Script Aliases

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Script Aliases in FreeFlyer allow you to create a simple abbreviation for more complex expressions in FreeForm script.

 

The following Sample Mission Plans (included with your FreeFlyer installation) demonstrate the use of the Script Aliases:

 

FreeFlyer Scripting Samples

Script Aliases

 

An Alias can refer to any of the following types of expressions:

 

Objects

Child Objects

Child Objects with type casting

Object Properties

Object Methods with set input arguments

Math expressions

 

Alias s = Spacecraft1;                 // Object

Alias g = GroundStation1;             // Object

 

Alias p1 = s.Propagator;               // Child Object

Alias p2 = (s.Propagator AsType RK89); // Child Object with type casting

 

Alias d = Spacecraft1.ElapsedTime;     // Object Property

Alias c = s.Contact(g);               // Object Method with set input argument

 

Alias e = (1 - Spacecraft1.Apogee/Spacecraft1.A);   // Math expression

 

Once the Alias has been created, it can be used anywhere in FreeFlyer script in place of the expression it was initialized with:

 

p1.StepSize = TIMESPAN(60 seconds); // The StepSize property is available to all Propagator types

p2.FixedStep = 1;                   // The FixedStep boolean property is only available to some Propagator types, such as the RK89

 

 

// Propagation loop:  View, Plot, and Step for three days

While (d < TIMESPAN(3 days));

 

     View s, g;

     Plot d, c;

     Step s;

 

End;

 

Aliases will behave just as if the expressions they refer to have been substituted directly into the FreeFlyer script. For example, Aliases that refer to properties or methods with a state are initialized separately for each time the Alias is used.

 

While (d < TIMESPAN(1 days));

     While (d < TIMESPAN(1 days));   // Each "d" is a separate instance of Spacecraft1.ElapsedTime

          View s;

          Step s;

     End;

End;

 

 

Scoping


Script Aliases can be created within loops, such as a While, For, or If loop, but an Alias created in a loop cannot be used outside the loop. In the following example, the Alias "a" can be used inside the While loop in order to generate a Plot, but if you attempt to Report the Alias "a" outside the While loop, a syntax error will be generated.

 

While (d < 3);

 

    Alias a = Spacecraft1.A;

    Plot d, a;

    Step s;

   

End;

 

Report a; // Syntax error

 

 

Limitations


Script Aliases cannot refer to expressions containing any FreeFlyer commands.

 

// The following is NOT valid:

 

Alias f = Step Spacecraft1;

 

Script Aliases can be passed into Procedures, but they cannot be used to Define a Procedure.

 

// The following is NOT valid:

 

Define Procedure AliasProcedure(Alias s);

 

// ...

 

EndProcedure;

 

// This is valid, but the user must ensure that the Alias "s" will

// deliver the input type expected by the Procedure.

 

Define Procedure AliasProcedure(Spacecraft sc);

 

// ...

 

EndProcedure;

 

Call AliasProcedure(s);   // "s" is an Alias that refers to "Spacecraft1"