Introduction to Autosequences

STK Premium (Space) or STK Enterprise
You can obtain the necessary licenses for this tutorial by contacting AGI Support at support@agi.com or 1-800-924-7244.

The results of the tutorial may vary depending on the user settings and data enabled (online operations, terrain server, dynamic Earth data, etc.). It is acceptable to have different results.

Capabilities Covered

This lesson covers the following STK Capabilities:

  • STK Pro
  • Astrogator

Problem Statement

You have a satellite in a near-circular, equatorial orbit. You want to raise the apogee, and then the perigee.

Solution

Use Automatic Sequences, available with STK's Astrogator capability, to model the satellite and raise its apogee and perigee. You will perform a 100 m/sec burn at every perigee until apogee altitude is greater than 10,000 km. After that, a 100 m/sec burn is performed along the velocity vector at apogee until the perigee altitude is greater than 10,000 km.

What You Will Learn

Upon completion of this tutorial, you will be able to:

  • Open and design a simple Automatic Sequence
  • Set UserDefined stopping conditions
  • Create and generate custom graphs

Video Guidance

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

Model the World!

Before you can begin with any analysis, you need to create a scenario and add a satellite. Let's do that now.

  1. Create a new scenario and call it PulsedOrbitRaise.
  2. Set the analysis time to the following:
    3. Option Value
    Analysis Start Time Default Analysis Time
    Analysis End Time +2 Days
  3. Insert a satellite () using the Define Properties () method.
  4. Change the name to TransferSat.

Configure the Initial State

The initial properties of the satellite are known so we use them in the Initial State segment.

  1. Set the Propagator to Astrogator.
  2. Select the Basic - Orbit page.
  3. Select the Initial State segment.
  4. Set the following values for the Initial State:
    5. Option Value
    Coordinate Type Keplerian
    Semimajor Axis 7000 km
    Eccentricity 0.001
    Inclination 0
    RAAN 0
    Argument of Periapsis 0
    True Anomaly 0
  5. Click Apply .

Create a Simple Autosequence

Autosequences are Mission Control Sequence (MCS) elements that are structurally similar to Sequence segments, but are not MCS segments, properly. Rather, autosequences can be assigned to Propagate and Maneuver (finite) segments, and function as subroutines by executing in response to specified stopping conditions of those segments.

  1. Click the Automatic Sequence Browser () button to create a new autosequence.
  2. Click the New button to create a new sequence called SmallBurn.

    3. You will notice that the Stop sequence is the only one available by default. It is called any time a Stopping Condition in the Propagator segment is matched. You'd like to create a new sequence and let it automatically run when well defined circumstances occur.

  3. Double-click SmallBurn to edit it. A new, blank MCS designer appears.
  4. Click the Insert Segment After button ().
  5. Click the Maneuver () segment type.
  6. Set the Attitude Control type to Thrust Vector.
  7. Set the X Velocity to 100 m/sec in the VNC(Earth) Reference Frame.
  8. Click OK to exit the Automatic Sequence Properties.
  9. Click OK to exit the Automatic Sequence Browser.

Raise Apogee Using the Created Sequence

In the satellite's Mission Control Sequence (MCS), you should see two items listed, an Initial State and a Propagate segment. We'll now begin modifying that Propagate segment.

  1. Rename the default propagation segment to RaiseApogee.
  2. Ensure the Trip value for the Duration Stopping Condition is set to 0.5 days.
  3. Add Periapsis as a new Stopping Condition type.
  4. On the Sequence field, click the ... button to change the Stop sequence to SmallBurn.
  5. Set the Max Trip Times value to two (2).

When the Periapsis stopping condition is met, instead of stopping the propagation and passing the control to the next segment, the Periapsis condition automatically starts the SmallBurn sequence. After the maneuver is executed, the control is passed back to the RaiseApogee segment that propagates the orbit to the next stopping condition, which again is Periapsis. The maneuver is executed for the second and last time, since the Max Trip Times field is set to two (2). The control is passed back to the propagate sequence for the remainder of the propagation duration.

Since the Periapsis condition has been met twice, the Propagate segment does not stop the propagation at perigee anymore; instead, it is stopped when the Duration is met. In this case, the Duration we set was a half day.

Run the Mission Control Sequence

  1. Run () the MCS.
  2. Clear () the Graphics.

Maneuver Summary

Let's verify the results visually through a Maneuver Summary report.

  1. Select TransferSat () in the Object Browser.
  2. Open the Report & Graph Manager ().
  3. In the Styles list, select the Maneuver Summary report.
  4. Click Generate... .

