Using Chains and Constellations

STK Pro, STK Premium (Air), 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.

This lesson requires STK 12.8 or newer to complete in its entirety. If you have an earlier version of STK, you can view a legacy version of this lesson.

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. 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

Creating a scenario

Create a new STK Scenario () object using the STK: New Scenario Wizard tool. The Scenario object defines the context in which the properties and behavior of other objects are defined.

  1. Launch STK ().
  2. Click Create a Scenario in the Welcome to STK dialog box.
  3. Enter the following in the STK: New Scenario Wizard:
    4. Option Value
    Name Chains
    Location Default
    Start 1 Nov 2023 16:00:00.000 UTCG
    End + 1 day
  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 dialog box.
  7. Click Save.

Save () often during this lesson!

Loading the GPS constellation

You will insert a Satellite () object using the Load GPS Constellation () method. STK will propagate the GPS satellites using orbital elements from GPS almanac files and place them in a Constellation () object.

  1. Select Satellite () in the Insert STK Objects Tool.
  2. Select the Load GPS Constellation () method.
  3. Click Insert... .

Creating the ground site

You will create a ground site with a targeted sensor using both a Place () object and a Sensor () object.

Inserting a Place object

You will insert a Place () object using the From City Database () method. The city database contains thousands of cities around the world.

  1. Insert a Place () object using the From City Database () method.
  2. Type Colorado Springs in the Name: field when the Search Standard Object Data dialog box opens.
  3. Click Search.
  4. Select Colorado Springs Colorado in the Results: list.
  5. Click Insert.
  6. Click Close to close the Search Standard Object Data dialog box.

Inserting a Sensor object

You will insert a Sensor () object.

  1. Insert a Sensor () object using the Insert Default () method.
  2. Select Colorado Springs () in the Select Object dialog box.
  3. Click OK to close the Select Object dialog box.
  4. Right click on Sensor1 () In the Object Browser.
  5. Select Rename in the shortcut menu.
  6. Rename Sensor1 () to Relay.

Updating Relay's properties

Update Relay's () cone half angle.

  1. Open Relay's () properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.
  3. Enter 70 deg in the Cone Half Angle field in the Simple Conic frame.
  4. Click Apply to accept your change and to keep the Properties Browser open.

GPSConstellation pointing type

The Targeted pointing type causes the sensor to point to other objects in the scenario. The sensor can point to different objects over time. If two objects are specified at the same time, the sensor points to the first object it sees.

  1. Select the Basic - Pointing page.
  2. Open the Pointing Type: shortcut menu.
  3. Select Targeted.
  4. Move () GPSConstellation () from the Available Targets list to the Assigned Targets list.
  5. Click Apply to accept your change and to keep the Properties Browser open.

Sensor projection

2D Projection Graphics for sensors control the display of sensor projection graphics in the 2D and 3D Graphics windows.

  1. Select the 2D Graphics - Projection page
  2. Enter 20850 km in the Extension Distances > Projection Altitude > Maximum Altitude: field.

    3. This sets the sensor's field-of-view projection in the 2D and 3D Graphics windows to the highest GPS altitude achieved in the scenario.

  3. Click OK to accept your changes and to close the Properties Browser.

Visualizing Relay in the 3D Graphics window

You will watch Relay () in the 3D Graphics window as it tracks the satellites in GPSConstellation ().

  1. Bring the 3D Graphics window to the front.
  2. Using your mouse, zoom out until you can see Relay's () field of view and the individual satellites.
  3. Click Start () in the Animation toolbar to animate the scenario.
  4. Watch as Relay () tracks the satellites.

    5. Relay () will target the first satellite it sees. When it loses tracking on that satellite, it will automatically target the next satellite that it sees.

  5. When finished, click Reset () in the Animation toolbar.

Inserting a Chain object

A chain is a list of objects (either individual or grouped into constellations or satellite collection subsets).

  1. Insert a Chain () object using the Insert Default () method.
  2. Rename Chain1 () to CommChain.

Defining the start and end objects

Start by choosing the start object and end object in your chain.

