Carrier to Noise Ratio in the LEO, MEO, and GEO Zones

STK Premium (Air) and STK Premium (Space) or STK Enterprise
You can obtain the necessary licenses for this training 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:

  • Analysis Workbench
  • Aviator
  • Communications
  • Coverage
  • STK SatPro
  • STK Pro
  • Volumetrics

Problem statement

If you are an engineer or analyst, you require an efficient way to create constellations of satellites with similar payloads. You may also need to determine if receivers on those satellites will sustain interference from other communication devices.

For instance, suppose you need to evaluate an aircraft's communications with a constellation of Low Earth Orbiting (LEO) satellites as the aircraft flies a search pattern. The aircraft is equipped with a new phased-array communication antenna. Additionally, you need to evaluate the additional RF energy which may interfere with other LEO, Medium Earth Orbit (MEO) and Geostationary Earth Orbit (GEO) satellites.

Solution

You can evaluate the scenario above using STK and its capabilities. In this tutorial, you will use the Walker tool to create a constellation of satellites. Next, you will create an Aircraft object that flies a search pattern in a designated area of interest using the STK Aviator capability. You will use the Comm System object to determine possible communication interference with the LEO satellites. Finally you will build a Volumetrics object to visualize potential radio frequency interference in the LEO, MEO and GEO volume of space.

What you will learn

Upon completion of this lesson, you will understand the following:

  • The Walker tool
  • The Volumetric object
  • The Spatial Analysis tool
  • Aviator Area Target Search patterns
  • Transmitter and Receiver objects

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

You will create a new scenario with a run time of five (5) hours.

  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: Volumetrics_CN
    Start: 15 Mar 2022 16:00:00.000 UTCG
    Stop: +5 hrs
  4. Click OK when you finish.
  5. Click Save () once the scenario loads. A folder with the same name as your scenario is created for you.
  6. Verify the scenario name and location.
  7. Click Save (). It is best to save your scenario often.

Low earth orbit constellation

You will be using a walker constellation to model the constellation of LEO satellites. Each satellite will have a hemispherical nadir pointing antenna. In order to define the walker constellation, you first need to create a "seed" satellite to define the general orbital parameters.

Walker constellations

The Walker Tool makes it easy to generate a Walker constellation using the Two Body, J2, J4, or SGP4 orbit propagators. First, you can define a satellite with the characteristics and orbit you need. Then, you can open the Walker tool by highlighting the satellite in the Object Browser and selecting Walker... from the Satellite menu.

A Walker constellation consists of a group of satellites (t) that have the same period and inclination. The pattern of the constellation consists of evenly spaced satellites (s) in each of the orbital planes (p) specified so that t=sp. The ascending nodes of the orbital planes are also evenly spaced over a range of right ascensions (RAAN).

The way in which spacing between the ascending nodes that define the orbital planes is calculated depends on the Type of Walker constellation you choose. In addition to specifying the number of satellites in each plane, you must also specify the location of the first satellite in each plane relative to the first satellite in adjacent planes. The way to specify the position of the first satellite depends on the type of Walker constellation you choose.

Type Description
Delta Delta configurations have orbit planes distributed evenly over a span of 360 degrees in right ascension. Requires an integer value of f for inter-plane phasing.
Star Star configurations have orbit planes distributed over a span of 180 degrees. Requires an integer value of f for inter-plane phasing.
Custom A Custom configuration allows for explicit input of the span over which ascending nodes should be distributed and allows for the explicit specification of inter-plane phasing in terms of a true anomaly offset.

Creating a seed satellite

The original satellite that is used to create the Walker constellation is referred to as the “Seed” satellite, while the satellites generated using the Walker tool are referred to as children. Use the Orbit Wizard to create the “seed” satellite from which the other satellites will be derived.

