Communications Part 3: Introduction to Analyzing Digital and Analog Transponders

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.

This lesson requires STK 12.8 or newer to complete it in its entirety.

Capabilities covered

This lesson covers the following STK Capabilities:

  • STK Pro
  • Communications

Problem

You require a fast and easy way to analyze digital and analog transponders. You need to up link data from a communications site to a satellite transponder, which will then down link the data to a separate ground site.

Solution

You will use STK to model digital and analog transponders for link budget analysis and mission planning. You will then generate a digital repeater comm link report and a bent pipe report to analyze your communications.

If you are new to STK, review the following Level 1 - Beginner tutorials first: Part 1: Build Scenarios and Part 2: Objects and Properties.

What you will learn

Upon completion of this tutorial, you will understand:

  • How to model communications involving digital transponders.
  • How to model communications involving analog transponders.
  • How to generate a Digital Repeater Comm Link report using the Chain object and the Report & Graph Manager.
  • How to generate a Bent Pipe report using the Chain object and the Report & Graph Manager.

Using the starter scenario (*.vdf file)

To speed things up and enable you to focus on this lesson's main goal, you will use a partially created scenario. The partially created scenario is saved as a visual data file (VDF) in your STK install.

Retrieving the starter scenario

  1. Launch STK ().
  2. Click Open a Scenario () in the Welcome to STK dialog box.
  3. Go to <STK install folder>\Data\Resources\stktraining\VDFs\.
  4. Select STK_Communications.vdf.
  5. Click Open.

Visual data files versus Scenario files

You must make sure that you save your work in STK as a scenario file (.sc) and not a visual data file (.vdf) by selecting Save As from the STK File menu. A VDF is a compressed version of an STK scenario, which makes them great for sending your work in STK to others. However, you want to use a scenario file while working with STK on your machine.

If you open a VDF file, STK keeps it as a VDF and does not automatically convert it to a scenario file. This means, STK does not change the file type of your scenario when you launch your scenario. You need to convert the VDF to a Scenario file using Save As.

Saving a VDF file as a Scenario file

Use Save As from the STK File menu to convert the VDF file that you opened into a scenario file.

  1. Open the File menu.
  2. Select Save As.
  3. Select the STK User folder on the left side of the Save As window.
  4. Click New Folder.
  5. Rename New Folder to match the title of the scenario.
  6. Open the folder you just created.
  7. Enter the name of the folder into File name field.
  8. Open the Save as type drop-down menu.
  9. Select Scenario Files (*.sc).
  10. Click Save.

Selecting relevant objects

In this scenario, you will analyze communications between a communications ground site located in Southern California, a geosynchronous satellite (GEO), and a communications ground site located in Southern Germany. Each ground site has a servo system that is tracking the GEO satellite which you can see in the 3D Graphics window. You will use local analytical terrain files.

  1. Select the check box for the following objects in the Object Browser:
    • Communication_Site ()
    • UL_Servo ()
    • Raisting ()
    • DL_Servo ()
    • GEO_Sat_East ()
  2. Bring the 3D Graphics window to the front.
  3. Right-click on GEO_Sat_East () in the Object Browser.
  4. Select Zoom To.
  5. Use your mouse and zoom out to view the servo motor connections between both ground sites and the satellite.

Servo Motor View

Configuring terrain and atmospheric models to your scenario

You will configure your scenario to include the ITU-R rain model and the ITU-R atmospheric absorption model. Later in this lesson, you will determine how these conditions affect your communications.

Loading the ITU-R rain model

You will use the ITU-R rain model for your analysis.

  1. Right-click on your Scenario object () in the Object Browser.
  2. Select Properties ().
  3. Select the RF - Environment page when the Properties Browser opens.
  4. Select the Rain, Cloud & Fog tab.
  5. Select the Use check box in the Rain Model frame.
  6. Leave the default ITU-R (International Telecommunication Union) model.
  7. Click Apply to accept the changes and keep the Properties Browser open.

Loading the ITU-R Atmospheric Absorption Model

You will use the ITU-R atmospheric absorption model.

