Geometry of GPS relative positioning

GPS positioning is often assimilated to trilateration and even to triangulation methods. Both comparisons are wrong because GPS observations are pseudoranges containing clock errors. The geometric interpretations of GPS relative positioning and trilateration method are presented. Receiver clock parameter is also analyzed from a geometric point of view. The generalization of positioning solutions is made without and with observations redundancy. The geometry of the propagation of systematic errors into positioning solutions is also shown, especially the tropospheric (and the ionospheric) delay.