  1. Select Satellite () in the Insert STK Objects tool.
  2. Select the Orbit Wizard () method.
  3. Click Insert... .
  4. Set the following in the Orbit Wizard:
    5. Option Value
    Type: Repeating Ground Trace
    Satellite LEO_Sat
    Approximate Altitude: 800 km
    Color: White
  5. Click OK.

Modeling the LEO satellite receiver

Insert a Receiver () object which will function as the receiver on the LEO satellite and all of its children. Use a Complex Receiver Model.

  1. Insert a Receiver () object using the Define Properties () method.
  2. Select LEO_Sat () in the Select Object dialog box.
  3. click OK.
  4. Select the Basic - Definition page.
  5. Click the Reciever Models Component Selector ().
  6. Select the Complex Receiver Model in the Select Component dialog box.
  7. Select the Antenna tab.
  8. Click the Antenna Models Component Selector ().
  9. Select Hemispherical.
  10. Click Apply.

Visualizing the Antenna Pattern

  1. Select the 3D Graphics – Attributes page.
  2. Enable the Show Volume option.
  3. Click OK.
  4. Right-click on Receiver1 () in the Object Browser.
  5. Select Rename the shortcut menu.
  6. Rename Receiver1 () to LEO_Rx.
  7. Right-click on LEO_Sat () in the Object Browser.
  8. Select Zoom To.

Hemispherical Antenna Pattern

Creating a walker constellation

Using the seed satellite, create a constellation with orbital characteristics that provide global coverage. Design a constellation that has at least one satellite in view of the aircraft at all times.

  1. Right-click on LEO_Sat () in the Object Browser.
  2. Select Satellite in the shortcut menu.
  3. Select Walker... in the second shortcut menu.
  4. Enter the following in the Walker Tool:
    5. Option Value
    Type: Delta
    Number of Sats per Plane 10
    Number of Planes: 10
    Color by Plane Off
  5. Click Create / Modify Walker.
  6. Click Close to close the Walker Tool.

If the seed satellite has sub-objects such as Receiver () objects, the sub-objects are also created for each of the child satellites.

Walker satellite relationships

When a Walker constellation is created, each child has the same base name as the seed satellite plus two numbers. The first number identifies the plane in which the satellite resides and the second identifies the satellite's position in the plane. For instance, here we define a Walker constellation with two planes and ten satellites per plane, LEO_Sat102 would be the second satellite in the first plane.

If you keep the seed satellite in the scenario, two identically configured satellites (the seed satellite and the first satellite in the first plane) will be considered in your analysis. To prevent duplicate analysis, let’s remove the seed satellite, now.

  1. Save () your scenario ().

    2. When you save the scenario, all objects in the scenario are also saved. It is important that you save the scenario before you remove LEO_Sat in case you need to reload it later for further analysis.

  2. Select LEO_Sat () in the Object Browser.
  3. Click Delete ().

Viewing the LEO satellite constellation

  1. Bring the 3D Graphics window to the front.
  2. Click Home View () in the 3D Graphics window toolbar.
  3. Use your mouse to turn the Earth in order to view the constellation of LEO satellites.

LEO Constellation

Creating the test area

Use an Area Target () object to create the training area in which the test aircraft will fly a standard search pattern.

  1. Insert an Area Target () object using the Define Properties () method.
  2. Select the Basic - Boundary page
  3. Click Add four (4) times in the Points frame.
  4. Set the following in the order shown:
    5. Latitude Longitude
    34 deg -121 deg
    34 deg -119 deg
    32 deg -119 deg
    32 deg -121 deg
  5. Select the 2D Graphics - Attributes page.
  6. Disable Inherit from Scenario in the Inheritable Settings frame.
  7. Disable Show Label, and Show Centroid.
  8. Click OK.
  9. Rename AreaTarget1 () to Test_Area.

Creating the test aircraft

Insert the test aircraft that will fly a search pattern inside of the designated test area. The aircraft will use the Aviator propagator.

