Using Communication Constraints to Design Communication Links

STK Pro, STK Premium (Air), 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.

This lesson requires STK 12.9 or newer to complete in its entirety. If you have an earlier version of STK, you can complete a legacy version of this lesson.

The results of the tutorial may vary depending on the user settings and data enabled (online operations, terrain server, dynamic Earth data, etc.). It is acceptable to have different results.

Capabilities covered

This lesson covers the following STK Capabilities:

  • STK Pro
  • Communications
  • Coverage
  • Analysis Workbench

Problem statement

Engineers and Operators require a quick way to model communications devices, add one or more constraints to them, and see how adjustments affect link performance.

Solution

Using STK's Communications capability, you can constrain access between a transmitter and a receiver to satisfy a variety of RF criteria. You will set up a link between a ground‐based receiver and a transmitter on a communications satellite and then impose certain communication constraints to observe the effects.

What you will learn

Upon completion of this tutorial, you will understand how to:

  • Apply communication constraints
  • Use communication constraints in Coverage

Video guidance

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

Creating a new scenario

Create a new scenario with an analysis period of 24 hours.

  1. Launch STK ().
  2. Click Create a Scenario in the Welcome to STK dialog box.
  3. Enter the following in the STK: New Scenario Wizard:
    4. Option Value
    Name STK_Comm_Constraints
    Location Default
    Start 15 Mar 2024 16:00:00.000 UTCG
    Stop 16 Mar 2024 16:00:00.000 UTCG
  4. Click OK when finished.
  5. Click Save () when the scenario loads. A folder with the same name as your scenario is created for you in the location specified above.
  6. Verify the scenario name and location in the Save As window.
  7. Click Save.

Save () often!

Disabling Terrain Server

Analytical and visual terrain is not required in this analysis.

  1. Right-click on STK_Comm_Constraints () in the Object Browser.
  2. Select Properties ().
  3. Select the Basic - Terrain page in the Properties Browser.
  4. Clear Use terrain server for analysis.
  5. Click Apply to accept your change and to keep the Properties Browser open.

Specifying the radio frequency (RF) environment

Before defining parameters that are specific to the transmitter and receiver, it will be useful to select environmental models applicable to any communications link in the scenario. Environmental factors can affect the performance of a communications link.

Applying the rain model

Apply rain and atmospheric absorption models to the analysis. Rain models are used to estimate the amount of degradation (or fading) of the signal when passing through rain.

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

Modeling atmospheric absorption

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

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

Inserting a ground station

Use a Facility () object, from the Standard Object Database, that is located in the central portion of the United States.

  1. Insert a Facility () using the From Standard Object Database () method.
  2. Type Offutt in the Name: field of the Search Standard Object Data dialog box.
  3. Click Search.
  4. Select Offutt AFB SATCOM Terminal in the Results field with the Local Database data source.
  5. Click Insert.
  6. Click Close to close the Search Standard Object Data dialog box.

Inserting a Satellite object

Add a new Satellite object to the scenario using the Orbit Wizard.

  1. Insert a Satellite () object using the Orbit Wizard () method.
  2. Set the following parameters in the Orbit Wizard:
    OptionValue
    Satellite NameCommSat
    Altitude1500 km
  3. Click OK.

Creating communication equipment

In this scenario, you will have a Transmitter () object attached to the Satellite () object that can downlink information to the Facility () object. The Facility () object will have a Receiver () object that receives the downlink transmission. You can target both the Receiver () and Transmitter () objects. In STK, you will use a Sensor () object as the targeting device.

Modeling the targeted sensor on CommSat

First, you will model a sensor () object on CommSat that targets Offutt_AFB_SATCOM_Terminal ().

Inserting a Sensor object on CommSat

Attach a Sensor () object to CommSat ().

  1. Insert a Sensor () object using the Insert Default () method.
  2. Select CommSat () when the Select Object dialog box opens.
  3. Click OK.
  4. Right click on Sensor1 () in the Object Browser.
  5. Select Rename.
  6. Rename Sensor1 () to TgtOffutt.

