STK Ephemeris

The STKEphemeris attribute is attached to four objects in ODTK: the Simulator, the Filter, the Smoother, and the Variable Lag Smoother (VLS). The processes associated with these objects can generate satellite ephemeris that, via this attribute, you can save to STK-compatible ephemeris (*.e) files or three other ephemeris formats. The options available depend on the process involved.

During process

This option 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 ODTK generates after the process.

During Process Settings
Setting Description
Generate Set this to true to write ephemeris generated during the process to the file; otherwise, set it to false.
TimeGrid Defines how often ODTK writes ephemeris to the file. The 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 attribute 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 nonuniform 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 on any grid times inserted for ephemeris generation.

This setting is not an option for the VLS. The VLS writes out information (only) on the VLS grid.
UniformTimeStep Least Squares uses this, but only when TimeGrid = "Uniform".
MaxTimeStep Least Squares uses this, but only when TimeGrid = "Filter". It 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 ODTK will include predicted ephemeris in the file. ODTK uses the state at the last time of the Filter or Smoother run to predict ephemeris into the future, and it adds this prediction into the ephemeris file(s).

Predict Settings
Setting Description
Generate Set this to true to include predicted ephemeris in the file.
TimeStep This is the uniform time step at which ODTK will generate ephemeris.
AlignTimeGrid Controls the alignment of ephemeris prediction output nodes to a preferred, evenly spaced grid that is defined based on the requested time step. The nodes on the preferred, evenly spaced grid have the property that represents an integer number of time steps from 00:00Z. Set this 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 Enables you to specify how long the predict period will be. The options are:
  • TimeSpan: You define 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, ODTK does not generate ephemeris.

CoordFrame

This defines the 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

This is the coordinate frame for STK to use. Selection choices depend on the central body. For Earth, choose from ICRF, J2000, TEMEofDate, or Fixed. For Sun, choose from ICRF, J2000, or Fixed. For satellites with initial conditions referenced to the Moon, choose from ICRF, J2000, Fixed, Mean Earth, and Inertial.

The coordinate frame setting initially has a default value. You can reset it via the scenario level SatEphemeris.CoordFrame attribute. Reference this attribute for more information.

EphemerisArchiving

This attribute controls for which iterations ODTK should create the ephemeris (*.e) files during a Least Squares run. The options are:

  • All Iterations - ODTK creates ephemeris files during each iteration of each stage.
  • Final Iteration - ODTK creates ephemeris files during the last successful iteration.
  • Initial and Final - ODTK creates ephemeris files during the first and last iterations.

Acceleration

This attribute is a boolean flag that causes ODTK to write the acceleration 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 ODTK can add the 3x3 position covariance or 6x6 position-velocity covariance 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 considered state parameters. Set the Generate property to true to write covariance to the ephemeris file, and select the desired type using the CovarianceType attribute.

The CovarianceCoordSystem may specify the reference frame to be used for covariance representation in the ephemeris file. Choose between the same coordinate system used for the ephemeris representation and the Gaussian (RIC) frame. The RIC frame designation is synonymous with the UVW frame designation. The CovarianceCoordSystem option is only available when you select the CCSDS Ephemeris file format.

State Error Transition matrix

Set the StateErrorTransition attribute to True to have the Filter, Simulator, Smoother, or Variable Lag Smoother generate a State Error Transition matrix as part of the output. This matrix provides a linear mapping of small deviations in position and velocity from the reference epoch to the time specified with the matrix.

File format

The file format of the output ephemeris file(s) is as follows:

  • STK Ephemeris is in the STK *.e format.
  • STK Binary Ephemeris is in the STK *.be binary format.
  • CCSDS Ephemeris is in the CCSDS 502 format. Files will have a *.oem extension.
  • SPICE BSP is the SPICE ephemeris format. Files will have a *.bsp extension.
  • SPICE IDs for satellite objects are specified using the SPICE.ID property on the satellites.

FileNamingOption

This feature gives you partial control over the names of ephemeris files that are automatically generated by the Filter, Smoother, and Simulator. You can include in the file name a string containing time information of one of the following types:

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 you can access visually or via a user-defined script to see what files were created.

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

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