STK Astrogator Plugin Points

STK Astrogator provides the following plugin points:

• Calculation Objects
• Custom Engine Models
• Propagators

For definitions of certain terms used in describing inputs and outputs, see Parameter Definitions.

Additional plugin points utilizing Microsoft COM technologies are available for Astrogator. These plugin points include HPOP Force Model plugins, Search Profile plugins, Engine Model plugins and Attitude Controller plugins. For information on developing and using COM-based plugin scripts, see "Extend AGI Products->Extend the Engine->COM Based Engine Plugins" in the STK Programming Interface help system. To access this help system, select Programming Interface Help from the STK Help menu. Examples of these plugins in multiple programming languages are located under CodeSamples\Extend in the STK install directory.

Custom Functions

Many of the Astrogator plugin points are implemented using Custom Functions. A Custom Function is an Astrogator Component in the Component Browser that calls a script function defined in a file. There are four types of Custom Functions: MATLAB, VBScript, Python and Perl. A Custom Function has only one attribute: the filename containing the function to be called. Each time a Custom Function is initialized, STK checks the timestamp of the file. If the file has been edited since its last loading, then it is automatically reloaded; otherwise, reloading is unnecessary and is not performed.

For more information, see Scripting Environments.

Calculation Objects

Calc Objects can utilize scripting through the use of:

• Inline functions.
• Custom Functions.

Each of these types is found in the Scripts folder under the Astrogator Components->Calculation Objects folder in the Component Browser. The Component Browser is available from the Utilities menu.

Inline Functions

Inline functions are used for very simple computations that are not currently available as Calculation Objects in the Component Browser. They have three attributes:

Attributes of Inline Functions

There are four types of inline script functions available: MATLAB, VBScript, JavaScript, and Perl. When an inline Calculation Object is used, STK creates a function with the inputs listed in CalcArguments, returning the value computed by the InlineFunc, with a name derived from the CalculationObject name itself.

Example:

     CalcObject Name:        Incl cos Raan
     CalcArguments:          Inclination, RAAN
     InlineFunc:             Inclination*cos(RAAN)
     UnitDimension:          AngleUnit

An inline function named CalcObject_Incl_cos_Raan is created, with Inclination and RAAN as inputs. The InlineFunc expression must use the correct syntax of the scripting language, as follows:

The MATLAB Calculation Object uses the MATLAB inline function to define the function:

CalcObject_Incl_cos_Raan  = inline('Inclination*cos(RAAN)', 'Inclination',
     'RAAN');

The VBScript Calculation Object creates the function:

Function CalcObject_Incl_cos_Raan (Inclination, RAAN)
       Dim value
       value = Inclination*cos(RAAN)
       CalcObject_Incl_cos_Raan  =  value
End Function

The JavaScript Calculation Object creates the function:

function CalcObject_Incl_cos_Raan (Inclination, RAAN)
{
      return eval(“Inclination*cos(RAAN)”);
}

Perl defines functions differently from MATLAB and VBScript. The Perl Calculation Object equivalent function is defined as:

      sub CalcObject_Incl_cos_Raan
      {
           return $_[0]*cos($_[1]);
      }

Note that there is no method for declaring the arguments as Inclination and RAAN: they have been replaced by $_[0] and $_[1], respectively, based on the knowledge that the order of the arguments is Inclination followed by RAAN. You can enter the InlineFunc value in proper Perl notation as shown above (complete with the scalar prefix $) but then must translate each argument to the function by the proper symbol $_[n], where n is the index of the argument (beginning with 0 in Perl).

If you are not familiar with Perl syntax but want to use Perl, The Perl Calculation Object will create the proper Perl syntax automatically from an InlineFunc written using the names—thus, you can enter Inclination*cos(RAAN) as the InlineFunc even for Perl. When utilizing this behavior, be careful not to mix the two approaches: $Inclination*cos($RAAN) will result in a syntax error.

Caution: Because the name of the Calculation Object is used in the name of the function being defined, it must be a valid function name. Spaces in the name are allowed because STK will substitute underscores (i.e., '_'); MATLAB, VBScript and Perl each allow underscores in function names. Symbols such as '(' or '*' etc. are not allowed however.

Custom Function Calc Objects

Calculation Objects based upon Custom Functions are available for use with computations that are lengthy or involved. Unlike the inline functions, Custom Function Calc Objects require you to write a script stored in a file. See Custom Functions.

