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.

Note: You will need a valid STK Professional Edition license for this tutorial.

Create 12 Satellites and a Targeted Sensor

1. Create a scenario and name it Chains.
2. Insert the Colorado Springs facility from the Facility Database.

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

4. Create a satellite and name it Comsat. Open its Orbit page and make the following settings:
5. • Mean Motion: 4 revs/day
   • Inclination: 55 deg

   Tip: To expose the Mean Motion field, open the dropdown list that currently displays Semimajor Axis.

6. Leave other orbital elements at their default values, and click OK to dismiss the Properties Browser.
7. Right-click the satellite in the Object Browser, and select Satellite - Walker. This launches the Walker tool, which enables you easily to create multiple satellites with specified spacing and distribution from a 'seed' satellite.
8. In the Walker dialog, enter the following values:
9. • Type: Delta
   • Number of Planes: 3
   • Number of Sats per Plane: 4
   • Inter Plane Spacing: 1
   • RAAN Spread: 360 deg
10. Click Create Walker.
11. Click Close to dismiss the Walker dialog.
12. Open the 2D Graphics - Global Attributes page for the scenario, uncheck the Vehicles - Show Ground Tracks/Route option, and click OK.
13. Attach a sensor to the facility and name it Relay. Open its Definition page and increase the Cone Angle to 70 deg.
14. Open the Basic - Pointing page for the sensor. Set the Pointing Type to Targeted, and assign as targets all of the 12 satellites that were added with the Walker tool (that is, all satellites except the one named 'Comsat').

15. Tip: To assign all 12 satellites at once, select the first one (Comsat11), hold down the Shift key, and select the last one (Comsat34). Then click the right arrow to move them to the Assigned Targets list.

16. Click OK to dismiss the Properties Browser, then 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. Select the icon in the Object Catalog to add a constellation to the scenario. Name it ComNet.
2. Open the constellation's Definition page, and move all 12 of the satellites that you created with the Walker tool to the Assigned Objects list.
3. Now open the constellation's Constraints - Basic page, make certain that the Restriction field is set to Any Of, and then click OK.
4. Select the icon in the Object Catalog to add a chain to the scenario. Name it ComChain.
5. On the chain's Definition page, add the Relay sensor and the ComNet constellation - in that order - to the Assigned Objects list. Click OK.

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

7. Open the Report & Graph Manager for the chain, and create a Complete Chain Access report. As you can see, the sensor has access to at least one satellite in the constellation throughout the scenario time period.
8. Animate the scenario and observe that access graphics appear continuously in the 2D and 3D Graphics windows.
9. Now return to the ComNet's Constraints - Basic page, set the Restriction field to At Least N, and enter 3 in the textbox. Click OK.
10. Refresh the Complete Chain Access report. The report now lists the periods during which the sensor has access to at least three of the satellites in the constellation.
11. 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 three satellites.
12. Try to set other constraints on the constellation, such as Exactly N with a value of 2.

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