Modeling a limited field-of-view

Set a limited field-of-view for the Sensor () object to provide situational awareness in the 3D Graphics window.

  1. Open TgtOffutt's () properties ().
  2. Select the Basic - Definition page.
  3. Notice the Simple Conic is the default Type.
  4. Enter 2 deg in the Cone Half Angle: field.
  5. Click Apply.

Targeting the facility

Use the Targeted pointing type to point the sensor.

  1. Select the Basic - Pointing page.
  2. Select Targeted for the Pointing Type.
  3. Move () Offutt_AFB_SATCOM_Terminal () from the Available Targets list to the Assigned Targets list.
  4. Click OK.

Modeling the transmitter on CommSat's sensor

Inserting a Transmitter object

Attach a Transmitter () object to TgtOffutt ().

  1. Insert a Transmitter () object using the Insert Default () method.
  2. Select TgtOffutt () in the Select Object dialog box.
  3. Click OK.
  4. Rename Transmitter1 () to DLXmtr.

Using a Complex Transmitter model

The Complex Transmitter 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 DLXmtr'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 in the Select Component dialog box.
  5. Click OK to close the Select Component dialog box.
  6. Select the Model Specs tab.
  7. Set the following:
    8. Option Value
    Frequency 4.5 GHz
    Power 5 dBW
  8. Click Apply.

Modeling the Transmitter's antenna

Use the default Gaussian Antenna Model. The Gaussian Antenna Model uses an analytical model of a Gaussian beam. The model is similar to a parabolic antenna.

  1. Select the Antenna tab.
  2. Enter 4.5 GHz in the Design Frequency field.
  3. Click OK.

Modeling the receiver on the facility

Inserting a Receiver object

Attach a Receiver () object to Offutt_AFB_SATCOM_Terminal ().

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

Using a Medium Receiver model

The Medium Receiver model provides more flexibility than the Simple model by enabling you to specify the components of system temperature. The Medium Receiver model uses an isotropic, omnidirectional antenna which is an ideal spherical pattern antenna with constant gain.

  1. Open DLRcvr's () properties ().
  2. Select the Basic - Definition page.
  3. Click the Receiver Model Component Selector ().
  4. Select Medium Receiver Model () in the Receiver Models list in the Select Component Dialog box.
  5. Click OK to close the Select Component Dialog box.
  6. Enter 20 dB in the Gain: field.
  7. Enter 0.010 in the Outage Percent: field in the Rain Model frame.
  8. Click Apply.

    9. The Outage Percentage expresses, as a percentage, how much time you will allow to be sacrificed to rain outage during the year or, conversely, how much time the link must be maintained despite rain. In this case, you are specifying that the link must be maintained 99.99 percent of the year, rain or no rain, and other parameters, such as power or frequency, may need to be adjusted to meet this requirement.

Setting the system noise temperature

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

  1. Select the System Noise Temperature tab.
  2. Select the Compute option.
  3. Enter 1.2 dB in the Noise Figure: field in the LNA frame.

    4. LNA is low noise amplifier. The noise figure represents the contribution to the total system noise by the gain stages of the receiver.

  4. Select the Compute option in the Antenna Noise frame.
  5. Select the following check boxes:

    • Sun

    • Atmosphere

    • Rain

  6. Click Apply.

Modeling atmospheric refraction

Atmospheric refraction is the bending of an RF signal as it travels through the atmosphere, caused by the changing refractive index at different altitudes.

  1. Select the Basic - Refraction page.
  2. Select Use Refraction in Access Computations.
  3. Select the ITU model for the Refraction Model.
  4. Click OK.

    5. This model computes the refracted elevation on the basis of inputs consisting of the nonrefracted elevation angle and the mean sea-level altitude of the receiver, using empirical criteria contained in ITU Recommendation. This completes the setup of the receiver.

Calculating Access

You can now calculate a link budget between DLXmtr () and DLRcvr ().

  1. Right click on DLRcvr () in the Object Browser.
  2. Select Access... () in the shortcut menu.
  3. Expand () CommSat () in the Associated Objects list in the Access Tool.
  4. Expand () TgtOffutt ().
  5. Select DLXmtr ().
  6. Click .

