Space Mission Analysis and Design (SMAD): Communications Design (Part 3 of 3)

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.

Optional Product Install: This lesson uses the Ansys Electronics Desktop (AEDT)™ electronics systems design platform. You can obtain a license for this product 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.

For this lesson, you will work with Far Field Data files (*.ffd) only. The STK software does not support the use of Near Field Data files (*.nfd).

Capabilities covered

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

  • STK Pro
  • Communications

Problem statement

Engineers and operators want to design a mission to characterize the steam, ash, and lava emissions of active volcanoes in the United States. The objective of the satellite, STKSat, is to image these volcanoes to provide early indications of volcanic eruption and to transmit the images to a ground site for analysis. They want optimize the design of the communications system to ensure the strength of the communications link between the satellite and the ground site allows data to be transmitted and received successfully. The mission requirements for the communication system are as follows:

  • The satellite shall downlink health and imagery data at an ultra-high frequency (UHF) of around 435 MHz.
  • The satellite shall receive uplink data from the ground site at a very-high frequency (VHF) of around 145 MHz.
  • The satellite shall operate with a link margin greater than or equal to 3 dB for imagery and health data to be considered acceptable.
  • The satellite shall communicate with a bit error rate of less than or equal to 1e-03 for imagery and health data to be considered acceptable.

Solution

Use the STK software's Communications capability to model a complete RF link between the example mission ground site in Exton, PA and the STKSat spacecraft. You will compare properties such as power and modulators to optimize the RF link, establish a link and telemetry budget for the STKSat mission and analyze the impact of environmental losses on the RF link. Finally, you will vary parameters to optimize the link for the mission requirements.

What you will learn

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

  • Use the Communications capability to model an RF communications link between a spacecraft and a ground site in real-world conditions.
  • Import custom-generated antenna pattern files into the STK software and use them analytically.
  • Analyze the effects of various design parameters on a communications link to optimize the design of a communications system.

Video guidance

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

Downloading required files

This lesson uses a surface geometry file modeling the STKSat satellite and four antenna files produced using the Ansys HFSS™ high-frequency electromagnetic simulation software, which is a part of the Ansys Electronics Desktop (AEDT)™ electronics systems design platform, that you will need to download below.

  1. Download the zipped folder, CommDesign.zip, here: https://support.agi.com/download/?type=training&dir=sdf/help&file=CommDesign.zip

    2. If you are not already logged in, you will be prompted to log in to agi.com to download the file. If you do not have an agi.com account, you will need to create one. The user approval process can take up to three (3) business days. Please contact support@agi.com if you need access sooner.

  2. Note the location of the downloaded folder.

Using the starter scenario

The starter scenario contains the constellation of volcanoes to be studied, the imaging satellite and its camera, a Chain object for analysis, and a ground site that you developed in the Orbit Design tutorial and continued building on in the Solar and Power Design tutorial in the Space Mission Analysis and Design series. If you wish to continue building on your scenario, save a copy named in a new folder named CommDesign and skip to the Extracting the required files section.

Downloading the starter VDF

Download the starter VDF file and open it in the STK application.

  1. Download the starter VDF file, CommDesignStarter.vdf, here: https://support.agi.com/download/?type=training&dir=sdf/help&file=CommDesignStarter.vdf
  2. Launch the STK () application.
  3. Click Open a Scenario in the Welcome to STK dialog box.
  4. Browse to location of your extracted VDF file.
  5. Select CommDesignStarter.vdf.
  6. Click Open.

Saving the VDF as a Scenario File

When you save a scenario with the STK application, it will save in the from in which it originated. In other words, if you open a VDF, the default save format will be a VDF file (.vdf); the same is true for a scenario file (*.sc). If you want to save a VDF file as a SC file (or vice-versa), you must change the file format when you are performing the Save As procedure.

  1. Open the File menu.
  2. Select Save As.
  3. Select the STK User folder in the navigation pane when the Save As dialog box opens.
  4. Click the New Folder option in the selector bar.
  5. Name the folder CommDesign.
  6. Select the Enter key.
  7. Click Open.
  8. Select Scenario Files (*.sc) in the Save as type drop-down list.
  9. Enter CommDesign.sc as the File name.
  10. Click Save.

Save () often!

Extracting the required files

Extract the downloaded ZIP file into your scenario folder.

  1. Navigate to the downloaded ZIP file, CommDesign.zip.
  2. Right-click on CommDesign.zip.
  3. Select Extract All... in the shortcut menu.
  4. Set the Files will be extracted to this folder path to be within your scenario folder (e.g. C:\Users\username\Documents\STK 12\CommDesign).
  5. Click Extract to unzip the contents of CommDesign.zip to your scenario folder.
  6. The following files are found in the zipped CommDesign folder:
    1. STKSatModel.stl
    2. HFSS_Antenna_Files:
      • Exton_YagiUda_UHF.ffd
      • Exton_YagiUda_VHF.ffd
      • STKSatModel_UHF.ffd
      • STKSatModel_VHF.ffd

Updating the scenario analysis period

Unlike the scenarios in the first and second parts of the Space Mission Analysis and Design series, you won't need the constellation of volcanoes or the imaging camera for this portion of the analysis. For the purposes of this exercise, you will focus your analysis on a one-day period. On your own, you can extend the scenario time to the entire year and analyze the RF link.

Update your scenario's time Start and Stop times and turn off the display of the objects not used in this tutorial. They will still be available analytically, but this will help clear up your workspace to focus on the communications design goals of this lesson.

  1. Right-click on CommDesign () in the Object Browser.
  2. Select Properties () in the shortcut menu.
  3. Select the Basic - Time page when the Properties Browser opens.
  4. Modify the start and stop times of your scenario to the values listed in the table below:
    5. Field Value
    Start 1 Jan 2024 00:00:00.000 UTCG
    Stop + 1 day
  5. Click OK to accept your changes and to close the Properties Browser.
  6. Clear the checkbox for Imager () in the Object Browser.
  7. Clear the checkboxes for each of the 10 volcanoes ().
  8. Clear the checkbox for Observe ().

Modeling a satellite tracking sensor

Typically, many ground sites have satellite tracking capabilities. The ground site’s antennas are attached to an antenna rotor system that can be used to change the azimuth and elevation of the antennas thus allowing directional pointing and tracking.

Inserting the Sensor object

Insert a sensor object to function as the antennas' servo motor.

  1. Click Insert Object ().
  2. Insert a Sensor () object using the Define Properties () method when the Insert STK Objects tool opens.
  3. Select Exton () in the Select Object dialog box.
  4. Click OK to insert the sensor and to close the Select Object dialog box.
  5. Right-click on Sensor1 () in the Object Browser.
  6. Select Rename in the shortcut menu.
  7. Rename Sensor1 () to AntennaRotor.

Configuring the Sensor object

Modify AntennaRotor's properties so that it has a narrow field of view and points at the STKSat satellite.

  1. Right-click on AntennaRotor ()
  2. Select Properties () in the shortcut menu.
  3. Select the Basic – Definition page when the Properties Browser opens.
  4. Change the Cone Half Angle to 2 deg.

    5. In this case, the cone half angle is only for aesthetic purposes to show the pointing and tracking capabilities of the ground site.

  5. Select the Basic – Pointing page.
  6. Change the Pointing Type from Fixed to Targeted.
  7. Select STKSat () in the Available Targets list.
  8. Move () STKSat () to the Assigned Targets list.
  9. Click OK.

Visualizing the antenna link

You can visualize the sensor tracking STKSat in the 3D graphics window.

  1. Bring the 3D graphics window to the front.
  2. Click Start () on the Animation Toolbar to animate your scenario.
  3. Watch as the AntennaRotor sensor tracks STKSat as it passes overhead.
  4. Click Reset () when finished.

Exton's Sensor tracking STKSat

Modeling the ground site's VHF transmitter

Modeling transmitters involves configuring the output power of the system, the modulation technique being utilized, system losses, data rate, and properties of the antenna connected to the transmitter.

Inserting a Transmitter object

Attach a Transmitter () object to AntennaRotor (). The ground site will be sending commands up to the satellite for this mission.

  1. Insert a Transmitter () object using the Insert Default () method.
  2. Select AntennaRotor () in the Select Object window.
  3. Click OK.
  4. Rename Transmitter1 () VHF_ExtonTx.

