Spacecraft Propagators

Top  Previous  Next

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.


If an Integrator is used, the forces acting on the Spacecraft are defined by its ForceModel object. There are many high-fidelity Integrators available in FreeFlyer to propagate the Spacecraft, applying the accelerations from the user-selected forces.

If an Ephemeris File is used, the ForceModel object is not used. The Spacecraft's initial state (epoch, position, velocity, etc.) will be overwritten with the first state in the ephemeris file.


For more information on the relationship of Propagators, Integrators, and Ephemeris Objects in FreeFlyer, see Type Casting and the Object Hierarchy list.


Spacecraft Propagator Editor

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.


See Working with Integrators and the Choosing an Integrator page for more information.


Force Model

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).


See Working with Ephemerides and Ephemerides and Attitude History Files for more information.