User Variables

STK's Astrogator capability can incorporate extra variables during propagate and finite maneuver segments. Within Astrogator, these variables are called "user variables". These variables carry through the MCS with the rest of the spacecraft state, and are saved at each time step as part of the ephemeris.

To incorporate a user variable, you must define it in the MCS Options window of the Astrogator satellite and in an EOM Function plugin. In the MCS Options window, use the User Variables tab to define the variable by name and unit dimension; the name must exactly match the variable name registered in the plugin.

After you have defined a user variable in the MCS Options window, you can set its initial value in an Initial State, Follow, or Launch segment and you can change its value using an Update segment. You can also select the initial value and updated values as independent variables in a Target Sequence search profile.

In propagate and finite maneuver segments, you can add an EOM Function plugin to the propagator that modifies the user variable. The value of the user variable can either be set directly by the plugin or the plugin can give the derivative of the variable and Astrogator will integrate it. For more information, refer to the Help for EOM Function in the Programming Interface Help and the example plugin scenario provided with STK.

You can use the user value calculation object to report the value of a user variable.