Part 6: Using Terrain, 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 tutorial requires version 12.9 of the STK software or newer.

Capabilities covered

This lesson covers the following capability of the Ansys Systems Tool Kit® (STK®) digital mission engineering software:

  • STK Pro

Problem statement

Engineers and operators require an expedient way to determine if local terrain is affecting visibility between ground sites and satellites for purposes such as communications, imaging, and general situational awareness. You are conducting a line-of-sight test between a constellation of GPS satellites, a ground-based test team located in mountainous terrain, a CubeSat communications satellite, and a base team located in Morton, Washington. The effects of the terrain must be taken into account. This test could be used to schedule times to transmit location information if someone has access to GPS and a simple FM transmitter and wants to schedule prearranged communications with the base team using a low-Earth-orbiting satellite.

Solution

Use the STK application to load a USGS Digital Elevation Model (DEM) file and analyze the impact of local terrain on accesses between the GPS constellation, the test team, a CubeSat satellite, and the base team. First, change the DEM file into a STK terrain file (.pdtt), which can be used to visualize terrain in the 3D Graphics window. Then, create a constellation of GPS satellites, use Place objects as the teams' locations, factor in the impact of local terrain using an azimuth-elevation (AzEl) mask, and utilize a Chain object to create connections between all the nodes.

What you will learn

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

  • Create an STK terrain file (.pdtt) from the local terrain file
  • Use local terrain files for analysis
  • Model a Constellation object
  • Model a Chain object

Video guidance

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

Creating a new scenario

Create a new scenario with a run time of 24 hours.

  1. Launch the STK application ().
  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

    TerrainChainsConstellations
    Start Default
    Stop Default
  4. Click OK when you finish.
  5. Click Save () when the scenario loads. A folder with the same name as your scenario is created for you.
  6. Verify the scenario name and location in the Save As dialog box.
  7. Click Save.

    8. Save () often!

Locating the U.S. Geological Survey DEM file

Find and copy the preinstalled USGS Digital Elevation Model (DEM) file for use in your scenario. The USGS Digital Elevation Model is the standard set forth by the US Geological Survey (USGS) for handling digital elevation data for the United States and Puerto Rico.

The DEM file used in this tutorial is located inside a compressed (zipped) file in the install directory. Take a moment to copy the file out of the zipped file archive. Do not extract the entire archive. Follow the instructions in the tutorial to use the file.

  1. Using File Explorer, browse to the location of the DEM file in the install directory:
    • STK 12: C:\Program Files\AGI\STK 12\CodeSamples\CodeSamples.zip\SharedResources\Scenarios\Events
    • STK 13: C:\Program Files\AGI\STK_ODTK 13\CodeSamples\STKCodeSamples.zip\SharedResources\Scenarios\Events
  2. Copy the file named hoquiam-e.dem.
  3. Paste the hoquiam-e.dem file on your desktop.
  4. Close File Explorer.

Using the DEM file for analysis and visualization

The STK application provides a foundation for analyzing and visualizing complex systems in the context of their missions. You'll use the DEM terrain data file you copied out of the zipped Code Samples file for both analytical and visual terrain in your scenario. The file contains digital elevation data for the Mount Saint Helens vicinity near the city of Hoquiam, Washington.

Turning off streaming terrain

By turning off streaming terrain, you're simulating what you'd see in a setting that doesn't have an internet connection.

  1. Right-click on TerrainChainsContellations () in the Object Browser.
  2. Select Properties () in the shortcut menu.
  3. Select the Basic - Terrain page when the Properties Browser opens.
  4. Clear the Use terrain server for analysis check box in the Terrain Server panel.
  5. Click Apply to accept your changes and keep the Properties Browser open.

Loading the local DEM file

Now, load the hoquiam-e.dem terrain data file on your local machine into the scenario for visualization and analysis.

  1. Click Add in the Custom Analysis Terrain Sources panel.
  2. Open the file type drop-down list when the Open dialog box opens.
  3. Select USGS DEM (DEM).
  4. Browse to your desktop.
  5. Select hoquiam-e.dem.
  6. Click Open to select the file and to close the Open dialog box.
  7. Click Apply to accept your changes and keep the Properties Browser open.

Updating the local satellite database

Database properties enable you to set the defaults for the city, facility, satellite, and star databases. You can specify a stock STK database or one of your own that meets the STK software's format requirements.

If you do not have an internet connection, you can disregard this task. You'll still be able to insert GPS satellites using the default satellite database that is installed with your version of the STK application. The two-line element (TLE) data will be older, but it will suffice for this training. Skip to Creating a terrain inlay using the Imagery and Terrain Converter.

