Where is the International Space Station

STK Pro

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 STK 12.9 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 operators require a quick way to predict when a satellite will be visible to them in the future. It’s exciting to be able to see a satellite moving in the night sky - even more so when it’s the International Space Station (ISS). You’d like to determine when you can see the ISS from your present location (call it MyHouse) when it flies overhead within the next three (3) weeks.

Solution

Build a scenario that allows you to calculate a simple line-of-sight access between MyHouse and the International Space Station (ISS) that considers lighting and elevation constraints.

What you will learn

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

  • A Satellite object from the Standard Object Database

  • A Place object using the Insert by Address tool

  • Lighting constraints

  • Elevation constraints

 

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.

  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 Where_is_ISS
    Description When will ISS be visible over the next three (3) weeks?
    Location Default
    Start Default
    End + 21 days
  4. Click OK when finished.
  5. Click Save () when the scenario loads. A folder with the same name as your scenario is created for you in the location specified above.
  6. Verify the scenario name and location in the Save As window.
  7. Click Save.

    8. Save () often!

Breaking it down

You have some information that may be helpful. Here’s what you know:

  1. You will start looking for ISS today.
  2. You are interested in passes that occur over the next three weeks (21 days).
  3. You can see ISS with the naked eye from the ground when it is illuminated by sunlight.
  4. You can see ISS with the naked eye when it is dark on the ground.
  5. When you are on the ground trying to see something in space, the lower you look along the horizon, the more atmosphere you have to look through and the better the chance that something will be in the way.

Disabling Terrain Server

Analytical and visual terrain is not required in this analysis.

  1. Right-click on Where_is_ISS () in the Object Browser.
  2. Select Properties ().
  3. Select the Basic - Terrain page in the Properties Browser.
  4. Clear Use terrain server for analysis.
  5. Click OK to accept your change and to close the Properties Browser.

Inserting a Place object

You will insert a Place () object and configure it to be located at your address. An easy way to introduce a location is to create a Place () object using the Insert by Address tool. If you are at AGI headquarters, select Insert Default as the insert method. This method will position the facility at the latitudinal and longitudinal coordinates of AGI headquarters near Philadelphia.

To use the Search By Address feature, you will need an Internet connection. If you don't have an internet connection, you can use the City Database tool to select the nearest city to your location.

  1. Bring the Insert STK Objects tool () to the front.
  2. Select Place () in the Select An Object To Be Inserted: list.
  3. Select the Search by Address () method in the Select A Method: list.
  4. Click Insert... .

Using the STK: Insert by Address tool

For this example, it will be sufficient to mark the location of your house. You will call the location, MyHouse. An easy way to introduce a location is to create a Place () object using the Insert by Address tool. The Insert by Address tool offers a variety of methods for introducing different types of locations.

  1. Enter your address in the Enter an address or other search criteria below: field in the STK: Insert By Address tool.
  2. Select your address in the Results: list.
  3. Click Insert Place(s).
  4. Click Close to close the STK: Insert By Address tool.
  5. Right click on the Place () object in the Object Browser bearing your address.
  6. Select Rename in the shortcut menu.
  7. Rename the Place () object to MyHouse.

Viewing MyHouse in the 3D Graphics window

This option is only available for those who have an internet connection. It is not required to complete this exercise. If you don't have an internet connection, you can skip to the next section.

Now that you have your house inside STK, take a look at what it might look like in the 2D and 3D Graphics window using Microsoft Bing Maps.

Through AGI’s partnership with Microsoft, STK/TIM users can stream global high-resolution Microsoft Bing Maps raster imagery onto their STK, AGI Viewer, and STK Engine 3D globes. You can select from different data sets (e.g. aerial imagery, roads, etc.) to provide enhanced situational awareness.

  1. Bring the 3D Graphics window to the front.
  2. Right click on MyHouse () in the Object Browser.
  3. Select Zoom To.
  4. Use your mouse to set the view to your liking.

    5. 3D View: MyHouse

Viewing MyHouse in the 2D Graphics window

Selecting Zoom To for an object in the Object Browser doesn't work in the 2D Graphics window. You have to manually zoom into MyHouse ().