Viewing access lines in the 2D Graphics window

Access highlights in the 2D Graphics window will show portions of the satellite's ground track where there is access between the transmitter and receiver.

  1. Bring the 2D Graphics window to the front.
  2. Notice the access lines on the 2D Graphics window.

    3. 2D Graphics window access lines

Generating a Link Budget report

In the remainder of this exercise, when you impose communication constraints that limit the periods of access on the basis of given RF criteria, it will be interesting to observe corresponding changes in the access graphics.

  1. Return to the Access Tool.
  2. Click Report & Graph Manager... located under the Reports frame.
  3. Select the Link Budget - Detailed report () in the Installed Styles list in the Report & Graph Manager.
  4. Click Generate....
  5. Scroll across the report to become familiar with the data provider elements in the report.

Changing Receiver's constraints and properties

The first part of this tutorial focuses on the Receiver () object. With the link budget report open, you can change communication constraints and properties, then refresh the report to see the effects your changes create on the link performance.

Applying a received isotropic power constraint

Start by looking at received isotropic power. Received isotropic power (Rcvd. Iso. Power (dBW), is the product of the effective isotropic radiated power (EIRP) of the transmitter and the propagation losses (atmospheric, rain, and free space losses) without any contribution by the receiver.

Observing the current received isotropic power values

View the current received isotropic power values in the Link Budget - Detailed report.

  1. Return to the Link Budget - Detailed report ().
  2. Scroll to the Rcvd. Iso. Power (dBW) column.

    3. Suppose it is desirable to exclude any link with an received isotropic power value of less than –140 dBW.

Modeling the received isotropic power constraint

Set the received isotropic power constraint on the receiver.

  1. Open DLRcvr's () properties ().
  2. Select the Constraints - Active page.
  3. Click Add new constraints () in the Active Constraints toolbar.
  4. Select Rcvd Isotropic Power in the Constraint Name list in the Select Constraints to Add dialog box.
  5. Click Add.
  6. Click Close to close the Select Constraints to Add dialog box.
  7. Notice that the Min: check box is selected in the Rcvd Isotropic Power frame in the Constraint Properties section.
  8. Enter -140 dBW in the Min: field.
  9. Click Apply.

Viewing access lines in the 2D Graphics window

View the updated access lines in the 2D Graphics window.

  1. Bring the 2D Graphics window to the front.
  2. Notice that the constraint you imposed reduced the access lines.

    3. RIP Constraint

Applying the constraint change to the Link Budget - Detailed report

The Link Budget - Detailed report is open. You will refresh the report in order to apply the constraint changes.

  1. Return to the Link Budget - Detailed () report.
  2. Focus on the Rcvd. Iso. Power (dBW) column.
  3. Click Refresh (F5) () in the report's tool bar.
  4. Notice that no time entries for received isotropic power values less than –140 dBW appear in the report.

Disabling the received isotropic power constraint

Remove the received isotropic power constraint.

  1. Return to DLRcvr's () properties ().
  2. Select the Constraints - Active page.
  3. Clear the Enable check box for Rcvd Isotropic Power in the Active Constraints list.
  4. Click Apply.
  5. Return to the Link Budget - Detailed () report
  6. Click Refresh (F5) ().
  7. Notice the time entries with received isotropic power values of less than –140 dBW now appear back in the report.

Applying a Doppler Shift constraint

The Doppler Shift constraint relates to the ability of the receiver to adjust to increases in the frequency of the incoming signal as the satellite approaches its closest point to the receiver and to adjust to decreases in that frequency as the satellite moves away from that point. Recall that you specified a transmitter frequency of 4.5 GHz.

Observing the current Rcvd. Frequency values

View the current received isotropic power values in the Link Budget - Detailed report.

  1. Return to the Link Budget - Detailed report ().
  2. Scroll to the Rcvd. Frequency (GHz) column.
  3. Notice that the receiver must accommodate a shift in frequency to avoid losing any of the incoming signal.

