Start Time, Stop Time, Step Size & Epochs

In the Orbit, Route or Trajectory page of a vehicle's Basic properties, use the time controls to set the time period of interest. The default value for vehicle start and stop times is the scenario analysis period . Step size (default = 60 sec) is the interval between calculated ephemeris output points.

You can change the Step Size, but we recommend at least 50 ephemeris points per orbit for accurate interpolation.

If you change the scenario's epoch on the Time Period page of the scenario's Basic properties, any newly defined satellites have a default start time and orbit epoch equal to the new epoch. In addition, relative start times (in epoch seconds) are adjusted for all previously defined vehicles.

Orbit Epoch

The orbit epoch defines the time at which the established orbit elements (on the right side of the page) are true.

Coordinate Epoch

The Coordinate Epoch specifies the epoch of the input coordinate system. Some STK coordinate systems require an epoch in order to be fully defined. If you select a coordinate system with a pre-established coordinate epoch (such as J2000 or B1950), the Coord Epoch option is disabled.