Designing Satellite Constellations with ModelCenter

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.

Required product install: The Ansys ModelCenter® model-based systems engineering software and the STK Plugin for ModelCenter are required to complete this tutorial.

ModelCenter installation prerequisites: The ModelCenter software requires the installation of a 64-bit version of Java, a 64-bit implementation of Python 3.x, and the installation of the thrift and six Python packages. See the ModelCenter Installation Prerequisites for more information.

This tutorial was written using version 2026 R1 of the Ansys ModelCenter® model-based systems engineering software.

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

  • STK Pro
  • STK SatPro
  • Coverage
  • STK Analyzer

Problem statement

Engineers and operators require a quick and easy way to study future satellite constellation designs. You are designing a Walker constellation, which is a group of satellites in circular orbits that have the same period and inclination. Your Walker constellation will employ synthetic-aperture radars (SAR) to create three-dimensional reconstructions of the Earth's land masses. The analysis will require the use of shapefiles, which store data for geographic features. You don't have software developers on your team, so another requirement is a simple trade study tool, which you need to quickly perform "what if" analysis on, collect data from, and subsequently optimize your model.

Solution

Use the STK software's SatPro and Coverage capabilities and the Analyzer capability, which is part of the Ansys ModelCenter® model-based systems engineering software, to determine what your coverage of the Earth's land masses will be over a 24-hour period, considering different variations of altitude on your constellation's satellites.

What you will learn

Upon completion of this tutorial, you will understand how to:

  • Create a Walker satellite collection
  • Use shapefiles with the Coverage capability
  • Analyze a satellite constellation using the ModelCenter software

Creating a new scenario

First, you must create a new STK scenario and then build from there.

  1. Launch the STK application ().
  2. Click Create a Scenario in the Welcome to STK dialog box.
  3. Enter the following in the New Scenario Wizard:
    4. Option Value
    Name Walker_Analyzer
    Location Default
    Start Default
    Stop Default
  4. Click OK when you finish.
  5. Click Save () when the scenario loads. The STK software creates a folder with the same name as your scenario for you.
  6. Verify the scenario name and location in the Save As window.
  7. Click Save.

Save () often during this lesson!

Disabling streaming terrain

Streaming terrain will not be used in your analysis. You can turn it off for your analysis.

  1. Right-click on Walker_Analyzer () in the Object Browser.
  2. Select Properties ().
  3. Select the Basic - Terrain page when the Properties Browser opens.
  4. Clear the Use terrain server for analysis check box in the Terrain Server panel.
  5. Click OKto accept your change and to close the Properties Browser.

Creating a Walker satellite constellation

Walker constellations are based on a simple design strategy for distributing the satellites. There are two main Walker constellation variants: Walker-Delta constellations and Walker-Star constellations. The two variants differ in the distribution of the ascending nodes between the planes of the constellation: in a Delta configuration, the ascending nodes of the planes are distributed over a full 360-degree range, while in a Star configuration, the ascending nodes are distributed over a 180-degree span.

Creating a seed satellite

The Walker Tool, a satellite engineering tool, which is part of the SatPro capability, makes it easy to generate a Walker constellation. First, define a satellite with the characteristics and orbit you need. The original satellite that you defined is the seed satellite, and the satellites generated using the Walker tool are the children.

  1. Bring the Insert STK Objects Tool () to the front.
  2. Select Satellite () in the Insert STK Objects tool.
  3. Select the Orbit Wizard () method.
  4. Click Insert....
  5. Set the following in the Orbit Wizard:
    6. Option Value
    Type Repeating Ground Trace
    Satellite Name SAR_Sat
  6. Look at the Approximate Altitude value of 400 km in the Position panel. This will be discussed later in the tutorial.
  7. Click OK.

Attaching a Sensor object to your seed satellite

If your seed satellite has sub-objects (child objects) such as sensors, the Walker Tool creates the same sub-objects for each of the child satellites. So, if you want the same grouping of sensors on each of satellites in the constellation, create them on the seed satellite, and the Walker Tool will copy them into the constellation. Insert a Sensor object that will be used on your seed satellite and on the child satellites in the Walker constellation.

  1. Insert a Sensor () object using the Define Properties () method.
  2. Select SAR_Sat () in the Select Object dialog box.
  3. Click OK.