Configuring the VHF transmitter model

The ground site will transmit in the very high frequency (VHF) range with a custom power and data rate. Configure the transmitter to use a Complex Transmitter model.

  1. Open VHF_ExtonTx ()'s Properties ().
  2. Select the Basic – Definition page when the Properties Browser opens.
  3. Click the Transmitter Model Component Selector ().
  4. Select Complex Transmitter Model () from the Transmitter Models list when the Select Component dialog box opens.
  5. Click OK to accept your changes and to close the Select Component Dialog.
  6. Select the Model Specs tab.
  7. Set the following options:
    8. Option Value
    Frequency 145 MHz
    Power 20 dBW
    Data Rate 9600 b/sec
  8. Click Apply to save your changes and to keep the Properties Browser open.

Configuring the antenna polarization

Modify the polarization of the transmitter's antenna to Right-hand Circular to model a Yagi-Uda antenna. Yagi antennas antennas are typically either right-hand or left-hand circularly polarized.

  1. Select the Antenna tab.
  2. Select the Polarization subtab.
  3. Select the Use checkbox.
  4. Change the polarization to Right-hand Circular.
  5. Click Apply.

Setting the transmitter's modulation technique

Set the transmitter to use MSK modulation.

  1. Select the Modulator tab.
  2. Select MSK from the Name drop-down list as the modulating technique.
  3. Ensure the Auto Scale checkbox is selected in the Signal Bandwidth panel.

    This option automatically calculates the bandwidth of the signal based on the frequency, data rate, and modulation technique.

  4. Click OK.

Modeling the ground site's UHF receiver

Modeling receivers involves configuring the internal and external system losses, the system noise temperature, the demodulation technique to be used, and properties of the antenna connected to the receiver.

Inserting a Receiver object

Attach a Receiver () object to AntennaRotor (). The ground site will be receiving images from the satellite during this mission.

  1. Insert a Receiver () object using the Insert Default () method.
  2. Select AntennaRotor () in the Select Object window.
  3. Click OK.
  4. Rename Receiver1 () UHF_ExtonRx.

Configuring the UHF receiver model

Use a Complex Receiver model for the ground site's reiver.

  1. Open UHF_ExtonRx's () Properties ().
  2. Select the Basic – Definition page.
  3. Click the Receiver Model Component Selector ().
  4. Select Complex Receiver Model () in the Select Component dialog box.
  5. Click OK.
  6. Select the Model Specs tab.
  7. Clear the Auto Track checkbox.
  8. Set the following options:
    9. Option Value
    Frequency 435 MHz
    Antenna to LNA Line Loss 3 dB
    LNA Gain 20 dB
    LNA to Receiver Line Loss 2 dB
  9. Click Apply.

Configuring the receiver antenna's polarization

Modify the polarization of the receiver's antenna to model a Yagi-Uda antenna.

  1. Select the Antenna tab.
  2. Select the Polarization subtab.
  3. Select the Use checkbox.
  4. Change the polarization to Right-hand Circular.
  5. Click Apply.

Setting the system noise temperature

The Receiver's System Noise Temperature allows you to specify the system's inherent noise characteristics. These settings can more accurately simulate real-world RF conditions.

  1. Select the System Noise Temperature tab.
  2. Select the Compute checkbox.
  3. Set the following options:
    4. Option Value
    Antenna to LNA Transmission Line - Temperature 290 K
    LNA - Noise Figure 2 dB
    LNA - Temperature 60 K
    LNA to Receiver Transmission Line - Temperature 130 K
  4. Select the Constant checkbox in the Antenna Noise panel.
  5. Set the Antenna Noise - Constant to 1200 K.
  6. Click Apply.

For a higher-fidelity analysis, using the Compute option is preferred; for the sake of consistency in this exercise, you are using a constant value instead.

Configuring the demodulator and filter

The demodulator and the filter are the remaining settings that you will change.

  1. Select the Demodulator tab.
  2. Clear the Auto-select Demodulator checkbox.
  3. Select MSK in the Name drop-down list as the demodulating technique.
  4. Select the Filter tab.
  5. Ensure the Auto Scale checkbox is selected in the Receiver Bandwidth panel.
  6. Click OK.

Modeling the satellite's UHF transmitter

The STKSat satellite uses a CPUT VUTRX transceiver, comprised of a VHF uplink receiver and a UHF downlink transmitter.

The transmitter specs are as follows:

  • Transmitter Output Power: adjustable, 27–33 dBm
  • Transmit Power Draw: 4–10 W (27–33 dBm)
  • Data Rate: 9,600 baud (same as b/sec)
  • Modulation: GMSK/MSK

Use these values to model the UHF transmitter portion of the transceiver with a Transmitter () object.

Inserting a Transmitter object

Attach a Transmitter () object to STKSat ().

  1. Insert a Transmitter () object using the Insert Default () method.
  2. Select STKSat () in the Select Object dialog box.
  3. Click OK.
  4. Rename Transmitter2 () UHF_SatTx.

Configuring the UHF transmitter model

STKSat's transceiver transmits in the ultra high frequency (UHF) range, with a custom power, and data rate. Use a Complex Transmitter model to model configured for these specifications.

  1. Ooen UHF_SatTx's () Properties ().
  2. In the Properties window, select the Basic – Definition page.
  3. Change the Transmitter Model to a Complex Transmitter Model () in the Component Selector.
  4. Select the Model Specs tab.
  5. Set the following options:
    6. Option Value
    Frequency 435 MHz
    Power 3 dBW
    Data Rate 9600 b/sec
  6. Click Apply.

Configuring the transmitter antenna's polarization

Modify the polarization of the transmitter's antenna to model an onboard Yagi-Uda monopole antenna. Change the polarization to Right-hand Circular.

  1. Select the Antenna tab.
  2. Select the Polarization subtab.
  3. Select the Use checkbox.
  4. Change the polarization to Right-hand Circular.
  5. Click Apply.

Setting the transmitter's modulation

Set the transmitter to use the MSK modulation technique.

  1. Select the Modulator tab.
  2. Select MSK in the Name drop-down list.
  3. Ensure the Auto Scale checkbox is selected in the Signal Bandwidth panel.
  4. Click OK.

Modeling the satellite's VHF receiver

The STKSat satellite uses a CPUT VUTRX transceiver, comprised of a VHF uplink receiver and a UHF downlink transmitter.

The receiver specs are as follows:

  • Receiver Sensitivity: -120 dBm
  • Receiver Power Draw: 250 mW
  • Data Rate: 9,600 baud
  • Modulation: GMSK/MSK

Use these values to model the VHF uplink receiver portion of the transceiver with a Receiver () object.

Inserting a Receiver object

Attach a Receiver () object to STKSat (). The Satellite will be receiving commands from the ground site during the mission.

  1. Insert a Receiver () object using the Insert Default () method.
  2. Select STKSat () in the Select Object dialog box.
  3. Click OK.
  4. Rename Receiver2 () VHF_SatRx.

Configuring the VHF receiver model

Use a Complex Receiver model for the satellite's receiver.

  1. Open VHF_SatRx's () Properties ().
  2. Select the Basic – Definition page.
  3. Change the Receiver Model to a Complex Receiver Model () in the Component Selector.
  4. Select the Model Specs tab.
  5. Clear the Auto Track checkbox.
  6. Set the Frequency to 145 MHz.
  7. Click Apply.

Configuring the receiver antenna's polarization

Modify the polarization of the receiver to model an onboard Yag-Uda monopole antenna. Change the polarization to Right-hand Circular.

  1. Select the Antenna tab.
  2. Select the Polarization subtab.
  3. Select the Use checkbox.
  4. Change the polarization to Right-hand Circular.
  5. Click Apply.

Setting the system noise temperature

Specify the receiver's inherent noise characteristics to better simulate real-world RF situations.

  1. Select the System Noise Temperature tab.
  2. Select the Compute checkbox.
  3. Set the following options:
    4. Option Value
    Antenna to LNA Transmission Line - Temperature 290 K
    LNA - Noise Figure 1 dB
    LNA - Temperature 1000 K
    LNA to Receiver Transmission Line - Temperature 290 K
  4. Select the Constant option in the Antenna Noise panel.
  5. Set the Antenna Noise - Constant to 1200 K.
  6. Click Apply.