  1. Bring the 2D Graphics window to the front.
  2. Click Zoom In () in the 2D Graphics toolbar.
  3. Place your cursor near MyHouse () on the 2D Graphics window map.
  4. Hold down the left mouse button.
  5. Drag your mouse cursor across the area of the 2D map window that includes MyHouse ().
  6. Repeat those steps or move your mouse scroll wheel up to zoom in until MyHouse () is clearly visible.

    7. 2D View: MyHouse

Inserting a Satellite object from the Standard Object Database

You can insert a Satellite () object from the online or locally installed Standard Object Databases. STK supports the use of nine-digit SSC numbers in the GP data files. You will use the SSC number associated with the International Space Station (ISS).

  1. Insert a Satellite () object using the From Standard Object Database () method.
  2. Enter 25544 in the Name or ID: field in the Search Standard Object Data dialog box.
  3. Click Search.
  4. Look at the Common Name column in the Results: list.

    5. There are two choices. ISS (ZARYA) and ZARYA.

  5. Look at the Data Source column.

    • ISS (ZARYA) is associate with AGI's Standard Object Data Service. If you have an Internet connection, this is a good way to propagate a Satellite () object. It will be downloaded into your scenario based on the analysis period you selected using the correct Two Line Element set (TLE) and propagated using the SGP4 propagator. The database is published by USSTRATCOM. This database contains positional data for thousands of satellites.

    • ZARYA is associated with the Local Database. This choice uses a TLE stored on your local machine. There is an online database and a locally installed database. It's important that you have updated your local Satellite Database with the correct TLEs when using this choice.

  6. Select ISS (ZARYA) in the Common Name column.
  7. Click Insert.
  8. Click Close when ISS_ZARYA_25544 () is propagated and appears in the Object Browser to close the Search Standard Object Data dialog box.

Viewing ISS's TLE and orbital parameters

Look at the properties for ISS_ZARYA_25544 ().

  1. Open ISS_ZARYA_25544's () properties ().
  2. Select the Basic - Orbit page in the Properties Browser.
  3. Click Preview... in the TLE Source Frame.
  4. Look at the data in the TLE Preview dialog box.

    5. You can preview or modify the GP element information that is used to propagate the SGP4 satellite in the TLE Preview dialog box.

  5. Click Close to close the TLE Preview dialog box when finished.
  6. Leave ISS_ZARYA_25544's () properties () open.

Calculating when you can see ISS from MyHouse

You have two objects in your scenario: ISS_ZARYA_25544 () and MyHouse (). You need to know when ISS_ZARYA_25544 () is within MyHouse's () line-of-sight. Calculate an access to determine when this takes place.

  1. Right click on MyHouse () in the Object Browser.
  2. Select Access... () in the shortcut menu.
  3. Select ISS_Zarya_25544 () in the Associate Objects list in the Access tool.
  4. Click .
  5. Click Access... in the Reports frame.
  6. Scroll through the access report. The report shows you every time that ISS_ZARYA_25544 () is visible to MyHouse () during the next 21 days.
    1. How many accesses are there?
    2. What is the total duration of all the accesses?
  7. Leave the access report open.

Creating an Azimuth Elevation Range (AER) report

Knowing when to look for ISS is helpful, but you also want to know where to look for ISS_ZARYA_25544 (). You will be looking for an elevation as close to 90 degrees as possible. This represents when ISS_ZARYA_25544 () is directly overhead. Perfect for viewing the bright space station. See if you can determine where the ISS_ZARYA_25544 () is located with respect to MyHouse () when access occurs.

  1. Return to the Access tool.
  2. Click AER... in the Reports frame.
  3. Scroll through the AER report. This report tells you where to look based on azimuth, elevation and range during access.
    1. What is the range of elevation angle over all accesses?
  4. Bring the Access tool to the front.
  5. Click Close to close the Access tool.
  6. Leave the AER report open.

Modeling lighting conditions

According to the information that you have, you can only see ISS_ZARYA_25544 () with the naked eye from the ground when it is in direct sun and when it is dark at MyHouse ().

In order to model realistic system limitations, STK provides constraints. Constraints allow you to place limitations on the performance of objects. STK provides several different types of constraints. The types of constraints available will depend on the object on which the constraint is being imposed.