If you're using an internet connection, use the following steps:

  1. Select the Basic - Database page.
  2. Make sure Satellite is showing in the Database Type drop-down list.
  3. Click Update Database Files....
  4. Click Update when the Update Satellite Database dialog box opens.
  5. Click OK to close the Information dialog box when the database update is complete.
  6. Click Close to close the Update Satellite Database dialog box.
  7. Click OK to accept your changes and to close the Properties Browser.

Creating a terrain inlay using the Imagery and Terrain Converter

Use the Imagery and Terrain Converter to create a STK terrain file (.pdtt) for a specific region.

Selecting the DEM file for conversion

Select your input data file for the source of the terrain data.

  1. Open the Utilities menu.
  2. Select Imagery and Terrain Converter....
  3. Select the Terrain Region page when the Imagery and Terrain Converter opens.
  4. Open the Terrain Source drop-down list in the Input Data panel.
  5. Select the path to the hoquiam-e.dem file.

Setting the terrain Output Data

Specify the file location and the filename of the terrain file to be created.

  1. Click the Directory ellipsis ().
  2. Navigate to the location of your scenario (e.g. C:\Users\<username>\Documents\STK_ODTK 13\TerrainChainsConstellations when the Directory dialog box opens.
  3. Click Select Folder to confirm your selection and to close the Directory dialog box.
  4. Enter StHelensTerrain in the Filename field in the Output Data panel.
  5. Click Convert.
  6. Click Close to close the Imagery and Terrain Converter when finished.

Adding terrain and imagery files in Globe Manager

Globe Manager allows you to customize scenario globes with imagery and terrain data and to manage that data once it has been applied.

Opening Globe Manager

Open Globe Manager from the Globe Manager Toolbar in the 3D Graphics window.

  1. Bring the 3D Graphics window to the front.
  2. Click Globe Manager () in the 3D Graphics window toolbar.
  3. Click Add Terrain/Imagery () In the Hierarchy toolbar when Globe Manager opens.

    4. The Hierarchy tab is used to add central bodies, image, and terrain items to a scenario.

  4. Select Add Terrain/Imagery... () in the shortcut menu.

Selecting the files to display in the 3D Graphics window

You can use the Globe Manager: Open Terrain and Imagery Data dialog box to select imagery and terrain data to display in the 3D Graphics window.

  1. Open the Path drop-down list when the Globe Manager: Open Terrain and Imagery Data dialog box opens.
  2. Select the path to your scenario (e.g. C:\Users\<username>\Documents\STK_ODTK 13\TerrainChainsConstellations).
  3. Select the check box for StHelensTerrain.pdtt.
  4. Click Add.
  5. Click No when the Use Terrain for Analysis dialog box opens.

    6. You're already using the hoquiam-e.dem file for analysis.

Viewing the terrain inlay in the 3D Graphics window

Gain situational awareness by viewing the terrain overlay in the 3D Graphics window.

  1. Bring the 3D Graphics window to the front.
  2. Right-click on StHelensTerrain.pdtt in Globe Manager.
  3. Select Zoom To () in the shortcut menu.
  4. Use your mouse to view the image and surrounding terrain.

    5. Mount Saint Helens Terrain

Modeling the GPS satellite constellation

The Global Positioning System (GPS) is deployed and operated as a constellation of navigational satellites. A Constellation () object in the STK application allows you to group objects together for use with other analysis tools available in the STK software, such as Chains and Coverage capabilities. Constellation objects can be used to group together not just satellite constellations, but also ground station networks, groups of targets, and multiple sensors. A Constellation object allows you to apply constraints and a routing file that determine the criteria as to how the Constellation is used in Chain computations.

Inserting a Satellite object

Start by inserting a Satellite () object using the From Standard Object Database () method.

  1. Bring the Insert STK Objects tool () to the front.
  2. Select Satellite () in the Insert STK Objects tool ().
  3. Select the From Standard Object Database () method.
  4. Click Insert. . .

Inserting active GPS satellites

If you were analyzing navigational accuracy, dilution of precision, and so on, you would want to use the From GPS Almanac method to propagate your GPS satellites using the GPS propagator. Since you are analyzing accesses between the test team and GPS satellites, you can use your local database of satellites and TLE sets.

  1. If you have an Internet connection, clear the Data Sources - Online check box when the Search Standard Object Data dialog box opens. This will only show satellite selections from your local database.