    5. The Maneuver Summary is a great place to verify the MCS run when there are multiple burns, especially if they are executed from within autosequences. The segment column is useful to see which segment fired the maneuver and the Total Fuel Used is useful for fuel budget information.

  5. Leave the Maneuver Summary report open.

Add a Constraint for the Apogee Radius

Instead of limiting the number of SmallBurn maneuvers to execute, we can allow Astrogator to continue the RaiseApogee segment until a certain threshold is met. We can do this by setting a constraint to check when we reach an apogee greater than 10,000 km.

  1. Open TransferSat's () properties.
  2. Select the RaiseApogee propagation segment.
  3. From the list of Stopping Conditions locate the Periapsis Stopping Condition.
  4. Set the Max Trip Times field to 10000 (the original default value).
  5. Delete () the Duration Stopping Condition.

Add a Stopping Condition

  1. Add a new Stopping Condition called Apoapsis.

    2. We need to stop the sequence just when the apoapsis radius is greater than 10000 km, so we need to add a constraint.

  2. With the Apoapsis stopping condition selected, click the ellipsis button in the Constraints section.
  3. Move () the UserDefined constraint.
  4. Under Component Details, double-click on the field beside the Criteria option.
  5. Set the Criteria to Greater Than.
  6. Beside the CalcObject field, double-click on the field to open the Embedded Component Link Selection window.
    1. In the Embedded Component Link Selection window, set the Component Name field by clicking Change... .
    2. Expand the Keplerian Elems category from the list of possible CalcObjects and select Radius of Apoapsis.
    3. Click OK .
  7. Click OK to close the window.
  8. Bring the Select Constraints window to the front and change the ComponentName field to Apoapsis GT 10000.
  9. Set the Value field to 10000 km.
  10. Your settings should look like the following:
    11. Option Value
    Component Name Apoapsis_GT_10000
    Criteria Greater Than
    CalcObject Radius Of Apoapsis
    Use Absolute Value false
    Value 10000 km
  11. Click OK .

Run the Mission Control Sequence

  1. Run () the MCS.
  2. Clear () the Graphics.
  3. Click OK to close the properties panel.
  4. Refresh the Maneuver Summary report.

Create a New Graph

You can create a new graph style that will graph the apogee and perigee on one graph.

  1. Open the Report & Graph Manager ().
  2. Create a New Graph.
  3. Name the new graph Apo Peri Graph.
  4. Move the Classical Elements - ICRF Apogee Radius to the Y Axis.
  5. Click OK .
  6. Generate the graph.

In summary, the RaiseApogee segment propagates until:

  • An apogee is reached. If the apogee radius is greater than 10,000 km, control is passed onto the next segment in the MCS. Otherwise nothing happens and the control is sent back the propagator.
  • A perigee is reached. In this case, the autosequence is fired and the control is sent back to the propagator.

Raise the Perigee (with Constraint)

Now you can raise the perigee up to a minimum radius of 10,000 km. To accomplish this, you need to create a new propagation segment with similar logic to our RaiseApogee segment.

  1. Open TransferSat's () properties.
  2. Add a new Propagate () segment.
  3. Name the new segment, RaisePerigee and set the color to red.
  4. Add a new Apoapsis stopping condition.

    5. To raise the perigee, you need to burn at apogee. You can use the SmallBurn sequence previously defined as the triggered event.

  5. Set the Sequence to Small Burn for the Apoapsis stopping condition.
  6. Delete () the Duration Stopping Condition.
  7. Add a new Periapsis stopping condition.

    8. However, you'd like to stop the propagation only when the perigee radius is greater than 10,000 km. We can add that as a constraint.

  8. Click the ellipsis button beside the Constraints field of the Periapsis stopping condition.
  9. Add a UserDefined constraint and set the following options:
    10. Option Value
    Component Name Periapsis_GT_10000
    Criteria Greater Than
    CalcObject Radius Of Periapsis
    Use Absolute Value false
    Value 10000 km

Run the Mission Control Sequence

  1. Run () the MCS.
  2. Refresh the Maneuver Summary report.

Add Perigee Radius to the Custom Graph

  1. Open the Report & Graph Manager.
  2. Open the Properties for the custom Apo Peri Radius Graph.
  3. Expand the Classical Elements option.
  4. Expand the ICRF option.
  5. Move Perigee Radius to the Y axis.
  6. Generate the Apo Peri Radius Graph.