Configuring the receiver's demodulator and filter

The demodulator and the filter settings.

  1. Select the Demodulator tab.
  2. Clear the Auto-select Demodulator checkbox.
  3. Select MSK in the Name drop-down list.
  4. Select the Filter tab.
  5. Ensure the Auto Scale checkbox is selected in the Receiver Bandwidth panel.
  6. Click Apply.

Including additional gains and losses

Model the pre-receive and pre-demodulation losses in STKSat's receiver. The first loss is due to antenna coupling, polarization, etc., due to the satellite's inability to point the receiver. The pre-modulation loss represents losses inside the receiver system between the antenna and the demodulator. This loss is due to size and technology constraints.

  1. Select the Additional Gains and Losses tab.
  2. Click Add in the Pre-Receive Gains/Losses panel.
  3. Click in the Gain cell.
  4. Update the Gain value to -15 dB.

    5. This represents a pre-receive loss due to antenna coupling, polarization, etc. This loss is greater due to the lack of pointing capability.

  5. Click Add in the Pre-Demodulation Gains/Losses panel.
  6. Change the Gain value to -8 dB.

    7. This represents losses inside the receiver system between the antenna and the demodulator. This loss is also greater for satellites due to size and technology constraints.

  7. Click OK.

Simulating the satellite’s antennas with the Ansys HFSS SBR+ software

Antennas operating on large host platforms can experience significant performance degradation due to electromagnetic interaction with their host structure. The installation placement of the antenna will significantly affect the overall efficiency of the wireless communication or radar systems that these antennas service.

You can simulate and study installed antenna applications using the Ansys HFSS SBR+™ asymptotic high-frequency electromagnetic (EM) simulator for modeling EM interaction in electrically large environments. The SBR+ software is part of the Ansys HFSS high-frequency simulation software suite, which itself is part of the Ansys Electronics Desktop (AEDT) electronics systems design platform. The SBR+ application uses shooting and bouncing rays to solve the antenna interaction with the satellite to which it is attached.

STKSat's antennas must take into account the obstructions from the satellite platform. Use the SBR+ software to see how the STKSat satellite model itself interferes with the antenna gain in all directions.

If you do not have access to the AEDT platform and the HFSS SBR+ software, skip to the Importing and configuring the antennas in the STK application section.

Setting up a new HFSS design

Create a new project in the AEDT platform for modeling the STKSat's antennas.

  1. Open the Ansys Electronics Desktop (AEDT) application.
  2. Select the Desktop tab.
  3. Click New on the Desktop ribbon to create a new AEDT project.
  4. Right-click on the project.
  5. Select Rename.
  6. Name the project STKSat.
  7. Right-click on the project.
  8. Select Insert in the shortcut menu.
  9. Select Insert HFSS Design in the Insert submenu. The 3D Model window will appear.

Setting up the Modeler

Set up your AEDT project to use the HFSS SBR+ software for modeling.

  1. Select the Modeler command in the menu bar.
  2. Select Units….
  3. Select meter in the Select units drop-down menu when the Set Model Unites and Max Extent dialog box appears.
  4. Click OK to change the units to meters and to close the Set Model Unites and Max Extent dialog box.
  5. Select the HFSS command in the menu bar.
  6. Select Solution Type….
  7. Select SBR+ in the Solutions Types panel when the Solution Types: Project STKSat - HFSSDesign1 dialog box apperas.
  8. Click OK to confirm your selection and to close the Solution Types: Project STKSat - HFSSDesign1 dialog box.
  9. Click Save to save your project.

Preparing the HFSS import options

Additional HFSS import options need to be enabled before you can import your satellite model.

  1. Select the Tools command in the menu bar.
  2. Select Options.
  3. Select General Options... in the Options submenu.
  4. Expand General in the Options list.
  5. Select the Desktop Configuration page.
  6. Click Beta Options. . ..
  7. Scroll to the HFSS Feature Names when the Beta Options dialog box opens.
  8. Select the checkboxes for HFSS Auto HPC NUMA suppport, HFS Cable Modeling, HFSS Nonlinear Drude for Implicit Transient, and HFSS PI.
  9. Click OK to confirm your selection and to close the Beta Options dialog box.
  10. Click OK to confirm your changes and to open the Ansys Electronics Desktop dialog box.

Restarting the Ansys Electronics Desktop platform

For the modified beta options to take effect, the AEDT platform needs to be restarted.

  1. Click Yes to close the Ansys Electronics Desktop dialog box and to restart the platform.
  2. Select the Desktop tab.
  3. Click the downward-facing arrow below Open.
  4. Select STKSat.aedt in the drop-down menu to reopen your project.

Importing the satellite model

Import the satellite model into the modeler for SBR simulation.

  1. Select the Modeler command in the menu bar.
  2. Select Import….
  3. Navigate to your scenario folder.
  4. Select the STKSatModel.stl stereolithography file modeling the surface geometry of the STKSat satellite you downloaded at the start of this lesson.
  5. In the Facet Import Options panel, select the checkboxes for Heal and Import as Lightweight Faceted Body. This will help speed things up, especially since this a shooting and bouncing ray simulation.

    6. Facet import options in the Import File dialog box

  6. Click Open to load the model and open the 3D Modeler window.
  7. The model will load into the Modeler View Area of the 3D Modeler window. The History Tree in the left of the 3D Modeler window lists the active model's structure and grid details.

STKSatModel.stl in the model view Area

Assigning properties to the model

In order to validate the analysis, the model needs to have it’s material and boundary defined.

  1. Expand Model in the History Tree.
  2. Click on Sheets - Unassigned - STKSatModel. This will highlight the entire satellite model.
  3. Right-click Modeler View Area.
  4. Select Assign Boundary.
  5. Select Perfect E… in the Assign Boundary submenu.
  6. Click OK to confirm your selection and to close the Perfect E Boundary dialog box.

highlighted STKSAT model and history tree

Adding a relative coordinate system for the UHF antenna

Define a new relative coordinate system for the antenna. Antennas are attached at the origin of their assigned coordinate system.

  1. Zoom into the underside of the satellite. You should zoom in very close.
  2. Click Relative CS in the Draw Ribbon.
  3. A small square will appear under the cursor that highlights the snap point at any vertex you hover over. Click on a vertex near the bottom-left corner of the satellite model.

    4. Underside of the STKSat Model showing the highlighted vertex for the new Relative CS

  4. The new relative coordinate system will appear with the Z axis pointing down. This needs to be fixed.

    5. relative coordinate system for the STKSAT Model

  5. Expand Coordinate Systems in the History Tree.

Updating the UHF coordinate system properties

It is important to ensure that you are modeling your antenna on the correct place on the satellite model and to ensure the axes are oriented correctly.

  1. Right-click on RelativeCS1.
  2. Select Properties... in the shortcut menu.
  3. Enter UHF_CS in the Name field when the Properties: Project STKSat - HFSSDesign1 - Modeler dialog box opens.
  4. In the Properties dialog, change the X Axis property to 0 ,1 ,0.
  5. Change the Y Point property to 1 ,0 ,0.
  6. Set the Origin to 0.6237762581643 ,-1.0219597668976 ,-0.57153988427925.

    7. UHF relative CS Properties

  7. Click OK to accept your changes and to close the Properties dialog box. The Z axis will now be correctly orientated.

    8. correctly oriented z axis

Updating the relative coordinate system for the VHF antenna

This first relative coordinate system was for the UHF antenna. Now, set up another reference coordinate system for the VHF antenna.

  1. Zoom in to the upper-right corner of the satellite.

    2. Node for the new VHF Relative CS

  2. Select Global under Coordinate Systems in the History Tree. This is to allow the next reference coordinate system to reference the Global coordinate system, and not the one you just made.
  3. Click Relative CS in the Draw Ribbon.
  4. Click on the vertex on the top-right corner of the satellite to create your new relative coordinate system.

Updating the VHF coordinate system's properties

Update the relative coordinate system's properties to ensure the antenna is modeled at the correct place on the satellite with the correct orientation.

