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Spacecraft motion modeling through time is accomplished in FreeFlyer via a Propagator object.
Propagators are used to evolve, or "step", a Spacecraft state forward or backward in time. Each Spacecraft has its own Propagator object, which can use an Integrator or an Ephemeris File to step the Spacecraft.
Spacecraft Propagator Editor
A numerical Integrator can be used to calculate the state of a spacecraft at a point in time using an initial state and a definition of the forces acting upon the spacecraft as input. It is used to step the spacecraft in time with a user-defined step size.
Choosing a variable step size (instead of fixed step size) for long duration propagation may improve performance. Using a variable step size allows FreeFlyer to increase or decrease the propagation step size based on a user-defined tolerance; however, output data will not be reported at a constant interval.
The Force Model object defines the forces that will be modeled in the accelerations that are applied when a Spacecraft or Formation are propagated using the Step or Maneuver commands.
Ephemerides and Attitude History Files
As an alternative to using a ForceModel, FreeFlyer can base calculations on an imported Ephemeris as specified by a location and file name. Similarly, as an alternative to modeling the Spacecraft attitude in FreeFlyer Script, the attitude can be specified using an Attitude History File (AHF).