a. Gravity effects shall be included in all orbit determination and orbit prediction processes, and in attitude determination and prediction processes for Earth and planetary orbiters.
b. The inclusion of different gravity sources, their associated model details, and corresponding model truncation errors shall be compliant with the requirements on orbit and/or attitude determination accuracy, and they shall be at least of the same perturbation order as considered perturbing accelerations due to non-gravitational effects.
c. The retained accuracy level of a gravity model shall be compliant with the accuracy of the position and orientation of the central body.
NOTE Harmonic coefficients can lead to resonance effects, if they have a degree or order close to some integer multiple of the ground track repeat cycle. For orbits that are known to be repetitive, it is then recommendable to include discrete resonant harmonics of degrees that normally fall outside the truncated expansion series.