Introduction to Satellite Maneuver Planning with Astrogator

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 Ansys Systems Tool Kit (STK®) digital mission engineering software capabilities:

  • STK Pro
  • Astrogator

Problem statement

Engineers and mission planners working for an imaging satellite company are planning to launch a new satellite. Having already identified an appropriate launch window, they need to determine what would be required to circularize the orbit of the satellite once it separates from the launch vehicle.

You have some information that may be helpful. Here’s what you know:

  • The desired orbit of the satellite is circular and at a 7,000-km radius to the Earth.
  • They would like to see the orbit propagated for three days.
  • The satellite should begin to separate from the launch vehicle at 2,300 epoch seconds.
  • The solar panels and antenna should automatically deploy by 2,550 and 2,650 epoch seconds, respectively.

Solution

Use the STK/Astrogator® capability to launch the spacecraft into the proper orbit, at the proper time, for the proper duration.

What you will learn

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

  • Attach a Satellite object to a Launch Vehicle object for a specified period of time using a Follow segment.
  • Place the Satellite object into a circular orbit using two impulsive Maneuver segments and two Propagate segments.
  • Generate and evaluate relevant data.
  • Load a new 3D Graphics model and its associated articulation file for realistic visualization in the 3D Graphics window.

Using a starter scenario (*vdf file)

To speed things up and to help you focus on satellite maneuver planning, a partially created starter scenario has been provided for you.

  1. Launch the STK application ().
  2. Click Open a Scenario in the Welcome to STK dialog box.
  3. Browse to <STK install folder>\Data\Resources\stktraining\VDFs.
  4. Select Astrogator_AtlasLaunch.vdf.
  5. Click Open.

Saving a VDF as a Scenario File

When you save a scenario in the STK application, it will save in the format in which it originated. In other words, if you open a VDF, the default save format will be a VDF (.vdf). The same is true for a scenario file (*.sc). If you want to save a VDF as a SC file (or vice-versa), you must change the file format when you are performing the Save As procedure.

  1. Open the File menu.
  2. Select Save As....
  3. Click STK User () on the left side of the Save As dialog box.
  4. Click the New Folder option in the selector bar.
  5. Name the folder Astrogator_IntroToSatelliteManeuverPlanning.
  6. Select the Enter key.
  7. Click Open.
  8. Open the Save as type shortcut menu.
  9. Select Scenario Files (*.sc).
  10. Click Save.

Save () often!

Understanding the starter scenario

Review the starter scenario. It contains only two objects: a Facility object, Vandenberg (), and a Launch Vehicle object, AtlasIIAS (). Vandenberg () models launchpad SLC-3E of Space Launch Complex 3 at Vandenberg Space Force Base (SFB). Its 3D model was imported from the STK Standard Object Database. The shadow on the ground you can see in the Bing Maps imagery is the shadow of the actual launch tower of the SLC-3E launchpad. In addition to the positioning being technically accurate, the scenario also uses a 3D model (atlas-lp.mdl) of the real Atlas launchpad at the site.

In the scenario, AtlasIIAS () is positioned on the launchpad for a southward launch, as it would be for a real launch from the site. It also uses a 3D model that looks similar to an actual Atlas IIAS launch vehicle.

Changing the analysis period

The starter scenario has a one-hour analysis period. Extend the analysis period so that it is long enough to model the three-day duration of the satellite's orbit.

  1. Right-click on Astrogator_AtlasLaunch () in the Object Browser.
  2. Select Properties () in the shortcut menu.
  3. Select the Basic - Time page when the Properties Browser opens.
  4. Enter + 3 days in the Stop field located in the Analysis Period panel.
  5. Click OK to accept your change and to close the Properties Browser.

Inserting a Satellite object

Insert a Satellite object that will model the specifications of your satellite.

  1. Click Insert Object ().
  2. Select Satellite () in the Select An Object To Be Inserted list when the Insert STK Objects tool opens.
  3. Select Insert Default () in the Select A Method list.
  4. Click Insert....
  5. Right-click on Satellite1 () in the Object Browser.
  6. Select Rename in the shortcut menu.
  7. Rename Satellite1 () ImageSat.