    4. Suppose the receiver is limited to a 50 kHz adjustment in either direction.

Modeling the Doppler Shift constraint

Limit the receiver to -50 kHz adjustment in frequency in either direction.

  1. Return to DLRcvr's () properties ().
  2. Select the Constraints - Active page.
  3. Click Add new constraints () in the Active Constraints toolbar.
  4. Select Doppler Shift in the Constraint Name list in the Select Constraints to Add dialog box.
  5. Click Add.
  6. Click Close to close the Select Constraints to Add dialog box.
  7. Select Min: and Max: in the Doppler Shift frame in the Constraint Properties section.
  8. Enter -50 kHz in the Min: Field.
  9. Enter 50 kHz in the Max: field.
  10. Click Apply.

Viewing access lines in the 2D Graphics window

View the updated access lines in the 2D Graphics window.

  1. Bring the 2D Graphics window to the front.
  2. Notice that the constraint you imposed reduced the access lines.

    3. Doppler Constraint

    The 2D Graphics window clearly reflects the impact of this constraint. In fact, if you alternatively disable and enable the Min and Max options, you will see that the Doppler shift limitations are reflected alternatively in the approaching and departing ground tracks, just as you would expect.

From here on, you'll only apply changes to the Link Budge - Detailed report. However, you can look at the 2D Graphics window if you desire to do so after each constraint change.

Applying the constraint change to the Link Budget - Detailed report

The Link Budget - Detailed report is open. You will refresh the report in order to apply the constraint changes.

  1. Return to the Link Budget - Detailed () report.
  2. Focus on the Rcvd. Frequency (GHz) column.
  3. Click Refresh (F5) () in the report's tool bar.
  4. Notice that time entries for Received Frequencies less than 4.499950 GHz and greater than 4.500050 GHz are excluded. You can also see this in the Freq. Doppler Shift (kHz) column.

Disabling the Min and Max Doppler Shift constraints

Remove the Doppler Shift constraint.

  1. Return to DLRcvr's () properties ().
  2. Select the Constraints - Active page.
  3. Clear the Enable check box for Doppler Shift in the Active Constraints list.
  4. Click Apply.
  5. Return to the Link Budget - Detailed () report.
  6. Click Refresh (F5) ().

Applying a Flux Density constraint

Flux Density, usually measured in dB (watts/m^2), is the expression of the transmitter's radiated power, reduced by any applicable atmospheric losses, divided by the surface area of a sphere with a radius equal to the distance between the transmitter and receiver:

Flux Density

Observing the Flux Density values

View the current Flux Density values in the Link Budget - Detailed report.

  1. Return to the Link Budget - Detailed report ().
  2. Scroll to the Flux Density (dBW/m^2) column.
  3. Note that entries for Flux Density should range between approximately -114 and –99 dBW/m^2.

    4. Suppose you wish to exclude from consideration links with a Flux Density less than –110 dBW/m^2.

Modeling the Flux Density constraint

Set a constraint that excludes any links with a Flux Density less than –110 dBW/m^2.

  1. Return to DLRcvr's () properties ().
  2. Select the Constraints - Active page.
  3. Click Add new constraints () in the Active Constraints toolbar.
  4. Select Flux Density in the Constraint Name list in the Select Constraints to Add dialog box.
  5. Click Add.
  6. Click Close to close the Select Constraints to Add dialog box.
  7. Enter -110 dBW/m^2 in the Min: field in the Flux Density frame in the Constraint Properties section.
  8. Click Apply.

Applying the constraint change to the Link Budget - Detailed report

The Link Budget - Detailed report is open. You will refresh the report in order to apply the constraint changes.

  1. Return to the Link Budget - Detailed () report.
  2. Focus on the Flux Density (dBW/m^2) column.
  3. Click Refresh (F5) () in the report's tool bar.
  4. Notice all Flux Density (dBW/m^2) values less than -110 dBW/m^2 are removed from the report.

Disabling the Flux Density Constraint

Remove the Flux Density constraint.

