Part 15: Introduction to the Advanced CAT Tool

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.

Capabilities covered

This lesson covers the following STK Capabilities:

  • STK Pro
  • Conjunction Analysis Tool

Problem statement

Engineers and operators need to easily address situations in which the launch or operation of a satellite or system of satellites, or a related Earth-based operation, may be affected by the actual or apparent proximity of other orbiting objects. Such effects range from a temporary delay or loss of access to the system to physical damage of space-based assets. In this scenario, you want to analyze seven days of data to determine the probability of your satellite having a conjunction with any other satellites or debris.

Solution

Use the Advanced Conjunction Analysis Tool (AdvCAT), which is a part of the STK's Conjunction Analysis capability, to carry out a close-approach analysis between a primary object (your satellite) and secondary objects (satellites possibly presenting a risk of collision). A conjunction occurs when two orbiting objects are closer than the specified minimum acceptable distance. You choose whether the distance is measured between the ellipsoidal threat volumes of the objects or by the range between the objects. In this scenario, you will complete a linear analysis using two-line element sets (TLEs) .

Video Guidance

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

Create a new scenario

  1. Launch STK ().
  2. In the Welcome to STK window, click Create a Scenario.
  3. Enter the following in the STK: New Scenario Wizard:
    4. Option Value
    Name: STK_CAT
    Location: Default
    Start: 1 Nov 2020 17:00:00.000
    Stop: + 7 days
  4. When you finish, click OK.
  5. When the scenario loads, click Save (). STK creates a folder with the same name as your scenario and places it in the location specified above.
  6. Verify the scenario name and location and click Save.

Prepare STK

For this scenario, you will not use the Timeline View nor the 2D Graphics window. Close them.

  1. Close the Timeline View at the bottom of the STK window.
  2. Close the 2D Graphics window.
  3. Maximize the 3D Graphics window.

Obtain an archived satellite database

AdvCAT does not add any objects to the scenario. It merely propagates ephemerides for the primary and secondary objects that you select for the specified time frame. Use a local database for the analysis.

With AdvCAT, you can use either satellite objects propagated from your TLE database or analytical objects downloaded and propagated from the AGI server .

  1. Select the scenario STK_CAT () and click properties ().
  2. In the properties window, select the Basic - Database page.
  3. Click Update Database Files.
  4. When the Update Satellite Database window opens, set the following:
    5. Option Value
    Option Obtain Archived Database
    Database Information Specific Database: stkAllTLE
    Database Scenario Location (typically C:\Users\username\Documents\STK 12\STK_CAT)
    Archive Date: 1 Nov 2020 17:00:00.000 UTCG
  5. Click Update.
  6. Click OK to close the Information window.
  7. In the Update Satellite Database window, click Close.
  8. Click OK to close STK_CAT's properties.

Primary satellite

The primary satellite used in the analysis is your satellite. For this scenario, your satellite is STARLINK-1611.

If you have your own ephemerides converted to *.e files, regardless of the reference frame, STK will handle all the conversions internally for whatever analysis needs to be performed. When using the AdvCAT, STK needs *.e files, manually entered satellite properties, or TLE sets to run CAT. Any reference frame is acceptable.

  1. Using the Insert STK Objects tool (), insert a Satellite () object using the From TLE File method.
  2. When the Select a TLE File window opens, browse to the location of the downloaded TLE file named stkAllTLE.tce, typically C:\Users\username\Documents\STK 12\STK_CAT.
  3. Select stkAllTLE.tce.
  4. Click Open.
  5. When the Question window opens, read the data and click OK or Yes. Be patient, as this can take a minute or two to load.

Modify the Satellite Database options

  1. When the Insert From Satellite Database window opens, click Modify.
  2. When the Satellite Database: TLE Source window opens, clear the On propagation, automatically retrieve elements checkbox.
  3. Click OK.
  4. When the Question pop up opens, click OK or Yes.
  5. When the Insert From Satellite Database window repopulates, enter 46147 in the SSC Number field.
  6. Click Search.
  7. In the Results list, select STARLINK-1611.
  8. Click Insert.
  9. After STARLINK-1611 () has propagated, click Close.

Add the AdvCAT Object to the Insert STK Objects tool

  1. Bring the Insert STK Objects tool () to the front.
  2. Click Edit Preferences.
  3. In the New Object list, select AdvCAT ().
  4. Click OK.

Create an Advanced CAT object

The AdvCat object () provides a convenient way for you to carry out close-approach analyses for multiple satellites and TLEs.

  1. Using the Insert STK Objects tool (), insert an AdvCAT () object using the Insert Default method.
  2. Open AdvCAT1's properties ().

Selecting and defining analysis objects

Most of the Main page of the AdvCAT Properties is comprised of two lists: a Primary List and a Secondary List. The Primary List contains satellites of interest to you, such as those that you own or wish to use. The Secondary List contains satellites that present a potential risk of collision with, or unacceptably close approach to, satellites in the Primary List.