    2. If you do not have an internet connection, the Online data source option will be unavailable and the Local option will be selected by default.

  2. Enter GPS in the Name or ID field.
  3. Select the Operational Status - Active check box. This selection will only show GPS satellites that are active or operational during your analysis time period.
  4. Click Search.
  5. Use CTRL+ Click to select only those GPS satellites whose Mission is listed as Navigation in the Results list.
  6. Select the Create Constellation from Selected check box in the Insert Options panel. This will automatically group all of the selected GPS satellites into a Constellation () object.
  7. Enter GPS_Satellites in the Name field in the Insert Options panel.
  8. Click Insert.
  9. Click Close to close the Search Standard Object Data dialog box after your satellites have been propagated.

Viewing the objects assigned to the Constellation

Open the Constellation object's definition page to view the objects that are grouped into the constellation.

  1. Open GPS_Satellites' () Properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.

    3. You can see that the GPS Satellite () objects were placed in the Assigned Objects list when you created the Constellation () object in the Search Standard Object Data dialog box.

Updating the Constellation object's constraints

Constellation constraints allow you to specify the criteria to be used when constellations are combined with other objects in a chain. Each pair of objects in the chain can be thought of as creating access pairs with a "from" object and a "to" object. The constellation constraints allow you to specify different logical and parent ownership constraints depending on where the constellation sits in the chain, either as the "from" object or the "to" object. In this instance, you are "sending" from the GPS satellites to the test team. Therefore, you will use the "From" access position logical restriction.

  1. Select the Constraints - Basic page.
  2. Open the From access position drop-down list in the Logical Restriction panel.
  3. Select At Least N.
  4. Enter 4 in the At Least N field.

    5. Although you aren't analyzing dilution of precision or navigation accuracy, you are testing the link for proper accesses. Therefore, you want to make sure that the test team can access at least four GPS satellites at all times. Anything less than four will not be considered a successful access.

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

Cleaning up the 2D and 3D Graphics windows

Although it's not required, you can adjust the properties of the 2D and 3D Graphics windows to have better situation awareness later in your scenario.

Clearing the satellite orbit tracks

Remove the visible orbit tracks from the 2D and 3D Graphics windows.

  1. Open TerrainChainsConstellations' () Properties ().
  2. Select the 2D Graphics - Global Attributes page when the Properties Browser opens.
  3. Clear the Show Orbits/ Trajectories check box in the Vehicles panel.
  4. Click OK to accept your change and to close the Properties Browser.

Decluttering 3D Graphics window labels

Objects located on the surface of the terrain could be covered by the terrain, which makes them unreadable. You can fix this by making a change to the 3D Graphics window's properties.

  1. Bring the 3D Graphics window to the front.
  2. Click Properties () on the 3D Graphics window Default toolbar.
  3. Select the Details page when the Properties Browser opens.
  4. Select the Enable check box in the Label Declutter panel.
  5. Click OK to accept your change and to close the Properties Browser.

Inserting the test team's location

Insert a Place () object, which will simulate the location of the test team.

  1. Bring the Insert STK Objects tool to the front.
  2. Insert a Place () object using the Insert Default () method.
  3. Right-click on Place1 () in the Object Browser.
  4. Select Rename in the shortcut menu.
  5. Rename Place1 () to TestTeam.

Updating the test team's position

The test team is located in very mountainous terrain. Update the position of TestTeam () to reflect this.

  1. Open TestTeam's () Properties ().
  2. Select the Basic - Position page when the Properties Browser opens.
  3. Set the following options in the Position panel:
    4. Option Value
    Latitude 46.304 deg
    Longitude -122.321 deg
    Height Above Ground 6 ft

    Height Above Ground represents the height of Test Team's antenna if you were analyzing communication devices.

  4. Click Applyto accept your changes and to keep the Properties Browser open.

Using the terrain mask constraint

Use the terrain mask constraint in your analysis. The STK application constrains access to the object to which access is being calculated by any terrain data in the line of sight. The terrain mask constraint determines instantaneous visibility based on detecting intersections of the instantaneous line of sight with the terrain surface.

  1. Select the Constraints - Active page.
  2. Click Add new constraints () in the Active Constraints toolbar.
  3. Select Terrain Mask in the Constraint Name list when the Select Constraints to Add dialog box opens.
  4. Click Add.
  5. Click Close to close the Select Constraints to Add dialog box.
  6. Click OK to accept your changes and to close the Properties Browser.

Inserting the base team's location

The base team is located in the city of Morton, Washington.

