Satellite Constellation Design with Analyzer

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.

Additional installation - Analyzer. You can obtain the necessary install by visiting http://support.agi.com/downloads or calling AGI support.

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
  • STK SatPro
  • Coverage
  • STK Analyzer

Problem statement

Engineers and operators require a quick and easy way to generate a Walker constellation to study future satellite constellation designs. A Walker constellation consists of a group of satellites that are in circular orbits and have the same period and inclination. A small constellation of satellites employing synthetic-aperture radars (SAR) will be used to create three-dimensional reconstructions of the Earth's land masses. The analysis will require the use of shapefiles. Another requirement is a trade study tool which they can use to quickly perform "what if" analysis, collect data from, and use to optimize the scenario. You don't have software developers on your team.

Solution

Use STK's SatPro, Coverage, and Analyzer capabilities to create an STK scenario and run trade studies. The trade studies will help determine what your coverage of the Earth's land masses will be over a 24-hour period, considering different variations of altitude on your satellites.

What you will learn

Upon completion of this tutorial, you will have a basic understanding of the following:

  • Satellite Collection object and the Walker Tool
  • Coverage and Shapefiles
  • Analyzer

Video guidance

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

Creating a new scenario

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

  1. Launch STK ().
  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. STK 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!

Turning off theTerrain Server

Terrain will not be used in 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 frame.
  5. Click OK to accept your change and to close the Properties Browser

Creating a seed satellite

The Walker tool makes it easy to generate a Walker constellation. First, define a satellite with the characteristics and orbit you need. The original satellite that is used to create the Walker constellation is referred to as the "Seed" satellite.

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

Synthetic Aperture Radar (SAR) sensor

The Synthetic Aperture Radar (SAR) sensor type synthesizes the aperture of a larger antenna than is actually present, using SAR pattern definitions designed to model the field of regard of a SAR sensor onto the surface of the earth.

Inserting a Sensor object

Insert a Sensor () object that will be used to model the synthetic aperture radar field of regard on the Satellite () object.

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

Defining the SAR

The SAR sensor synthesizes the aperture of a larger antenna than is actually present using 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. 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 the Sensor1 () to Sar_Sens.

Walker constellations

You have defined a satellite with the characteristics and orbit you need. You will use the Satellite Collection () object and the Walker tool to generate a Walker constellation. 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, but it's still available analytically.

Satellite Collection Reference objects

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, STK will associate the entries 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.), STK 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 (optional).
  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. AllSensors is now available.
  8. Click OK to accept your changes and to close the Properties Browser.

Inserting a Coverage Definition

You want to assess coverage over the Earth's land masses. To do this, you will assess coverage of the land masses that is based on the boundaries of a shapefile (.shp). A shapefile is a vector data file format used for geospatial analysis. You will need to specify the region being examined, how each grid point should be treated, and what assets will be used to examine the region. Use a Coverage Definition () object to model the area that you will analyze.

  1. Insert a Coverage Definition () object using the Insert Default () method.
  2. Rename CoverageDefinition1 () to Land_Cov.

Defining the Coverage Grid Area of Interest

You will define the Coverage Grid Area of interest with a Custom Region. You can choose to define the area using the boundary points of an existing area target, one or more region list files (*.rl) or shapefiles (*.shp), or both.

Shapefiles and region list files have an advantage over STK area targets because they 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.

  1. Open Land_Cov's () properties ().
  2. Select the Basic - Grid page when the Properties Browser opens.
  3. Open the Type drop-down menu in the Grid Area of Interest frame.
  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 STK, typically at C:\Program Files\AGI\STK 12\Data\Shapefiles\Land, when the Select Region File dialog box opens.
  8. Select Land.shp.
  9. Click Open.

Specifying Point Granularity

Define the point granularity with Lat/Lon spacing of four degrees.

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

Viewing the grid

View the Coverage grid in the 2D Graphics window.

  1. Bring the 2D Graphics window to the front.
  2. The Coverage grid points are only on the land masses to include islands.

Coverage Grid

Selecting 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

Advanced properties enable you to adjust the manner in which STK stores and computes access information. You will turn off the Automatically Recompute Accesses option. AGI recommends turning off the auto recompute access feature prior to making a number of asset modifications to avoid recomputing multiple times. Remember to either manually compute accesses or turn the auto recompute feature back on after you have applied all changes, so STK will update the access information.

  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 the accesses.

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

Applying Figures of Merit

STK 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 or not 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 () to Simple_Cov.

Viewing the 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. There are some holes in your coverage.

Simple Coverage

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 time 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 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 () list when the Report & Graph Manager opens.
  4. Click Generate....
  5. Note the % Satisfied value at the bottom of the report, for example, ~90 percent.

    6. You can see that the Walker constellation you created has left you short of covering 100 percent of the land mass. You could adjust your satellites and redo your analysis at a variety of altitudes. 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 Analyzer to automate such a study.

