Using Chains and Constellations

STK Pro, STK Premium (Air), STK Premium (Space), or STK Enterprise
You can obtain the necessary licenses for this training 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.

This lesson requires STK 12.0.1 or newer to complete it in its entirety.

Capabilities Covered

This lesson covers the following STK Capabilities:

  • STK Pro

Problem Statement

Engineers and technicians require a simple, quick way to analyze whether one object has access to a constrained group of objects, not just the individual objects. You need to analyze how many satellites a ground sensor can see during a 24 hour period. You also need to determine when you can see four or more of the satellites at any given time.

Solution

Using STK Pro, you will construct a 24 hour scenario that uses Chain and Constellation objects. You will first model a constellation of satellites and a ground-based sensor tracking any satellite in the constellation. Next, you will set up a Chain object that defines the access between the ground-based sensor and the constellation. Then, you will calculate when the sensor can see any of the satellites using the Chain object. Last, you will add the constraint that the sensor must see at least four satellites simultaneously, and recompute access.

What You Will Learn

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

  • Constrain Constellation objects
  • Create a Chain object
  • Generate and understand a Complete Chain Access Report

Video Guidance

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

Create a Scenario

  1. Click the Create a Scenario () button.
  2. Enter the following in the New Scenario Wizard:
    3. Option Value
    Name Chains
    Location C:\Users\<username>\Documents\STK 12\
    Start 1 Jul 2020 16:00:00.000 UTCG
    End + 1 day
  3. 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 .

 

Load the GPS Constellation

Insert the GPS constellation using the Load GPS Constellation method. STK will insert the GPS Satellites using orbital elements from GPS almanac files and place them in a Constellation () object.

  1. Using the Insert STK Objects Tool, insert a Satellite () object using the Load GPS Constellation method.
  2. Notice the GPS satellites () and the GPSConstellation () object are inserted into the scenario.

Add a Place and Attach a Targeted Sensor to the Place

  1. 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. Insert a Sensor () using the Insert Default method.
  3. When the Select Object window opens, select Colorado Springs ().
  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 GPSconstellation () and move () it to the Assigned Targets list .
  11. On the 2D Graphics - Projection page, set the Extension Distances Maximum Altitude to 20850 km. This sets the sensor's field-of-view projection in the 2D Graphics Window to the highest GPS altitude achieved in the scenario.
  12. Click OK to accept the changes and close 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 GPSConstellation'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 fields are set to Any Of
  4. Click OK to accept the changes and close the Properties Browser.

Add a Chain

  1. Insert a Chain () using the Insert Default method.
  2. In the Object Browser, rename the Chain object "ComChain".
  3. Open the ComChain's () properties ().
  4. Select the Basic - Definition page.
  5. Add () Relay () to the Assigned Objects list.
  6. Then add () GPSconstellation () to the Assigned Objects list.

    7. It is important to add the sensor to the chain first, then add the constellation. Order matters in Chain objects.

  7. Click OK to accept the changes.
  8. Close the Properties Browser.

Create a Report

  1. Open the Report & Graph Manager ().
  2. Select the following:
    3. Option Value
    Object Type Chain
    Object (Below Object Type) ComChain
    Show Reports On
    Show Graphs Off
    Style Complete Chain Access
    Generate as Report/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 GPSConstellation's () properties ().
  2. On the Constraints - Basic page, set 'To' access position: to At Least N and set it to four (4).
  3. Click OK to accept the changes and close the Properties Browser.
  4. Refresh () the Complete Chain Access report. The report now lists the periods during which the sensor has access to at least four (4) of the satellites in the constellation.
  5. 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 (4) satellites.
  6. Try to set other constraints on the constellation, such as Exactly N with a value of six (6).

  7. Save () your work.

Summary

During a 24 hour analysis period, you wanted to determine how many satellites in a constellation of satellites you could track both individually and in groups of four or more. You inserted the GPS constellation of satellites. Next, you created a ground site using a targeted sensor which locked onto the nearest satellite, but could still see other satellites inside its field of view. You then constrained the constellation so that the sensor had to see four or more satellites at any given time. You built a Chain object used to determine when you could see the satellites.

On Your Own

Try using the From City Database method to load all the United States capitals as Target () objects. Assign them to a Constellation () object. Put a Sensor () object on a Satellite () object, and determine when the satellite can see the capital cities. Remember, in order to calculate access to a group of assets, a constellation, you have to use a Chain object. Have fun!