Orienting ImageSat

You want to make sure ImageSat is not oriented upside down on AtlasIIAS. Use the ECF velocity alignment with radial constraint attitude profile to ensure the satellite is properly positioned on the launch vehicle. This profile aligns the vehicle's x axis with the Earth-fixed velocity vector direction and constrains the z axis in the radial direction (the direction along the position vector and opposite to geocentric nadir).

  1. Open ImageSat's () Properties ().
  2. Select the Basic - Attitude page.
  3. Open the Type drop-down list located in the Basic panel.
  4. Select ECF velocity alignment with radial constraint.
  5. Click Apply to accept your change and to keep the Properties Browser open.

Understanding the Astrogator capability

The Astrogator capability contains specialized analyses for interactive orbit maneuver and spacecraft trajectory design. The Astrogator capability acts as one of the propagators available for a satellite object. The Astrogator capability calculates the satellite's ephemeris by running a Mission Control Sequence (MCS), which you define according to the requirements of your mission.

Using the Astrogator capability enables you to model impulsive and finite maneuvers as well as high-fidelity orbit propagation. It provides multiple targeting methods, including:

  • A differential corrector used to find the necessary values of control parameters, such as launch epoch or burn duration, to meet desired mission goals
  • An optimizer used to change control parameters to achieve a goal, while applying a set of constraints that define the problem space.

Furthermore, using the Astrogator capability as the propagator allows you to define automatic sequences. These represent predefined sets of actions that can be performed whenever a specified event occurs, such as a maneuver that occurs at every periapsis.

The Astrogator capability also utilizes a component catalog and editor in the STK application called the Component Browser. The Component Browser enables you to define and customize engine models, force models, propagators, central bodies, atmospheric models, and other elements of a space mission analysis scenario. Also, the Component Browser contains a wide array of calculation objects. All these elements can then be used or adapted for any scenario where the Astrogator capability is used. These details highlight just some of the Astrogator capability's many features.

Propagating the satellite using the Astrogator capability

The satellite that you are designing will be used to image ground locations. You want to model the imaging satellite that is being used in this mission. It will be launched out of Vandenberg SFB using the existing Atlas IIAS launch vehicle. You want to circularize the orbit at a 7,000-kilometer radius to the Earth. You want your satellite to be jettisoned from the launch vehicle at the proper time according to the launch profile. Use the Astrogator capability as the satellite propagator to design the satellite to follow the same ephemeris as the launch vehicle for a specified time and then maneuver to a circular orbit.

  1. Select the Basic - Orbit page.
  2. Open the Propagator drop-down list.
  3. Select Astrogator.
  4. Click Apply.

Setting up the Mission Control Sequence

The Mission Control Sequence (MCS) is the core of your space mission scenario. The MCS functions as a graphical programming language, in which mission segments dictate how the Astrogator capability calculates the trajectory of the spacecraft based on the general settings that you specify for the MCS itself.

The MCS is defined by selecting and organizing MCS Segments in a manner that produces your desired trajectory. By default, an Astrogator satellite's MCS contains two segments: an Initial State () segment and a Propagate () segment.

Removing the default segments

You do not need the Initial State and Propagate segments that are provided for you. ImageSat will follow AtlasIIAS first,and then deploy. Delete these two segments so you can start fresh.

  1. Select Initial State () in the MCS.
  2. Click Delete Segment () in the MCS toolbar.
  3. Click Yes to confirm.
  4. Select Propagate () in the MCS.
  5. Click Delete Segment () in the MCS toolbar.
  6. Click Yes to confirm.

Inserting a Follow segment

ImageSat should follow AtlasIIAS for the initial launch segment and then propagate into a circular orbit. Use the Follow segment to set ImageSat to follow AtlasIIAS at a specified offset, and to separate from that launch vehicle upon meeting a specified condition. You can specify the configuration of the spacecraft, the conditions at which the spacecraft joins with the leader vehicle, and the conditions at which the spacecraft separates from the leader vehicle. The segment goes though the ephemeris points of the leader, adding the specified offset to them and then adding these points to the ephemeris. Ephemeris points are not added until the joining condition is met and are added until the separation condition is met.

