• Mainly gravitational forces act on satellites which can be categorized into two main groups:
  • central gravitational attraction
  • non-central gravitational (also called the perturbing forces)
• Magnitude of central gravitational forces is three order of magnitudes larger than non-central gravitational and all other combined forces. Hence, the modelled satellite motion can be considered by central gravitational field and all other forces are considered as perturbing or disturbing forces.
• Various perturbing forces include (Figure 9.1):
  • Non-central gravitational force
  • Third body effects (gravitational attraction of sun, moon, and planets)
  • Atmospheric drag force
  • Solar radiation pressure
  • Magnetic forces
  • Variable part of earth gravitational field arising from tidal and other deformation of solid earth and ocean.

Effect of some of these factors is significant which can be expressed as time dependent variation from mean motion and can be introduced as corrections. Resultant effect of various dynamic model shortcomings is treated as orbital bias.