To confirm an accurate position of a user terminal on the surface of the earth.

An earth station 38 communicates with a satellite 10, and the satellite 10 communicates with a selectable terminal from among plural user terminals 44. The position of the user terminal 44 on the surface of the earth 14 is measured, and the measurement accuracy of the position of the satellite 10 is improved by using all of the user terminals 44 as further trigonometrical survey points. The improved position is used for deciding the position of the user terminals 44 further, and the improvement is accumulated. Weighting is applied so that user terminals 44 with a larger error provide small importance, and user terminals 44 with a smaller error provide large importance in the trigonometrical survey.