  1. Return to DLRcvr's () properties ().
  2. Select the Constraints - Active page.
  3. Clear the Enable Flux Density check box in the Active Constraints list.
  4. Click Apply.
  5. Return to the Link Budget - Detailed () report.
  6. Click Refresh (F5) ().

Applying carrier-to-noise ratio constraints

One of the most commonly used criteria for assessing link performance is carrier-to-noise ratio (C/N (dB)). This can be expressed independently of bandwidth as:

CNR Expression

It is a common practice for a receiver vendor to stipulate a minimum required carrier -to-noise ratio that must be satisfied in order for the equipment to perform to specification. Imposing a C/N constraint on accesses between a receiver and a transmitter is an easy way to model this requirement in the design of communications links. For example, if the manufacturer specifies that its equipment requires a carrier-to-noise ratio of at least five (5) dB to function properly, you can simply enter a minimum value of five (5) for the C/N constraint, which will reduce the number and/or length of calculated periods of access between the receiver and transmitter.

For greater confidence in the quality and reliability of a link, it is a good idea to add a fade margin to other requirements that you must meet. A natural way to do this is to increase the minimum value of the C/N constraint to include that margin. Thus, to model a fade margin of three (3) dB for a receiver requiring a minimum carrier-to-noise ratio of five (5) dB, just set the minimum value for C/N to eight (8) dB. You can then enjoy a higher degree of confidence in the access periods.

Observing the current C/N ratio

View the current C/N values in the Link-Budget - Detailed report.

  1. Return to the Link Budget - Detailed report ().
  2. Scroll to the C/No (dB*Hz) and C/N (dB) columns.

    3. For the communications link in this exercise, C/No and C/N fall in the approximate ranges 75 to 95 dB/Hz and ‐1 to 20 dB, respectively. You can constrain link performance with respect to either of these criteria.

    Suppose you wish to exclude from consideration links with a carrier-to-noise ratio of less than 10 dB.

Setting the C/N constraint

Set a min C/N constraint of 10 dB.

  1. Return to DLRcvr's () properties ().
  2. Select the Constraints - Active page.
  3. Click Add new constraints () in the Active Constraints toolbar.
  4. Select C/N in the Constraint Name list in the Select Constraints to Add dialog box.
  5. Click Add.
  6. Click Close to close the Select Constraints to Add dialog box.
  7. Enter 10 dB in the Min: field in the C/N frame in the Constraints Properties section.
  8. Click Apply.

Applying the constraint change to the Link Budget - Detailed report

The Link Budget - Detailed report is open. You will refresh the report in order to apply the constraint changes.

  1. Return to the Link Budget - Detailed () report.
  2. Focus on the C/N (dB) column.
  3. Click Refresh (F5) () in the report's tool bar.
  4. Notice all C/N (dB) values are greater than 10 dB.

Adjusting the receiver gain

Making gain adjustments, you can improve C/No and C/N via receiver as well as transmitter adjustments, since receiver gain appears in the numerator of both equations.

  1. Return to DLRcvr's () properties ().
  2. Leave the C/N constraint in place.
  3. Select the Basic - Definition page.
  4. Select the Model Specs tab.
  5. Enter 25 dB in the Gain: field.
  6. Click Apply.

Applying the updated receiver gain

Refresh the Link Budget - Detailed report to see the effects of the gain change.

  1. Return to the Link Budget - Detailed () report.
  2. Click Refresh (F5) ().
  3. Note the changes in the C/No and C/N fields.

Disabling the constraints

Change the receiver gain back to 20 dB and remove the constraint.

  1. Return to DLRcvr's () properties ().
  2. Select the Basic - Definition page.
  3. Select the Model Spec tab.
  4. Enter 20 dB in the Gain: field.
  5. Select the Constraints - Active page.
  6. Clear the Enable C/N check box in the Active Constraints list.
  7. Click Apply.
  8. Return to the Link Budget - Detailed () report
  9. Click Refresh (F5) ().

Applying a refraction angle constraint

The calculation of the refracted elevation or range depends on the selected refraction model. As you may recall, you selected an ITU model satisfying empirical criteria.