  1. Right-click RelativeCS1 in the History Tree.
  2. Select Properties... in the shortcut menu.
  3. Rename RelativeCS2 to VHF_CS.
  4. Change the X Axis property to 0 ,1 ,0 when the Properties: Project STKSat - HFSSDesign1 - Modeler dialog box opens.
  5. Change the Y Point property to 1 ,0 ,0.
  6. Set the Origin to -0.62374598620885 ,1.0217968642179 ,-0.57154787877286.
  7. Click OK to accept your changes and to close the Properties dialog box.

VHF_CS Properties and axes

Defining the UHF far-field setup

Define your far-field setup before generating the radiation patterns for your UHF antenna.

  1. Expand HFSSDesign1 (SBR+).
  2. Right-click on Radiation in the Project Manager.
  3. Select Insert Far Field Setup in the shortcut menu.
  4. Select Infinite Sphere… in the Insert Far Field Setup submenu.
  5. Click on the Coordinate System tab once the Far Field Radiation Sphere Setup dialog box opens.
  6. Select Use local coordinate system.
  7. Select the UHF_CS in the Choose from existing coordinate systems... drop-down list.
  8. Click OK to confirm your selection and to close the Far Field Radiation Sphere Setup dialog box.
  9. Rename Infinite Sphere1 UHF.

Defining the VHF far-field setup

Repeat steps outlined in Defining the UHF far-field setup to create another infinite sphere referencing the VHF_CS coordinate system.

  1. Right-click on Radiation.
  2. Select Insert Far Field Setup in the shortcut menu.
  3. Select Infinite Sphere… in the Insert Far Field Setup submenu.
  4. Click on the Coordinate System tab.
  5. Select Use local coordinate system.
  6. Select the VHF_CS in the Choose from existing coordinate systems... drop-down list.
  7. Click OK.
  8. Rename Infinite Sphere2 VHF.

Defining UHF excitations

In the HFSS software, excitations are sources of electromagnetic fields in a design. By assigning excitations, you define sources of electromagnetic fields, including antennas. For this exercise you will model a parametric wire monopole antenna using a current-source representation. Parametric antennas create far-field pattern sources that can provide S-parameter outputs and composite far-field radiation patterns; when using a current-source representation of the wire monopole, the axial current sources radiate in all directions, and the SBR software can correctly predict the interaction with the placement structure directly underneath because of the high-order fields imbedded in the current-source representation.

  1. Select UHF_CS in the History Tree.
  2. Right-click on Excitations in the project manager.
  3. Select Create Antenna Component in the shortcut menu.
  4. Select Wire Monopole… in the Create Antenna Component submenu.
  5. Enter UHF in the Name field once the Create Wire Monopole dialog box opens.
  6. Select Current Source.
  7. Change the Resonant Frequency to 0.435 GHz.
  8. Click Next >.
  9. Click Finish to accept your changes and to close the Create Wire Monopole dialog box.

Updating the UHF ground plane visualization

Though the HFSS software lets you model a signal layer as a ground plane, it is not needed for this analysis. Turn off its display in the 3D Modeler window.

  1. Expand 3D Components in the Project Manager.
  2. Right-click on UHF1.
  3. Select Properties… in the shortcut menu.
  4. Select the Visualization tab when the Properties dialog box opens.
  5. Clear the Show Ground Plane - Value checkbox.
  6. Click OK to accept your changes and to close the Properties dialog box.

Defining VHF excitations

Create an excitation called VHF with a resonant frequency of 0.145 GHz.

  1. Select VHF_CS in the History Tree.
  2. Right-click on Excitations in the project manager.
  3. Select Create Antenna Component in the shortcut menu.
  4. Select Wire Monopole… in the Create Antenna Component submenu.
  5. Enter VHF in the Name field.
  6. Select Current Source.
  7. Change the Resonant Frequency to 0.145 GHz.
  8. Click Next >.
  9. Click Finish.

Updating the VHF ground plane visualization

Turn off the display of the VHF ground plane in the 3D Modeler window.

  1. Expand 3D Components in the Project Manager.
  2. Right-click on VHF1.
  3. Select Properties… in the shortcut menu.
  4. Select the Visualization tab when the Properties dialog box opens.
  5. Clear the Show Ground Plane - Value checkbox.
  6. Click OK to accept your changes and to close the Properties dialog box.

Adding the UHF solution setup

The last modeling step is to add the solution setup. Then, you can run the analysis.

  1. Right-click on Analysis in the Project Manager.
  2. Select Add Solution Setup… in the shortcut menu.
  3. Enter UHF in the Setup Name field when the SBR+ Solution Setup dialog box opens.
  4. Click in the Distribution field in the Frequency Sweeps panel.
  5. Select Single Point in the drop-down list.
  6. Change the frequency sweep to start at 0.435 GHz.
  7. Select the Compute Fields checkbox below the Frequency Sweeps panel.
  8. Select the Options tab.
  9. Select UHF from the drop-down list in the Field Observation Domain panel.
  10. Click OK to save your changes and to close the SBR+ Solution Setup dialog box.

Adding the VHF solution setup

Repeat these steps for a solution setup called VHF, with a frequency sweep of 0.145 GHz, and using the VHF field.

  1. Right-click on Analysis in the Project Manager.
  2. Select Add Solution Setup… in the shortcut menu.
  3. Enter VHF in the Setup Name field.
  4. Click in the Distribution field in the Frequency Sweeps panel.
  5. Select Single Point in the drop-down list.
  6. Change the frequency sweep to start at 0.145 GHz.
  7. Select the Compute Fields checkbox below the Frequency Sweeps panel.
  8. Select the Options tab.
  9. Select VHF from the drop-down list in the Field Observation Domain panel.
  10. Click OK.

Running the analysis

Now that you have set up solutions for both patterns, you can run your analysis.

  1. Select the Simulation tab.
  2. Click Analyze All on the Simulation ribbon.

Setting the post-process sources

Define the sources you will use to visualize the gain plots in the 3D Modeler window.

  1. Right-click on Excitations in the Project Manager.
  2. Select Edit Sources… in the shortcut menu.
  3. Select the Source Context tab when the Edit post process sources dialog box opens.
  4. Select the checkboxes for both UHF1_p1 and VHF1_p1
  5. Click OK to accept your changes and to close the Edit post process sources dialog box.

Generating the UHF gain pattern

Build a 3D polar plot to visualize the UHF antenna pattern.

  1. Right-click on Results in the Project Manager.
  2. Select Create Far Fields Report in the shortcut menu.
  3. Select 3D Polar Plot in the Create Far Fields Report submenu.
  4. Select UHF1_P1 in the Sources menu in Context panel when the New Report - New Traces(s) dialog box opens.
  5. Ensure that the solution is set to the UHF : Sweep and that Geometry is referencing the UHF infinite sphere.
  6. Ensure the category is Gain, and the quantity is GainTotal.
  7. Select dB in the Function list.
  8. Click New Report to generate the gain plot.
  9. Click Close to close the Report dialog box.
  10. Rename Gain Plot 1 UHF.

Generating the VHF gain pattern.

Repeat these steps for a solution setup called VHF, with a frequency sweep of 0.145 GHz, and using the VHF field.

  1. Right-click on Results in the Project Manager.
  2. Select Create Far Fields Report in the shortcut menu.
  3. Select 3D Polar Plot in the Create Far Fields Report submenu.
  4. Select VHF : Sweep in the Solution menu.
  5. Ensure that Geometry is referencing the VHF infinite sphere.
  6. Select VHF1_P1 in the Sources menu.
  7. Select dB in the Function list.
  8. Click New Report.
  9. Click Close.
  10. Rename Gain Plot 1 VHF.

UHF and VHF Gain patterns

Visualizing the antenna gain patterns on the satellite.

The antenna gain patterns can also be modeled on the satellite. This will help with orientation once brought into STK.

  1. Right-click on the UHF gain plot in the Project Manager.
  2. Select Show in Modeler Window in the shortcut menu.
  3. Repeat these steps for the VHF gain plot.

    4. The patterns are quite large and are overlapping, so they should be sized down. Note, this is purely for visuals, and is not changing anything about the actual results.

  4. Right-click on Field Overlays in the Project Manager.
  5. Select Plot Fields in the shortcut menu.
  6. Select Radiation Field....
  7. Reduce the scale for both plots to 0.1 when the Overlay Radiation Field dialog box opens.
  8. Click Apply to save your changes.
  9. Click Close to close the Overlay Radiation Field dialog box
  10. Click HFSSDesign1 (SBR+) in the Project Manager to view the overlaid patterns.