Using an SAR sensor type

The Sensor objects will model the field of regard of a synthetic aperture radar. A Synthetic Aperture Radar sensor type synthesizes the aperture of a larger antenna than is actually present. It uses SAR pattern definitions designed to model the field of regard of a SAR sensor onto the surface of the earth.

  1. Select the Basic - Definition page when the Properties Browser opens.
  2. Open the Sensor Type shortcut menu.
  3. Select SAR.
  4. Set the following Elevation Angles:
    5. Option Value
    Min 50 deg
    Max 90 deg
  5. Set the following Exclusion Angles:
    6. Option Value
    Forward 70 deg
    Aft 70 deg
  6. Select the Track parent altitude check box.

    7. Enabling this option will force the SAR sensor to track the altitude of its parent object, rather than assuming a constant altitude.

  7. Click OK to accept your changes and to close the Properties Browser.
  8. Right-click on Sensor1 () in the Object Browser.
  9. Select Rename in the shortcut menu.
  10. Rename Sensor1 () SAR_Sens.

Creating a Walker Satellite collection

You have defined a satellite with the characteristics and orbit you need. Use a Satellite Collection object and the Walker Tool to generate a Walker constellation. A Satellite Collection object models a group of satellites as a single object in the Object Browser. The associated satellites do not appear in the Object Browser, but are available for analysis purposes within other computational tools. For your preliminary analysis, two satellites in one orbital plane are required.

  1. Insert a SatelliteCollection () object using the Walker Tool () method.
  2. Click Select Object... when the Walker Tool opens.
  3. Select SAR_Sat () in the Select Object dialog box.
  4. Click OK to close the Select Object dialog box.
  5. Set the following values:
    6. Option Value
    Number of Sats per Plane 2
    Number of Planes 1
  6. Set the following Container Options:
    7. Option Value
    Select Option Create Satellite Collection
    Name SAR_Sats
  7. Click Create / Modify Walker.
  8. Click Close to close the Walker Tool.
  9. Clear the SAR_Sat () check box in the Object Browser.

    10. This turns SAR_Sat () off visually in both the 2D and 3D graphics windows; it's still available analytically.

Using a Satellite Collection reference object

When you use an entry of a satellite collection in your analysis, that entry will inherit the properties of a reference object. By default, the reference object is simply the default satellite object. However, if you choose a default subset reference object, the STK application will associate each member of a subset with that specific satellite in the scenario. Using a specified satellite provides a way to customize settings (attitude, access constraints, etc.) when you use the satellite collection member in an analysis. Moreover, when the reference object contains child objects (sensors, transmitters, receivers, etc.), The STK application also associates these children with the satellite entry. You can then use these children in the analysis tools.

  1. Open SAR_Sats' () Properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.
  3. Click Edit default subset reference object () for the Default Subset Reference Object.
  4. Select SAR_Sat () in the Select Object dialog box.
  5. Click OK to close the Select Object dialog box.
  6. Click Apply.
  7. Look at the Subsets list. The AllSensors subset is now available.
  8. Click OK to accept your changes and to close the Properties Browser.

Using the Coverage capability

The STK software's Coverage capability allows you to analyze the global or regional coverage provided by one or more assets while considering all available accesses. To address area coverage capabilities, Coverage provides you with two STK object classes: Coverage Definition objects and Figure of Merit objects. Before running a Coverage analysis, you must first insert a Coverage Definition object.

Inserting a Coverage Definition object

You want to assess coverage over the Earth's land masses. To do this, you will use a Coverage Definition object to model the area that you will analyze. Coverage Definition objects enable you to define and maintain an area of coverage, to define the STK objects providing coverage for the area (such as satellites, aircraft and sensors), to define the time period of interest, and to calculate accesses to the region.

  1. Bring the Insert STK Objects Tool () to the front.
  2. Insert a Coverage Definition () object using the Insert Default () method.
  3. Rename CoverageDefinition1 () Land_Cov.

Defining the Coverage Grid Area of Interest

Coverage analyses are based on the accessibility of assets (objects that provide coverage) and geographical areas. For analysis purposes, the geographical areas of interest are further refined using regions and points defined by the STK application. Points have specific geographical locations and are used in the computation of asset availability. Regions are closed boundaries that contain sets of points. The combination of the geographical area, the regions within that area, and the points within each region is called the coverage grid.