  1. Insert an Aircraft object () using the From STK Data Federate method ().
  2. Select the Search tab in the Open dialog box.
  3. Enter Orion in the Search terms: field.
  4. Click Search.
  5. Select the second P-3C_Orion.ac that shows the path /Sites/AGI/documentLibrary/STK Standard Objects/Aircraft/P-3C_Orion.ac in the results list.
  6. Click Open.

STK Area Target Site

You can use an STK Area Target site to define a search area.

  1. Open P-3C_Orion's () properties ().
  2. Select the Basic - Route page.
  3. Right-click on Phase 1 in the Mission List.
  4. Select Insert First Procedure for Phase... ().
  5. Select STK Area Target () in the Select Site Type: list.
  6. Click Next >.

Area Target Search procedure

An Area Target Search procedure conducts a search pattern flight within the selected area target site.

  1. Select AreaTargetSearch () in the Select Procedure Type: list.
  2. Set the following in the Search Options frame:
    3. Option Value
    Max Separation: 10 nm
    Course Mode: Override
    Centroid True Course: 180 deg
  3. Change Turn Factor: to 2 in the Enroute Options frame.
  4. Click Finish.
  5. Click OK.
  6. Select Optimize STK for Aviator when the Flight Path Warning window appears.
  7. Click OK.
  8. Bring the 2D Graphics window to the front.
  9. Zoom To Test_Area ().

Aircraft Search Pattern

Building an aircraft transmitter

The aircraft transmitter will use a phased array antenna.

  1. Insert a Transmitter () object using the Define Properties () method.
  2. Select P-3C_Orion () in the Select Object dialog box.
  3. Click OK.
  4. Select the Basic - Definition page.
  5. Select the Transmitter Models Component Selector ().
  6. Select the Complex Transmitter Model in the Select Component dialog box.
  7. Click OK.
  8. Select the Model Specs tab.
  9. Set Power: to 40 dBW.
  10. Click Apply.

Modeling the phased array transmitter antenna

The Phased Array Antenna model consists of many radiating elements. Each element is modeled as an isotropic pattern.

  1. Select the Antenna tab.
  2. Set the following:
    3. Option Value
    Type: Phased Array
    Number of Elements X: 9 and Y: 9

Beam Direction Provider

You will use the Beam Direction Provider tab to select where the antenna points its beam.

  1. Select the Beam Direction Provider sub-tab.
  2. Turn on Enabled in the Beam Steering frame.
  3. Enable Satellite () in the Selection Filter: list.
  4. Move () all of the satellites to the beam target list.
  5. Click Apply.

Orientation

The antenna uses a spherical Az/El coordinate system.

  1. Select the Orientation sub-tab.
  2. Set the Elevation to -90 deg. Positive Z points to nadir so -90 degrees points the antenna boresight up, not down.
  3. Set Z: to -7 ft in the Position Offset frame. This will place the antenna on top of the aircraft's fuselage.
  4. Click Apply.

Visualizing the antenna pattern

  1. Select the 3D Graphics - Attributes page.
  2. Set the following in the Volume Graphics frame:
    3. Option Value
    Show Volume Enabled
    Gain Scale (per dB): 0.0005 km
    Minimum Displayed Gain: -20 dB
  3. Enable Set azimuth and elevation resolution together in the Pattern frame.
  4. Click Apply.

Boresight vector

The boresight vector is a unit vector along the Body Z-axis. The Vector is fixed by its components in reference axes.

  1. Select the 3D Graphics - Vector page.
  2. Enable the Boresight Vector option.
  3. Click OK.
  4. Rename Transmitter1 () to Orion_Tx.
  5. Right click on P-3C_Orion () in the Object Browser.
  6. Select Zoom To.
  7. Bring the 3D Graphics window to the front.

Antenna Pattern and Boresight

Plotting the carrier to noise ratio along the flight route

There are a couple of ways to approach this problem. Mission planners could use a Chain () object and create a link between the Orion transmitter and whatever LEO receivers are in view. Or they could employ a Comm System () object and direct the phased array's main lobe gain towards the LEO satellite that is has the best overhead elevation angle when multiple satellites are in view. Mission planners have decided on the latter.