Analysis object attributes - primary satellite

In analyzing close approaches between a primary and secondary object, AdvCAT assigns to each a threat volume comprising an ellipsoidal shape enclosing the object. The following are key Analysis Object attributes:

  • Fixed class specifies the dimensions of the threat volume ellipsoid.
  • Tangential is the dimension of the threat volume ellipsoid along the X axis.
  • Cross Track is the dimension of the threat volume ellipsoid along the Y axis.
  • Normal is the dimension of the threat volume ellipsoid along the Z axis.

Since you're evaluating potential linear conjunctions and not using covariance settings, keep the default Class, Tangential, CrossTrack, and Normal values. Actual threat volumes would be set based on established ellipsoid and covariance screenings.

  1. In the Main page of the AdvCAT Properties, go to the Primary List and move () Satellite/STARLINK-1611_46147 to the Chosen: list.

  2. Use a typical Hard Body Radius of an A-Train satellite. Set the HardBodyRadius value to 3.0 m.
  3. Click Apply.

Analysis Object attributes - secondary satellites

The U.S. Strategic Command (USSTRATCOM) keeps track of thousands of space objects. These objects constitute the space object catalog. While most of the catalog is made available to the public, some information is restricted. AGI provides the publicly released information for use with STK in the form of satellite database files and TLEs. AGI updates this data at least once and up to three times a day. Your analysis will check for potential conjunctions between STARLINK-1611 and the complete satellite database.

  1. In the Secondary List, move () stkAllTLE.tce to the Chosen: list.
  2. Set the HardBodyRadius value to 2.0 m.
  3. Click Apply.

Threshold

Often, the threat volume represents the degree of uncertainty about an object's position at any given time. AdvCAT computes the range between the threat volumes of the two objects at specified time steps over a given interval. It gives a warning whenever the range falls below a user-selected threshold. A collision event occurs whenever the range between the two threat volumes becomes zero or negative.

At the top of the Basic - Main page, note the Threshold: value of 10 km. This can be changed manually when required.

Using prefilters

The basic idea behind close-approach processing is to start with all cataloged orbiting objects and then efficiently delete the members of the population that do not come within the specified distance of the reference object. The first step is eliminating as many members of the population as possible via geometric properties, which takes considerably less time than fully propagating these satellites and then computing close approaches. The final determination of the existence of close approaches is always done by sampling the ephemeris of the candidate and reference objects, but some prefilters are usually applied to limit the amount of ephemeris generation that is required.

  1. In the AdvCAT Properties, select the Basic - Advanced page.
  2. Note the selected Pre-Filters and their values.
    • Apogee / Perigee: AdvCAT uses the apogee/perigee prefilter to cut candidate close-approach objects having a range of altitude that does not overlap with that of the reference object.
    • Time: The goal of the time filter is to identify time intervals when each object in a pairing is close enough to the elliptical representation of the other object's trajectory to have a conjunction.

Compute possible conjunctions

When you compute possible conjunctions, any object threat volumes that fall within the threshold and prefilter distances appear as ellipsoids in the 3D Graphics window after creating a report.

  1. In the AdvCAT properties, select the Basic - Main page.
  2. Select the Display Acknowledgment when done checkbox.
  3. Click Compute. Be patient. This could take a minute. There is a progress bar visible in the lower right corner of STK.
  4. When the Advanced CAT message window appears, read the message and then click OK.

Close approach by minimum range report

Right now, you're interested in a few specific data providers. Remember, AdvCAT only reports objects with threat volumes that come within ten (10) kilometers and also pass the prefilters of your primary satellite.

  1. In the Object Browser, right-click AdvCAT1 () and select Report & Graph Manager.
  2. In the Styles section, in the Installed Styles folder, select Close Approach By Min Range.
  3. Click Generate.
  4. Look through the report and focus on Object Name, Time In (UTCG), Time Out (UTCG), Min Separation (km) and Min Range (km). The AdvCAT Data Provider is Events by Min Range. The following are the data provider elements used in the report:
    • Object Name: This is the name of the secondary object involved in the close-approach event.
    • Time In: This is the start time of the event. For this scenario, it is when the threat volume is within 10 kilometers of the primary's threat volume.
    • Time Out: This is the stop time of the event. For this scenario, it is when the threat volume moves beyond 10 kilometers of the primary's threat volume.
    • Min Separation: This is the separation distance between the primary and secondary threat volumes at the time of closest approach. If the ellipsoids intersect, then the value will be "Intersect".
    • Min Range: This is the distance between the center points of the primary and secondary objects at the time of closest approach.
  5. When finished, close the report.

Create a custom report

Add three more Events by Min Range elements to your report. This will add situational awareness.

  1. In the Installed Styles section, right-click Close Approach By Min Range and select Properties ().
  2. In the Report Style window, under Report Contents, select a value in Section 6.
  3. Under Data Providers, expand () the Events by Min Range folder.
  4. Move () Time of Close Appr (TCA) to the Report Contents list.
  5. Move () Relative Velocity to the Report Contents list.
  6. Move () Collision Probability (Analytic) to the Report Contents list.
  7. Click OK.
  8. Read the Warning message and then click OK.