You will use a custom region for your coverage grid. Custom regions are defined using the boundary points of an existing area target or one or more region list files (*.rl) or shapefiles (*.shp). A shapefile is a vector data file format used for geospatial analysis. Shapefiles were developed by ESRI for use with their ArcGIS software but can be imported into the STK application using the GIS tool to more easily define a precise geographical area on the Earth.

  1. Open Land_Cov's () Properties ().
  2. Select the Basic - Grid page when the Properties Browser opens.
  3. Open the Type drop-down list in the Grid Area of Interest panel.
  4. Select Custom Regions.
  5. Select Region File in the drop-down menu below Type.
  6. Click Load Region File ....
  7. Browse to the location of the land shapefiles installed with the STK software, typically at C:\Program Files\AGI\STK_ODTK 13\Data\Shapefiles\Land, when the Select Region File dialog box opens.
  8. Select Land.shp.
  9. Click Open.

Specifying the point granularity

When using custom regions, it is important to select adequate point granularity. If a single custom region is smaller than the resolution, the STK application will generate an alternative grid using a finer granularity to ensure at least three samples within the region.

  1. Enter 4 deg in the Lat/Lon field in the Point Granularity panel.
  2. Click Apply to accept your changes and to keep the Properties Browser open.

Viewing the Coverage grid

View the Coverage grid in the 2D Graphics window.

  1. Bring the 2D Graphics window to the front.
  2. Review the Coverage grid.

Coverage Grid

The Coverage grid points are only on the land masses and include islands. Unlike with Area Target objects, shapefiles support defining polygons with holes. The interior of a polygon is defined to be the area to the right side of the line made by following the points defining the polygon (clockwise). You can therefore create holes in polygons by creating a polygon that overlaps another and has its points defined in the opposite (counterclockwise) order. You can see one such example with the Caspian Sea, which is left out of the coverage definition.

Selecting the coverage assets

Set the satellite sensors as the coverage assets. Assets properties enable you to specify the STK objects used to provide coverage.

  1. Return to Land_Cov's () Properties ().
  2. Select the Basic - Assets page.
  3. Expand () SAR_Sats () in the Assets list.
  4. Select AllSensors ().
  5. Click Assign.
  6. Click Apply.

Turning off Automatically Recompute Accesses

By default, the STK application is set to automatically recompute accesses every time an object on which the coverage definition depends (such as an asset) is updated. In an effort to manage your resources more efficiently, turn off recomputing access automatically every time you make a change.

  1. Select the Basic - Advanced page.
  2. Clear Automatically Recompute Accesses.
  3. Click OKto accept your changes and to close the Properties Browser.

Computing accesses

The ultimate goal of coverage is to analyze accesses to an area using assigned assets and applying necessary limitations upon those accesses. Now that your Coverage Definition object is defined, compute accesses by opening the Compute Accesses tool.

  1. Right-click Land_Cov () in the Object Browser.
  2. Select CoverageDefinition in the shortcut menu.
  3. Select Compute Accesses in the CoverageDefinition submenu.
  4. Observe coverage displaying accesses in the 2D Graphics window.

Inserting a Figure of Merit object

The STK software enables you to specify the method by which the quality of coverage is measured using a Figure Of Merit object. The default figure of merit type is Simple Coverage. Simple Coverage measures whether a point is accessible by any of the assigned assets.

  1. Insert a Figure Of Merit () object using the Insert Default () method.
  2. Select Land_Cov () in the Select Object dialog box.
  3. Click OK.
  4. Rename FigureOfMerit1 () Simple_Cov.

Viewing the quality of coverage

View the coverage in the 3D Graphics window.

  1. Bring the 3D Graphics window to the front.
  2. Observe the Simple Coverage assessment over the land masses.

Simple Coverage

Notice there are some holes in your coverage. You can quantitatively gauge just how much with a Percent Satisfied report.

Determining static satisfaction

Generate a Percent Satisfied report to determine the amount of land that is covered by at least one sensor over the 24-hour analysis period. The Static Satisfaction data provider reports the amount of the coverage analysis grid that satisfies satisfaction criteria defined in the figure of merit based on the static definition of the figure of merit. The data provider elements are Percent Satisfied and Area Satisfied.