  1. Right-click on the Return segment () in the MCS.
  2. Select Insert Before... in the shortcut menu.
  3. Select Follow () in the Segment Selection dialog box.
  4. Click OK to close the Segment Selection dialog box.

Selecting the leader

The first step is to select the vehicle the satellite being propagated will follow.

  1. Select the General tab.
  2. Click the Leader ellipsis ().
  3. Select AtlasIIAS () in the Select Leader dialog box.
  4. Click OK to confirm your selection and to close the Select Leader dialog box.

Positioning ImageSat on the launch vehicle.

In order for ImageSat to be in the correct position on AtlasIIAS, you need to use the Offset From Leader parameter. This specifies the distance that the spacecraft will be offset from the leader's body frame by entering X, Y, and Z offset values. You'll place ImageSat on AtlasIIAS's nose.

  1. Select the General tab.
  2. Enter 33.9 m in the Leader's Body Frame X field located in the Offset From Leader panel.

    3. 3D Graphics translational offsets specify the model translation in the object body frame's X, Y and Z directions. The value of the X offset used here corresponds directly to the translational offsets that were used to position AtlasIIAS's 3D Graphics model in the 3D Graphics window.

Understanding joining conditions

Joining and Separation conditions are stopping conditions (which are Astrogator capability components) that define the point of joining or separation between the spacecraft and its leader.

  1. Open the Joining drop-down list in the Additional Options panel.
  2. Review the options, but leave the selection as the default Join at Beginning of Leader’s Ephemeris option.

    3. When using Join at Beginning of Leader's Ephemeris, the spacecraft follows the leader from the beginning of the leader's ephemeris.

Selecting separation conditions

ImageSat begins to separate from AtlasIIAS 2,300 epoch seconds after launch. Use that value to set the duration of the Follow segment. The Follow segment will end when ImageSat separates from AtlasIIAS.

  1. Select the Separation tab.
  2. Click New... () in the Separation toolbar.
  3. Select Epoch () in the New Stopping Conditions dialog box.
  4. Click OK to close the New Stopping Conditions dialog box.
  5. Select Duration in the stopping condition list.
  6. Click Delete () in the Separation toolbar.
  7. Select Epoch in the stopping condition list.
  8. Enter 2300 EpSec in the Trip field.
  9. Click Apply to accept your changes and to keep the Properties Browser open.

Inserting a Target Sequence segment

Use a Target Sequence as a structural element to define maneuvers and propagations in terms of the goals they are intended to achieve.

  1. Right-click on the Return () segment in the MCS.
  2. Select Insert Before... in the shortcut menu.
  3. Select Target Sequence () in the Segment Selection dialog box.
  4. Click OK to close the Segment Selection dialog box.
  5. Right-click on Target Sequence () in the MCS.
  6. Select Rename in the shortcut menu.
  7. Rename Target Sequence () to Circularize Orbit.

Inserting a Maneuver segment

Use a Maneuver segment to model an Impulsive, Finite, or Optimal Finite maneuver. Your Maneuver segment will be impulsive.

  1. Right-click on the Return () segment nested in Circularize Orbit ().
  2. Select Insert Before... in the shortcut menu.
  3. Select Maneuver ().
  4. Click OK.

Starting with this Maneuver segment, make sure this and all the following segments are nested within Circularize Orbit (). You can click and drag segments if you incorrectly place them in the MCS.

Designing an impulsive maneuver

For an impulsive maneuver, the Astrogator capability calculates the new state of the spacecraft by adding a Delta-V vector to the final state velocity of the previous segment. This new state is then added to the ephemeris and passed to the next segment.

You need to circularize ImageSat's orbit at a 7,000-km radius to the Earth. Use the Circularize Orbit Target Sequence to achieve the desired orbit. Use an impulsive maneuver that calculates a state by adding the defined Delta-V vector to the velocity of the final state of the previous segment and then passes it on to the next segment.

