Part 11: Introduction to Communications

STK Pro, STK Premium (Air), 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.

Capabilities Covered

This lesson covers the following STK Capabilities:

  • STK Pro
  • Communications

Problem Statement

Engineers and operators need to quickly determine communication link budgets. Factors such as communications unobstructed or obstructed by terrain, trees or man made structures need to be taken into consideration. Other factors that need to be included and analyzed are rain models, atmospheric losses, and RF interference sources. In this scenario, a team of scientists is monitoring glacial meltwater in a remote, mountainous location. Prior to setting up camp, they need to determine how their location will impact a link budget between them and a low earth orbit (LEO), Earth observation satellite which is downloading data to the team.

Solution

Use STK Pro and STK's Communications capability to model and analyze a link budget between the ground site and the Earth observation satellite. The satellite transmitter will be analyzed using an isotropic, omnidirectional antenna pattern. The ground team will employ a small parabolic antenna steered by a servo motor that can track the satellite. After establishing a link budget based on line of sight only, terrain obstruction, rain, atmospheric absorption and system noise temperature will be factored in until the final link budget analysis is complete.

What you will learn

Upon completion of this tutorial, you will understand:

  • Receiver and Transmitter objects
  • STK Antenna models
  • RF Environment properties
  • Simple and detailed link budgets

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

Create a new scenario for a twenty four (24) hour time period starting 1 Jul 2020 06:00:00.000 UTCG.

  1. Launch STK ().
  2. In the Welcome to STK window, click Create a Scenario.
  3. Enter the following in the New Scenario Wizard:
    4. Option Value
    Name: STK_Communications
    Location: Default
    Start: 1 Jul 2020 06:00:00.000 UTCG
    Stop: 2 Jul 2020 06:00:00.000 UTCG
  4. When you finish, click OK.
  5. When the scenario loads, click Save (). 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 and click Save.

Save () often!

Add Analytical and Visual Terrain

An STK Terrain File (pdtt) located in the STK install area will be used for analysis and situational awareness in the 3D Graphics window.

  1. Right-click on STK_Communications () and select Properties ().
  2. Select the Basic - Terrain page.
  3. Disable Use terrain server for analysis.
  4. Click OK to accept the changes and close the Properties Browser.
  5. Bring the 3D Graphics window to the front.
  6. In the 3D Graphic window toolbar, click Globe Manager ().
  7. In the Globe Manager's Hierarchy toolbar, click Add Terrain/Imagery ().
  8. Select the Add Terrain/Imagery () option in the drop-down.
  9. In the Globe Manager: Open Terrain and Imagery Data window, click the ellipsis button () in the Path field.
  10. Navigate to <STK Install Folder>\Data\Resources\stktraining\imagery (e.g. This PC then C:\Program Files\AGI\STK 12\Data\Resources\stktraining\imagery).
  11. Click OK.
  12. Select RaistingStation.pdtt.
  13. Click Add .
  14. When the Use Terrain for Analysis prompt displays, click Yes to enable terrain for analysis.

Declutter the Labels

Label Declutter is used to separate the labels on objects that are in close proximity for better identification in small areas.

  1. Bring the 3D Graphics window to the front.
  2. Open the 3D Graphics window properties ().
  3. On the Details page, locate the Label Declutter section.
  4. Select Enable.
  5. Click OK to accept the changes and close the Properties Browser.

Insert the Earth Observation Satellite

The Earth observation satellite is in a sun-synchronous orbit. Sun-synchronous orbits are designed to utilize the effect of the Earth's oblateness, causing the orbit plane to precess at a rate equal to the mean orbital rate of the Earth around the Sun. Sun synchronous orbits have the property that their nodes maintain constant local mean solar times.

  1. Using the Insert STK Objects tool, insert a Satellite () object using the From Standard Object Database method.
  2. In the Search Standard Object Data window, enter Rapideye in the Name or ID: field.
  3. Click Search.
  4. In the Results: field, select RapidEye 2 using the AGI's Standard Object Data Service Data Source.
  5. Click Insert.
  6. Close the Search Standard Object Data window.

