establishing the geodetic relationships between points by determining the positions of artificial earth satellites in space using the laws of their motion in the earth’s gravitational field. Use of this method makes it unnecessary to conduct observations at all points at the same moment.

The dynamic methods of space geodesy include determining the parameters of the earth’s gravitational field by investigating changes in certain elements of the orbits of artificial space satellites, computed on the basis of the results of systematic positional and distance-measuring observations of the satellites.