  1. Select Maneuver () in the MCS.
  2. Click Segment Properties () in the MCS toolbar.
  3. Enter Thrust To Apoapsis in the Name field of the Edit Segment dialog box.
  4. Open the Color picker drop-down.
  5. Select White.
  6. Click OK to close the Edit Segment dialog box.

    7. A Maneuver segment isn't visible in the 2D and 3D Graphics windows. Making Maneuver segments white is just a way to focus on Propagate segment colors when you build them. Propagate segments are visible in the 2D and 3D Graphics windows.

Specifying the attitude

The Attitude Control field enables you to select the mode in which the maneuver pointing direction is prescribed. Using the Thrust Vector attitude control setting, you specify the Delta-V vector in some reference frame using either Cartesian or spherical components. Delta-V Magnitude is the change in spacecraft velocity due to the burn (distance/time). The Astrogator capability then computes the attitude so that the total thrust vector in the body frame, as specified by the thruster set or engine model, aligns with this vector in the reference axes. You want to specify the total Delta-V Magnitude in Cartesian coordinates with respect to the thrust axes. You can achieve this condition by setting the Attitude Control to Thrust Vector.

  1. Select the Attitude tab.
  2. Open the Attitude Control drop-down list.
  3. Select Thrust Vector.

    4. You want the maneuver to thrust in the velocity direction out to the desired radius (apoapsis).

  4. Click the X (Velocity) target ().

    5. Notice the target now has a check mark (). Selecting the target makes Delta-V Magnitude an independent variable. This tells the Astrogator capability to determine the value used to achieve the desired result, which you will determine shortly.

Maintaining the selected attitude

You want to satellite to maintain the attitude that you previously defined on the Basic - Attitude page.

  1. Click More Options....
  2. Select the Use attitude page definition for other STK functions option when the More Attitude Options dialog box opens.

    3. When selected, all other calculations in the STK application, including the 3D Graphics window, will use the attitude specified by the satellite object’s Basic - Attitude properties, ignoring the actual attitude during the maneuver.

  3. Click OK to confirm your selection and to close the More Attitude Options dialog box.

Changing mass due to fuel usage

The Engine tab defines the magnitude and the nature of the propulsion. The engine parameters specified on this tab are used primarily to define the maneuver direction when using a thruster set to seed a finite maneuver and to update the fuel mass. When you select Update Mass Based on Fuel Usage, the Astrogator capability updates the mass of the spacecraft as fuel consumed delta-M.

  1. Select the Engine tab.
  2. Notice that the engine model defaults to Constant Thrust and Isp.
  3. Select the Update Mass Based on Fuel Usage checkbox.
  4. Click Apply to accept your changes and to keep the Properties Browser open.

Inserting a Propagate segment

Use a Propagate segment to model the movement of the spacecraft along its current trajectory until meeting specified stopping conditions. The segment uses the defined propagator and integrator to propagate the state, adding each point to the ephemeris as it goes. After each step, the segment checks to see if any stopping conditions were met during the step. If so, it then finds the exact point, within tolerance, where the stopping condition is satisfied. From that point, the segment either executes an automatic sequence or stops the propagation and passes the state at that point to the next segment. The Astrogator capability also adds an ephemeris point at the time that the stopping condition is triggered.

You want to propagate ImagSat to the proper apoapsis, which has been defined as a 7,000-km radius. Use a Propagate segment to model this.

  1. Right-click on Thrust To Apoapsis () in the MCS.
  2. Select Insert After... in the shortcut menu.
  3. Select Propagate () in the Segment Selection dialog box.
  4. Click OK to close the Segment Selection dialog box.
  5. Click Segment Properties () in the MCS toolbar.
  6. Enter Prop To Apoapsis in the Name field of the Edit Segment dialog box.
  7. Open the Color picker drop-down.
  8. Select a color that's different from the Follow () segment.
  9. Click OK.

Setting the Propagate segment's stopping condition

Create an Apoapsis stopping condition. This will stop at the farthest point from the origin.

