Optimum orbit of the geodetic satellite GS-1.

Abstract

The optimum orbits of the geodetic satellite GS-1, expected to be launched after 1982, are here investigated. The satellite is a balloon, approx. 10 m in diameter, which can be observed by both photography and laser ranging so that its direction and distance can be determined simultaneously. The role of the satellite is to determine the positions of the isolated islands around Japan, with an accuracy of ±1～2 m, with respect to an unified geodetic system for establishing the marine geodetic network which is to be a positioning reference system in all scientific investigations in the sea area, and to correct the national triangulation network on land. In order to accomplish these programs the satellite orbit should be selected in such a way that will satisfy the following conditions : i) the satellite can be photographed with a moderate size astrograph to determine its direction and time, ii) laser ranging is possible with a comparatively simple device, iii〕there are plenty of chances for simultaneous observation from two stations approx. 2000 km apart, iv) the satellite will remain in orbit for five years or more during which the observations can he completed as scheduled, and v) there is little difficulty in the problems of safety and technique needed the satellite into the orbit. From the standpoint of planning the observation program, and the efficiency of the observations, it is desirable that the orbit be circular. The optimum values of the orbital height and inclination, which directly affect the chances of observation, have been investigated through numerical analysis and simulation, taking the conditions enumerated above into account. Through the analysis the relationships among the satellite irradiation, observable area on the earth’s surface, and the occurrence of observation chances, have been evaluated by providing a varied orbital height and observable elevation angle limit. During the simulation the positioning accuracy and the observation times have been evaluated by using the sham observation values derived from the normal random numbers. The conclusions are as follows : Concerning the orbital height, although the positioning accuracy of the observation stations depends on the accuracy of a single observation, due to the satellite height and direction, such an effect is smaller than the effect which the height has on the number of observation chances. The number of the observation chances also depends on the optical capacity of the astrograph. The lower the optical limit of the observable elevation angle the more the observation chances, although scintillation becomes significant in the lower elevations. If the astrograph is effective for all areas above some fixed angle between 20°and 30°, it is better to adopt a higher orbit, within the limit of the laser ranging. If there is a lower limit in the elevation angle, at approx. 30°, there is not so much difference in the observation chances for heights of 1400～1600 km. If the optical capacity of the astrograph is insufficient, it appears to be better to adopt a lower orbit. But this is no more desirable because of the decrease of the absolute amount of the chances. For the inclination, the simulation shows that there are no remarkable differences in the chances due to an inclination between 45° and 55° in the case of observation in Japan and its surrounding area. Hence an inclination should be decided, which will minimize any error in height at the launching, optimize the period of observable seasons, and, if necessary, enable the use of the satellite by stations in other regions of higher latitudes. Key words: satellite geodesy.