  1. Right-click on Simple_Cov () in the Object Browser.
  2. Select Report & Graph Manager... () in the shortcut menu.
  3. Select the Percent Satisfied () report in the Installed Styles () folder in the Styles list when the Report & Graph Manager opens.
  4. Click Generate....
  5. Note the % Satisfied value at the bottom of the report — for example, approximately 90 percent.

You could adjust your satellites and redo your analysis at a variety of altitude until you reached 100 percent coverage, but it would be tedious to repeat the entire scenario time and time again for each altitude that you want to test. Instead, you can use the ModelCenter software to automate your study.

Closing out of the STK application

Before opening the ModelCenter application, clean up and close out of the STK application.

  1. Close the Percent Satisfied report.
  2. Close the Report & Graph Manager.
  3. Save () your scenario.
  4. Close the STK application.

Creating a new ModelCenter project

The Ansys ModelCenter software is designed for multidisciplinary analysis and design optimization. It allows you to automate complex workflows, integrate multiple engineering tools, and optimize designs within model-based systems engineering frameworks. Before opening the ModelCenter application, however, save your scenario and close out of the STK application.

  1. Open the ModelCenter () application.
  2. Click Start a New Model in the Welcome to ModelCenter dialog box.
  3. Click Process when the What type of model would you like to create? dialog box opens.
  4. Navigate to your scenario folder (for example, C:\Users\<username>\Documents\STK_ODTK 13\Walker_Analyzer.
  5. Enter Walker_Analyzer in the File name field.
  6. Ensure the Save as type is set to the ModelCenter Model (Zip) (*.pxcz).
  7. Click Save.

Launching the STK Plugin for ModelCenter

The STK Plugin for ModelCenter imports a copy of the desired Scenario and opens it in an instance of the STK application. Launch the plugin by adding it to your workflow's analysis view from the Server Browser.

  1. Select favorites () in the Server Browser at the bottom of the window.
  2. Click and drag the STK component () into the dashed circle underneath "Drop items here to build the model" in the workflow's Analysis View.
  3. Select Walker_Analyzer.sc when the Open STK Scenario file dialog box opens.
  4. Click Open.
  5. After a few moments, the STK Analyzer window will open.

The Walker_Analyzer scenario file will open in the STK application in the background.

You can add any of the STK variables as ModelCenter input or output variables through the STK Analyzer window that appears. If you change the value of a variable in your scenario through the STK interface or the ModelCenter Component Tree, you should re-add the variable into ModelCenter or re-run the workflow before running any trade studies with the new value.

Setting up your analysis with Analyzer

Use the STK Analyzer window to configure the input and output variables available for further analysis with the Analyzer capability.

Selecting the input variables

Use all nine Walker variables as the input variables for your study.

  1. Select the SAR_Sats () in the STK Variables tree.

    2. When you select an object in the STK Variables tree, all possible input variable candidates for that object are listed under the General tab and the Active Constraints tab in the STK Property Variables panel.

  2. Select the Walker () property.
  3. Move () Walker () to the Analyzer Variables list.

    4. Note that the nine Walker variables () are all listed under Inputs in the Analyzer Variables list.

Selecting the output variables

The same data providers that are available in the Report & Graph Manager in the STK application are available in the Data Provider Variables tree.

  1. Expand () Land_Cov () in the STK Variables tree.
  2. Select Simple_Cov ().
  3. Expand () the Static Satisfaction () data provider in the Data Provider Variables tree.
  4. Select the Percent Satisfied () data provider element.

    5. Just like you did with the Figure Of Merit object in your STK scenario, you're looking for changes in percent satisfied at each semi major axis change.

  5. Move () Percent Satisfied () into the Analyzer Variables list.

    6. Note that Percent Satisfied () is listed under Outputs in the Analyzer Variables list.

  6. Click OK to confirm your selections and to close the STK Analyzer window.

    7. This will also close the STK application, which had been running in the background.

Determining the impact of satellite altitude on coverage

You want to determine the altitude that results in your satellite sensors providing 100-percent coverage of the Earth's land masses. Create a Parametric Study to find this value.