All constraints work in an AND configuration meaning that when multiple constraints are set, they must all be satisfied in order for an access to occur (e.g., Basic constraints AND Sun constraints AND Temporal constraints... must all be satisfied in order for an access to occur). If any single constraint is violated, that access is invalidated.

Constraining the ISS

Your list says you can only see ISS_ZARYA_25544 () with the naked eye from the ground when the spacecraft is in direct sun. You need to exclude accesses that occur when ISS_ZARYA_25544 () is NOT in direct sunlight because although it will pass over you, it won’t be illuminated such that you will be able to see it from the ground with the naked eye.

You can use a Sun constraint to model this limitation. Sun constraints enable you to impose lighting constraints based on the position of the Sun and Moon. When a lighting option is set, it indicates that access to an object, which uses one or more constraints, is valid only under these specified lighting conditions.

  1. Bring ISS_ZARYA_25544’s () properties to the front.
  2. Select the Constraints - Active page.
  3. Click Add new constraints () in the Active Constraints toolbar.
  4. Select Lighting in the Constraint Name list in the Select Constraints to Add dialog box.
  5. Click Add.
  6. Click Close to close the Select Constraints to Add dialog box.
  7. Ensure Direct Sun is set as Lighting constraint in the Constraint Properties section. The default Lighting constraint setting is Direct Sun, and that is what you require. Direct sun is total sunlight.
  8. Click OK to accept your changes and to close the Properties Browser.

Applying the ISS's lighting constraint to your reports

When you change properties for any objects which are part of a report, you will refresh the report to view the changes in your data.

  1. Bring the access report to the front.
  2. Click Refresh () in the report toolbar or press F5 on your keyboard.
  3. Bring the AER report to the front.
  4. Click Refresh () in the report toolbar or press F5 on your keyboard.

Changing the reports to local time

Your default report access times are in Coordinated Universal Time displayed in Gregorian format (UTCG) or Zulu time. You want to change the time to the local time or Gregorian Local (LCLG). This will simulate the current time at MyHouse ().

  1. Bring the access report to the front.
  2. Click Report Units () in the access report's toolbar.
  3. Select DateFormat in the Dimension column in the Units: Access dialog box.
  4. Select Gregorian LCL (LCLG) in the New Unit Value list.
  5. Click OK to close the Units: Access dialog box.

    6. Gregorian LCL is dependent on your system clock and time zone. If MyHouse () is located in a time zone other than the one your computer is set to, the resulting access times would be invalid.

    When you changed the report's units, the report automatically refreshed. You still have the same number of accesses, but now they are being reported based on local time instead of Zulu time.

  6. Bring the AER report to the front.
  7. Click Report Units () in the access report's toolbar.
  8. Select DateFormat in the Dimension column in the Units: Access dialog box.
  9. Select Gregorian LCL (LCLG) in the New Unit Value list.
  10. Click OK to close the Units: Access dialog box.

Analyzing the effect of the ISS's lighting constraint

You are including only accesses that occur when ISS_ZARYA_25544 () is in direct sunlight. Look at the report to see the lighting constraint has on access.

  1. View the refreshed access report.
  2. Answer the following questions:
    • How many accesses are there?
    • What is the total duration of the accesses?
    • Did the number and length of the accesses decrease from the original report?
    • Do accesses occur during daylight hours? Next, we will exclude any access that occurs when MyHouse () is in daylight hours.

Constraining MyHouse's lighting

Your list says you can only see ISS_ZARYA_25544 () with the naked eye when it is dark at MyHouse (). You need to exclude accesses that occur during daylight hours at MyHouse () because, although the ISS_ZARYA_25544’s () will pass over you, it won't be dark enough at MyHouse () to spot ISS_ZARYA_25544 () from the ground with the naked eye.

For your analysis, you need MyHouse () to be available only at night. To model the lighting conditions you need to impose a separate constraint on MyHouse () based on the position of the Sun.

  1. Open MyHouse’s () properties ().
  2. Select the Constraints - Active page.
  3. Click Add new constraints () in the Active Constraints toolbar.
  4. Select Lighting in the Constraint Name list in the Select Constraints to Add dialog box.
  5. Click Add.
  6. Click Close to close the Select Constraints to Add dialog box.
  7. Select Umbra as the Lighting constraint in the Constraint Properties section.