Antenna patterns modeled on the satellite

Defining the source of the UHF pattern for export

.Before you can generate a pattern file for export, you must limit the source to just the UHF excitations.

  1. Right-click on Excitations in the Project Manager.
  2. Select Edit Sources… in the shortcut menu.
  3. Set the magnitude for the UHF excitation to 1 and the magnitude for the VHF excitation to 0.
  4. Click OK.

Exporting the UHF gain pattern as a Far Field Data file

Now that you have defined your sources, you can compute the antenna parameters and produce a Far Field Data file modeling the UHF antenna gain pattern that can be imported into the STK application.

  1. Expand Radiation in the Project Manager.
  2. Right-click on UHF.
  3. Select Compute Antenna Parameters... in the shortcut menu.
  4. Ensure that the Solution is set to UHF : Sweep and Intrinsic Variation is set to Freq=0.435GHz when the Antenna Parameters dialog box opens.
  5. Click Export Fields
  6. Name the file STKSatModel_UHF.ffd.
  7. Click Save.
  8. Click Close to close the Antenna Parameters dialog box.

Defining the source of the VHF pattern for export

.Limit the excitation source just to the VHF frequency.

  1. Right-click on Excitations in the Project Manager.
  2. Select Edit Sources… in the shortcut menu.
  3. Set the magnitude for the VHF excitation to 1 and the magnitude for the UHF excitation to 0.
  4. Click OK.

Exporting the VHF gain pattern as a Far Field Data file

Now that you have defined your sources, you can compute the antenna parameters and produce the Far Field Data file modeling the VHF antenna gain pattern.

  1. Expand Radiation in the Project Manager.
  2. Right-click on VHF.
  3. Select Compute Antenna Parameters... in the shortcut menu.
  4. Ensure that the Solution is set to VHF : Sweep and Intrinsic Variation is set to Freq=0.145GHz.
  5. Click Export Fields
  6. Name the file STKSatModel_VHF.ffd
  7. Click Save.
  8. Click Close.
  9. Click Save in the ribbon.
  10. Close the AEDT platform.

Importing and configuring the antennas in the STK application

You can import Far Field Data antenna pattern files simulated using the Ansys HFSS software into your scenario. For this exercise, you will need patterns for each of the four communications objects in your scenario:

  • Exton_YagiUda_UHF.ffd
  • Exton_YagiUda_VHF.ffd
  • STKSatModel_UHF.ffd
  • STKSatModel_VHF.ffd

If you used the SBR+ software to simulate STKSat's antennas yourself, use the Far Field Data files you produced in the Simulating the satellite’s antennas with the Ansys HFSS SBR+ software section.

Importing the ground site UHF receiver antenna pattern

Import the antenna pattern for Exton's UHF receiver.

  1. Open UHF_ExtonRx ()'s Properties ().
  2. Select the Basic - Definition page.
  3. Select the Antenna tab.
  4. Select the Model Specs subtab.
  5. Change the Antenna Model to ANSYS ffd Format () in the Component Selector.
  6. Click the ellipsis () next to External Filename.
  7. Browse to the location of your Far Field Data files.
  8. Select Exton_YagiUda_UHF.ffd when the Select File dialog box opens.
  9. Click Open to set the antenna model to use the external pattern file and to close the Select File dialog box.
  10. Click Apply.

Orienting the ground site UHF receiver antenna pattern

The pattern needs to be rotated to align with the axes in the STK application. You want the antenna gain pattern to match the direction of the sensor.

  1. Select the Orientation subtab.
  2. Select YPR Angles from the Orientation drop-down list
  3. Enter the following parameters:
    4. Parameter Position Offset
    Yaw 90 deg
    Pitch 90 deg
    Roll 0 deg
  4. Click Apply.

Visualizing the ground site UHF receiver antenna pattern

The hot part of the gain should point along the sensor pointing direction. In order the check this, the graphics for the volume have to be turned on.

  1. Select the 3D Graphics - Attributes page.
  2. Select Show Volume in the Volume Graphics panel.
  3. Set the Minimum Displayed Gain to -50 dB.

    This value corresponds to the minimum gain displayed on the 3D GainTotal graph in HFSS.

    4. Antenna Gain Pattern as Visualized in the HFSS software

  4. In the Pattern panel, set the Elevation - Stop angle to 180 deg.
  5. Click OK to save your changes.

Reviewing your changes

  • Animate the scenario to see when the sensor points at the satellite to check if the gain pattern is properly orientated.
    1. Click Start () on the Animation Toolbar to animate your scenario.
    2. Pause () the animation when the satellite is overhead.
    3. View the antenna pattern in the 3D Graphics window.
    4. If this orientation does not work, keep changing the angle in increments of 90 degrees until it aligns.

      5. visualized Ground site Antenna pattern

    Importing the ground site VHF transmitter antenna pattern

    Import the antenna pattern for the Exton's VHF transmitter.

    1. Open VHF_ExtonTx ()'s Properties ().
    2. Select the Basic - Definition page.
    3. Select the Antenna tab.
    4. Select the Model Specs subtab.
    5. Change the Antenna Model to ANSYS ffd Format () in the Component Selector.
    6. Click the ellipsis () next to External Filename.
    7. Browse to the location of your Far Field Data files.
    8. Select Exton_YagiUda_VHF.ffd when the Select File dialog box opens.
    9. Click Open.
    10. Click Apply.

    Orienting the ground site VHF transmitter antenna pattern

    The pattern needs to be rotated to align with the axes in the STK application.

    1. Select the Orientation subtab.
    2. Select YPR Angles from the Orientation drop-down list.
    3. Enter the following parameters:
      4. Parameter Position Offset
      Yaw 90 deg
      Pitch 90 deg
      Roll 0 deg
    4. Click Apply.

    Visualizing the ground site VHF transmitter antenna pattern

    As with the UHF pattern, hot part of the gain should point along the sensor pointing direction. In order the check this the graphics for the volume have to be turned on.

    1. Select the 3D Graphics - Attributes page.
    2. Select Show Volume in the Volume Graphics panel.
    3. Set the Minimum Displayed Gain to -47 dB.
    4. In the Pattern panel, set the Elevation - Stop angle to 180 deg.
    5. Click OK to save your changes.

    Importing the satellite UHF transmitter antenna pattern

    Import the antenna pattern for STKSat's UHF transmitter.

    1. Open UHF_SatTx ()'s Properties ().
    2. Select the Basic - Definition page.
    3. Select the Antenna tab.
    4. Select the Model Specs subtab.
    5. Change the Antenna Model to ANSYS ffd Format () in the Component Selector.
    6. Click the ellipsis () next to External Filename.
    7. Browse to the location of your Far Field Data files.
    8. Select STKSatModel_UHF.ffd when the Select File dialog box opens.
    9. Click Open to set the antenna model to use the external pattern file and to close the Select File dialog box.
    10. Click Apply.

    Orienting the satellite UHF transmitter antenna pattern

    The pattern's position on the satellite needs to be offset to the location where it was modeled.

    1. Select the Orientation subtab.
    2. Select YPR Angles from the Orientation drop-down list.
    3. Leave the YPR angles at zero and enter the following position offset parameters:
      4. Parameter Position Offset
      X 0.4 m
      Y -0.7 m
      Z 0.8 m
    4. Click Apply.

    Visualizing the UHF satellite transmitter antenna pattern

    Visualize the antenna pattern on STKSat in the 3D Graphics window.

    1. Select the 3D Graphics - Attributes page.
    2. Select Show Volume in the Volume Graphics panel.
    3. Set the Minimum Displayed Gain to -20 dB.
    4. In the Pattern panel, set the Elevation - Stop angle to 180 deg.
    5. Set Gain Scale (per dB) to 3e-05 km. This decreases the size of the pattern for visualization purposes.
    6. Click OK to save your changes.

    Importing the satellite VHF receiver antenna pattern

    Import the antenna pattern for STKSat's VHF receiver.