  1. Bring the Insert STK Objects tool to the front.
  2. Insert a Place () object using the From City Database () method.
  3. Enter Morton in the Name field when the Search Standard Object Data dialog box opens.
  4. Click Search.
  5. Select Morton - Washington in the Results list.
  6. Click Insert.
  7. Click Close to close the Search Standard Object Data dialog box.

Raising Morton's height above the ground

Although you aren't using an actual antenna in your analysis, the location of the antenna on the base team's building is located on the building's roof, which is 25 feet above ground level.

  1. Open Morton's () Properties ().
  2. Select the Basic - Position page when the Properties Browser opens.
  3. Enter 25 ft in the Height Above Ground field in the Position panel.
  4. Click Apply to accept your changes and to keep the Properties Browser open.

Defining an azimuth-elevation mask for analysis

Using the azimuth-elevation (AzEl) mask is another way of using analytical terrain in your analysis. The AzElMask properties, which are a part of the Basic properties for facilities, places and targets, enable you to define an AzEl mask for the facility, place, or target. When computing the AzEl Mask from terrain, terrain blockage is only modeled up to the ground distance specified by the maximum range that was considered when generating the mask. If the AzEl Mask constraint is used when doing access to an object, and the ground distance to the object is larger than the maximum range that was considered when computing the mask, then the mask may fail to correctly model the terrain blockage. The AzEl Mask constraint leverages a provided or computed AzEl Mask to determine visibility. The mask may be computed from terrain information to be representative of terrain-based visibility restrictions.

You can construct terrain-based AzEl masks by extending a number of rays in directions of constant azimuth outwards from the facility, place, or target location. Obstruction information is stored along each ray. During visibility computations, the STK software uses obstruction information from the two rays that bound the current direction of interest to compute an interpolated visibility metric.

Defining Morton's AzEk mask

Define an AzEl mask for Morton through its Properties.

  1. Select the Basic - AzElMask page.
  2. Set the following options:
    3. Option Value
    Use Terrain Data
    Max range to consider 160 km
    Use Mask for Access Constraint Selected
  3. Click Applyto accept your changes and to keep the Properties Browser open.

    4. Using Terrain Data automatically creates and stores an AzEl mask file, which is an ASCII text file that is formatted for compatibility with the STK software and ends in an .aem extension, into your scenario folder. Selecting Use Mask for Access Constraint enables the AzEl Mask constraint located on the Constraints - Active page. Using the AzElMask constraint constrains access to a 360-degree field of view around the object being constrained.

Displaying Morton's AzEl mask

For situational awareness, you can display the AzEl mask in both the 2D Graphics and 3D Graphics windows at a specified number of steps from the minimum to the maximum range from Morton's antenna.

  1. Select the 2D Graphics - AzElMask page.
  2. Set the following properties in the At Range panel:
    3. Option Value
    Show Selected
    Number of Steps 16
    Minimum Range 0 km
    Maximum Range 160 km
  3. Click OKto accept your changes and to close the Properties Browser.

Viewing the AzEl mask in the 3D Graphics window

View the AzEl mask in context in the 3D Graphics window.

  1. Bring the 3D Graphics window to the front.
  2. Right-click on Morton () in the Object Browser.
  3. Select Zoom To in the shortcut menu.
  4. Using your mouse, zoom out until you can see the visual representation of the AzEl mask.

    5. Morton's Azimuth-Elevation Mask

Adding a communications satellite

When looking at the AzEl mask in the 3D Graphics window, it's clear that the base team cannot access test team directly through line-of-sight communication. In order for the test team to access the base team, a CubeSat communications satellite, which is located in a low Earth orbit (LEO), is needed.

Inserting a satellite object

Insert a Satellite () object to model the CubeSat satellite.

  1. Bring the Insert STK Objects tool to the front.
  2. Insert a Satellite () object using the Insert Default () method.
  3. Rename Satellite1 () to CubeSat.

Propagating the CubeSat satellite

You can set the orbital parameters of the CubeSat satellite using CubeSat's properties.

  1. Open CubeSat's () Properties ().
  2. Select the Basic - Orbit page when the Properties Browser opens.
  3. Enter the following orbital parameters:
    4. Option Value
    Semimajor Axis 6997 km
    Eccentricity 0.02
    Inclination 64.8 deg
    Argument of Perigee 267 deg
    RAAN 2 deg
    True Anomaly 302 deg
  4. Click OK to propagate CubeSat () and to close the Properties Browser.