  1. Click New... () in the Stopping Conditions toolbar.
  2. Select Apoapsis () in the New Stopping Condition dialog box.
  3. Click OK.
  4. Select Duration in the Stopping Conditions list.
  5. Click Delete () in the Stopping Conditions toolbar.
  6. Click Apply to accept your changes and to keep the Properties Browser open.

Adding a result to the Propagate segment

Use Radius Of Apoapsis as the result. You will set the 7,000-km radius as a dependent variable for the Target Sequence later.

  1. Click Results... below the MCS.
  2. Expand () Keplerian Elems () in the Available Components list located in the User-Selected Results - Prop To Apoapsis dialog box.
  3. Select Radius of Apoapsis ().
  4. Move () Radius of Apoapsis () to the Selected Components list.
  5. Click OK to close the User-Selected Results - Prop To Apoapsis dialog box.
  6. Click Apply to accept your changes and to keep the Properties Browser open.

Adding another Maneuver segment to the MCS

ImageSat's orbital trajectory requires one more Maneuver segment that thrusts the spacecraft into a circular orbit.

  1. Select PropToApoapsis () in the MCS.
  2. Click Insert Segment After () in the MCS toolbar.
  3. Select Maneuver () in the Segment Selection dialog box.
  4. Click OK to close the Segment Selection dialog box.
  5. Click Segment Properties () in the MCS toolbar.
  6. Type Circularize in the Name field of the Edit Segment dialog box.
  7. Open the Color picker drop-down.
  8. Select white.
  9. Click OK to close the Edit Segment dialog box.

Setting the attitude

Specify the altitude control and direct STKSat to maintain the same altitude during the maneuver.

  1. Select the Attitude tab.
  2. Open the Attitude Control drop-down list.
  3. Select Thrust Vector.
  4. Click the X (Velocity) target () to set your control parameter.
  5. Click More Options....
  6. Select the Use attitude page definition for other STK functions option in the More Attitude Options dialog box.
  7. Click OK to close the More Attitude Options dialog box.
  8. Click Apply to accept your changes and to keep the Properties Browser open.

Setting the engine parameters

Select Update Mass Based on Fuel Usage to calculate the mass of the spacecraft as fuel is consumed.

  1. Select the Engine tab.
  2. Select the Update Mass Based on Fuel Usage checkbox.
  3. Click Apply to accept your changes and to keep the Properties Browser open.

Adding a result to the maneuver

You need to add an eccentricity result to Circularize so that you can circularize the orbit by targeting an eccentricity value of zero (0).

  1. Click Results....
  2. Expand () Keplarian Elems () in the Available Components: list of the User-Selected Results - Circularize dialog box.
  3. Move () Eccentricity () to the Selected Components list.
  4. Click OK to close the User-Selected Results - Circularize dialog box.
  5. Click Apply to accept your changes and to keep the Properties Browser open.

Inserting another Propagate segment

You need another propagate segment to ensure the satellite maintains the circularized orbit for three days.

  1. Select the Circularize () in the MCS.
  2. Click the Insert Segment After () in the MCS toolbar.
  3. Select Propagate () in the Segment Selection dialog box.
  4. Click OK to close the Segment Selection dialog box.

Updating the Propagate segment's properties

Rename the new Propagate segment and choose a new color for it.

  1. Click Segment Properties () in the MCS toolbar.
  2. Enter Maint Circ in the Name field of the Edit Segment dialog box.
  3. Open the Color picker drop-down.
  4. Select a color that isn't being used by any other segment.
  5. Click OK to close the Edit Segment dialog box.

Setting the Propagate segment's stopping condition

Update the Propagate segment's default Duration stopping condition to stop after three days.

  1. Select Duration in the Stopping Conditions list.
  2. Enter 3 day in the Trip field.
  3. Click Apply to accept your changes and to keep the Properties Browser open.

Using the differential corrector profile

The differential corrector profile targets specific values, which are defined as independent variables. The Target Sequence will change the value of independent variables as needed to achieve the goal defined by the dependent variables, utilizing a differential correction algorithm.

