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Overview 

Data property available for use by commands

Description 

Parameters are named resource properties that can be used to obtain data for use by Mission Sequence commands. Unlike resource fields, which can be set in the initialization portion of a script (or in the GUI's Resources tree),  parameters are derived properties that can only be used to get data, not store it. There is some overlap between the two categories, however: the ECC property of Spacecraft is both a field (i.e. settable during initialization) and a parameter (i.e. usable by Mission Sequence commands).

Parameters are classified as one of three types: central-body-dependent parameters, coordinate-system-dependent parameters, and standalone parameters. Standalone parameters are the simplest type, in that they have no dependencies. The ElapsedSecs parameter of Spacecraft is an example of this; it is simple referenced as Spacecraft.ElapsedSecs.

Central-body-dependent parameters, as the name suggests, have a value that is dependent on the chosen celestial body. The Altitude parameter of Spacecraft is an example of this. To reference this parameter, you must specify a central body, such as Spacecraft.Mars.Altitude. Any built-in central body or user-defined Asteroid, Comet, Moon, or Planet is valid as a dependency.

Likewise, coordinate-system-dependent parameters have a value that is dependent on the chosen coordinate system. The DEC parameter of Spacecraft is an example of this. To reference this parameter, you must specify the name of a CoordinateSystem resource, such as Spacecraft.EarthFixed.DEC. Any default or user-defined CoordinateSystem resource is valid as a dependency.

The nuances of fields and parameters in GMAT are explored in more detail in the script language documentation, and the individual parameters are documented along with the resources their parent resources. The GUI, however, has a parameter selection interface that is common to all parameters. This interface is documented in GUI, below.
 
See Also: Script Language, FuelTank, ImpulsiveBurn, Spacecraft

GUI 

Parameters can be used as input in several places throughout GMAT, such as the ReportFile and XYPlot resources and the If/Else, Propagate, and Report commands. In the GUI, all of these use a common interface called the ParameterSelectDialog that allows for interactive parameter selection. A basic ParameterSelectDialog window looks like the following:

The ParameterSelectDialog window is used to build a parameter, along with any dependencies, for use in a command or resource. Some resources and commands have different requirements for the types of parameters that can be used, so the ParameterSelectDialog can take slightly different forms, depending on where it's used. This section will describe the generic interface, then mention any resource- or command-specific exceptions.

General Usage

The first step in choosing a parameter is to select the object (or resource) type from the Object Type list in the upper right. Five types can appear in this list: Spacecraft, ImpulsiveBurn, Variable, Array, and String.

Once you've selected a type, The Object List box is populated with all existing resources of that type. Use this list to choose the specific resource you'd like to reference.

If the Spacecraft type is selected, the Attached Hardware List appears below the Object List. This list displays any hardware (such as FuelTank resources) attached to the selected Spacecraft. If the Array type is selected, Row and Col boxes appear. Use these to specify a row and column to select an individual array element, or check Select Entire Object to choose the entire array.

Once a resource is selected, the Object Properties list is populated with all available parameters provided by that resource. Some resources, such as instances of Variable or Array, are themselves parameters, so this list remains empty.

Parameters with different dependency types are commingled in the Object Properties list. When you select one, the appropriate dependency (if any) appears below the list. For example, after selecting the Spacecraft AOP parameter, a Coordinate System list appears. After selecting the Spacecraft Apoapsis parameter, a Central Body list appears. And after selecting the Spacecraft Cd parameter, no dependency list appears.

To select a parameter, select the appropriate Object Type, the specific resource from the Object List or Attached Hardware List, the desired parameter from the Object Properties list, and the required dependency, and add it to the Selected Value(s) list on the right. There are six buttons available to control this list:

• UP: Move the selected item in the Selected Value(s) list up one position (if allowed).
• DN: Move the selected item in the Selected Value(s) list down one position (if allowed).
• ->: Add the selected item in the Object Properties list to the Selected Value(s) list.
• <-: Remove the selected item in the Selected Value(s) list.
• =>: Add all items to the Selected Value(s) list.
• <=: Remove all items from the Selected Value(s) list.

When finished, the Selected Value(s) list contains the final selected parameters. Click OK to accept the selection.

Special Considerations

Some resources and commands (such as the Propagate command Parameter argument) only accept a single parameter as input; in this context the ParameterSelectDialog only allows one parameter in the Selected Value(s) list and does not allow use of the UP, DN, and => buttons.

In some instances (such as in the Vary command), only parameters that are also fields (and so can be set in the Mission Sequence) can be used. In this case only the allowed parameters will be shown in the Object Properties list.

In the Propagate command Parameter argument, only parameters of Spacecraft can be used. In this case only Spacecraft will be shown in the Object Type list.

Parameters

Spacecraft

ImpulsiveBurn

FuelTank

Examples 

Describe the example with a short sentence two and include minimal script for example below

Create Spacecraft aSat;
Create ForceModel aForceModel;
Create Propagator aProp;
aProp.FM              = aForceModel;
aProp.Type            = PrinceDormand78;
aProp.InitialStepSize   = 60;
aProp.Accuracy        = 1e-011;
aProp.MinStep         = 0;
aProp.MaxStep         = 86400;
aProp.MaxStepAttempts = 50;
aProp.StopIfAccuracyIsViolated = true;
 
BeginMissionSequence
 
Propagate aProp(aSat) {aSat.ElapsedDays = .2};

Test Procedures

Assumptions 

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Existing Tests

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Recommended Additional Tests 

Nominal Tests