Using the Parametric Study tool

The Parametric Study tool runs a workflow through a sweep of values for some input variable. You can plot the resulting data to view trends.

  1. Expand () all the elements in the Component Tree.
  2. Click Parametric Study () in the Standard toolbar.
  3. Click and drag SemiMajorAxis () from the Component Tree to the Design Variable field when the Parametric Study tool opens.
  4. Set the following Design Variable values:
    5. Option Value
    starting value 6900
    ending value 7700
    step size 50

    Values are in kilometers, the default value for the satellites' semi-major axes. Setting the step size to 50 automatically sets the number of samples to a value of 17, which means your scenario will be analyzed 17 times in 50-kilometer increments between 6,900 and 7,700 kilometers. You want to find the lowest semi-major axis that provides 100-percent satisfaction.

  5. Click and drag Percent_Satisfied () from the Component Tree to the Responses field.
  6. Click Run....

Clicking Run... will open the Data Explorer, which is a tool used by Trade Study tools to display data while they are being collected from your Model. While data are being collected, the Data Explorer displays a progress meter, a halt button, and the data. The Table page of the Data Explorer displays trade study data in a tabular form. It is the default window that is present for all trade studies. Cells are shaded differently depending on the associated variable's state. Input variables are shown with green text, valid values are displayed with black text, invalid values are displayed with gray text, and modified values are displayed with blue text. From the table it is possible to view and edit all values in your trade study and even to add and remove whole runs.

Creating a 2D Line Plot

Once the trade study is complete and all data have been collected, the Data Explorer toolbar becomes active. The Data Explorer stores values for all variables in a workflow and special variables from the trade study. Any variable in the workflow can be plotted against any other variable. Some trade study tools will automatically launch a default plot window when the trade study runs. For other plots, you can create them from the Add View menu. For this study, you will create a 2D Line Plot. A 2D Line Plot displays an X-Y plot for variables in your model.

  1. Close the 2D Scatter Plot that opened when the trade study finished running.
  2. Click Add View () on the Table Page toolbar.
  3. Select 2D Line Plot () in the drop-down menu.
  4. Review the 2D Line Plot.

Percent Satisfied versus Semimajor Axis 2D Line Plot

You can see that coverage satisfaction reaches 100 percent at approximately 7,450 kilometers.

Reviewing the Data Explorer table

Look at the Data Explorer table, which provides more specific data about the points on the graph.

  1. Bring the Table page to the front.
  2. Select the Chart menu in the Menu Bar.
  3. Select Show Only Design Variables.

    4. This will hide all but the variables you are studying with the Parametric Study to make viewing the data easier.

  4. Scroll until you find the first run where the Percent Satisfied is 100.

    5. Tabular Data

    Your results may vary from those shown above.

  5. Take a look at the summary of the input and output values.

You can see that you reach 100-percent satisfaction with a semi-major axis of around 7,450 kilometers for the 24-hour analysis period, which matches what you saw in the 2D Line Plot. You could repeat your Parametric Study, focusing on the range of 7,400 and 7,500 kilometers with smaller step sizes, to find a closer approximation.

Saving your work

Save your work and close out ModelCenter application.

  1. Close out any open plots, tools, and the Data Explorer window.
  2. Click No when prompted to close your trade study without saving.
  3. Click Save () to save your ModelCenter workflow.
  4. Close the ModelCenter application.

Summary

This tutorial was an introduction to using the Satellite Collection object and the Walker tool in conjunction with the ModelCenter software. Simple values were used in order to expose you to their functionality. You began the scenario by placing a Satellite object in a repeating ground trace orbit. You attached a Sensor object to the satellite that modeled a synthetic-aperture radar's field of regard. Using the Walker tool to make, you made your Satellite object the seed satellite for a Satellite Collection object to created a simple constellation of two satellites in opposition to each other in one orbital plane. You inserted a Coverage Definition object and assigned a shapefile as your grid area of interest, which covered all the Earth's land masses, and assigned the Sensor objects as the coverage assets. Using a Figure Of Merit object and a Simple Coverage definition, you determined that approximately 90 percent of the Earth's land mass was accessed by the two SAR sensors during a 24-hour analysis period. Using the ModelCenter software's Parametric Study tool, you varied the Satellite object's semi-major axis to study satisfaction percentage at various altitudes.