    1. Open VHF_SatRx ()'s Properties ().
    2. Select the Basic - Definition page.
    3. Select the Antenna tab.
    4. Select the Model Specs subtab.
    5. Change the Antenna Model to ANSYS ffd Format () in the Component Selector.
    6. Click the ellipsis () next to External Filename.
    7. Browse to the location of your Far Field Data files.
    8. Select STKSatModel_VHF.ffd when the Select File dialog box opens.
    9. Click Open to set the antenna model to use the external pattern file and to close the Select File dialog box.
    10. Click Apply.

    Orienting the satellite VHF receiver antenna pattern

    The pattern's position on the satellite needs to be offset to the location where it was modeled.

    1. Select the Orientation subtab.
    2. Select YPR Angles from the Orientation drop-down list.
    3. Leave the YPR angles at 0 and enter the following position offset parameters:
      4. Parameter Position Offset
      X -0.4m
      Y 0.7 m
      Z 0.8 m
    4. Click Apply.

    Visualizing the satellite VHF receiver antenna pattern

    Visualize the antenna pattern on STKSat in the 3D Graphics window.

    1. Select the 3D Graphics - Attributes page.
    2. Select Show Volume in the Volume Graphics panel.
    3. Set the Minimum Displayed Gain to -14 dB.
    4. In the Pattern panel, set the Elevation - Stop angle to 180 deg.
    5. Set Gain Scale (per dB) to 4e-05 km.
    6. Click OK to save your changes.

    Reviewing your changes

    View the antenna patterns on STKSat in the 3D Graphics window.

    1. Right-click on STKSat () in the Object Browser.
    2. Select Zoom To in the shortcut menu.
    3. Move around the satellite to view the antenna patterns in place on the satellite model.

      Visualized UHF and VHF gain patters on STKSat

    Computing access between the transmitters and receivers

    Access must be computed between two endpoints of an RF link in order to analyze the communications system. Compute access for both the VHF uplink and UHF downlink components.

    Computing access for the VHF uplink

    Compute access between the VHF transmitter on the ground site and the VHF receiver on the spacecraft.

    1. Right-click on the VHF_ExtonTx () in the object browser.
    2. Select Access... () in the shortcut menu.
    3. Expand () STKSat () in the Associated Objects list once the Access Tool opens.
    4. Select VHF_SatRx ().
    5. Click .

    Computing access for the UHF downlink

    Compute access between the UHF transmitter on the spacecraft and the UHF receiver on the ground site.

    1. Click Select Object to select a new object for which to compute access and to open the Select Object dialog box.
    2. Select the UHF_SatTx () in the Select object list and to close the Select Object dialog box.
    3. Expand () Exton () in the Associated Objects list once the Access Tool opens.
    4. Expand () AntennaRotor ().
    5. Select UHF_ExtonRx ().
    6. Click .
    7. Close the Access window.

    Generating detailed link budgets

    Now that access has been computed between the UHF and VHF components of the communications system, you can generate and analyze your link budget for each link.

    Generating a detailed link budget for the VHF uplink

    Start by generating a detailed link budget report for the VHF uplink.

    1. Open the Analysis menu.
    2. Select Report & Graph Manager...().
    3. Select Access in the Object Type drop-down list.
    4. Select the Facility-Exton-Sensor-AntennaRotor-Transmitter-VHF_ExtonTx-To-Satellite-STKSat-Receiver-VHF_ SatRx Access () object.
    5. Select the Link Budget - Detailed report () in the Installed Styles list.
    6. Click Generate.

    Reviewing the detailed link budget report

    Review the results of the VHF link in the Link Budget - Detailed report.

    1. Scroll to the right of the report. Note the values of C/N, Eb/No, BER, and so on. Notice that the bit error rate (BER) values are very low numbers.

      2. There are some very small bit error rates in the report. These outliers are somewhat unrealistic but indicate that the RF link between the ground site transmitter and the receiver on the spacecraft is strong at that point in time. These extremely small values indicate that there may be some loss not being accounted for. However, this is an acceptable report on this RF link since it gives us a comprehensive overview of the link as a whole.

    2. Scroll back through the report and find the columns titles Freq. Doppler Shift (kHz) and Rcvd. Frequency (GHz).
    3. Change the units of the Rcvd. Frequency (GHz) column to MHz.
      1. Right click on the Rcvd. Frequency (GHz) title.
      2. Select Rcvd. Frequency from the shortcut menu.
      3. Select Units... () in the Received Frequency submenu.
      4. Clear the Use Defaults checkbox when the Units dialog box opens.
      5. Select Megahertz (MHz) in the New Unit Value list.
      6. Click OK to apply your changes and to close the Units dialog box.
    4. Scroll down through the report and notice how the Rcvd. Frequency value shifts over time. This shift is called Doppler Shift. The Doppler Shift is the difference between the design frequency and the received frequency.
    5. Finally, scroll through the report and find the EIRP (dBW) and Prop Loss (dB) columns. Recall that the EIRP is the effective isotopic radiated power and that Prop Loss is signal loss as it travels from the transmitter to the receiver. In this case, the Prop Loss should be equal to the Free Space Loss as other environmental losses have not been modeled yet. The received isotropic power (RIP) is the difference between the EIRP and the propagation loss. The RIP is the received power at the receiver’s antenna.

      6. In the STK software the RIP includes both the effect of propagation loss and the effect of bandwidth overlap. This overlap is caused by a mismatch between the transmitted signal’s bandwidth and the receiver bandwidth. This is another column shown in the link budget report style. This overlap may cause additional loss in calculations and thus a slightly weaker RIP.

    6. Keep the report open.

    Generating a detailed link budget for the UHF downlink

    Generate a detailed link budget report for the UHF downlink.

    1. Return to the Report & Graph Manager ().
    2. Select the Satellite-STKSat-Transmitter-UHF_SatTx-To-Facility-Exton-SensorAntennaRotor-Receiver-UHF_ExtonRx Access () object.
    3. Select Link Budget - Detailed ().
    4. Click Generate.

    Reviewing the detailed link budget report

    Review the results of the UHF link in the Link Budget - Detailed report.

    1. Scroll through the report and note the values of C/N, Eb/No, BER, and so on. Notice that the bit error rate (BER) values are larger. This is because the transmitter puts out much less power than a ground site transmitter.
    2. Scroll through the report and compare the link values from the UHF link to the VHF link analyzed previously. Note changes in Freq. Doppler Shift, Rcvd. Iso. Power, C/N, Eb/No, BER, and so on.
    3. Change the units of the Rcvd. Frequency (GHz) column to MHz.
      1. Right click on the Rcvd. Frequency (GHz) title
      2. Select Rcvd. Frequency from the shortcut menu
      3. Select Units... () in the Received Frequency submenu.
      4. Clear the Use Defaults checkbox.
      5. Select Megahertz (MHz) in the New Unit Value list.
      6. Click OK.
    4. Keep the report open.

    Modeling environmental losses

    An important aspect to modeling any RF link is modeling its losses. There are several built-in environmental models that allow modeling of losses due to rain, clouds and fog, atmospheric absorption, urban propagation, and so on. For satellites, rain, cloud, and atmospheric absorption models are the most applicable. Some of the models used are based on recommendations from the International Telecommunication Union (ITU). These recommendations overview the valid ranges and base equations of these models.

    Enabling the environmental models

    Enable the ITU RF environment models in your scenario.

    1. Open CommunicationsDesign's () Properties ().
    2. Select the RF - Environment page.
    3. Select the Environmental Data tab.
    4. Select the Use ITU-R P.618 Section 2.5 checkbox in the Total Propagation Calculation tab.

      This option allows the use of some other ITU models and calculates the total propagation loss in accordance with the ITU recommendation.

    5. Select the Rain, & Cloud & Fog tab.
    6. Select the Use checkbox in the Rain Model section. Ensure that the ITU-R P618-13 model is selected.
    7. Select the Use checkbox in the Clouds and Fog Model section. Ensure the ITU-R P840-7 model is selected.
    8. Select the Atmospheric Absorption tab.
    9. Select the Use checkbox. Ensure the ITU-R P676-9 model is selected.
    10. Click OK.

    Verifying rain model settings in the receiver objects

    Ensure that the Use Rain Model check box is marked in both the receiver objects.

    1. Right-click on UHF_ExtonRx () and click Properties ().
    2. Select the Model Specs tab on the Basic - Definition page.
    3. Ensure that the Use checkbox is selected in the Rain Model panel.
    4. Click OK.
    5. Right-click on VHF_SatRx () and click Properties ().
    6. Select the Model Specs tab on the Basic - Definition page.
    7. Ensure that the Use checkbox is selected in the Rain Model panel
    8. Click OK.