  1. Select the Atmospheric Absorption tab.
  2. Select the Use check box.
  3. Leave the default ITU-R model.
  4. Click OK to accept your changes and to close the Properties Browser.

Configuring the uplink transmitter on the communications site

You will analyze the communications from the ground site to the satellite's transponders. In order to do that, you will create an uplink transmitter on a ground site first.

Inserting an uplink transmitter

Your parabolic antenna is directional. You can steer the antenna by attaching the Transmitter () object to a Sensor () object.

  1. Select Transmitter () in the Insert STK Objects tool.
  2. Select the Insert Default () method.
  3. Click Insert...
  4. Select UL_Servo () in the Select Object dialog box.
  5. Click OK.
  6. Right-click on Transmitter1 () in the Object Browser.
  7. Select Rename in the shortcut menu.
  8. Rename Transmitter1 () to Uplink_Tx.

Using the complex transmitter model

You will use a complex transmitter model to analyze the uplink portion of your link analysis. The complex transmitter model enables you to select among a variety of analytical and realistic antenna models and to define the characteristics of the selected antenna type.

  1. Open Uplink_Tx's () properties ().
  2. Select the Basic - Definition page.
  3. Click the Transmitter Model Component Selector ().
  4. Select Complex Transmitter Model in the Transmitter Models list, once the Select Component dialog box opens.
  5. Click OK to close the Select Component dialog box.
  6. Set the following:
    7. Option Value
    Frequency: 18 GHz
    Power: 10 dBW
    Data Rate: 10 Mb/sec
  7. Click Applyto accept your changes and to keep the Properties Browser open.

Using a parabolic antenna

The communications site uses a parabolic antenna.

  1. Select the Antenna tab.
  2. Select the Model Specs sub-tab.
  3. Click the Antenna Model Component Selector ().
  4. Select Parabolic in the Antenna Models list once the Select Component dialog box opens.
  5. Click OK to close the Select Component dialog box.
  6. Set the following:
    7. Option Value
    Design Frequency: 15 GHz
    Diameter: 3.5 m
  7. Click Apply to accept your changes and to keep the Properties Browser open.

The design frequency doesn't match the uplink frequency. The antenna works in a frequency range of 12 - 18 GHz.

Enabling refraction

Atmospheric refraction is the bending of an RF signal as it travels through the atmosphere.

  1. Select the Basic - Refraction page.
  2. Select the Use Refraction in Access Computations check box.
  3. Open the Refraction Model: shortcut menu.
  4. Select the ITU-R model.
  5. Click OKto accept your changes and to close the Properties Browser.

Configuring a Digital Transponder

In a digital (or regenerative) transponder, the transponder demodulates and then re-modulates the incoming signal before the transponder sends the signal back out. Essentially, the transponder cleans and then re-sends the signal.

DIGITAL TRANSPONDER SCHEMATIC

The image above shows the path of a signal from a ground station transmitter to a satellite's digital transponder and then from the satellite's digital transponder to a ground site's receiver. The satellite's digital transponder is marked by the dashed box.

The digital transponder receives the signal and cleans out the radio frequency (RF) environmental noise during the demodulation of the incoming signal. Then the digital transponder re-modulates the signal and transmits the “clean” signal to the ground receiver.

The advantage of a digital transponder over an analog transponder is the demodulation/remodulation that cleans up the signal. When the ground site receives a signal from the satellite’s digital transponder, the signal has less RF environmental noise because the digital transponder has cleaned out the RF environmental noise that occurred during the uplink and is downlinking a clean signal.

Using an Uplink Satellite Receiver

When creating a transponder in STK, you will use a Receiver () object and a Transmitter () object on the same parent object.

  1. Insert a Receiver () object using the Insert Default () method.
  2. Select GEO_Sat_East () in the Select Object dialog box.
  3. Click OK to close the Select Object dialog box.
  4. Rename Receiver1 () to Uplink_Rx.

Specifying the receiver model

You will use a Complex Receiver model for your transponder.

