Attitude Coverage: Step by Step

The following is a step by step guide to setting up a relatively simple illustration of how Attitude Coverage can be applied to a common analytical and planning task: to determine the best direction in which to point an Earth-based sensing device over a period of time to have access to the maximum number of satellites in a constellation.

This walkthrough assumes that you are basically familiar with the STK user interface and that you know, for example, that changes you make on a property page or in a popup dialog are not captured until you click Apply or OK. If you are not, try the STK Tutorial for a basic walkthrough of STK.

Note: You will need a valid license of STK Professional Edition and STK/Coverage.

1. Set the Scene

The context for the attitude coverage analysis is a ship crossing the equator near the South American coast, equipped with a sensor that needs to maintain access to as many satellites as possible in a constellation of middle-earth orbiting (MEO) satellites. An attitude sphere centered on the ship will be used for visualization of the results of pointing the sensor in various directions over time.

1. Create a scenario and set its time period from 1 Jun 2003 to 2 Jun 2003.
   
   
2. Select the Ship in the Select an Object field.
   
   
3. Select Define Properties in the Select a Method field.
   
   
4. Click Insert....
   
   
5. Create a ship. On its (Basic) Route page, select Smooth Rate as the Route Calculation Method, and enter two waypoints: 5 deg latitude, -95 deg longitude; and -5 deg latitude, -85 deg longitude. Click Apply.
   
   
6. Open the ship's Attitude Sphere page and turn on the Show option. Click OK.
   
   
7. Dismiss the Insert STK Objects window.
   
   
8. Attach a sensor to the ship. On its Definition page, set Cone Angle to 70 deg.
   
   
9. On the sensor's 3D Graphics Attributes page, set % Translucency to 100. Click OK. This is to avoid displaying the sensor cone in 3D, which would interfere with the display of the attitude sphere.
10. Use the Orbit Wizard to create a satellite with a Circular Orbit and an Altitude of 6000 km.
    
    
11. Use the satellite you just created as the seed for a Walker constellation of 6 planes with 4 satellites each. Check the Create Constellation box and give the constellation a name.

2. Set up Attitude Coverage

The points and regions defined for attitude coverage represent directions in which the attitude-dependent object (here, a sensor) can be pointed.

1. With the ship selected in the Object Browser, create an AttitudeCoverage object.
   
   
2. Open the Properties for the AttitudeCoverage object.
   
   
3. On the Grid page, select Latitude Bounds as the Type, set Min. Latitude to 5 deg, and leave Max. Latitude at 90 deg. This excludes from consideration all directions less than 5 degrees above the horizon.
   
   
4. Click Grid Constraint Options..., check the Use Object Instance for Constraints, select Sensor as the Object Type, highlight the sensor, and click OK.
   
   
5. On the Basic - Assets page, assign the constellation and make certain that the selected Grouping option is Separate.
   
   
6. On the 3D Graphics Attributes page, check the Show Regions box and leave other options unchecked.
   
   
7. This completes the setup of the AttitudeCoverage object. With it still highlighted in the Object Browser, select Compute Accesses from the AttitudeCoverage menu.
   
   

3. Define a Figure of Merit

The purpose of the analysis is to determine the best directions in which to point the sensor, over the relevant time interval, to maximize its access to the satellites in the constellation.

1. Select the Attitude Coverage object in the Object Browser.
   
   
2. Create an AttitudeFigureOfMerit object. On the Definition page, set the Type to N Asset Coverage and the Compute option to Minimum. Do not enable Satisfaction.
   
   
3. On the 3D Graphics Attributes page, turn Static Graphics off and Animation Graphics on. Set Accumulation to Current Time, and check the Fill Points box.
   
   
4. On the 3D Graphics Contours page, select Static as the Type, and check the Show box. Using Start, Stop, Step as the Add Method, set a Start value of 0, a Stop value of 6, and a Step of 1, and click Add Levels. Assign a color to each level, or use the Color Ramp method to assign a segment of the spectrum over the contour levels. Click Apply. Optionally set up a Legend for static contours.
   
   

   Note: Although you will first look at dynamic contours, it is more efficient to go ahead and set up static contours at this point, since dynamic (animation) contour settings can be inherited from the static settings.

5. Now change the contours Type to Animation, and check the Show and Use Static Contours boxes. Optionally set up a legend for dynamic contours.

4. Display Dynamic Contours

As you animate the 3D Graphics window, the attitude sphere will display a changing contour pattern, reflecting, at each time step, the quality of coverage (i.e., the number of satellites to which the sensor would have access) in each direction.

1. In the 3D Graphics window toolbar, click View From/To, and set the view from and to the ship.
   
   
2. Animate the scenario and observe the changing contour pattern.
   
   
3. Stop the animation, open the sensor's Constraints - Sun page, and set a 10 deg Boresight Solar Exclusion Angle.
   
   
4. Accesses will automatically be recomputed.
   
   
5. Animate again and note the effect of the position of the Sun on coverage.

5. Display Static Contours

As you may recall, the Compute option for the figure of merit definition is set to Minimum. Static contours based on this figure of merit will therefore indicate the minimum number of satellites to which the sensor has access at any time during the analysis interval in each direction included in the coverage definition. That is, a pessimistic picture is painted.

1. Return to the 3D Graphics Attributes page for the figure of merit, turn off Animation Graphics and turn on Static Graphics. Check the Fill Points box.
   
   
2. Now look at the 3D Graphics window.

A rather large area, shown here in light green (your colors may differ), has a coverage level of 2, i.e., if the sensor is pointed in any direction within that area, it will always have access to at least two satellites. Note the path (shown here in red) in which coverage is at level 0 because of the solar exclusion angle constraint.

Try other settings for the Attitude Coverage Compute option. For example, suppose it is essential for the sensor to have access to at least 4 satellites, even if for only 25 percent of the time. Set the Compute option to Percent Above, and enter 25 as the Percent Level. You will find that a large number of pointing directions satisfy that criterion.

This walkthrough illustrates only one of many available figures of merit, including two -- Pointing Accuracy and Pointing DOP (Dilution of Precision) -- designed specifically for attitude coverage.

This walkthrough only touches on a few of the possibilities offered by Attitude Coverage. In addition to 3D graphics display, there are several reporting and graphing options for the AttitudeCoverage and AttitudeFigureOfMerit objects. Try some of those out before closing your scenario.