Part 18: Design Trajectories with Astrogator

This training requires additional licenses to complete. You can obtain the necessary license for the training by visiting http://licensing.agi.com/stk/evaluation or calling AGI support.

Astrogator

STK Astrogator is a specialized analysis module for interactive orbit maneuver and spacecraft trajectory design. It supports an unlimited series of events for modeling and targeting a spacecraft's trajectory, including impulsive and finite burns and high-fidelty orbit propagation, while providing the ability to target specified and optimized orbit states that reference customizable control and result parameters.

STK Astrogator was used to plan a series of maneuvers to get NASA's LADEE spacecraft to the Moon. Learn more in the case study: AGI Software for NASA Ladee Flight Dynamics System.

A Mission Control Sequence (MCS) - defines the trajectory as a sequence of events ("segments"), functioning as a graphical programming language in which each segment dictates how Astrogator calculates the trajectory before passing the spacecraft's state on the next segment. Some of the MCS segments available include:

  • Initial State () - This segment can be used to define the initial conditions of your MCS, or of a subsequence within the MCS.
  • Launch () - This segment can be used to model a simple spacecraft launch from Earth or another central body.
  • Follow () - This segment can be used to set the spacecraft to follow another vehicle (Satellite, Launch Vehicle, Missile, Aircraft, Ship, or Ground Vehicle) at a specified offset, and to separate from that vehicle upon meeting specified conditions.
  • Maneuver () - This segment can be used to model a spacecraft maneuver.
  • Propagate () - This segment can be used to model the movement of the spacecraft along its current trajectory until meeting specified stopping conditions


The Component Browser provides access to all available, customizable elements needed to construct a trajectory with Astrogator. This includes MCS segments, propagators, stopping conditions, central bodies, engine models, and more.

Model a Spacecraft Object Using STK Astrogator

Watch the following video, then follow the steps below incorporating the systems and missions you work on (sample inputs provided).

  1. Create a new scenario with a two (2) day analysis interval.Closed
    1. Click the Create a new scenario () button.
    2. In the New Scenario Wizard, set the following options:
      • Enter a name for the scenario (e.g. STK_Astrogator).
      • Define the analysis start and stop times so the stop time is two (2) days after the start time.
    3. Click OK.
  2. Insert a new satellite () with the Astrogator Propagator.Closed
    1. Insert a new satellite () using the Define Properties method.
    2. Open the Properties () of the Satellite ().
    3. Select the Basic - Orbit page, and set the Propagator to Astrogator.
  3. Set the Satellite's Initial State in the MCS to Keplerian with a 0.015 Eccentricity.Closed
    1. Select the Initial State in the Mission Control Sequence (MCS).
    2. Select Keplerian from the Coordinate Type drop-down list.
    3. Enter 0.015 in the Eccentricity text field.
  4. Create a new Periapsis stopping condition for the existing propagate () segment and remove the duration stopping condition.Closed
    1. Create a new Periapsis stopping condition.Closed
      1. Select the existing propagate () segment in the MCS.
      2. Click the New... icon () in the Stopping Conditions section on the right or right-click on the existing Duration stopping condition and select New... ().
      3. Select Periapsis in the New stopping condition window and click OK.
    2. Remove the Duration stopping conditionClosed
      1. Select the Duration stopping condition in the Stopping Conditions section on the right.
      2. Click the Delete icon () or right-click on the Duration stopping condition and select Delete ().
    3. Right-click on the propagate segment in the MCS, select Rename, and rename it PropToPeriapsis.

    You can set multiple stopping conditions for a propagate segment. Astrogator stops propagating the satellite when it meets one of the stopping conditions.

  5. Click Run Entire Mission Control Sequence () on the MCS toolbar to run the MCS.
  6. Insert a new maneuver segment() after the first propagate segment () with a Delta V Magnitude of 1,000 m/sec along the Velocity Vector.Closed
    1. Click on the first propagate segment to highlight it and click Insert Segment () on the MCS toolbar or right-click on the first propagate segment and select Insert After... from the menu.
    2. Select Maneuver in the Segment Selection window and click OK.
    3. Set the Delta V Magnitude to 1,000 m/sec.
    4. Keep the default for the Attitude Control as Along Velocity Vector option.

    There are two different types of Maneuvers available, impulsive and finite. The Impulsive maneuver calculates a state by adding the defined delta-v vector to the velocity of the final state of the previous segment. The finite maneuver is basically a propagate segment with thrust.

