STK Ephemeris

The STKEphemeris attribute is attached to four objects in ODTK: the simulator, the filter, the smoother, and the variable lag smoother. The processes associated with these objects can generate satellite ephemeris which, via this attribute, can be saved to STK-compatible ephemeris (*.e) files. The options available depend on the process involved.

During Process

This section of the STKEphemeris attribute controls whether or not ephemeris generated during the process is written to the ephemeris file (versus writing predicted ephemeris to the file, which is generated following the process).

DuringProcess Settings
Setting Description
Generate Set to true to write ephemeris generated during the process to the file, otherwise to false.
TimeGrid Defines how often ephemeris is written to the file. Options are:
  • Uniform: Write information on an even (uniform) grid. The uniform grid is defined by the ProcessNoiseUpdateInterval attribute for the Filter, and by the UniformTimeStep for Least Squares.
  • Filter: Write information at every time possible.

The filter runs on a union of multiple grids -- the uniform time update grid, the non-uniform measurement grid and the maneuver time grid. The simulator runs on a uniform grid set by a single time step. The smoother processes the same times as the filter. Least Squares runs on a union of the measurement grid and the maneuver time grid as well as any grid times inserted for ephemeris generation.

Note: This setting is not an option for the VLS. The VLS writes out information (only) on the VLS grid.

UniformTimeStep Used by LS only when TimeGrid = "Uniform"
MaxTimeStep Used by LS only when TimeGrid = "Filter". Defines the maximum time step between ephemeris outputs.

Predict

This option is available for the filter and the smoother. This section controls whether or not predicted ephemeris will be included in the file. The state at the last time of the filter or smoother run can be used to predict ephemeris into the future, and is added into the ephemeris file(s).

Predict Settings
Setting Description
Generate Set to true to include predicted ephemeris in the file.
TimeStep The uniform time step at which ephemeris will be generated.
AlignTimeGrid Controls the alignment of ephemeris prediction output nodes to a preferred evenly spaced grid which is defined based on the requested time step. The nodes on the preferred evenly spaced grid have the property that represent an integer number of time steps from 00:00Z. Set to true to align to the preferred grid or to false to enforce prediction at purely even time steps from the initial prediction time. In both cases, the specified initial and final prediction times are contained in the resulting ephemeris. In the case where AlignTimeGrid is true, the first and last time steps in the resulting ephemeris may differ from the requested time step.
StopMode Allows you to specify how long the predict period will be. Options are:
  • TimeSpan: You define for how many minutes past the last filter or smoother time to generate a predicted ephemeris.
  • Stop Time: You specify an explicit stop time for the predicted ephemeris. If this stop time precedes the process stop time, no ephemeris is generated.

CoordFrame

Defines ephemeris coordinate frame settings for each satellite in the scenario.

CoordFrame Settings
Setting Description
Satellite Satellite name (read only)
CBName " Central body name associated to this satellite's initial conditions (read only)
CoordFrame

Coordinate frame to be used. Selection choices depend on the Central Body. For Earth, choose between ICRF, J2000, TEMEofDate or Fixed. For Sun, choose between ICRF, J2000, or Fixed. For satellites with initial conditions referenced to the Moon, choose between ICRF, J2000, Fixed, Mean Earth, and Inertial.

Note: The coordinate frame setting is defaulted and can be reset via the scenario level SatEphemeris.CoordFrame attribute. Reference this attribute for more information.

EphemerisArchiving

This attribute controls for which iterations the ephemeris (*.e) files are to be created during a Least Squares run. Options are:

Acceleration

This attribute is a boolean flag that causes the acceleration to be written to the STK ephemeris (*.e) file. The information is used during interpolation.

Covariance

This attribute is defined for least squares, filter and smoother. These processes produce covariance information with the ephemeris, and the 3x3 position covariance or 6x6 position-velocity covariance can be added into the ephemeris file(s). Covariance applies to ephemeris created either during the process run interval or during the prediction period. For least squares, the covariance will include the effects of any consider state parameters. Set the Generate property to true to write covariance to the ephemeris file, and select the desired type using the CovarianceType attribute.

FileFormat

The file format of the output Ephemeris file(s).

FileNamingOption

This feature allows you partial control over the names of ephemeris files that are automatically generated by the filter, smoother and simulator. A string containing time information of one of the following types can be included in the file name.

FileNamingOption Choices
Choice Description
ProcessStart Start time of the given process (filter, smoother or simulator) regardless of the ephemeris span.
ProcessStop Stop time of the given process, not including prediction requests.
PredictStart Start time of the predicted ephemeris.
PredictStop Stop time of the predicted ephemeris.

Files

This is a list of the results of writing ephemeris files. Each satellite involved in a process (simulator, filter, smoother) will have its own ephemeris file. The names of the files are based on the satellite names and the epoch of the ephemeris data in the file. Each entry in the list contains a satellite name and its associated ephemeris filename, which can be accessed visually or via a user-defined script to see what files were created.

Note: the epoch of the ephemeris file will be in UTC time, regardless of the setting for Scenario.Units.DateFormat.

The OutputDirectory attribute lets you specify the location at which the ephemeris files will be stored.

ODTK 6.5