This added new elements to the custom report:

  • Time of Close Appr (TCA): This is the time when the minimum range occurs between the primary and secondary bodies.
  • Relative Velocity: This is the magnitude of the difference in the inertial velocities of the primary and secondary objects at a given time.
  • Collision Probability (Analytic): This is the probability of collision computed using an analytic method derived from the book Spacecraft Collision Probability by F. Kenneth Chan.

Rename and run the report

  1. In the Styles section, expand () the My Styles folder.
  2. Rename the custom report to be "My Close Approach By Min Range".
  3. Click Generate.
  4. Look through the report.

Identify and insert a close approach

Focus on the first satellite in the report.

  1. In the report, copy the SSC number (Satellite Catalog Number) of the first satellite in the report, for example 45540.
  2. Using the Insert STK Objects tool (), insert a Satellite () object using the From TLE File method.
  3. When the Select a TLE File window opens, browse to the location of the downloaded TLE file named stkAllTLE.tce, which is typically C:\Users\username\Documents\STK 12\STK_CAT.
  4. Select stkAllTLE.tce.
  5. Click Open.
  6. When the Question pop up opens, read the data and click OK or Yes. Be patient, this can take a minute or two to load.

Modify the Satellite Database options

  1. When the Insert From Satellite Database window opens, click Modify.
  2. When the Satellite Database: TLE Source window opens, clear On propagation, automatically retrieve elements.
  3. Click OK.
  4. When the Question window opens, click OK or Yes.
  5. When the Insert From Satellite Database window repopulates, paste or type in the first satellite's SSC number from the report into SSC Number field, for example 45540.
  6. Click Search.
  7. In the Results list, select the satellite, for example STARLINK-1368.
  8. Click Insert.
  9. After the Satellite () object has propagated, click Close.

Visualize the close approach

  1. In the report, right-click on the first Time In (UTCG), select Time In, and then select Set Animation Time.
  2. Bring the 3D Graphics window to the front.
  3. Right-click on STARLINK-1611_46147 () and select Zoom To.
  4. In the Animation Tool Bar, click Decrease Time Step () until your Time Step: value is 0.01 sec.
  5. Click Step in Reverse () one time.
  6. Using your mouse, zoom out until you can see the primary and secondary ellipsoids.

    7. Primary and Secondary Ellipsoids

    All of your ellipsoids are green. When they enter the threshold of 10 kilometers, they turn yellow. When they intersect, they turn red. Other satellites that enter the threshold are represented by ellipsoids. They aren't analytical objects.

  7. Click Start () to animate the scenario.
  8. Run the animation through the conjunction. After the conjunction, when the two satellites turn green again, click Pause ().

Set the animation time to the Time of Close Approach

  1. Return to the report.
  2. Set the animation time to the Time of Close Appr (UTCG) of the two satellites.
  3. Return to the 3D Graphics window.
  4. Using your mouse, zoom in so that you can get a good view of both satellites.
  5. When finished, Reset () the scenario.
  6. In the 3D Graphics window, click Home View ().
  7. Close the report and the Report & Graph Manager.

Visualize all the satellites

At this time, only the satellites having threat volumes that fell within the 10 kilometer threshold and passing the prefilters are visible in the 3D Graphics window. You can display all secondary ellipsoids if desired.

  1. Return to AdvCAT1's () properties ().
  2. Select the 3D Graphics - Attributes page.
  3. In the Visibility section are the primary ellipsoid and secondary ellipsoids that have conjunctions. Enable All.
  4. Click OK.
  5. In the Object Browser, uncheck both Satellite () objects.

    6. All the Satellites in the 3D Graphics window

  6. In the Animation Tool Bar, click Increase Time Step () until your Time Step: value is 10.00 sec.
  7. Click Start () to animate the scenario. Remember, the satellites are represented by ellipsoids that are 20 by 10 by 5 kilometers in size!
  8. Use your mouse to zoom out until you see the geostationary belt.

Save your work

  1. When you're finished, Reset () the scenario.
  2. Save () your work.

Summary

You began by updating your satellite database with an archived database of satellites that matched your scenario time period. Next, you inserted the primary satellite into the scenario. After inserting the AdvCAT object, you moved the primary satellite from the Primary List to the Chosen list. Then you moved the complete satellite database (stkAllTLE.tce) from the Secondary List to the Chosen list. You left the threat volumes and threshold at default values, and then launched the close approach computation process. Using the Events by Time In data provider and elements, you created a custom report that allowed you to view important information concerning possible conjunctions with your primary satellite. You ended by visualizing a close approach and by visualizing all the satellite ellipsoids from the stkAllTLE.tce database.

On your own

Throughout the tutorial are hyperlinks that point to in-depth information concerning the AdvCAT. Now is a good time to go back through this tutorial and view that information. Experiment with other satellites, data providers, and prefilters. Explore and have fun!

If you're interested in a more in-depth tutorial that uses covariance data and performs a nonlinear conjunction analysis, try the Advanced CAT Tool: Assessing the Threat of a Collision tutorial