  7. Insert a new apoapsis propagate segment () named PropToApoapsis after the maneuver segment ().Closed
    1. Insert a new Propagate segment in the MCS.Closed
      1. Click on the Maneuver segment to highlight it and click Insert Segment () on the MCS toolbar or right-click on the Maneuver segment and select Insert After... from the menu.
      2. Select Propagate in the Segment Selection window and click OK.
    2. Create a new Apoapsis stopping condition.Closed
      1. Select the new Propagate () segment in the MCS.
      2. Click the New icon () in the Stopping Conditions section or right-click the existing Duration stopping condition and select New...
      3. Select Apoapsis in the New stopping condition window and click OK.
    3. Remove the Duration Stopping Condition.Closed
      1. Select the Duration stopping condition in the Stopping Conditions section on the right.
      2. Click the Delete icon () or right-click on the Duration stopping condition and select Delete ().
    4. Right-click on the propagate segment in the MCS, select Rename, and rename it PropToApoapsis.
    5. Give each segment a unique color.Closed
      1. Double-click the segment in the MCS or right-click the segment and select Properties...
      2. Select the desired color in the Color drop-down menu from the Edit Segment window.
      3. Click OK to dismiss the Edit Segment window.
  8. Click Run Entire Mission Control Sequence () on the MCS toolbar to run the MCS.

Target Sequences

Target Sequences () are used to calculate and subsequently define the required maneuver characteristics necessary to meet specified or optimal mission parameters. Once a target sequence is inserted into the MCS, it is defined in three steps:

  • Insert the segments that define the controls or results of the targeting calculation inside the target sequence.
  • Select the target sequence and define one or more profiles. These target profiles set the type of search algorithm that is used or could alter the properties of the targeted segments to affect the course of the MCS run without halting the trajectory creation.
  • Configure the target sequence by accessing the properties of each profile.

A target sequence runs the segments nested within it and applies profiles to the run according to its configuration. When applying a search profile such as a Differential Corrector, the target sequence adjusts the targeted values over multiple iterations in an attempt to converge at a solution within the defined tolerance. The results of a target sequence are applied to the MCS to produce a trajectory that meets the goals you need to achieve.

Use a Target Sequence to Raise Your Orbit

Watch the following video, then follow the steps below incorporating the systems and missions you work on (sample inputs provided).

  1. Add a target sequence (), Raise Orbit, after the PropToApoapsis segment ().Closed
    1. Click on the PropToApoapsis segment to select it.
    2. Click Insert Segment () on the MCS toolbar or right-click PropToApoapsis and select Insert After... from the menu.
    3. Select Target Sequence in the Segment Selection window and click OK.
    4. Right-click on theTarget Sequence in the MCS, select Rename, and rename it RaiseOrbit.
  2. Add the maneuver and PropToApoapsis segments to the new target sequence.Closed
    1. Drag and drop the Maneuver () segment into the Target Sequence to nest it.
    2. Drag and drop the PropToApoapsis segment into the Target Sequence after the Maneuver to nest it.
  3. Select the maneuver's Delta V Magnitude as a Control Parameter.Closed
    1. Click on the Maneuver segment within the Target Sequence in the MCS.
    2. Click the Target icon () to the right of the Delta V Magnitude to make it the independent variable. The selection is confirmed by the appearance of a check mark over the icon ().
  4. Add the Altitude of Apoapsis as a new Result for the PropToApoapsis segment.Closed
    1. Click on the PropToApoapsis segment within the Target Sequence in the MCS.
    2. Click Results... below the MCS to bring up the User-Selected Results window.
    3. Expand () Keplerian Elems and select Altitude of Apoapsis.
    4. Click Insert Component () to select it as the dependent variable.
    5. Click OK to close the User-Selected Results window.
  5. Configure the Target Sequence.Closed
    1. Access the Differential Corrector profile's properties for the Target Sequence.Closed
      1. Select the Target Sequence, RaiseOrbit, in the MCS.
      2. In the Profiles section on the right, click the Properties icon () to open the Variables page or right-clicking the Differential Corrector and select Properties... from the menu.
    2. Set up the Targeter.Closed
      1. Select the Use option in the Control Parameters and Equality Constraints (Results) sections.
      2. Set the Desired Value for the Altitude of Apoapsis to 7500 km.
      3. Click OK to dismiss the Differential Corrector Properties window.
      4. Change the Action to Run Active Profiles in the drop-down list.
  6. Click Run Entire Mission Control Sequence () on the MCS toolbar to run the MCS.

    7. Once you have applied the changes you can set the Action to Run Nominal Sequence. This runs the mission control sequence using the results of the Targeter.

  7. Click Apply Changes in the Profiles and Corrections section of the parameters to apply the current values.
  8. Click Run Entire Mission Control Sequence () on the MCS toolbar to run the MCS to look at the final trajectory design.

Use a Target Sequence to Raise the Altitude at Periapsis

Watch the following video, then follow the steps below incorporating the systems and missions you work on (sample inputs provided).

