The Global Positioning System (GPS) is currently being used for a wide
variety of applications. A GPS receiver aboard a spacecraft can provi
de the means for autonomous navigation and also allows a very accurate
reconstitution of the trajectory of the spacecraft when onboard recor
ded measurements are combined with ground-based measurements. This art
icle outlines some of the basic concepts involved and presents the act
ivities that the European Space Operations Centre is carrying out in t
he field of satellite navigation using GPS.