Satellite dependent EE are most difficult to completely model, correct and compensate for because many forces acting on the predicted orbit of a satellite are difficult to measure directly. Satellite orbital bias is defined as the discrepancy between the true position (and velocity) of a satellite and its broadcast ephemeris .

Ephemeris information to calculate GPS satellite positions is generated from the tracking data collected by five monitor stations of the control segment. The collected tracking data is processed at the MCS and the satellite navigation message information is uploaded to every satellite. Errors in the prediction of a satellite position are transmitted to the user in the satellite data message and are available to GPS users at the time of observation.

Satellite motion is controlled by Keplerian laws which in mean sense considers motion about ideal earth having perfectly radial gravitational field and no atmosphere (Nassar, 1994).

• Ideal earth : Spherical earth having axis of rotation passing through its centre of gravity (CG) and having homogeneous mass distribution.
• Real earth : Earth is non-homogeneous body of random mass distribution and non radial gravitational field. Hence, motion is governed by Newton's laws where satellite position in space described by six Keplerian elements.