GPS Broadcast Orbits

The user receiver computes the satellites coordinates from the information broadcast by the GPS satellites in the navigation messages. The orbits are seen as Keplerian in first approximation, and the perturbations are treated as temporal variations in the orbital elements. Indeed, an extended set of sixteen quasi-Kleperian parameters is broadcast to the user in the navigation message and regularly updated. This expanded set consists of the six orbital elements plus three rate parameters to account for the linear changes with time, three pairs of sinusoidal corrections, and the reference ephemeris epoch.