    8. Umbra is total shadow.

  8. Click Apply to accept your changes and to keep the Properties Browser open.

Applying the ground lighting constraint to your reports

Constraining MyHouse () ensures that it will be considered in your analysis only when it is totally shadowed from the sun. Refresh the report to view the changes in your data.

  1. Bring the Access report to the front.
  2. Click Refresh () in the report toolbar or press F5 on your keyboard.
  3. Bring the AER report to the front.
  4. Click Refresh () in the report toolbar or press F5 on your keyboard.

Analyzing the effect of the ground lighting constraint

You are now including only accesses that occur when ISS_ZARYA_25544 () is in direct sunlight and MyHouse () is totally shadowed from the sun. Check to see how the additional constraint has affected access to ISS_ZARYA_25544 ().

  1. Bring the Access report to the front.
  2. Answer the following questions.
    • How many accesses are there?
    • What is the total duration of the accesses?
    • How did applying a lighting constraint to MyHouse () affect accesses?
  3. Look at the AER report.
  4. Note at what elevation the first access occurs.

Setting elevation constraints

You know that when you are on the ground trying to see something in space the lower you look along the horizon the more atmosphere you have to look through and the higher the chance that something will be in the way. To help avoid the elevation angle problem, STK allows you to put an elevation angle constraint on a ground-based location. A good typical minimum elevation is 6-8 degrees, but it can be more depending on the area, artificial lighting conditions, the surrounding terrain, and even buildings.

To model a more realistic representation of MyHouse (), impose a minimum elevation angle constraint of 6 deg.

  1. Return to MyHouse’s () properties ().
  2. Select the Constraints - Active page.
  3. Click Add new constraints () in the Active Constraints toolbar.
  4. Select Elevation Angle in the Constraint Name list in the Select Constraints to Add dialog box.
  5. Click Add.
  6. Click Close to close the Select Constraints to Add dialog box.
  7. Enter 6 deg in the Min: field in the Elevation Angle - Constraint Properties section.
  8. Click OK to accept your changes and to close the Properties Browser.

Applying the elevation angle constraint to your reports

Constraining MyHouse () ensures that it will be considered in your analysis only when the elevation angle to ISS_ZARYA_25544 () is greater than 6 deg. Refresh the report to view the changes in your data.

  1. Bring the Access report to the front.
  2. Click Refresh () in the report toolbar or press F5 on your keyboard.
  3. Bring the AER report to the front.
  4. Click Refresh () in the report toolbar or press F5 on your keyboard.

Analyzing the effect of the elevation angle constraint

The elevation angle constraint along with the lighting constraint model a more realistic representation of MyHouse’s () ability to see ISS_ZARYA_25544 (). Check to see how that has affected the number of accesses and the minimum elevation angles of your AER report.

  1. Bring the Access report to the front.
  2. Answer the following questions.
    • How many accesses are there?
    • What is the total duration of the accesses?
    • How did applying an elevation constraint to MyHouse () affect accesses?
  3. Look at the AER report.
    • At what elevation does the first access occur?
  4. Close () the AER report.
  5. Keep the Access report open.

    6. The access report is telling you when you can see ISS_ZARYA_25544 () from MyHouse () and the AER report is telling you where to look.

Understanding where to look

You have data that lets you know exactly where and when to look for ISS_ZARYA_25544 () from MyHouse (). Another outstanding STK capability is its ability to provide situational awareness of your analysis in the 2D and 3D Graphics windows. Both windows currently show shading based on sunlight, penumbra and umbra. You can add more visual clues if you desire.

Adding lighting transitions to the 2D Graphics window

You can set lighting conditions in the 2D Graphics window that will provide a visual representation of how the lighting constraints that you set are affecting the objects in your scenario.

Adding lighting transitions on the ground

When you display outlines on the map in the 2D Graphics window, lighting conditions are displayed on the surface of the Earth. ISS_ZARYA_25544 () is approximately 400 kilometers above the Earth, so the times at which it crosses the solar terminator in the 2D Graphics window will not be exact, but they do provide a general idea of the lighting conditions for ISS_ZARYA_25544 (). They're exact for MyHouse ().