  1. Open Uplink_Rx's () properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.
  3. Click the Receiver Model Component Selector ().
  4. Select Complex Receiver Model in the Receiver Models list once the Select Component dialog box opens.
  5. Click OK to accept your selection and to close the Select Component dialog box.

Adding a parabolic antenna to the receiver

Your satellite receiver will have a parabolic antenna.

  1. Select the Antenna tab.
  2. Select the Model Specs sub-tab.
  3. Click the Antenna Model Component Selector ().
  4. Select Parabolic in the Antenna Models list once the Select Component dialog box opens.
  5. Click OK to accept your selection and to close the Select Component dialog box.
  6. Enter 15 GHz in the Design Frequency: field.
  7. Click Apply to accept your changes and to keep the Properties Browser open.

Orienting the antenna using the Az-El orientation method

The Az-El (azimuth-elevation) orientation method specifies the location of the antenna boresight in relation to the azimuth and elevation in the local coordinate frame of the parent object. The receive antenna points in the general direction of the communications site.

  1. Select the Orientation sub-tab.
  2. Set the following:
    3. Option Value
    Azimuth: 216.3 deg
    Elevation: 81.45 deg
  3. Click Apply to accept your changes and to keep the Properties Browser open.

Computing the system noise temperature

Compute system noise temperature.

  1. Select the System Noise Temperature tab.
  2. Select the Compute option.
  3. Select the Compute option in the Antenna Noise frame.
  4. Select the following:
    • Earth
    • Sun
    • Atmosphere
    • Rain
    • Cosmic Background
  5. Click Apply to accept your changes and to keep the Properties Browser open.

Enabling refraction

Use refraction in your analysis.

  1. Select the Basic - Refraction page.
  2. Select the Use Refraction in Access Computations check box.
  3. Open the Refraction Model: shortcut menu.
  4. Select the ITU-R model.
  5. Click OK to accept your changes and to close the Properties Browser.

Configuring a Digital Satellite Transmitter

Insert the satellite transmitter.

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

Using a complex transmitter model

You will use a complex transmitter model for the downlink portion of your satellite's transponder.

  1. Open Digital_Downlink_Tx'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 in the Transmitter Models list, once the Select Component dialog box opens.
  5. Click OK to accept your selection and to close the Select Component dialog box.
  6. Select the Model Specs tab.
  7. Set the following:
    8. Option Value
    Frequency: 16 GHz
    Power: 5 dBW
    Data Rate: 5 Mb/sec
  8. Click Apply to accept your changes and to keep the Properties Browser open.

The downlink frequency is lower than the uplink frequency. The path loss of the downlink frequency is smaller and the link needs less transmit power to achieve the required energy per bit to noise density ratio Eb/No and its associated bit error rate (BER).

Adding a parabolic antenna to the transmitter

The satellite transmitter employs a parabolic antenna.

  1. Select the Antenna tab.
  2. Select the Model Specs sub-tab.
  3. Click the Antenna Model Component Selector ().
  4. Select Parabolic in the Antenna Models list once the Select Component dialog box opens.
  5. Click OK to accept your selection and to close the Select Component dialog box.
  6. Enter 15 GHz in the Design Frequency: field.
  7. Click Apply to accept your changes and to keep the Properties Browser open.

Using the Az-El orientation method to orient the antenna

The transmit antenna points in the general direction of Raisting.

  1. Select the Orientation sub-tab.
  2. Set the following:
    3. Option Value
    Azimuth: 308.5 deg
    Elevation: 81.44 deg
  3. Click Apply to accept your changes and to keep the Properties Browser open.

Enabling refraction

  1. Select the Basic - Refraction page.
  2. Select the Use Refraction in Access Computations check box.
  3. Open the Refraction Model: shortcut menu.
  4. Select the ITU-R model.
  5. Click OKto accept your changes and to close the Properties Browser.

Configuring Raisting's downlink receiver

You will use a complex receiver model for Raisting.

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

Specifying the receiver model

You will use a complex receiver model.

