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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 be an Analytic Propagator, an Integrator, or an Ephemeris.
Spacecraft Propagator Editor
Analytic Propagators use pre-defined equations of motion to compute Spacecraft motion, rather than directly modeling forces as an Integrator does. These propagators tend to be very computationally efficient, but can be lower fidelity than integrators. Examples within FreeFlyer are the TwoBody, J2Mean, and SGP4 propagators.
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).
Choosing a propagator appropriate to the task can significantly improve FreeFlyer's performance. FreeFlyer can utilize several propagators with varying degrees of accuracy and speed, from a simple Two Body, to the robust Runge-Kutta 8(9). The following table summarizes details about the propagators that are provided for the Spacecraft. For detailed information on a specific propagator, follow the links shown below. In every propagator's case, it is especially helpful for Spacecraft in specific orbits and orbital regimes such that its advantages come into play. Because of that, some insight is provided below into what each propagator is recommended for and what it isn't recommended for. Speed and accuracy were determined by varying the propagator type and step size in sample LEO, GPS, and GEO orbit regimes, illustrating common propagation scenarios. Individual results will vary. All speed values are percentages relative to the run time of the RK89 integrator. All accuracy values are expressed as orders of magnitude relative to the accuracy of the Two Body propagator.
Note: The specific Runge-Kutta integration method used in FreeFlyer is the Runge-Kutta-Verner method.
•Integrator Properties and Methods