ABSTRACTSince NAVSTAR/GPS has extensive application, users may have diverse requirements. Here we study how to select the satellites at every instant.In GPS navigation, users usually obtain their positions and clock biases from four visible satellites. The selection of these four satellites has a great influence on the accuracy of a navigation fix. The degree of this influence is well represented by GDOP. In this paper GDOP is expressed by the simplest form of a ratio of polynomials using a well‐chosen coordinate. Using this expression, it is shown that the minimum GDOP is realized when the four satellites are located in symmetric directions with respect to the user position, and that the minimum achievable value of GDOP is 1.5811. The reason GDOP tends to be inversely proportional to the volume of the tetrahedron formed by the points of unit vectors from user toward the satellites is also revealed. An optimization under a visible condition is discussed. On the basis of these analyses, a user's practical method of selecting the four satellites at every instant is proposed and it's performance is evaluated by simulation. As the result of this evaluation, it is clarified that the proposed method provides sufficiently small GDOP in spite of its small requirement of computatio