  6. Close the report.
  7. Close the Report & Graph Manager.
  8. Save () your scenario.

Analyzer

STK's Analyzer capability is integrated into the STK workflow to help you automate and analyze STK trade studies to better understand the design of your system. For purposes of this tutorial, you will use Analyzer to:

  • Parametrically explore the STK design space in order to analyze your scenario.
  • Perform parameter studies that vary an input variable through a range of values and plot one or more output variables.

Launching Analyzer

Turn on the Analyzer toolbar.

  1. Open the View menu.
  2. Select Toolbars in the first shortcut menu.
  3. Select Analyzer in the second shortcut menu.
  4. Click Analyzer... () in the Analyzer toolbar.

Assigning the input variables

Set up Analyzer to use the appropriate input variables for your analysis.

  1. Select SAR_Sats () in the STK Variables list.
  2. Double click on Walker () in the STK Property Variables list. This moves the Walker tool variables to the Analyzer Variables list as your inputs.

Assigning the output variable

Set up Analyzer to use the appropriate output variable for your analysis.

  1. Expand () Land_Cov () in the STK Variables list.
  2. Select Simple_Cov ().
  3. Expand () the Static Satisfaction () data provider in the Data Provider Variables list.
  4. Double Click Percent Satisfied (). This moves the Percent Satisfied element to the Analyzer Variables list as your output.

The Parametric Study Tool

You have selected your input and output variables. You only want to vary one parameter, so you can use the Parametric Study Tool to run a model through a sweep of values for a specific input variable. You can plot the resulting data to view trends.

You want to:

  • Adjust the altitude of your satellites.
  • Determine the altitude that results in your satellite sensors providing 100 percent coverage of the Earth's land masses.

Defining the design variable

You will run a study starting at a semi major axis of 6900 through 7700 kilometers in 50 kilometer increments.

  1. Click Parametric Study... () on the Analyzer toolbar.
  2. Note the SemiMajorAxis altitude in the Components Tree when the Parametric Study Tool opens.

    3. You will use SemiMajorAxis as the design variable for your study. In the Orbit Wizard, the altitude of Sar_Sat () was approximately 400 km, which is measured from the surface of the WGS84 ellipsoid. Analyzer coverts this to a semi major axis of ~6853 kilometers.

  3. Select SemiMajorAxis () in the Components Tree.
  4. Drag and drop SemiMajorAxis () to the Design Variable field on the right.
  5. Set the following Design Variable values:
    6. Option Value
    starting value: 6900 (km)
    ending value: 7700 (km)
    step size: 50 (km)

    Setting the step size to 50 kilometers 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 6900 and 7700 kilometers. You want to find the lowest semi major axis that provides 100 percent coverage.

Defining the response

Just like you did with the Figure Of Merit () object, you're looking for changes in percent satisfied at each semi major axis change. Set percent satisfied as the response for your parametric study.

  1. Select Percent_Satisfied () in the Components tree.
  2. Drag and drop Percent_Satisfied () to the Responses list.

Running the parametric study

Now that you defined the design variable, Walker variable and response, run your study.

  1. Click Save ().
  2. Click Run.... Be patient. It will take several minutes to analyze the different scenarios.

Reviewing your results

Take a look at the 2D Scatter Plot graph in the Data Explorer. You can edit the graph to make it easier to read.

  1. Bring the 2D Scatter Plot to the front.
  2. Click Add View in the 2D Scatter Plot window.
  3. Select 2D Line Plot.
  4. Look at the 2D Line Plot. You can see that you reach 100 percent coverage at ~7450 kilometers.

Percent Satisfied versus Semimajor Axis

Reviewing at the table data

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

  1. Bring the Table to the front.

    2. Table Data

    The numbers listed above are meant to show you want the results summary looks like and are not intended as specific Analyzer results. Your results may vary from those shown.

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

    3. You can see that you reach 100 percent coverage at a semi major axis of 7450 kilometers for the 24 hour analysis period which matches what you see in the 2D Line Plot.

  3. Close the Data Explorer. This will also close all your plots.
  4. Select No in the Save dialog box.
  5. Close the Parametric Study Tool.
  6. Close Analyzer.

Summary

This tutorial was an introduction to using the Satellite Collection () object and the Walker tool with Analyzer. 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 () object that created a synthetic aperture radar field of regard. Using the Satellite () object as a seed satellite, a Satellite Collection () object and the Walker tool, you created a simple constellation of two satellites opposite of each other in one orbital plane. Using a Coverage Definition () object, you assigned a shapefile as your grid area of interest, which covered all the Earth's land masses and assigned the Sensor () objects as the assets. Using a Figure Of Merit () object and Simple Coverage, you determined that approximately 90 percent of the Earth's land mass was accessed by the Sensor () objects during a twenty four hour analysis period. Using Analyzer's Parametric Study Tool, you varied the Satellite () object's semi major axis to study percent satisfied at various altitudes.

Saving your work

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