  1. Open Downlink_Rx's () properties ().
  2. Select the Basic Definition page when the Properties Browser opens.
  3. Click the Receiver Model Component Selector ().
  4. Select Complex Receiver Model in the Receiver Models list once the Select Component dialog box opens.
  5. Click OK to accept your selection and to close the Select Component dialog box.

Specifying the receiver antenna model

You will use the default Gaussian antenna model. The Gaussian antenna model uses an analytical model of a Gaussian beam. The model is like a parabolic antenna within about -6 dB relative to the boresight.

  1. Select the Antenna tab.
  2. Select the Model Specs sub-tab.
  3. Set the following:
    4. Option Value
    Design Frequency: 15 GHz
    Diameter 3.5 m
  4. Click Apply to accept your changes and to keep the Properties Browser open.

Computing system noise temperature

Compute system noise temperature.

  1. Select the System Noise Temperature tab.
  2. Select the Compute option.
  3. Select the Compute option in the Antenna Noise frame.
  4. Select the following:
    • Sun
    • Atmosphere
    • Rain
    • Cosmic Background
  5. Click Apply to accept your changes and to keep the Properties Browser open.

Enabling refraction

  1. Select the Basic - Refraction page.
  2. Select the Use Refraction in Access Computations check box.
  3. Open the Refraction Model: shortcut menu.
  4. Select the ITU-R model.
  5. Click OK to accept your changes and to close the Properties Browser.

Computing accesses for the digital transponder

To compute access, you will create a chain object and then generate a digital repeater comm link report.

Creating the Chain object

A chain is a list of objects in order of access. You will assign objects to the chain and define the order in which the objects are accessed.

  1. Insert a Chain () object using the Insert Default () method.
  2. Rename Chain1 () to Digital_Link.

Define the start and end objects

Start by choosing the start object and end object in your chain.

  1. Open Digital_Link's () properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.
  3. Click the Start Object: ellipses ().
  4. Select Uplink_Tx () in the Select Object dialog box.
  5. Click OK to close the Select Object dialog box.
  6. Click the End Object: ellipses ().
  7. Select Downlink_Rx () in the Select Object dialog box.
  8. Click OK to close the Select Object dialog box.

Create the Chain Object's first connection

After you choose the start and end objects in your chain, you need to build the chain's connections.

  1. Click Add in the Connections frame.
  2. Click the From Object: ellipses ().
  3. Select Uplink_Tx () in the Select Object dialog box.
  4. Click OK to close the Select Object dialog box.
  5. Click the To Object: ellipses ().
  6. Select Uplink_Rx () in the Select Object dialog box.
  7. Click OK to close the Select Object dialog box.

Create the Chain Object's second connection

  1. Click Add in the Connections frame.
  2. Click the From Object: ellipses ().
  3. Select Uplink_Rx () in the Select Object dialog box.
  4. Click OK to close the Select Object dialog box.
  5. Click the To Object: ellipses ().
  6. Select Digital_Downlink_Tx () in the Select Object dialog box.
  7. Click OK to close the Select Object dialog box.

Create the Chain Object's third connection

  1. Click Add in the Connections frame.
  2. Click the From Object: ellipses ().
  3. Select Digital_Downlink_Tx () in the Select Object dialog box.
  4. Click OK to close the Select Object dialog box.
  5. Click the To Object: ellipses ().
  6. Select Downlink_Rx () in the Select Object dialog box.
  7. Click OK to close the Select Object dialog box.
  8. Click OK to accept your changes and to close the Properties Browser.

Generating the Digital Repeater Comm Link Report

You will use the Report & Graph Manager to analyze the performance of your communication link using the Digital Repeater Comm Link report.

  1. Right-click on Digital_Link () in the Object Browser.
  2. Select Report & Graph Manager... () in the shortcut menu.
  3. Select Digital Repeater Comm Link () in the Installed Styles list once the Report & Graph Manager opens.
  4. Click Generate...
  5. Scroll to the BER Tot . 2 column when the report opens.

    6. For each reported access, the first line of the report is the uplink and the second line the downlink. BER Tot . 2 is the composite. You'll see that for several elements of performance, such as BER, while there may be degradation in the uplink or downlink, minimal degradation or increase is noted in the composite link. If technicians at Raisting are basing reception on a BER of 1.000000e-10 or lower, they can use this report to determine when they have good communications during the 24 hour analysis period.