    Reviewing the changes in the link budget reports

    You incorporated some environmental effects in your scenario. See the impact of these changes in your RF link in the detailed link budget reports.

    1. Return to the VHF Link Budget - detailed report window.
    2. Click the Refresh () at the top of the window to refresh the report data.

      3. Notice the changes in C/N, Eb/No, and BER, the values should decrease because the environmental conditions have impacted the RF link. Scroll through the report and find the Atmos Loss (dB), Rain Loss (dB), and CloudsFog Loss (dB). Notice how these columns are now populated with dynamic data that changes over time.

    3. Return to the UHF Link Budget - detailed report window.
    4. Click the Refresh () at the top of the window to refresh the report data.

      Notice the changes in C/N, Eb/No, and BER, the values should decrease because the environmental conditions have impacted the RF link. Scroll through the report and find the Atmos Loss (dB), Rain Loss (dB), and CloudsFog Loss (dB). Notice how these columns are now populated with dynamic data that changes over time.

    Calculating link margins and generating custom reports

    Link margins are used to describe the strength of a signal. Typically, a threshold value for some link parameter such as EIRP, Eb/No, received power, etc. is set. Then, value of that link parameter at a point in time is compared to the threshold. The difference between the actual link value and the threshold is the link margin. Usually, the link margin threshold is set to a minimum value where the link closes. For example, receiver sensitivity defines the minimum signal strength a receiver can handle to maintain an RF link. When creating a link margin, the receiver sensitivity would be the link margin threshold. Then, the link margin value would tell you how close you are to that minimum strength. In the STK software, this can be done with communication constraints and custom link reports.

    Setting the link margin threshold for VHF_SatRx

    Change the settings for the VHF_SatRx ().

    1. Right-click on VHF_SatRx () and click Properties ().
    2. Select the Model Specs tab on the Basic - Definition page.
    3. Select the Enable checkbox in Link Margin panel.
    4. Select Rcvd Carrier Power in the Type drop-down list.
    5. Set the Threshold field to -150 dBW
    6. Click OK.

    Setting the link margin threshold for UHF_ExtonRx

    Set the link margin for both receivers. First, change the settings for the UHF_ExtonRx ().

    1. Open UHF_ExtonRx ()'s Properties ().
    2. Select the Model Specs tab on the Basic - Definition page.
    3. Select the Enable checkbox in Link Margin panel.
    4. Select Rcvd Carrier Power in the Type drop-down list.
    5. Set the Threshold field to -156 dBW.

      6. This is the sensitivity of the receiver on the ground site.

    6. Click OK.

    Examining the VHF uplink

    Examine the Carrier to Noise Ratio (CNR) and Bit Error Rate (BER) for the two RF links. Since the RF links are represented as access calculations, examine those from the Report & Graph manager.

    1. Return to the Report & Graph Manager ().
    2. Ensure that Object Type is set to Access.
    3. Select the VHF Access () object.
    4. Generate the following graphs ():
      • Carrier_to_Noise_Ratio
      • Bit_Error_Rate
    5. In the Carrier to Noise Ratio graph, click and hold to draw a selection rectangle to around one of the peaks to zoom into one of the spikes.

      6. VHF Carrier to Noise Ratio Report

    6. Decrease the time Step to 10 sec.

      7. VHF Carrier to Noise Ratio Report zoomed in to a peak

    7. The BER graph has spikes that represent areas of high bit error rates.

      8. VHF BER Report

      These brief moments represent when a link is just starting or ending. In other words, the connection is very bad when the satellite first comes into or is leaving the view of the sensor.

      VHF BER Report showing twin SPIKES in the BER

    Investigating the results

    You can see from your data that the BER graph has spikes that represent areas of high bit error rates. These brief moments represent when a link is just starting or ending. In other words, the connection is very bad when the satellite first comes into or is leaving the view of the sensor.

    1. Right-click on the top of one of the spikes.
    2. Select Set Animation Time in the shortcut menu.
    3. Right-click on STKSat () in the Object Browser
    4. Select Zoom To in the shortcut menu.
    5. Bring the 3D Graphics window to the front.
    6. Zoom out from the satellite until you see the full sensor field of view.
    7. Move back and forth through time in the scenario. Watch as the satellite either comes just into the sensor's field of view or leaves it entirely.

      8. STKSat leaving the sensor's field of view

    Examining the UHF downlink and investigating the results

    Repeat these steps to generate and analyze the CNR and BER graphs for the UHF access object.

    UHF Carrier to Noise Ratio report

    UHF Carrier to Noise Ratio report zoomed in on a peak

    UHF BER report

    Creating custom link budget reports

    Create a custom report using data providers that are important to your analysis.

    Creating a new report style

    Add a new report style for your custom link budget.

    1. Return to the Report & Graph Manager ().
    2. Ensure that Object Type is set to Access.
    3. Select Scenario Styles in the Styles list.
    4. Click Create new report style () in the Styles toolbar.
    5. Name the report Custom Link Budget.
    6. Select the Enter key to open the report's properties to open the Report Style dialog box.

    Adding link Information data provider and elements

    The Custom Link Budget report will include the Time, EIRP, Propagation Loss, Received Isotropic Power, Carrier Power at Receiver Input, Receiver Gain, Bit Error Rate, and Link Margin.

    1. Expand () the Link Information data provider ().
    2. Select the following data items () in the Data Providers list to Insert () them into the Report Contents list in the order shown:
      • Time
      • EIRP
      • Prop Loss
      • Rcvd. Iso. Power
      • Carrier Power at Rcvr Input
      • Rcvr Gain
      • BER
      • Link Margin

    Setting options for data providers

    Set the option for the data providers. The BER notations defaults to a Floating Point, change this to use Scientific notation.

    1. Select Link Information-BER in the Report Contents list.
    2. Click Options....
    3. Select Scientific (e) in the Notion drop-down menu in the Data Format panel when the Options: Section 1, Line 1, Link Information-BER dialog box opens.
    4. Click OK to accept your change and to close the Options: Section 1, Line 1, Link Information-BER dialog box.
    5. Click OK to accept your changes and to close the Report Style dialog box.

    Generating the custom link budget reports

    Examine the results for both RF links.

    1. Select the UHF Access () object.
    2. Select Custom Link Budget () in the Scenario Styles folder located in the Styles panel.
    3. Click Generate....
    4. Notice the format of the custom report and the new columns Carrier Power at Rcvr Input (dBW) and Link Margin (dB).
    5. Return to the Report & Graph Manager () window.
    6. Select the VHF Access () object.
    7. Select the Custom Link Budget () report style.
    8. Click Generate....
    9. Notice the Carrier Power at Rcvr Input and Link Margin values for this RF uplink.

    Adding communications constraints

    Constraints can be used to limit access calculations. For example, by default, there is a line-of-sight (LOS) constraint on all objects. Therefore, access is only determined to be possible when there is line of sight between objects. Additional communications constraints can be used to limit access to times when "acceptable” RF conditions exist. Common practice is requiring a link margin of at least 3 dB and a BER of at least 1e-03 to be maintained for the link to +be considered valid. These values can be used to constrain the data set to only show accesses that satisfy these requirements.

    Constraining the UHF downlink

    Add constraints to UHF_ExtonRx downlink receiver.

    Adding constraints in versions 12.9 and later of the STK software

    Add constraints to UHF_ExtonRx on the Constraints - Active page when using versions 12.9 and later of the STK software.

    1. Open UHF_ExtonRx's () Properties ().
    2. Select the Constraints - Active page.
    3. Click Add New Constraints ().
    4. Add the following constraints:
      • Bit Error Rate
      • Link Margin
      • Power at Receiver Input
    5. Click Close.
    6. Select Bit Error Rate.
      1. Select the Max checkbox.
      2. Set the value to 1e-03.
      3. Clear the Min checkbox. Note that the Max constraint is used because a smaller BER is better.
    7. Select Link Margin.
      1. Select the Min checkbox.
      2. Set the value to 3 dB.
      3. Ensure the Max checkbox is cleared.
    8. Select Power at Receiver Input.
      1. Select the Min checkbox.
      2. Set the value to -156 dBW. This sets the minimum received power equal to the ground site receiver sensitivity.
      3. Ensure the Max checkbox is cleared.
    9. Click OK.