Communication system

STK's Communications capability provides a CommSystem () object that models dynamically configured communications links between constellations of transmitters and receivers.

To set up a CommSystem () object, you must first organize the relevant communication assets:

  • the transmitter(s) in the communications link of interest
  • the receiver(s) in the communications link of interest
  1. Insert two (2) Constellation () objects into the scenario using the Insert Default () method.
  2. Rename the new Constellation () objects Transmitter and Receivers.

Receivers constellation

  1. Open Receivers' () properties ().
  2. Select the Basic - Definition page.
  3. Enable Receiver () in the Selection Filter: list.
  4. Move () all the receivers to the Assigned Objects list.
  5. Click OK.

Transmitter constellation

  1. Open Transmitter’s () properties ().
  2. Select the Basic - Definition page.
  3. Select Orion_Tx () in the Available Objects list.
  4. Move () Orion_Tx () to the Assigned Objects list.
  5. Click OK.

Building the communications system

  1. Insert a Comm System () object using the Insert Default () method.

    2. If the Comm System () object does not appear in the Insert Objects tool, click the Edit Preferences... button and add it.

  2. Open CommSystem1’s () properties ().
  3. Select the Basic - Transmit page.
  4. Move () Transmitter () from the Available Constellations list to the Assigned Constellation list.
  5. Select the Basic - Receive page.
  6. Move () Receivers () from the Available Constellations list to the Assigned Constellation list.
  7. Select the Basic - Link Definition page.
  8. Enable Transmit in the Constraining Constellations frame.
  9. Leave the Link Selection Criteria set at Minimum Range.

    10. Minimum Range selects the non-constraining object (satellite receiver) with the minimum distance to the constraining object. Basically, you're forcing the phased array antenna to talk to the nearest satellite.

  10. Click OK.

Computing the CommSystem and plotting the C/N

  1. Right-click on the CommSystem1 () in the Object Browser.
  2. Open the CommSystem shortcut menu.
  3. Click Compute Data.

    4. Be patient! Due to the amount of receivers in the scenario, the computation can take a few minutes.

  4. Right-click on CommSystem1 () in the Object Browser.
  5. Select Report & Graph Manager... ().
  6. Select the Carrier to Noise vs Time () graph in the Installed Styles list.
  7. Click Show Step Value when the graph opens.
  8. Change Step: to 1 sec.
  9. Click Refresh (F5) () in the graph toolbar.

    10. C/N Over Time

    You can see that there is one instance where C/N is affected.

  10. Close the graph and the Report and Graph manager.

Analyzing the RF output throughout MEO

You would like to ensure that the RF energy used for communications doesn’t interfere with other satellite communications beyond MEO to GEO. In order to evaluate the C/N across the MEO, you can define a scalar calculation to a grid template object, and have that grid template object move to all the grid point locations defined in the volumetric object.

Creating the Volumetric object

The Volumetric () object defines a 3-dimensional grid of points using various coordinate definitions, with respect to various reference coordinate systems from the Vector Geometry tool. It also defines the conditions and calculations that depend on locations in 3D space and evaluates these conditions and calculations across grid points.

  1. Insert a Volumetric () object using the Insert Default () method.
  2. Rename Volumetric1 () to Vol_CN.
  3. Open Vol_CN's () properties ().
  4. Select the Basic - Definition page.

Selecting a Volume Grid component

Select a Volume Grid component from Analysis Workbench.