  6. Click Save as quick report () in the report tool bar.

Preparing for the next section

  1. Close any open reports, properties and the Report & Graph Manager.
  2. Clear the Digital_Link () check box in the Object Browser.

Configuring an analog transponder

In an analog transponder, the transmitted signal is essentially a reflection of the received signal, with the added possibility of frequency translation or power amplification:

ANALOG TRANSPONDER SCHEMATIC

Analog transponders do not clean up a received signal before transmitting it out again. Whatever RF environmental noise goes into the analog transponder during the uplink is what you get during the re-transmission of the signal plus any RF environmental noise that’s accumulated during the downlink.

You can use STK Communications to determine how RF environmental noise will affect your link budget during your analytical period.

Inserting an analog transmitter

You will reuse Digital_Downlink_Tx () with some modifications.

  1. Select Digital_Downlink_Tx () in the Object Browser.
  2. Click Copy () in the Object Browser toolbar.
  3. Select GEO_Sat_East ().
  4. Click Paste ().
  5. Rename Digital_Downlink_Tx1 () to Analog_Downlink_Tx.

Using the complex retransmitter model

The complex retransmitter 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. Open Analog_Downlink_Tx's () properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.
  3. Click the Transmitter Model Component Selector ().
  4. Select Complex Re-Transmitter Model in the Transmitter Models list once the Select Component dialog box opens.
  5. Click OK to accept your selection and to close the Select Component dialog box.
  6. Select the Model Specs tab.
  7. Set the following:
    8. Option Value
    Sat. Power: 15 dBW
    Sat. Flux Density: -120 dBW/m^2
  8. Click Apply to accept your changes and to keep the Properties Browser open.
    • Sat. Power or Saturation Output Power is the RF Power output of the transmitter as measured at the input to the antenna when the amplifier is at its saturated state.
    • Sat. Flux Density or Saturation Flux Density is the amplifier's saturation point by the input flux density in dBW/m2. This represents the per carrier flux density for systems supporting multiple carriers per transmitter.

Specifying the analog transmitter antenna

Use the default Gaussian antenna model.

  1. Select the Antenna tab.
  2. Insert 15 GHz in the Design Frequency: field.
  3. Click Apply to accept your changes and to keep the Properties Browser open.

Using the Az-El orientation method to orient the antenna

The transmit antenna points in the general direction of the Raisting.

  1. Select the Orientation sub-tab.
  2. Set the following:
    3. Option Value
    Azimuth: 308.5 deg
    Elevation: 81.44 deg
  3. Click Apply to accept your changes and to keep the Properties Browser open.

Transfer Function - Frequency

Frequency coefficients specify the transmitted frequency as a function of the received frequency. They can only be entered in polynomial form. The coefficient order displays in the left column of the table and updates automatically as coefficients are added or removed. Input and output units are in Hz. The default coefficients of -7.0e+08 and 1.0 are used to model a 700 MHz down conversion. Your downlink transmitter is transmitting at 16 GHz so you need to adjust the frequency coefficient.

  1. Select the Transfer Functions tab.
  2. Select the Frequency sub-tab.
  3. Change the Index 0 Coefficient to -2e+09.
  4. Click OK to accept your changes and to close the Properties Browser.

Computing accesses for the analog transponder

To compute access, you will create a chain object and then generate a bent pipe report.

Inserting a Chain object

You will insert a Chain object. You will assign objects to the chain and define the order in which the objects are accessed.

  1. Insert a Chain () object using the Insert Default () method.
  2. Rename Chain2 () to Analog_Link.

Define the start and end objects

Start by choosing the start object and end object in your chain.

  1. Open Analog_Link's () properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.
  3. Click the Start Object: ellipses ().
  4. Select Uplink_Tx () in the Select Object dialog box.
  5. Click OK to close the Select Object dialog box.
  6. Click the End Object: ellipses ().
  7. Select Downlink_Rx () in the Select Object dialog box.
  8. Click OK to close the Select Object dialog box.

