Targeting Examples

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This page presents several examples demonstrating key features of Targeting in FreeFlyer:

 

Example 1: Target Semi-Major Axis

Example 2: Target Orbital Period using Customized DifferentialCorrector

Example 3: Target Semi-Major Axis and Eccentricity

Example 4: Target using ResetTargetingLoop Command

Example 5: RuntimeSetup Targeting

 

For more background information, see the Targeting page.

 

Note: As of FreeFlyer 7.3, the default timing precision mode is nanosecond precision mode. For older Mission Plans that have not yet been converted from millisecond precision mode, the syntax for working with epochs and calendar date/time strings is different. See the timing precision mode page for more information.

 

 

Example 1: Target Semi-Major Axis


A targeting loop is used to determine the duration of a finite burn necessary to reach a semi-major axis of 8000 km, with an error tolerance of 50 km.

 

FiniteBurn FiniteBurn1;

 

Target;
      Iterate Spacecraft1;

      Vary FiniteBurn1.BurnDuration = TIMESPAN(30 seconds) + TIMESPAN(2 secondswithin <TIMESPAN(24 seconds), TIMESPAN(36 seconds)>;
      Maneuver Spacecraft1 using FiniteBurn1;
      Achieve Spacecraft1.A = 8000 +/- 50;
End;

 

Where:

30 is the seed value for FiniteBurn1.BurnDuration

2 is the perturbation for FiniteBurn1.BurnDuration

24 is the minimum for FiniteBurn1.BurnDuration

36 is the maximum for FiniteBurn1.BurnDuration

8000 is the goal value for Spacecraft1.A

50 is the allowable tolerance value for Spacecraft1.A

 

 

Example 2: Target Orbital Period using Customized DifferentialCorrector


A DifferentialCorrector object, with a maximum of ten iterations, is used to target the x-component of an impulsive burn required to achieve an orbital period of 110 minutes. The DifferentialCorrector object provides access to the status of the targeting process, enabling you to report properties such as "Iterations" and "Iteration Mode" to the output screen.

 

DifferentialCorrector DifferentialCorrector1;

ImpulsiveBurn ImpulsiveBurn1;

 

DifferentialCorrector1.MaxIterations = 10;

 

Target using DifferentialCorrector1;

      Iterate Spacecraft1;

      Vary ImpulsiveBurn1.BurnDirection[0] = 0.1 + 0.001;

      Maneuver Spacecraft1 using ImpulsiveBurn1;

      Step Spacecraft1;

      Achieve Spacecraft1.Period = 110 +/- 0.001;

      Report DifferentialCorrector1.Iterations, DifferentialCorrector1.IterationMode, ImpulsiveBurn1.BurnDirection[0], Spacecraft1.Period;

End;

 

Output:

 

Iterations

IterationMode

Burn X-Component

Period

0.000000000

Nominal

0.100000000

 99.933751603

0.000000000

Perturbation

0.101000000

 99.944445593

1.000000000

Nominal

1.041299526

113.801624904

1.000000000

Perturbation

1.042299526

113.821178065

2.000000000

Nominal

0.846874441

110.223565758

2.000000000

Perturbation

0.847874441

110.240876468

3.000000000

Nominal

0.833959560

110.000972154

3.000000000

Converged

0.833959560

110.000972154

 

 

Example 3: Target Semi-Major Axis and Eccentricity


This 2x2 Targeting example determines the burn duration and maneuver start time that will result in a semi-major axis of 7300 km while maintaining an eccentricity of 0. Following the maneuver, the Spacecraft is propagated for 0.5 days.

 

Spacecraft1.A    = 7100.0904823510;

Spacecraft1.E    = 0.00094973584523096;  

Spacecraft1.I    = 0.99755409493606;

Spacecraft1.RAAN = 298.41050399768;

Spacecraft1.W    = 112.90849010369;

Spacecraft1.TA   = 290.38727083823;

 

Spacecraft1.AddTank("tank1""SphericalTank");

Spacecraft1.AddThruster("thruster1");

Spacecraft1.Thrusters[0].AddTankToValve(0, Spacecraft1.Tanks[0]);

 

FiniteBurn FiniteBurn1;

 

Target;

      Iterate Spacecraft1;

      Vary FiniteBurn1.BurnDuration = TIMESPAN(30 seconds) + TIMESPAN(10 seconds);

      Vary FiniteBurn1.ManeuverStartTime = TIMESPAN(10 seconds) + TIMESPAN(1 secondswithin <TIMESPAN(0 seconds), TIMESPAN(1 days)>;

 

      Maneuver Spacecraft1 using FiniteBurn1 generating "BurnFile.txt";

 

      Achieve Spacecraft1.A = 7300 +/- 5;

      Achieve Spacecraft1.E = 0 +/- 0.1;

      Report FiniteBurn1.BurnDuration.ToSeconds(), FiniteBurn1.ManeuverStartTime, Spacecraft1.A, Spacecraft1.E;

      Plot FiniteBurn1.BurnDuration.ToSeconds(), Spacecraft1.A, Spacecraft1.E;

      Plot FiniteBurn1.ManeuverStartTime, Spacecraft1.A, Spacecraft1.E;

End;

 

Output: Excerpt from "BurnFile.txt"

 

Maneuver for spacecraft Spacecraft1

 

Finite maneuver start epoch:

  Modified Julian: 28849.635549178

  Calendar:        Jan 01 2020 03:15:11.449

                   20200101 03:15:11.449

 

Finite maneuver end epoch:

  Modified Julian: 28849.692357536

  Calendar:        Jan 01 2020 04:36:59.691

                   20200101 04:36:59.691

 

Maneuver Duration:   4908.2420900580 seconds

 

Premaneuver Keplerian State (ICRF):

    A      7100.1790031293 km

    E      0.00074475030402244  

    I      0.99619300838106 deg

 RAAN      297.34415545127 deg

   AP      108.71160088047 deg

   TA      285.64750365563 deg

 

Postmaneuver Keplerian State (ICRF):

    A      7301.2185335398 km

    E      0.0066203579216475  

    I      0.99582346870238 deg

 RAAN      296.82788286011 deg

   AP      193.15669142569 deg

   TA      133.77544219585 deg

 

 

Example 4: Target using ResetTargetingLoop Command


This Mission Plan demonstrates the use of the ResetTargetingLoop command in FreeFlyer. The Target command is used to achieve a certain orbit apogee by varying the impulsive delta V, but the seed and perturbation fed into the Targeter are set too high.