Building a Chain object

You are now ready to test access starting from the GPS satellite constellation and ending at Morton. You need a Chain () object to test all the links in the access. "Links" in the Chain can be individual objects like satellites, sensors, and places, or grouped objects, like constellations. By defining a Start object, an End object, and sets of paired object connections, the STK software can compute times when one object can access another through connections to one or more other objects.

Inserting the Chain object

Assign objects to the chain and define the order in which objects are accessed.

  1. Bring the Insert STK Objects tool to the front.
  2. Insert a Chain () object using the Insert Default () method.
  3. Rename Chain1 () to GPS_to_Morton.

Defining the start and end objects

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

  1. Open GPS_to_Morton's () Properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.
  3. Click the Start Object ellipsis ().
  4. Select GPS_Satellites () when the Select Object dialog box opens.
  5. Click OK to close the Select Object dialog box.
  6. Click the End Object ellipsis ().
  7. Select Morton () when the Select Object dialog box opens.
  8. Click OK to close the Select Object dialog box.

Creating the Chain object's first connection

After you choose the start and end objects in your chain, you need to build the chain's connections. It doesn't matter in which order you place the connections in the Connections list. What matters is the From Object must be able to access the To Object.

  1. Click Add in the Connections panel.
  2. Click the From Object ellipsis ().
  3. Select GPS_Satellites () when the Select Object dialog box opens.
  4. Click OK to close the Select Object dialog box.
  5. Click the To Object ellipsis ().
  6. Select TestTeam () when the Select Object dialog box opens.
  7. Click OK to close the Select Object dialog box.

Creating the Chain object's second connection

When the test team accesses at least four GPS satellites, it will upload its fix to the CubeSat satellite.

  1. Click Extend in the Connections panel.
  2. Click the To Object ellipsis ().
  3. Select CubeSat () when the Select Object dialog box opens.
  4. Click OK to close the Select Object dialog box.

Creating the Chain object's final connection

The CubeSat satellite will download test team's fix to the base team located in Morton.

  1. Click Extend in the Connections panel.
  2. Click the To Object ellipsis ().
  3. Select Morton () when the Select Object dialog box opens.
  4. Click OK to close the Select Object dialog box.
  5. Click OK to accept your changes and to close the Properties Browser.

Computing the Chain object accesses

You are ready to compute the Chain object's accesses.

  1. Select GPS_to_Morton () in the Object Browser.
  2. Select the Chain menu.
  3. Select Compute Accesses.

Generating a Complete Chain Access report

Generate a Complete Chain Access report to analyze whether or not you have accesses during your scenario analysis period. A complete chain access reports the time intervals for which the chain is completed. These intervals are computed by overlapping all the strand accesses.

  1. Right-click GPS_to_Morton () in the Object Browser.
  2. Select Report & Graph Manager... () in the shortcut menu.
  3. Select the Complete Chain Access () report in the Installed Styles folder () in the Styles panel when the Report & Graph Manager opens.
  4. Click Generate....

    5. You could use this report to schedule times during the next 24 hours, during which time you will have windows of opportunity to transmit test team's location to the base team.

  5. Right-click on the Start Time of the longest access duration.
  6. Select Start Time in the shortcut menu.
  7. Select Set Animation Time in the Start Time submenu.

Viewing the accesses in the 3D Graphics window

You can view the complete chain access in the 3D Graphics window.

  1. Bring the 3D Graphics window to the front.
  2. Zoom to Morton ().
  3. Using your mouse, maneuver your view to get an idea of all the connections in your chain.

    4. Complete chain access

    Your view will be different than the one shown, but it will be similar. You can see the downlink accesses from at least four GPS satellites to the test team. Next, you can see the uplink from the test team to the CubeSat satellite. Finally you can see the downlink from the CubeSat satellite to Morton.

Saving your work

Clean up and close out your scenario.

  1. Close any open reports, properties, and the Report & Graph Manager.
  2. Save () your work.

Summary

You conducted a line-of-sight test between a constellation of GPS satellites, a ground based test team located in mountainous terrain, a CubeSat and a base team located in Morton, Washington. You loaded a USGS DEM file to analyze the impact of local terrain on accesses between the GPS constellation, the test team, the CubeSat and the base team. Using the Terrain Region Converter, you changed the DEM file into a terrain inlay file and used that to visualize the terrain in the 3D Graphics window and to create an azimuth-elevation mask. Next, you created a constellation of GPS satellites, used Place objects to represent the teams' locations, and a Chain object to create connections between all the objects. Finally, you visualized the complete chain access linking the components together and generated a Complete Chain Access report, which could be used for further mission planning.