Create the Chain Object's first connection

After you choose the start and end objects in your chain, you need to build the chain's connections.

  1. Click Add in the Connections frame.
  2. Click the From Object: ellipses ().
  3. Select Uplink_Tx () in the Select Object dialog box.
  4. Click OK to close the Select Object dialog box.
  5. Click the To Object: ellipses ().
  6. Select Uplink_Rx () in the Select Object dialog box.
  7. Click OK to close the Select Object dialog box.

Create the Chain Object's second connection

  1. Click Add .
  2. Click the From Object: ellipses ().
  3. Select Uplink_Rx () in the Select Object dialog box.
  4. Click OK to close the Select Object dialog box.
  5. Click the To Object: ellipses ().
  6. Select Analog_Downlink_Tx () in the Select Object dialog box.
  7. Click OK to close the Select Object dialog box.

Create the Chain Object's third connection

  1. Click Add .
  2. Click the From Object: ellipses ().
  3. Select Analog_Downlink_Tx () in the Select Object dialog box.
  4. Click OK to close the Select Object dialog box.
  5. Click the To Object: ellipses ().
  6. Select Downlink_Rx () in the Select Object dialog box.
  7. Click OK to close the Select Object dialog box.
  8. Click OK to accept your changes and to close the Properties Browser.

Generating a Bent Pipe report

You are transmitting data from the uplink transmitter to the uplink receiver. The uplink receiver is transferring the data to the downlink transmitter. Then the downlink transmitter is sending the data to the downlink receiver like a bent pipe. The only processing performed is to retransmit the signal. Any signal degradation is passed on to the downlink receiver. Analyze the reception at Raisting using a bent pipe report.

  1. Right-click on Analog_Link () in the Object Browser.
  2. Select Report & Graph Manager... () in the shortcut menu.
  3. Select the Bent Pipe Comm Link () report in the Installed Styles list once the Report & Graph Manager opens.
  4. Click Generate...

    5. The report contains link performance data for the uplink (first line), downlink (second line), and the combined link (third line). Degradation in downlink and composite link performance can readily be perceived. For example, if you look at the first access line Bit Error Rate (BER) is ~ 2.4 x 10‐14 in the uplink, 1 x 10‐30 in the downlink, and BER Tot .2 is ~ 9.5 x 10‐13 in the composite link. If basing your communications on BER, you would use BER Tot .2 to determine when you can receive data.

  5. Leave the Bent Pipe Comm Link report open.
  6. Open the Quick Report Manager... () shortcut menu in the Data Providers toolbar at the top of STK.
  7. Select Digital Repeater Comm Link.
  8. Display the Bent Pipe Comm Link and Digital Repeater Comm Link reports so that you can view them simultaneously on the screen.

You can see that uplink performance for the analog and digital links are similar and that the analog link downlink performance is improved over the digital downlink performance. However, the composite link performance is better for the digital transponder. This is due to, among other things, the fact that the noise in the uplink channel is propagated through the downlink by the analog transponder but not by the digital transponder.

Summary

When you are using STK for communications analysis, it's not only important to know how to create a link budget between a transmitter and a receiver, but you need to understand how to model transponders.

In this lesson, you learned how to create digital and analog transponders, the difference being the satellite transmitter model used in your analysis. In your analysis, the digital transponder on the satellite used a complex transmitter model (but in reality, you could have used simple or medium transmitter models). You used a Chain object and generated a Digital Repeater Comm Link report and saw that link performance may degrade in the downlink but minimal degradation occurred in the composite link. Basically, the signal was reprocessed and cleaned up prior to retransmission.

For the analog link, you used a re-transmitter model for your satellite's transmitter. Next, you generated a Bent Pipe Comm Link report and saw that link performance was good in the downlink but degraded in the composite link due to noise in the uplink channel which was propagated through the downlink.

Save your work

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

What's next?

You are now ready to move onto the next tutorial Communications Part 4: Introduction to the Terrain Integrated Rough Earth Model.