Insert the Scientists' Camp Site

The camp site sits in a valley next to a river being fed by mountain glaciers.

  1. Using the Insert STK Objects tool, insert a Place () object using the Define Properties method.
  2. On the Basic - Position page, set the following:
    3. Option Value
    Latitude: 47.5605 deg
    Longitude: 11.5027 deg
    Height Above Ground: 6 ft (simulates antenna height)
  3. Click OK to accept the changes and close the Properties Browser.
  4. Rename the Place () object Camp_Site.

Simple Transmitter Model

The Simple Transmitter model is convenient when you do not have all the information necessary to model the transmitter in detail; for example, during the system engineering process.

  1. Using the Insert STK Objects tool, insert a Transmitter () object using the Insert Default method.
  2. When the Select Object window appears, select Rapideye2_33312 ().
  3. Click OK.
  4. Rename the Transmitter () object Download_Tx.

Transmitter Model Specs

Using a Simple Transmitter Model type, the Model Specs tab allows you to set the transmitter's frequency, EIRP (Effective Isotropic Radiated Power), data rate, and polarization. The Simple Transmitter model defaults to an isotropic antenna pattern. An isotropic antenna pattern is an ideal spherical pattern antenna with constant gain.

  1. Open Download_Tx's () properties.
  2. On the Basic - Definition page, ensure the Model Specs tab is selected.
  3. Set the following:
    4. Option Value
    Frequency: 1.7045 GHz
    EIRP: 10 dBW
    Data Rate: 4.2 Mb/sec
  4. Select Use in the Polarization field.
  5. Set the polarization to Right-hand Circular.
  6. Click OK to accept the changes and close the Properties Browser.

The default modulation for the Transmitter () object is Bi-phase shift keying (BPSK). Additional Gains and Losses is used to model gains and losses that affect performance but are not defined using built-in analytical models.

Receiver Antenna Orientation

The receiver antenna is steerable. To create a steering device (e.g. servo motor) in STK, you use a Sensor () object.

  1. Zoom To Camp_Site ().
  2. Using the Insert STK Objects tool, insert a Sensor () object using the Insert Default method.
  3. When the Select Object window appears, select Camp_Site ().
  4. Click OK.
  5. Rename the Sensor () object Servo_Motor.

Half Power Sensor Patterns

Half Power sensor patterns are designed to visually model parabolic antennas. The sensor half angle is determined by frequency and antenna diameter.

Half Power Sensor

  1. Open Servo_Motor's properties ().
  2. On the Basic - Definition page, set the following:
    3. Option Value
    Sensor Type: Half Power
    Frequency: 1.7045 GHz
    Diameter: 1.6 m
  3. Click Apply to accept the changes and keep the Properties Browser open.

Targeted Sensor

The Targeted pointing type causes the sensor to point to other objects in the scenario.

  1. Select the Basic - Pointing page.
  2. Set the Pointing Type: to Targeted.
  3. Set the Track Mode: value to Receive.
  4. Move () Rapideye2_33312 () from the Available Targets list to the Assigned Targets list.
  5. Click Apply to accept the changes and keep the Properties Browser open.

Target Times

Use Target Times to schedule or view access times from the sensor to the selected targets. This is a quick way to determine an access without using the Access Tool.

  1. Click Target Times.
  2. There are seven (7) opportunities to download data from the satellite to the ground site.
  3. Click Cancel to close the Target Times window.
  4. Click OK to close Servo_Motor's () properties.
  5. Bring the 3D Graphics window to the front.
  6. In the Timeline View, slowly move the gray pointer until the sensor accesses the satellite. Use your mouse to change the view so that you can view the access between Servo_Motor () and Rapideye2_33312 ().

    7. You can see that Servo_Motor () is targeting Rapideye2_33312 (). Rapideye2_33312 () has two (2) Sensor () objects attached to it that were downloaded with the Satellite () object if the satellite was inserted from AGI's Standard Object Data Service Data Source.

Sensor Targeting Satellite

Complex Receiver Model

The Complex Receiver model allows you to select among a variety of analytical and realistic antenna models, and to define the characteristics of the selected antenna type.

