Study of a GPS satellite selection policy to improve positioning accuracy

The four dimensional accuracy of GPS positioning using four satellites at every instant is reexamined. User receivers usually select suitable four satellites that have best geometry based on PDOP or GDOP. However, it is more reasonable to select a set of four satellites that has minimum VDOP for measuring vertical position, while to select a set of four satellites that has minimum HDOP for measuring horizontal position. In this studies, we propose a method of using VDOP and HDOP as the criteria to select the set of four satellites used for measuring vertical and horizontal positions respectively, and compare its performance with that of the conventional method by computer simulation. It is clarified that the combinations of the four satellites that minimize VDOP, HDOP and PDOP are generally different each other, and limited amount of improvement in VDOP and HDOP is obtained by optimizing them separately. Dependence of the amount of the improvement on latitude and mask angle is also examined