Using Chains & Constellations

While Access is a great tool to analyze pair-wise visibility between objects, you may often be more interested in whether an object has access to a group of objects. In this exercise you will construct a simple example that uses the chain and constellation objects to model a situation in which a ground-based sensor tracks satellites in a constellation.

STK Professional.

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.

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

Create a Scenario

  1. Click the Create a New Scenario button.
  2. Enter the following in the New Scenario Wizard:
    3. OptionValue
    NameChains
    LocationC:\Documents and Settings\<user>\My Documents\STK 11 (x64)\
    Start1 Jul 2016 16:00:00.000 UTCG
    End+ 1 day
  3. When you finish, click OK.
  4. When the scenario loads, click Save (). A folder with the same name as your scenario is created for you in the location specified above.
  5. Verify the scenario name and location and click Save.
  6. If you plan to complete the L3 quiz at the end of the tutorial, turn off the Terrain Server.Closed
    1. Right-click on the scenario () and open the Properties ().
    2. Select the Basic - Terrain page.
    3. Disable the Use terrain for analysis option.
    4. Click OK to apply changes and dismiss the Properties Browser.

Obtain an Archived Satellite Database

  1. Open the Scenario's () Properties ().
  2. Select the Basic - Database page.
  3. Click the "Update Database Files..." button.
  4. Select the following options:
    5. OptionValue
    OptionObtain Archived Database
    Database InformationSpecific Database
    Specific Database FilestkAllTLE
    Archive Date1 Jul 2016 16:00:00.000 UTCG
  5. Click Update.
  6. Click OK on the "Database update complete" pop up notification.
  7. Close the Update Satellite Database panel, then click OK to dismiss the Properties Browser.

Load the GPS Constellation

  1. Bring the Insert STK Objects tool () to the front.

    If the Insert STK Objects window is not already up, open it using Insert - New...

  2. Choose Satellite () as your object to be inserted and select From TLE File in the Method pane.
  3. Click Insert.
  4. Select stkAllTLE.tce and click Open.
  5. Click Modify... and de-select the "On propagation, automatically retrieve elements" option.
  6. Click OK.
  7. In the Common Name text box, replace the * with "GPS" and click Search.
  8. Select all of the BIIF, BIIR and BIIRM satellites except for "GPS BIIRM-7" as it is a reserve satellite. There should be 31 satellites selected.
  9. Click the option to Create Constellation from Selected Satellites and name the constellation"GPS_Constellation".
  10. Click Insert.
  11. Click Close to dismiss the Insert From Satellite Database panel.

Add a Place and Attach a Targeted Sensor to the Place

  1. Using the Insert STK Objects tool, insert Colorado Springs as a Place object () using the From City Database () method.

    You can easily find the Colorado Springs facility by using the asterisk (*) as a wildcard character. For example, enter Colorado* in the City Name field.

  2. Using the Insert STK Objects tool, insert a Sensor () using the Insert Default method.
  3. When the Select Object window opens, select the Place () object.
  4. Click OK.
  5. In the Object Browser, rename the Sensor object "Relay".
  6. Open Relay's () Properties ().
  7. On the Basic - Definition page, increase the Cone Half Angle to 70 deg.
  8. Select the Basic - Pointing page.
  9. Change the Pointing Type to Targeted.
  10. Select the GPS constellation () and move () it to the Assigned Targets list by clicking the right arrow.
  11. On the 2D Graphics - Projection page, set the Extension Distances Maximum Altitude to 20800 km. In the 2D Graphics Window, the sensor's field-of-view projection is set to the highest GPS altitude achieved in the scenario.
  12. Click OK to dismiss the Properties Browser.
  13. Animate () the scenario and watch as the sensor tracks the satellites in the 2D and 3D Graphics windows.

Use Chain and Constellation Objects to Analyze and Constrain Visibility

  1. Open the constellation's () Properties ().
  2. Browse to the Basic - Definition page, and verify that all of the GPS satellites appear in the Assigned Objects list.
  3. Browse to constellation's Constraints - Basic page, make certain that the Restriction field is set to Any Of, and then click OK.

Add a Chain

  1. Using the Insert STK Objects tool, insert a Chain () using the Insert Default method.
  2. In the Object Browser, rename the Chain object "ComChain".
  3. Open the Chain's () Properties ().
  4. Select the Basic - Definition page.
  5. Add () the Relay sensor () and the GPS constellation () - in that order - to the Assigned Objects list by clicking the right arrow. Click OK to dismiss the Properties panel.

    6. It is important to add the sensor to the chain first, then add the constellation.

Create a Report

  1. Open the Report & Graph Manager ().
  2. Select the following:
    3. Option Value
    Object TypeChain
    Object (Below Object Type)ComChain
    Show ReportsOn
    Show GraphsOff
    StyleComplete Chain Access
    Generate asReport/Graph
  3. Click Generate.

    4. As you can see, the sensor has access to at least one satellite in the constellation throughout the scenario time period.

    Depending on the sampling time step used in the calculations, you may see some very brief access gaps when the sensor pointing switches from one satellite to another.

  4. Leave the Complete Chain Access report open.
  5. Animate () the scenario and observe that access graphics appear continuously in the 2D and 3D Graphics windows.

Restrict the GPS Constellation

  1. Open the GPS constellation's () Properties ().
  2. On the Constraints - Basic page, set the Logical Restriction field to At Least N and enter four (4) in the textbox.
  3. Click OK.
  4. Refresh () the Complete Chain Access report. The report now lists the periods during which the sensor has access to at least four of the satellites in the constellation.

    5. If you animate () the scenario again, you will see that access graphics are periodically interrupted, since there are times when the sensor has access to fewer than four satellites.

  5. Try to set other constraints on the constellation, such as Exactly N with a value of six (6).

  6. Save () your work.

This exercise gives only a small sample of the many analytical tasks that you can perform with this useful and versatile module.

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