Real-time Propagators

The Real-time propagator allows you to import vehicle telemetry dynamically and to generate look ahead ephemeris automatically in a real-time operations environment. Position data can be updated in STK via Connect using the SetPosition Connect command. Once received, this data is considered historical data. Look ahead ephemerides are generated after every telemetry update using the last position sent, and are computed for a user-defined look ahead duration. The Real-time propagator is also capable of pruning historical data automatically when that data exceeds a user-defined look behind Duration. This enables you to continue updating the vehicle's telemetry without overloading system memory resources. STK can also display the historical, drop-out and look ahead segments of Real-time ephemeris with the Real Time option of the vehicle's Graphics Attributes.

Vehicles with this propagator will not be acknowledged by analysis performed with STK Coverage capability.
Use the Real-time propagator in an operational environment with the Animation Time Step set to Real Time in the Animation page of the scenario's Basic properties.

To use the Real-time propagator:

Select the Real-time Propagator. (Because data is provided to STK via a Connect socket, there is no need to set the Start Time, Stop Time and Step Size normally used to propagate ephemeris points.)
Choose a look ahead propagator and define the remaining Look Ahead Properties.
Define the History Properties (i.e., interpolation order).
Use the Modify Values... button to adjust the Durations if necessary.

Look ahead properties

Option	Description
Look Ahead Propagator	Choose a propagator to be used for calculating ephemeris for look ahead purposes. Not all propagators are available for all vehicles. HoldCBIPosition HoldCBFPosition DeadReckon Two-Body J2 Perturbation J4 Perturbation Ballistic
Time Step	The interval between computed ephemeris output points.
Time Out Gap	The time after which look ahead values are considered to be "stale" (i.e., the data has dropped out). For instance, if the current time is 12:15, the last data point was received at 12:00 and the Time Out Gap is 300.00 sec, the Look Ahead ephemeris points have been in drop-out for 10 minutes. This value is used to provide a visual indicator of stale data on the 2D Graphics window.

History Properties

Interpolation Order. History segments interpolate using Lagrangian interpolation, which is based on polynomials. Higher order interpolation (three or higher) is more appropriate for smooth data entered at nearly regular time steps - the interpolating polynomial aims to smoothly transition between grid points by sampling points ahead of and behind the time in question. Lower order interpolation is more appropriate for data entered at irregular time steps - while it cannot smooth the data well, it avoids undesirable "ringing" that may be produced by a higher order interpolation.

"Ringing" occurs when a polynomial oscillates with high amplitude when off the grid points--in such a case, the interpolating polynomial is unlikely to model the actual motion of the vehicle well.

Durations

The options in the Durations area are not editable. You must use the Modify Values... button to change Look Ahead and Look Behind Duration.

Option	Description
Look Ahead Duration	STK calculates ephemeris for future times relative to the most current data point received for the extent of the Look Ahead duration. For instance, if the time of the last data point is 12:00 and the Look Ahead Duration is 1800.0 seconds, STK would calculate future orbit ephemeris until 12:30.
Look Behind Duration	STK stores ephemeris data points that are transmitted via Connect into temporary memory. The Look Behind Duration specifies how long the temporary data points are to be stored by subtracting the Look Behind duration from the time of the most current data point. For instance, if the time of the last data point is 12:00 and the Look Behind Duration is 1800.0 seconds, STK would prune data received prior to 11:30. The bigger this value, the more memory is required to store temporary data
Modify Values...	Use this button to edit either or both duration values. Please note that any edits made in the Modify Real Time Values window are applied to real-time values for Orbit, Attitude and Articulations.

DeadReckon Look Ahead Propagator

The DeadReckon look ahead propagator projects the motion of the vehicle along a straight line, along the last velocity vector.