  1. Select Circularize Orbit () in the MCS.
  2. Select Differential Corrector in the Profiles panel.
  3. Click Properties... () in the Profiles toolbar.

Choosing the Control Parameters

The Control Parameters are independent variables that you marked for inclusion while setting up the Target Sequence.

  1. Select the Variables tab.
  2. Set the following in the Control Parameters panel when the Differential Corrector dialog box opens:
    3. Control Parameter Object Use Max. Step
    ImpulsiveMnvr.Pointing.Cartesian.X Thrust_to_Apoapsis Selected 500 m/sec
    ImpulsiveMnvr.Pointing.Cartesian.X Circularize Selected 500 m/sec

Updating the Equality Constraints (Results)

Equality Constraints in the search profile outline dependent variables to be considered in your analysis.

  1. Set the following in the Equality Constraints (Results) panel:
    2. Equality Constraint Use Desired Value Tolerance
    Radius_Of_Apoapsis Selected 7000 km 0.1 km
    Eccentricity Selected 0 0.0001
  2. Click OK to close the Differential Corrector dialog box.

    3. Setting the Radius of Apoapsis value means the targeter will not stop iterating until the radius of apoapsis is 7,000 km and the eccentricity is within 0.0001 of the desired value of zero (0).

Running the entire Mission Control Sequence

To calculate the trajectory of the spacecraft you must run the Mission Control Sequence. The Astrogator capability will proceed through the MCS and run each segment, generating an ephemeris for the spacecraft. As it runs the MCS, the Astrogator capability carries the trajectory and state of the spacecraft determined so far from one segment to the next.

  1. Open the Action drop-down list.
  2. Select Run active profiles.

    3. Running active profiles runs the mission control sequence allowing the active profiles to operate.

  3. Click Run Entire Mission Control Sequence () in the MCS toolbar.
  4. Read the data in the targeting status window.
  5. When you finish, close the targeting status window.

Clearing iterations from the 3D Graphics window

To erase the iteration graphics after finishing a run, click Clear Graphics on the MCS toolbar.

  1. Click Clear Graphics () in the MCS toolbar.
  2. Click Apply to accept your changes.
  3. Click Run Entire Mission Control Sequence () in the MCS toolbar.
  4. Close the targeting status window.
  5. Click OK to accept your changes and to close the Properties Browser.

Generating a Maneuver Summary report

The Maneuver Summary report style is available only for satellites propagated using the Astrogator capability. This report shows a summary of the maneuver segments in the MCS that have been run.

  1. Right-Click on ImageSat () in the Object Browser.
  2. Select Report & Graph Manager... () in the shortcut menu.
  3. Right-click on Maneuver Summary () in the Installed Styles () list in the Styles panel of the Report & Graph Manager.
  4. Select Duplicate () in the shortcut menu to copy the report and to open its Properties.

Changing the report units

Change the units for the date format so that it is consistent with the scenario time units, which is set to EpSec (epoch seconds).

  1. Select Maneuver Summary-Start Time in the Report Contents list of the Properties Browser.
  2. Click Units....
  3. Clear the Use Defaults checkbox in the Units: Section 1, Line 1, Maneuver Summary-Start Time dialog box.
  4. Select Epoch Seconds (EpSec) in the New Unit Value list.
  5. Click OK to close the Units: Section 1, Line 1, Maneuver Summary-Start Time dialog box.
  6. Select Maneuver Summary-Stop Time in the Report Contents list.
  7. Click Units....
  8. Clear the Use Defaults checkbox in the Units: Section 1, Line 1, Maneuver Summary-Stop Time dialog box.
  9. Select Epoch Seconds (EpSec) in the New Unit Value list.
  10. Click OK to close the Units: Section 1, Line 1, Maneuver Summary-Stop Time dialog box.
  11. Click OK to accept your changes and to close the Properties Browser.

Renaming the customized report

Rename your custom Maneuver Summary report.

  1. Expand () My Styles ().
  2. Right-click on Maneuver Summary ().
  3. Select Rename in the shortcut menu.
  4. Rename Maneuver Summary () to My Maneuver Summary.