  1. Add a target sequence () named RaiseAltitude after the RaiseOrbit target sequence ().Closed
    1. Click on the RaiseOrbit segment to highlight it and click Insert Segment () on the MCS toolbar or right-click on RaiseOrbit segment and select Insert After... from the menu.
    2. Select Target Sequence in the Segment Selection window and click OK.
    3. Right-click on the new Target Sequence in the MCS, select Rename, and rename it RaiseAltitude.
  2. Insert a new maneuver segment () within the RaiseAltitude target sequence ().Closed
    1. Click on the target sequence's Return Segment () to select it and click Insert Segment () or right-click on the target sequence's return segment and select Insert After... from the menu.
    2. Select Maneuver in the Segment Selection window and click OK. Keep the default Delta V Magnitude and Attitude Control.
  3. Select the maneuver's Delta V Magnitude as a Control Parameter.Closed
    1. Click on the Maneuver segment within the Target sequence in the MCS.
    2. Click the Target icon () to the right of the Delta V Magnitude to make it the independent variable. The selection is confirmed by the appearance of a check mark over the icon ().
  4. Insert a new periapsis propagate segment () named PropToPeriapsis after the target segment's maneuver segment ().Closed
    1. Insert a new Propagate Segment in the RaiseAltitude target sequence.Closed
      1. Click on the Maneuver segment to highlight it and click Insert Segment () on the MCS toolbar or right-click on the Maneuver segment and select Insert After... from the menu.
      2. Select Propagate in the Segment Selection window and click OK.
    2. Create a new Periapsis stopping condition.Closed
      1. Select the new Propagate () segment in the MCS.
      2. Click on the New icon () in the Stopping Conditions section or right-click on the existing Duration stopping condition and select New...
      3. Select Periapsis in the New Stopping Condition window and click OK.
    3. Remove the Duration stopping condition.Closed
      1. Select the Duration stopping condition in the Stopping Condition section on the right.
      2. Click the Delete Segment icon () or right-click on the Duration stopping condition and select Delete.
    4. Right-click on the new propagate segment in the MCS, select Rename, and rename it PropToPeriapsis.
  5. Add the Altitude of Periapsis as a new Result for the PropToPeriapsis segment.Closed
    1. Click on the PropToPeriapsis segment within the Target Sequence in the MCS.
    2. Click Results... below the MCS to bring up the User-Selected Results window.
    3. Expand () Keplerian Elems and select Altitude of Periapsis.
    4. Click Insert Component () to select it as the dependent variable.
    5. Click OK to close the User-Selected Results window.
  6. Give each segment a unique color.Closed
    1. Double-click the segment in the MCS or right-click the segment and select Properties...
    2. Select the desired color in the Color drop-down menu from the Edit Segment window.
    3. Click OK to dismiss the Edit Segment window.
  7. Configure the Target Sequence.Closed
    1. Access the Differential Corrector profile's properties for the Target Sequence.Closed
      1. Select the Target Sequence, RaiseAltitude, in the MCS.
      2. Select the Default Profile (Differential Corrector).
      3. In the Profiles section on the right, click the Properties icon () to open the Variables page or right-click the Differential Corrector and select Properties... from the menu.
    2. Set up the Targeter.Closed
      1. Select the Use option in the Control Parameters and Equality Constraints (Results) option.
      2. Set the Desired Value for the Altitude of Periapsis to 3000 km.
      3. Click OK to dismiss the Differential Corrector Properties window.
      4. Change the Action to Run Active Profiles in the drop-down list.
  8. Click Run Entire Mission Control Sequence () on the MCS toolbar to run the MCS.

    9. By default, the 3D Graphics window displays the iterations of a search profile during an MCS run before it converges on a final solution.

  9. Click Apply Changes in the Profiles and Corrections section of the parameters to apply the current values.

    10. Once you have applied the changes you can set the Action to Run Nominal Sequence. This runs the mission control sequence using the results of the Targeter.

  10. Click Run Entire Mission Control Sequence () on the MCS toolbar to run the MCS to look at the final trajectory design.
  11. Add a final propagate segment at the end of the MCS that ends after a half day.Closed
    1. Insert a new Propagate segment in the MCS.Closed
      1. Click on the RaiseAltitude target sequence in the MCS and click Insert Segment ()on the MCS toolbaror right-click on the RaiseAltitude target sequence and select Insert After... from the menu.
      2. Select Propagate in the Segment Selection window and click OK. The default Duration stopping condition is a half day (43200 sec).
    2. Click Run Entire Mission Control Sequence () on the MCS toolbar to run the MCS.
  12. Generate a Maneuver Summary report of the mission.Closed
    1. Right-click on the Satellite () in the Object Browser.
    2. Select the Report & Graph Manager ().
    3. Expand () the Installed Styles directory and select the Maneuver Summary report.
    4. Click Generate...

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