Updating the Refraction Model

Set the Refraction Model to the Effective Radius Method.

  1. Return to DLRcvr's () properties ().
  2. Select the Basic - Refraction page.
  3. Note the Use Refraction in Access Computations option is selected.
  4. Select Effective Radius Method for the Refraction Model.

    5. This computes the apparent elevation due to refraction.

  5. Click Apply.

Applying the new model to the report

The Link Budget - Detailed report is open. You will refresh the report in order to apply the changes.

  1. Return to the Link Budget - Detailed () report.
  2. Focus on the Eb/No (dB) and BER (bit error rate) columns.
  3. Click Refresh (F5) ().

    4. When you select the Use Refraction in Access Computations check box, STK computes the object visibility, range, elevation angle, and link angle of the antenna boresight with refraction taken into account.

Modeling the ITU model

Change the Refraction Model back to the ITU model.

  1. Return to DLRcvr's () properties ().
  2. Select the Basic - Refraction page.
  3. Select ITU model for the Refraction Model.
  4. Click Apply .

Applying refracted elevation and range constraints

You can set constraints in terms of the refracted elevation and range of the transmitter with respect to the receiver. As a reminder, you are currently using the ITU refraction model in your analysis.

Applying the Elevation Angle constraint

Apply a 5 deg min elevation angle constraint.

  1. Return to DLRcvr's () properties ().
  2. Select the Constraints - Active page.
  3. Click Add new constraints () in the Active Constraints toolbar.
  4. Select Elevation Angle in the Constraint Name list in the Select Constraints to Add dialog box.
  5. Click Add.
  6. Click Close to close the Select Constraints to Add dialog box.
  7. Enter 5 deg in the Min: field in the Elevation Angle frame in the Constraint Properties section.
  8. Click Apply.

Applying the constraint change to the Link Budget - Detailed report

The Link Budget - Detailed report is open. You will refresh the report in order to apply the constraint changes.

  1. Return to the Link Budget - Detailed () report.
  2. Focus on the BER column.
  3. Click Refresh (F5) () in the report's tool bar.

    4. This excludes links with satellites deemed to be too close to the horizon, which can be unreliable due to the relatively long path through the atmosphere to be traversed by the signal.

  4. Notice most, if not all, of your BER fall within acceptable range, which is at or below 1 x 10‐6.

Removing the Elevation Angle constraint

Remove the min elevation angle constraint.

  1. Return to DLRcvr's () properties ().
  2. Select the Constraints - Active page.
  3. Clear the Enable Elevation Angle check box.
  4. Click OK to accept your changes and to close the Properties Browser.

At the end of this tutorial is a section called On Your Own. If you want to practice further, go to that section and continue making changes to your Receiver object using the steps you learned in this tutorial.

Transmitter Constraints and Property Changes

This section of the tutorial will focus on the Transmitter () object and how constraint and property changes affect your link budge.

Changing the transmitter modulation

An interesting transmitter adjustment to try out is a change in the Modulation Type. In digital modulation, minimum-shift keying (MSK) is a type of continuous-phase frequency-shift keying.

Modeling an MSK modulator

Model the MSK modulator.

  1. Open DLXmtr's () properties ().
  2. Select the Basic - Definition page.
  3. Select the Modulator tab.
  4. Select MSK as the Name.
  5. Click Apply.

Refreshing the Link Budget - Detailed report

The Link Budget - Detailed report is open. You will refresh the report in order to apply the modulator change.

  1. Return to the Link Budget - Detailed () report.
  2. Focus on the C/N (dB) column.
  3. Click Refresh (F5) ().
  4. Note the increase in C/N.

Setting the Modulation back to BPSK

The default modulation in STK is binary phase-shift keying (BPSK).

  1. Return to DLXmtr's () properties ().
  2. Select the Basic - Definition page.
  3. Select the Modulator tab.
  4. Open the Name: shortcut menu.
  5. Select BPSK.
  6. Click Apply.
  7. Return to the Link Budget - Detailed () report.
  8. Click Refresh (F5) ().