  1. Click the Volume Grid: ellipsis ().
  2. Click Create new Volumetric Grid () in the Select Volume Grid for CN_Vol dialog box.
  3. Set the following in the Add Volumetric Component dialog box:
    4. Option Value
    Type: Cartographic
    Name: MEO_West
  4. Click Set Grid Values... .
  5. Set the following in the Longitude frame when the Grid Values dialog box opens:
    6. Option Value
    Minimum: -180 deg
    Maximum: 0 deg
    Number of Steps: 10
  6. Set the following in the Altitude frame:
    7. Option Value
    Minimum: 2000 km
    Maximum: 36000 km
    Number of Steps: 5

Cleaning up your workspace

  1. Click OK to close the Grid Values dialog box.
  2. Click OK to close the Add Volumetric Component dialog box.
  3. Select MEO_West in the Volume Grids for: Earth list in the Select Volume Grid for Vol_CN dialog box.
  4. Click OK.
  5. Click Apply .

Viewing the volume grid

  1. Bring the 3D Graphics window to the front
  2. Click Home View () in the 3D Graphics window toolbar.
  3. Use your mouse to view the Cartographic Grid (MEO_West).

MEO_West Cartographic Grid

Changing the view

Use the 3D Graphics Grid page to define the 3D Graphics Volumetric grid properties.

  1. Return to Vol_CN's () properties ().
  2. Select the 3D Graphics - Grid page.
  3. Change Size: to 2 in the Show grid points: field.
  4. Disable Show grid lines:.
  5. Click Apply.
  6. Return to the 3D Graphics window.

MEO_West Grid Points Only

Defining the volumetric grid

You can use one of the satellites' receivers as the volumetric grid template object. In order to do this, you will need to compute an access from the aircraft transmitter to that receiver so that the scalar calculation for C/N will exist.

  1. Right-click on Orion_Tx () in the Object Browser.
  2. Click Access... ().
  3. Expand () LEO_Sat0101in the Associated Objects list when the Access Tool opens.
  4. Select LEO_Rx1().
  5. Click Compute.
  6. Close the Access tool.

Spatial Analysis Tool

Use the Spatial Analysis Tool to create calculations and conditions that depend on locations in 3D space which are, in turn, provided by user-definable volume grids.

You have computed access from the Orion transmitter to the LEO satellite receiver. STK's Analysis Workbench capability has automatically created scalar calculations for all the available communications calculations, including C/N. Next, create a spatial calculation that will move the template object to each point in the volumetric grid and compute the C/N at those locations.

  1. Right-click on LEO_Sat0101 () in the Object Browser.
  2. Select Analysis Workbench... ().
  3. Select the Spatial Analysis tab when Analysis Workbench opens.

Spatial Calculation

A Spatial Calculation is a calculation that depends on both time and location.

  1. Click Create new Spatial Calculation ().
  2. Ensure that the Type: is set to Scalar At Location.
  3. Enter CN in the Name: field.
  4. Click the Scalar: ellipsis ().
  5. Select Aircraft-P-3C_Orion-Transmitter-Orion_Tx-To-Satellite-LEO_Sat0101-Receiver-LEO_Rx1 () in the objects list when the Select Reference Scalar Calculation dialog box opens.
  6. Expand () CommLinkInformation () in the Scalar Calculations for: Aircraft-P-3C_Orion-Transmitter-Orion_Tx-To-Satellite-LEO_Sat0101-Receiver-LEO_Rx1 list.
  7. Select C/N ().
  8. Click OK to close the Select Reference Scalar Calculation dialog box.
  9. Click OK to close the Add Spatial Analysis Component dialog box.
  10. Click Close to close the Analysis Workbench.

Updating the volumetric object to use the C/N spatial calculation

Now that you have the C/N spatial calculation, configure the Volumetric object to use that spatial calculation, compute the data, and configure the graphics.

  1. Return to Vol_CN's () Properties ().
  2. Select the Basic - Definition page.
  3. Enable the Spatial Calculation: option.
  4. Click the Spatial Calculation: ellipsis ().
  5. Select LEO_Sat0101 () in the object list when the Select Spatial Calculation for Vol_CN dialog box opens.
  6. Select CN () in the Spatial Calculations for: LEO_Sat0101 list.
  7. Click OK to close the Select Spatial Calculation for Vol_CN dialog box.
  8. Click Apply.