  1. Open CommChain's () properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.
  3. Click the Start Object: ellipsis ().
  4. Select Relay () in the Select Object dialog box.
  5. Click OK to close the Select Object dialog box.
  6. Click the End Object: ellipsis ().
  7. Select GPSConstellation () in the Select Object dialog box.
  8. Click OK to close the Select Object dialog box.
  9. Click Apply to accept your changes and to keep the Properties Browser open.

Creating the Chain object's connection

After you choose the start and end objects in your chain, you need to build the chain's connections.

  1. Click Add in the Connections frame.
  2. Click the From Object: ellipsis ().
  3. Select Relay () in the Select Object dialog box.
  4. Click OK to close the Select Object dialog box.
  5. Click the To Object: ellipsis ().
  6. Select GPSConstellation () in the Select Object dialog box.
  7. Click OK to close the Select Object dialog box.
  8. Click OK to accept your changes and to close the Properties Browser.

Creating a complete chain access report

A complete chain access report shows the time intervals where the chain is completed. The chain is complete when the relay detects a satellite in the GPSConstellation (). These intervals are computed by overlapping all the strand accesses.

  1. Right-click on CommChain () in the Object Browser.
  2. Select Report & Graph Manager... () in the shortcut menu.
  3. Select the Complete Chain Access () report in the Installed Styles list once the Report & Graph Manager opens.
  4. Click Generate...
  5. View the data in the report to determine when complete chain accesses occur during your scenario.

    6. As you can see Relay () 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.

  6. Leave the Complete Chain Access report open.

Visualizing accesses in the 3D Graphics window

You can visualize Relay's () accesses to the satellites in GPSConstellation () in the 3D Graphics window.

  1. Bring the 3D Graphics window to the front.
  2. Using your mouse, zoom out until you can see Relay () and the individual satellites.
  3. Click Start () in the Animation toolbar to animate the scenario.
  4. Watch as Relay () tracks the satellites.

    5. Relay () will target the first satellite it sees. When it loses tracking on that satellite, it will automatically target the next satellite that it sees. However, any satellite in Relay's () field of view will be accessed.

  5. When finished, click Reset () in the Animation toolbar.

Between the report and the 3D Graphics window, you can know exactly when the Relay can view a satellite.

Viewing at least four satellites

You will use the At Least N constraint to determine when Relay () accesses at least four satellites in GPSConstellation (), rather than just one satellite. The At Least N constraint uses AND/OR criterion. The constraint is satisfied if at least the specified number of objects in the constellation meet the conditions for chain access.

  1. Open GPSConstellation's () properties ().
  2. Select the Constraints - Basic page when the Properties Browser opens.
  3. Open the 'To' access position: shortcut menu in the Logical Restriction frame.
  4. Select At Least N.
  5. Enter 4 in the 'To' access position: field.
  6. Click OK to accept your changes and to close the Properties Browser.

Refreshing the complete chain access report

You will refresh the report and it will show you when Relay () accesses at least four satellites in GPSConstellation ().

  1. Bring the complete chain access report to the front.
  2. Click Refresh (F5) () the Complete Chain Access report toolbar.

    3. The report now lists the periods during which the sensor has access to at least four of the satellites in the constellation.

  3. Right click on the first access Start Time.
  4. Select Start Time in the first shortcut menu.
  5. Select Set Animation Time in the second shortcut menu.

Visualizing accesses in the 3D Graphics window

In the 3D Graphics window, you can visualize Relay's () accesses to at least four satellites in GPSConstellation (). When there are less than four satellites visible, there are no accesses because of the At Least N constraint.

  1. Bring the 3D Graphics window to the front.
  2. You can see that four satellites are visible in Relay's () field of view.
  3. Click Start () in the Animation toolbar to animate the scenario.

    4. You can see that access graphics are periodically interrupted since there are times when the sensor has access to fewer than four satellites.

  4. When finished, click Reset () in the Animation toolbar.

Summary

During a 24 hour analysis period, you wanted to determine the number of satellites from a constellation that you could track. You determined the amount you could track individually and in groups of four or more.

In STK, 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 created a chain which you used to determine when you could access 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!