  1. Using the Insert STK Objects tool, insert a Receiver () object using the Insert Default method.
  2. When the Select Object window appears, select Servo_Motor ().
  3. Click OK.
  4. Rename the Receiver () object Download_Rx.

Receiver Model Specs

  1. Open Download _Rx's () properties ().
  2. On the Basic - Definition page, click the ellipsis button (Component Selector) () beside the Type field.
  3. Select Complex Receiver Model.
  4. Click OK.

By default, the frequency is set to Auto Track. The Frequency Auto Track option allows a receiver to track and lock onto the transmitter's carrier frequency with which it is currently linking, including any Doppler shift. LNA refers to Low Noise Amplifier. If you have those specifications, you can add them.

Receiver Antenna

You can select to embed an antenna model from the Component Browser or you can link to an antenna object. You will use a parabolic antenna.

  1. Select the Antenna tab.
  2. Click the ellipsis button (Component Selector) () beside the Type field.
  3. Select Parabolic.
  4. Click OK.
  5. Set the following:
    6. Option Value
    Design Frequency: 1.7 GHz
    Diameter: 1.6 m
  6. Click Apply to accept the changes and keep the Properties Browser open.

Receiver Antenna Polarization

The receiver polarization type is the same as the transmitter's polarization.

  1. Select the Polarization sub-tab.
  2. Select Use.
  3. Change the polarization to Right-hand Circular.
  4. Click OK to accept the changes and close the Properties Browser.

Simple Link Budget

Creating a Link Budget in the Access tool is referred to as a Simple Link Budget. The Link Budget Report is a specialized Access Report for basic link budget analysis and is available using the Link Budget button in the Reports frame of the Access window.

  1. In the Object Browser, right click on Download_Rx () and select Access ().
  2. When the Access Tool opens, in the Associated Objects list, expand () Rapideye2_33312 ().
  3. Select Download_Tx ().
  4. Click Compute ().
  5. In the Reports section, click Link Budget...

    6. Take some time to look at the Simple Link Budget report. As the satellite rises over the horizon of the central body (WGS84) you receive transmissions. When the satellite falls below the horizon, you lose transmissions.

  6. Leave the Link Budget Report open.

Take Terrain Into Consideration

Use Terrain Mask. If this check box is selected, access to the object is constrained by any terrain data in the line of sight to which access is being calculated.

  1. Open Download_Rx's () properties ().
  2. Select the Constraints - Basic page.
  3. Select Terrain Mask.
  4. Click OK to accept the changes and close the Properties Browser.
  5. Return to the Link Budget Report and click Refresh (F5) ().

    6. You'll see that all accesses blocked by analytical terrain (RaistingStation.pdtt) have been removed from the report. You began with seven (7) accesses but taking terrain into account you now have four (4) accesses.

  6. Close the Link Budget Report.
  7. Return to the Access Tool.

Detailed Link Budget

The Detailed Link Budget Report is a modified Link Budge Report with added Access Data Provider Elements.

  1. In the Access Tool, click Report & Graph Manager...
  2. When the Report & Graph Manager opens, go to the Styles - Installed Styles list and select Link Budget - Detailed.
  3. Click Generate.
  4. Change the report Step: to 30 sec.
  5. Press Enter on your keyboard.

    6. Take some time to look at the report. Looking at the first access, note the values for Atmos Loss (dB) and Rain Loss (dB). They're currently zero (0). You haven't loaded any RF Environment models. Also note the values in the C/N (dB) (carrier to noise ratio), Eb/No (dB) (energy per bit to noise ratio), and BER (Bit Error Rate) columns. For the purposes of your analysis due to downloading data, you require a BER equal to or lower than 1.000000e-13. You can see from the report that you only have a few minutes during each pass to download data.

  6. Leave the report open.

Rain Model

Environmental factors can affect the performance of a communications link. Apply rain and atmospheric absorption models to the analysis. Rain models are used to estimate the amount of degradation (or fading) of the signal when passing through rain.

