CG-1280 Rev. N 09/13 - General Dynamics SATCOM Technologies

Theory3.1.2 Orbit DeterminationAn orbital element set is composed by making an initial guess of the orbit and thengenerating positions from this guess using the propagators described previously.These positions are compared with the positions gathered by AST. The differencesbetween AST data and propagator-generated data are used to determine acorrection to the orbit. The process is repeated until the RMS error between theAST data and the propagator-generated data is minimized. The algorithm used toperform this is referred to as the solver.The orbit must be periodically re-solved in order to remain accurate. Theoretically,if all of the forces acting upon the spacecraft are known at any time, the orbitalelement set can be predicted forward to any desired time. However, all of theforces are not known at all times. The largest unknown force comes from thestation-keeping maneuvers performed in order to keep the spacecraft in the desiredorbit. Station-keeping is only performed periodically (typically once every fewdays). The validity of an orbit solution determined from data after a maneuver hastaken place is not adversely affected by station-keeping until a new station-keepingmaneuver is performed.The two-body propagator does not take into account any forces except thosegenerated by the Earth's gravity, and even then, a simplified model is used. Aposition prediction from a two-body propagator has an error that grows with timedue to the omission of these forces in the orbit determination.Neither propagator can account for varying effects on the ground. The primaryground errors affecting the system are:• Wind loading of the antenna.• Thermal distortion of the antenna, caused by non-uniform solar heating.Both of these factors affect the data collected by AST. However, OPT is able toseparate ground errors from the orbital solution because the spacecraft's motionmust follow Newton's laws. This information is retained as two error terms, oneper axis. These error terms are used to correct future position predictions.3.1.2.1 Short-term and Long-term OPT SolutionsOPT generates three distinct types of orbital solutions:• Short-term solution using two-body propagator (ST)• Long-term solution using two-body propagator (LT2b)• Long-term solution using multibody propagator (LTmb)3-2