Custom Function Calc Objects have two Entry Points that can be utilized. Use of any Entry Point is optional, though the value of the calc Object will be 0.0 if the Eval Entry Point is not used.

Entry Points for Custom Function Calc Objects

The Eval Entry Point has the following available inputs and outputs:

Custom Function Calc Objects -- Eval Entry Point Arguments and Keywords

In the above table, <name> indicates the name of an object of the given Type from the Component Browser, and <refAxes> indicates the name of Axes from the Component Browser.

The Reset Entry point has the same inputs as the Eval Entry point, but no outputs.

Custom Engine Models (Deprecated)

A COM-based Custom Engine Model plugin script is now available. We recommend that when a plugin point is available for both scripting and COM-based plugin scripts, that you use the COM-based plugin script. For information on COM-based scripting in STK, see the COM-based Engine Plugins topic in the Programming Interface Help, which is accessible from the Help menu.

To run a deprecated Custom Engine Model plugin script, use the Astrogator script driver plugin available from the Component Browser.

The Custom Engine Models has five Entry Points that may be utilized. Use of any Entry Point is optional.

Entry Points for Custom Engine Models

Inputs and Outputs

The Pre-Propagation and Post-Propagation Entry Points have no inputs or outputs. (The script function signature, however, must still allow for one argument).

The Update Entry point has the following available inputs and outputs:

Custom Engine Models -- Update Entry Point Arguments and Keywords

In the above table, <name> indicates the name of an object of the given Type from the Component Browser, and <refAxes> indicates the name of an Axes from the Component Browser.

The SegmentStart Entry Point has the same inputs that are available in the Update Entry Point, but has no outputs.

Custom Engine Models -- Eval Entry Point Arguments and Keywords

The Eval Entry point has the following available inputs and outputs:

In the above table, <name> indicates the name of an object of the given Type from the Component Browser, and <refAxes> indicates the name of an Axes from the Component Browser.

Propagators (Deprecated)

A COM-based Propagator plugin script is now available. We recommend that when a plugin point is available for both scripting and COM-based plugin scripts, that you use the COM-based plugin script. For information on COM-based scripting in STK, see the COM-based Engine Plugins topic in the Programming Interface Help, which is accessible from the Help menu.

To run a deprecated Custom Propagator plugin script, use the Astrogator script driver plugin available from the Component Browser.

All Propagators have five Entry Points that may be utilized. Use of any Entry Point is optional.

Inputs and Outputs

The Pre-Propagation and Post-Propagation Entry Points have no inputs or outputs. (The script function signature, however, must still allow for one argument).

The Update Entry point has the following available inputs and outputs:

In the above table, <name>indicates the name of an object of the given Type from the Astrogator Component Browser, and <refAxes> indicates the name of an Axes from the Astrogator Component Browser.

Note: When CD, CR, DragArea, SRPArea, DryMass and/or FuelMass are set as outputs, the values that are assigned permanently affect the satellite's physical properties as viewed on the GUI or reported in the MCS Summary. Thus, assigning values for these during Update will affect the force model computation, unless overridden by the Eval Entry Point. In addition, subsequent MCS segments will use these updated values as well.

The SegmentStart Entry Point has the same inputs that are available in the Update Entry Point, but has no outputs.

Propagators – Eval Entry Point Arguments and Keywords

The Eval Entry point has the following available inputs and outputs:

In the above table,<name> indicates the name of an object of the given Type from the Astrogator Component Browser, and <refAxes> indicates the name of an Axes from the Astrogator Component Browser.

Note: When CD, CR, DragArea, SRPArea, Density and/or SolarIntensity are set as outputs, the values that are assigned permanently affect the satellite's physical properties as viewed on the GUI or reported in the MCS Summary. Thus, assigning values for these during Update will affect the force model computation, unless overridden by the Eval Entry Point. In addition, subsequent MCS segments will use these updated values as well.

Information on Arguments

Representation and Units

In the following table, arguments are identified by the Name element and, where necessary, the Type element. For each argument, the Representation (essentially, the data type) and Units are given.

Entry Point Representation and Units

In the above table, <name> indicates the name of an object of the given Type from the Astrogator Component Browser.

Parameter Definitions

The following table explains some of the terms used in defining arguments to Entry Points for Astrogator Plugin Points:

STK Programming Interface 11.0.1