  1. Open STK_Communications () properties ().
  2. Select the RF - Environment page.
  3. Select the Rain & Cloud & Fog tab.
  4. Select Use In the Rain Model section. Leave the default ITU (International Telecommunication Union) model.
  5. Click Apply to accept the changes and keep the Properties Browser open.

Atmospheric Absorption Model

Atmospheric Absorption models estimate the attenuation of atmospheric gases on terrestrial and slant path communication signals.

  1. Select the Atmospheric Absorption tab.
  2. Select Use. Leave the default ITU model.
  3. Click OK to accept the changes and close the Properties Browser.
  4. Return to the Link Budget - Detailed report.
  5. Observe the C/N (dB), Eb/No (dB) and BER columns.
  6. Click Refresh (F5) (). There was a slight change in the C/N (dB) and Eb/No (dB) values.
  7. Scroll to the Atmos Loss (dB) and Rain Loss (dB) columns.

    8. Losses were minimal, therefore the slight losses in C/N (dB) and Eb/No (dB) values.

  8. Leave the report open.

System Noise Temperature

The Receiver's System Noise Temperature allows you to specify the system's inherent noise characteristics.

  1. Open Download_Rx's () properties ().
  2. On the Basic - Definition page, select the System Noise Temperature tab.
  3. Select Compute.
  4. Select Compute in the Antenna Noise section.
  5. Select Sun, Atmosphere, Rain and Cosmic Background.
  6. Click OK to accept the changes and close the Properties Browser.
  7. Return to the Link Budget - Detailed report.
  8. Observe the C/N (dB), Eb/No (dB) and BER columns.
  9. Click Refresh (F5) (). There were increases in the C/N (dB) and Eb/No (dB) values and decreases in BER.
  10. Look at the following columns and observe their values:
    • Tatmos (K)
    • Train (K)
    • Tsun (K)
    • Tearth (K)
    • Tcosmic (K)
    • Tantenna (K)
    • Tequivalent (K)

These are the calculated noise temperatures. Calculating system noise temperature improved your Link Budget Report and extended the time available for downloading data.

Summary

You inserted a Place () object into the scenario which simulated the location of a scientific team monitoring glacial meltwater in mountainous terrain. You used a local terrain file both analytically and visually for your analysis and situational awareness. You inserted and propagated an actual Earth Observation Satellite () object which the scientists need to communicate with for data download. You were introduced to the Simple Transmitter Model which was attached to the Satellite () object. The Simple Transmitter uses an isotropic antenna. Next, you created a servo motor using a Sensor () object which was used to steer the ground site receiver antenna by locking onto the Satellite () object. The Receiver () object used a Complex Receiver Model. This model allowed you to input a Parabolic Antenna Pattern. The first analysis, using the Access Tool, focused on a Simple Link Budget. The only constraint applied was line of sight which is basically horizon to horizon using the central body. Next, using Terrain Mask, you took into account local terrain and lost multiple accesses to the Satellite () object. You moved to the Report & Graph Manager and generated a Detailed Link Budget. You enabled an ITU rain model and atmospheric absorption model which had a slight detrimental impact on the Link Budget Report. Finally, you added System Noise Temperature analysis to the Link Budget Report which had a positive impact to the link budget.

Save Your Work

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

On Your Own

Throughout the tutorial, hyperlinks were provided that pointed to in depth information concerning Transmitter () and Receiver () objects. Now's a good time to go back through this tutorial and view that information.

For further study:

Modulator

Communications allows you to select from multiple modulators, including user-defined modulators. Each modulator has a defined modulation. Normally, you'll perform this function in the Transmitter () object. The Receiver () object defaults to the same modulation as the Transmitter () object when calculating a Link Budget Report. However, you can override this.

  1. Select the Modulator tab.
  2. To understand the settings on this page, click HERE.

Additional Gains and Losses

During communications analyses, it is often necessary to model gains and losses that affect performance but are not defined using built-in analytical models. STK allows you to model these by specifying miscellaneous gains and losses that can be added to the equation. This can be found in both the Transmitter () and Receiver () objects.

  1. Select the Additional Gains and Losses tab.
  2. To understand the settings on this page, click HERE.