Computing Volumetric Analysis

Volumetric Analysis is likely to be more computationally expensive that 2D analysis because extending grids to three dimensions can greatly increase the number of grid points.

  1. Right-click on Vol_CN () in the Object Browser.
  2. Select Volumetric in the shortcut menu.
  3. Click Compute in Parallel in the second shortcut menu.

Be patient! Due to the amount of receivers and grid points in the scenario, the computation can take a few minutes.

Viewing in 3D

To animate the scenario, Volumetrics will need to recompute the data at every time step. In order to compute all of the data beforehand to animate smoothly, adjust the Interval to use the "At times at step size" option.

  1. Return to Vol_CN's () properties ().
  2. Select the Basic - Interval page.
  3. Enable At times at step size: in the Evaluation of Spatial Calculation frame.
  4. Enter 5 min in the At times at step size: field.

    5. On your own, you can adjust this value to what works for you. Any changes you make will be applied to the spatial calculation limits in the 3D Graphics - Volume page. Therefore, you may want to redo your fill levels.

  5. Click Apply.

Be patient. This could take a few minutes.

Configuring volumetric graphics

Spatial Calculation Levels represent straight line distances from the parent object.

  1. Select the 3D Graphics - Volume page.
  2. Enable Spatial Calculation Levels.

    3. When the Spatial Calculation Levels radio button is selected, the minimum and maximum limits of the spatial calculation can be seen at the top of the page in the Limits field. Your values may be different. The following image is only an example.

    Spatial Calculation Limits

  3. Click Insert Evenly Spaced Values...
  4. Set the following in the Insert Evenly Spaced Values dialog box:
    5. Option Value
    Start Value: -10 (dB)
    Stop Value: Round down to the highest Max integer (in the above example, 7 dB)
    Step Size: 3 (dB)

    We used -10 as the start value but this could be any value you choose. In this instance, it's an arbitrary value. For this scenario we're saying a C/N of -10 dB or higher could create interference.

  5. Click Create Values.
  6. Click Apply.

3D Graphics Legends

Use the Volumetric 3D Graphics Legends page to adjust the legend display in your 3D Graphics window.

  1. Select the 3D Graphics – Legends page.
  2. Enable Show Legend.
  3. Set the following in the Text Options frame:
    4. Option Value
    Title: C/N (dB)
    Number Of Decimal Digits: 0
  4. Set the following in the Range Color Options frame:
    5. Option Value
    Max Color Squares per Row: 40
    Color Square Width (pixels): 50
  5. Click OK.

Viewing C/N in the 3D volume of space

  1. Bring the 3D Graphics window to the front to view your volumetric coverage results.
  2. Reset () your scenario.
  3. Click Normal Animation Mode () in the Animation toolbar.
  4. Decrease Time Step () to 30 seconds in the Animation Toolbar.
  5. Click the Home View ().
  6. Zoom out so you can see the whole airspace.
  7. Start () the animation.

    8. C/N 3D Graphics

    You can see C/N changes every five (5) minutes.

Conclusion

You started by creating a constellation of LEO satellites using Walker tool; each satellite had receiver. Next, you designed an Aircraft object that flew a search pattern in a designated area of interest using the Aviator propagator. You utilized the Comm System object to determine possible communication interference with the LEO satellites. Finally you will created a Volumetrics object using the Spatial Analysis tool to create a custom grid to visualize potential radio frequency interference in the LEO, MEO and GEO volume of space.

Save your work

  1. Reset () the scenario when you are finished.
  2. Close any reports or tools that are still open.
  3. Save () your work.

On your own

Throughout the lesson, hyperlinks were provided that pointed to in depth information. Now's a good time to go back through this tutorial and view that information. Create your own satellite constellation with receivers and a static ground site and transmitter to evaluate interference and apply it to your own custom grid.