Generating the custom report

Generate your custom Maneuver Summary report.

  1. Select My Maneuver Summary () in My Styles ().
  2. Click Generate....
  3. Review the My Maneuver Summary report.
    • What are the Delta-V values for the two maneuvers?
    • What is the estimated fuel usage?
    • What are the estimated burn durations for the two maneuvers?
  4. Close the report and the Report & Graph Manager.

Viewing AtlasIIAS and ImageSat in the 3D Graphics window

Look at ImageSat’s orbit in the 3D Graphics window.

  1. Bring the 3D Graphics window to the front.
  2. Right-click on ImageSat () in the Object Browser.
  3. Select Zoom To in the shortcut menu.
  4. Using your mouse to position your view so that you can see ImageSat () on top of AtlasIIAS ().

imagesat and the atlas Iias launch vehicle on the Vandenberg launchpad

The launch route appears to fly sideways. If you take a look at ImageSat's 3D Graphics - Orbit System page, you will notice that the object’s route in 3D is set to the Inertial by Window reference frame. You can change this to Fixed by Window if you want the route to appear in a different reference frame. To change the Launch Vehicle's reference frame, change the same properties on the 3D Graphics - Trajectory System page.

Animating the scenario

View the satellite thrusting to its circular orbit in 3D.

  1. Click Decrease Time Step () in the Animation toolbar.
  2. Se the Time Step to 0.50 sec.
  3. Click Start () to animate the scenario.

    4. Imagesat at the Point of separation from ATLASIIAS

  4. Watch as ImageSat is brought into orbit.
    • Did you see ImageSat () separate from AtlasIIAS ()?

      • AtlasIIAS () is not modeled after the point of separation; it will disappear once it reaches the end of its ephemeris at 2,300 EpSec.

    • At what segment of the orbit does this happen?

      • Match the colors in the 3D Graphics window orbit display to the colors that you assigned to the segments in the mission control sequence.

    • Does the orbit follow AtlasIIAS's () path until separation and circularization?
    • Can you see when the different maneuvers and propagate segments of the trajectory occur?
  5. When finished, click Reset () to reset your scenario.

Using 3D model solar panel articulations

While watching the animation, did you notice that the solar panels on the satellite are fully deployed for the duration of the scenario? This is not correct; the panels should deploy after the satellite has separated from the launch vehicle. You can change this behavior by animating the model's articulations.

Articulation keyword blocks and commands are an integral part of a model file. A separate file, called the model articulation file, contains the information needed to animate the articulation commands contained in the model file. The model articulation file contains a time-ordered list of data about each articulation that can be applied to the associated object. You can open an articulation file with a text-editing application such as Notepad to view the data.

Locating the articulation file

The STK software comes preinstalled with an articulation file that will animate the articulations in ImageSat that you can load into your scenario.

  1. Open Windows File Explorer.
  2. Go to <STK install folder>\Data\Resources\stktraining\samples.
  3. Copy ImageSat.sama.
  4. Go to the scenario directory (typically, C:\Users\username\Documents\12\Astrogator_AtlasLaunch).
  5. Paste ImageSat.sama into the scenario directory.
  6. Close Windows File Explorer.

Updating the satellite's model file

The .sama articulation file actually goes with a model that is different from the default one you are using.

  1. Open ImageSat's () Properties ().
  2. Select 3D Graphics - Model page when the Properties Browser opens.
  3. Click the Model File ellipsis () in the Model panel.
  4. Select satellite-classic.glb from the list of files in the File dialog box.
  5. Click Open to select the file and to close the File dialog box.
  6. Click Apply to accept your change and to keep the Properties Browser open.

Loading the articulation file

Though you placed the solar panel articulation file into the scenario's directory, the STK application doesn’t know to use it. You must load the articulation file into the scenario.

  1. Select Use Articulation File in the Articulations panel.
  2. Click the Use Articulation File ellipsis ().
  3. Go to the scenario directory.
  4. Select ImageSat.sama in the File dialog box.
  5. Click Open to select the file and to close the File dialog box.
  6. Click Reload to reload the articulations file.
  7. Click Apply to accept your changes and to keep the Properties Browser open.