  1. Bring the 2D Graphics window to the front.
  2. Click Properties () in the 2D Graphics toolbar.
  3. Select the Lighting page in the Properties Browser.
  4. Select Show Outline in the Sunlight/Penumbra frame.
  5. Select Show in the Subsolar Point frame.
  6. Click OK to accept your changes and to close the Properties Browser.

Adding lighting transitions for the ISS

Use the ISS's properties to display exact lighting for ISS_ZARYA_25544 () in the 2D Graphics window.

  1. Open ISS_ZARYA_25544's () properties ().
  2. Select the 2D Graphics - Lighting page.
  3. Select Umbra.
  4. Select a the color that is different than the object color.
  5. Click OK to accept the changes and close the Properties Browser.

    6. You can also see ISS_ZARYA_25544's () lighting conditions by zooming to ISS_ZARYA_25544 () in the 3D Graphics window.

Viewing the lighting transitions in the 2D Graphics window

Look at the 2D Graphics window to see the lighting transitions.

  1. Return to the 2D Graphics Window.
  2. Notice the Sunlight/Penumbra boundary.
  3. Notice the Subsolar Point.
  4. Notice the ISS_ZARYA_25544's ()'s ground track has two colors. It now displays the specified umbra color when the ISS is not in sunlight at all.

Viewing the first access in the 3D Graphics window

You can set your animation time to the first access in the access report. By zooming to MyHouse () you will see an access line between MyHouse () and ISS_ZARYA_25544 (). Since you know the surroundings at MyHouse (), you will have a pretty good idea of where to look when the first access takes place.

  1. Bring the access report to the front.
  2. Right click on the first start time.
  3. Select Start Time in the shortcut menu.
  4. Select Set Animation Time in the next shortcut menu.
  5. Bring the 3D Graphics window to the front.
  6. Zoom to MyHouse ().
  7. Click Flashlight () in the 3D Graphics toolbar to illuminate the globe and objects when they aren't located in sunlight.
  8. Use your mouse to position your view so you know where to look when ISS_ZARYA_25544 () is visible to MyHouse ().
  9. Click Home View () in the 3D Graphics toolbar when finished to reorient the 3D Graphics camera back on the default Earth-centered view.

Animating your scenario

The ISS_ZARYA_25544 () marker is positioned according to the current animation time in both the 2D and 3D Graphics windows, and its ground and orbit tracks are clearly visible.

  1. Extend the Window menu item.
  2. Select Tile Vertically.
  3. Click Zoom Out () in the 2D Graphics window until you can see the whole Earth.
  4. Click Reset () in the Animation toolbar to have the scenario return to the start time.
  5. Click Decrease Time Step () and set you Time Step: to 5.00 sec.
  6. Click Start () to animate your scenario.
  7. Watch as your objects move along in the 2D and 3D Graphics windows as the animation progresses.
  8. Click Pause () when you see an access.

    9. 2D View: Access From MyHouse to ISS

Viewing the 3D Graphics model

You can zoom to ISS_ZARYA_25544 (), enjoy looking at the 3D Graphics model and see the access to MyHouse ().

  1. Bring the 3D Graphics window to the front.
  2. Zoom to ISS_ZARYA_25544 ().
  3. Mouse around in the window to get a better look at ISS_ZARYA_25544 ().
  4. Set your animation time to the first access time in the access report.
  5. Notice the access line between ISS_ZARYA_25544 () and MyHouse ().
  6. Click Start () to animate the scenario.
    • ISS_ZARYA_25544 () is always in sunlight when accesses occurs.

    3D View: ISS Model

  7. Click Reset () when finished.

Save your work

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

Summary

Using STK, you wanted to determine when you could walk outside of your house, look up into the sky, and view the ISS as it passes overhead. To do this, you inserted the location of your house (or wherever you are viewing from). Next, you propagated the ISS for 21 days. The first analysis showed when you could access the ISS. Next you constrained the ISS to only be visible when it is in direct sunlight. The ISS is great at reflecting this light. Next, you constrained your house location to be in total darkness, and also applied an elevation constraint which took into affect things like buildings, artificial light sources, dust, dirt, mountains, etc. This left you with the times that you can go outside, look up into the sky based on the AER report, and possibly see the ISS as it passes overhead.