    Adding constraints in versions 12.8 and earlier of the STK software

    Add constraints to UHF_ExtonRx on the Constraints - Comm page when using versions of the STK software that are older than version 12.9.

    1. Open UHF_ExtonRx's () Properties ().
    2. Select the Constraints - Comm page
    3. Select the checkboxes for the following constraints:
      • Bit Error Rate
      • Link Margin
      • Power at Receiver Input
    4. Enter the following values for the constraints:
      5. Constraint Values
      Bit Error Rate - Max 1e-03. Note that the Max constraint is used because a smaller BER is better.
      Link Margin - Min 3 dB
      Power at Receiver Input - Min -156 dBW. This sets the minimum received power equal to the ground site receiver sensitivity.
    5. Click OK.

    Viewing the constrained UHF downlink data

    Review the custom link budget report for the UHF downlink to see the affects of the new constraints. If the report was previously opened, then simply refresh it to see the changes.

    1. Re-open the UHF downlink Custom Link Budget report.
    2. Click the Refresh () button to refresh the data.
    3. Notice that the number of accesses and data points has decreased. Also, notice that all of the BER values are less than 1e-03 and the Link Margin values are all greater than 3 dB.

    Constraining the VHF uplink

    Repeat the process above to apply constraints to the VHF receiver on the satellite, the only change being a Minimum value for the Power at Receiver Input constraint of -150 dBW.

    Adding constraints in versions 12.9 and later of the STK software

    Add constraints to VHF_SatRx on the Constraints - Active page when using versions 12.9 and later of the STK software.

    1. Open VHF_SatRx's () Properties ().
    2. Select the Constraints - Active page.
    3. Click Add New Constraints ().
    4. Add the following constraints:
      1. Bit Error Rate
      2. Link Margin
      3. Power at Receiver Input
    5. Click Close.
    6. Select Bit Error Rate.
      1. Select the Max checkbox.
      2. Set the value to 1e-03.
      3. Clear the Min checkbox.
    7. Select Link Margin.
      1. Select the Min checkbox.
      2. Set the value to 3 dB.
      3. Ensure the Max checkbox is cleared.
    8. Select Power at Receiver Input.
      1. Select the Min checkbox.
      2. Set the value to -150 dBW.
      3. Ensure the Max checkbox is cleared.
    9. Click OK.

    Adding constraints in versions 12.8 and earlier of the STK software

    Add constraints to VHF_SatRx on the Constraints - Comm page when using versions of the STK software that are older than version 12.9.

    1. Open VHF_SatRx's () Properties ().
    2. Select the Constraints - Comm page.
    3. Select the checkboxes for the following constraints:
      • Bit Error Rate: Max
      • Link Margin: Min
      • Power at Receiver Input
    4. Enter the following values for the constraints:
      5. Constraint Value
      Bit Error Rate - Max 1e-03.
      Link Margin - Min 3 dB
      Power at Receiver Input - Min -150 dBW
    5. Click OK.

    Viewing the constrained VHF uplink data

    Review the custom link budget report for the VHF uplink to see the affects of the new constraints. If the report was previously opened, then simply refresh it to see the changes.

    1. Re-open the VHF uplink Custom Link Budget report.
    2. Click the Refresh () button to refresh the data.
    3. Notice that the number of accesses and data points has decreased. Also, notice that all of the BER values are less than 1e-03 and the Link Margin values are all greater than 3 dB.

    Optimizing the communications links

    With the RF uplink and downlink established between the ground site and STKSat spacecraft, the various link parameters can be modified to optimize the link. These parameters include modulation technique, data rate, receiver sensitivity, and transmitter output power.

    Modifying the modulation technique

    In order to modify the modulation technique, the modulation settings for both the transmitter and receiver in a link must be modified.

    1. Open VHF_ExtonTx's () Properties ().
    2. Select the Basic – Definition page.
    3. Select the Modulator tab.
    4. Change the modulator Name field from MSK to FSK.
    5. Click OK.
    6. Return to the VHF uplink Custom Link Budget report.
    7. Refresh () the report.
    8. Notice that when the modulator and demodulator do not match, the report shows that it has no data available. This is because the VHF receiver on the spacecraft is still configured for MSK modulation.

    Modifying the demodulation technique

    Change the demodulator on the receiver to FSK and refresh your report.

    1. Open VHF_SatRx's () Properties ().
    2. Select the Basic – Definition page.
    3. Select the Demodulator tab.
    4. Change the Name field from MSK to FSK.
    5. Click OK.
    6. Return to the VHF uplink Custom Link Budget report and refresh the data using the Refresh () button.
    7. Notice the changes to the Bit Error Rate column. You now have better values for the BER. The Link Margin appears to be more evenly distributed across the data. This is due to the fact that the power spectral density (PSD) of FSK modulation is wider than that of MSK modulation

    Modifying the UHF downlink

    Repeat steps in Modifying the modulation technique and Modify the demodulator tehcninque for the RF downlink (that is, modify the UHF_SatTx () and UHF_ExtonRx () objects). Notice that when the modulator and demodulator do not match the report has no data available. Once both the modulator and demodulator are set to FSK, refresh and examine the report.

    Modifying the transmitter data rate

    Unlike the process of changing the modulation technique, the data rate options only need to be changed on transmitters in the RF link.

    1. Open UHF_SatTx's () Properties ().
    2. Select the Basic – Definition page.
    3. Select the Model Specs tab.
    4. Change the Date Rate from 9600 b/sec to 1200 b/sec.
    5. Click OK.
    6. Return to the UHF Custom Link Budget report.
    7. Refresh () the report.
    8. Notice the changes in BER and Link Margin. A lower data rate corresponds to a lower (better) bit error rate. However, the Link Margin values have also decreased. This illustrates the tradeoff between signal bandwidth and power.

    Modifying the VHF transmitter data rate

    Repeat the steps in the Modifying the data rate section for the RF uplink (i.e. modify the VHF_ExtonTx () object) and examine the report.

    Modifying receiver sensitivity

    The receiver sensitivity only needs to be changed for the receiver in an RF link. Because the ground site configuration is already established, only the receiver sensitivity on the STKSat spacecraft will be modified.

    1. Open VHF_SatRx's () Properties ().
    2. Select the Basic – Definition page.
    3. Select the Model Specs tab.
    4. Change the Link Margin - Threshold value from -150 dBW to -135 dBW.
    5. Click OK.
    6. Return to the VHF uplink Custom Link Budget report and refresh the data using the Refresh () button.
    7. Notice the decreased access times and smaller Link Margin values due to the less-negative sensitivity value. The more-negative the sensitivity, the stronger the link.

    Modifying transmitter output power

    Similar to changing the data rate, the output power only needs to be changed for the transmitter in an RF link. Because ground sites typically just transmit at their maximum output power, only the output power for the transmitter on STKSat will be modified.

    1. Open UHF_SatTx's () Properties ().
    2. Select the Basic – Definition page.
    3. Select the Model Specs tab.
    4. Change the Power value from 3 dbW to 1 dbW.
    5. Click OK.
    6. Return to the UHF Custom Link Budget report and refresh the data using the Refresh () button.
    7. Notice the link access times decrease due to the decreased transmit power. Also notice that the Link margin values decrease to very near the threshold value of 3 dB.

    Saving your work

    Clean up your workspace and save your scenario.

    1. Close all reports and windows except the 2D and 3D Graphics windows.
    2. Save () your work.
    3. Close the scenario when done.

    Summary

    You created a custom antenna for the VHF and UHF frequency ranges, imported the antenna patterns designs created in the Ansys HFSS software into the STK application, and modeled their behavior in a multidomain mission. Using the HFSS SBR+ asymptotic high-frequency electromagnetic simulator and the STK software, you modeled the influence of physical obstacles near your antennas on the satellite and saw them reflected in a report. You analyzed a VHF uplink and a UHF downlink with these antennas using custom link budget reports. You then analyzed the link margins of the receiver sensitivity by optimizing the communications devices. When used together, you can create and analyze high-fidelity antenna models with the Ansys HFSS software and the STK application.