Setting the articulation interval

In orbit, the solar panels always point toward the sun. This is a good thing when the satellite is in orbit, but not when it is stowed in the launch vehicle. The solar panels should remain motionless when sitting in the launch vehicle's payload bay. You want to make sure the solar panels do not deploy and point towards the sun until after the launch vehicle and satellite have separated.

The solar panels are programmed as an autosequence task to be fully deployed by 2,550 epoch seconds after the launch. Make the start time for the solar panel pointing to begin at 2,650 epoch seconds, when the antenna has fully deployed and the satellite is ready to receive commands.

  1. Select the 3D Graphics - Model Pointing page.
  2. Select Intervals in the Available Targets list.
  3. Click Interval List....
  4. Click Add in the Model Pointing Intervals dialog box.
  5. Enter 2650 EpSec in the Start Time field.
  6. Enter 259200 EpSec in the Stop Time field.

    7. The stop time matches the end of ImageSat's three-day propagation period. If you view the properties of the Maint Circ Propagate segment, it should read 259200 sec, or 3 days.

Changing the model pointing target

By default, the solar panels are targeted to point to earth during the interval you just defined. Change their target so they will point towards the Sun.

  1. Double-click on the Target field to open the drop-down list of available targets.
  2. Select Sun.
  3. Click OK to close the Model Pointing Intervals dialog box.
  4. Click OK to accept your changes and close the Properties Browser.

Viewing the updated model in action

ImageSat separates from AtlasIIAS at 2,300 epoch seconds. Set your animation time to 2,290 epoch seconds in order to see the separation and solar panel deployment.

  1. Enter 2290 in the Current Scenario Time field of the Animation toolbar.
  2. Select the Enter key.
  3. Bring the 3D Graphics window to the front.
  4. Zoom to ImageSat ().
  5. Click Start () in the Animation toolbar to animate the scenario.

    6. ImageSat () separates from AtlasIIAS () at 2,300 epoch seconds. Shortly after, the solar panels start to deploy. They will be fully deployed at 2,550 epoch seconds. As the satellite orbits the earth, they will turn, pointing at the Sun.

    Updated ImageSat with articulated, sun-pointing solar panels

  6. When finished, click Reset ().

Saving your work

Clean up your workspace and save your work.

  1. Close any open reports, properties and tools.
  2. Save () your work.

Summary

You loaded a starter scenario using a visual data file (VDF). The scenario had a Facility object whose location was a launch pad at Vandenberg Space Force Base. The 3D Graphics model had been changed to depict an Atlas launchpad. A Launch Vehicle object (AtlasIIAS) was prepositioned on the launch pad. You inserted a Satellite object (ImageSat) into the scenario. Switching ImageSat's propagator to Astrogator, you created a Follow segment that followed AtlasIIAS from launch to separation. Next, you inserted a Target Sequence with the following segments:

  • An impulsive Maneuver segment to thrust along the X (Velocity) vector as an independent variable or Control Parameter
  • A Propagate segment changing its stopping condition to Apoapsis and Setting the result or Equality Constraint to Radius of Apoapsis.
  • A second impulsive Maneuver segment to thrust along the X (Velocity) vector as an independent variable or Control parameter and set its result or Equality Constraint to Eccentricity.
  • Finally, you inserted a second Propagate segment setting the stopping condition to three days.

Using a differential corrector, you enabled both Control Parameters and both Equality Constraints, setting the radius of apoapsis to a desired altitude of 7,000 kilometers and an eccentricity of zero (0) with a tolerance setting of 0.0001. You ran the entire mission control sequence to obtain data to place ImageSat in its required orbit. Opening the Report & Graph Manager, you created a customized maneuver summary report. Finally, you loaded a prebuilt articulation file into your scenario and changed the 3D Graphics model file to match the articulation file. With the articulation and model files loaded, you were able to visualize the solar panels deploying after separation from AtlasIIAS.