 

If an unrealistically high delta V is attempted, FreeFlyer will reset the Targeting loop with new, lower, seeds.

 

ImpulsiveBurn ImpulsiveBurn1;

Variable seed = 100;

Variable perturbation = 10;

 

Target;

      Iterate Spacecraft1;

      Vary ImpulsiveBurn1.BurnDirection[0] = seed + perturbation;

 

      Maneuver Spacecraft1 using ImpulsiveBurn1;

      Achieve Spacecraft1.Apogee = 8000 +/- 10;

 

      Report ImpulsiveBurn1.BurnDirection[0], seed, perturbation, Spacecraft1.Apogee;

 

      If (ImpulsiveBurn1.BurnDirection[0] > 1000);

 

            // Decrease the seed and perturbation by a factor of 10

            seed = seed/10;

            perturbation = perturbation/10;

 

            // Reset the Targeter with the new seed and perturbation

            ResetTargetingLoop;

 

      End;

End;

 

Where:

seed is a Variable containing the seed value for ImpulsiveBurn1.BurnDirection[0]

perturbation is a Variable containing the perturbation for ImpulsiveBurn1.BurnDirection[0]

8000 is the goal value for Spacecraft1.Apogee

10 is the allowable tolerance value for Spacecraft1.Apogee

 

 

Example 5: RuntimeSetup Targeting


In this example, a For loop is used to loop over arrays storing information about the maneuvers to apply and the values to achieve. In an operational satellite ground system, this maneuver information could be read into FreeFlyer from a database or other external data source. Impulsive burns are used to achieve a semi-major axis of 8500 km and an inclination of 35 degrees.

 

FreeFlyer contains two separate timing precision modes, and the Targeter behaves slightly differently in each timing precision mode. As of FreeFlyer 7.3, the default timing precision mode is nanosecond precision mode. In millisecond precision mode, the Target command keywords "with RuntimeSetup" must be used in order to call Iterate, Vary, and Achieve commands inside loops (If, While, For, etc).

 

List<ImpulsiveBurn> Burns;

DifferentialCorrector differentialCorrector1;

TimeSpanArray eventEpochs[0];

Array eventTypes[0];

Array eventValues[0];

Variable i;

Variable j;

 

Spacecraft1.A = 8000;

Spacecraft1.I = 30;

 

eventEpochs = {"Jan 01 2020 00:00:00.000000000".ParseCalendarDate(),  // Maneuver

                            "Jan 01 2020 04:00:00.000000000".ParseCalendarDate(),  // SMA

                            "Jan 02 2020 00:00:00.000000000".ParseCalendarDate(),  // Maneuver 

                            "Jan 02 2020 12:00:00.000000000".ParseCalendarDate()}; // Inc

 

eventTypes = {-1,       // Maneuver

                            1,       // SMA

                          -1,       // Maneuver

                            2};      // Inc

 

eventValues = {1,       // Maneuver

                            8500,    // SMA

                            2,       // Maneuver

                            35};     // Inc

 

Target using differentialCorrector1;

 

      Iterate Spacecraft1;

      j = 0;

 

      For i = 0 to eventEpochs.Dimension-1;

            

            If (eventTypes[i] == -1);  

                  If (differentialCorrector1.IterationMode == "Nominal" and differentialCorrector1.Iterations == 0);

                        Burns.Count++;

                  End;

                  Burns[j].AttitudeSystem = 2;

                  Burns[j].BurnDirection[0] = 0; 

                  If (eventValues[i] == 1);  // X direction

                        Vary Burns[j].BurnDirection[0] = 0.1 + 0.01 within <-0.5, 0.5>;

                  ElseIf (eventValues[i] == 2);  // Y direction

                        Vary Burns[j].BurnDirection[1] = 0.1 + 0.01 within <-2, 2>;

                  End;

                  Step Spacecraft1 to (Spacecraft1.Epoch == eventEpochs[i]);

                  Maneuver Spacecraft1 using Burns[j];

                  j++;

 

            ElseIf (eventTypes[i] == 1); 

                  Step Spacecraft1 to (Spacecraft1.Epoch == eventEpochs[i]);

                  Achieve Spacecraft1.A = eventValues[i] +/- 1;

 

            ElseIf (eventTypes[i] == 2); 

                  Step Spacecraft1 to (Spacecraft1.Epoch  == eventEpochs[i]);

                  Achieve Spacecraft1.I = eventValues[i] +/- 0.1;

            End;

      End;

      Report differentialCorrector1.IterationMode, Burns[0].BurnDirection, Burns[1].BurnDirection, Spacecraft1.A, Spacecraft1.I;

End;

 

 

See Also


Targeting Guide

Target Command

Iterate Command

Vary Command

Achieve Command

Maneuver Command

ResetTargetingLoop Command

Properties and Methods with a State

Maneuvering Guide