Decreasing the transmitter data rate

You can improve performance by tweaking the transmitter parameters further.

Observing the current Eb/No values

Observe hte current Eb/No (dB) values in the Link Budget - Detailed report.

  1. Return to the Link Budget - Detailed () report.
  2. Focus on the Eb/No (dB) column.
  3. Note that as reflected in the Link Budget report, the communications link in this exercise exhibits Eb/No values ranging from approximately 2.6 to 23 dB.

Updating the data rate

Update the transmitter's data rate to 12 Mb/sec.

  1. Return to DLXmtr's () properties ().
  2. Select the Basic - Definition page.
  3. Select the Model Specs tab.
  4. Enter 12 Mb/sec in the Data Rate: frame.
  5. Click Apply.

Refreshing the Link Budget - Detailed report

The Link Budget - Detailed report is open. You will refresh the report in order to apply the change.

  1. Return to the Link Budget - Detailed () report.
  2. Focus on the Eb/No (dB) column.
  3. Click Refresh (F5) ().
  4. Notice that the Eb/No (dB) values increase.

    5. This change increased the bit period T, which leads directly to an improvement in Eb/No.

Applying Eb/No constraints

For digital communications systems, the receiver bit-energy-to-noise (Eb/No) ratio is defined as:

Digital System Definition

Where T = the bit period.

Modeling the Eb/No constraint

Model a min Eb/No of 15 dB.

  1. Return to DLXmtr's () properties ().
  2. Select the Constraints - Active page.
  3. Click Add new constraints () in the Active Constraints toolbar.
  4. Select Eb/No in the Constraint Name list in the Select Constraints to Add dialog box.
  5. Click Add.
  6. Click Close to close the Select Constraints to Add dialog box.
  7. Enter 15 dB in the Min: field in the Eb/No frame in the Constraint Properties section.
  8. Click Apply.

Refreshing the Link Budget - Detailed report

The Link Budget - Detailed report is open. You will refresh the report in order to apply the change.

  1. Return to the Link Budget - Detailed () report.
  2. Focus on the Eb/No (dB) column.
  3. Click Refresh (F5) ().
  4. Notice that all instances of Eb/No values below 15.00 dB are removed from the report.

Disabling the Eb/No Constraint

Remove the min Eb/No constraint.

  1. Return to DLXmtr's () properties ().
  2. Select the Constraints - Active page.
  3. Clear the Enable Eb/No check box in the Active Constraints list.
  4. Click Apply.

Updating the transmitter's data rate

Change DLXmtr's () data rate back to 16 Mb/sec.

  1. Select the Basic - Definition page.
  2. Select the Model Specs tab.
  3. Enter 16 Mb/sec in the Data Rate: field.
  4. Click Apply.

Refreshing the Link Budget - Detailed report

The Link Budget - Detailed report is open. You will refresh the report in order to apply the change.

  1. Return to the Link Budget - Detailed report.
  2. Click Refresh (F5) ().

Applying Bit Error Rate constraints

A direct measure of link performance for a digital system is BER, which expresses the probability that a bit will be received in error. According to the Link Budget report, BER values in this exercise range from approximately 1 x 10‐30 to 3.2 x 10‐2. A typical desired BER is 1 x 10‐6 or lower.

Modeling the BER constraint

Set at max BER of 1e-10.

  1. Return to DLXmtr's () properties ().
  2. Select the Constraints - Active page.
  3. Click Add new constraints () in the Active Constraints toolbar.
  4. Select Bit Error Rate in the Constraint Name list in the Select Constraints to Add dialog box.
  5. Click Add.
  6. Click Close to close the Select Constraints to Add dialog box.
  7. Clear the Min: check box in the Bit Error Rate frame in the Constraint Properties section.
  8. Select the Max: check box.
  9. Enter 1e-10.
  10. Click Apply.

Refreshing the Link Budget - Detailed report

The Link Budget - Detailed report is open. You will refresh the report in order to apply the change.

  1. Return to the Link Budget - Detailed () report.
  2. Focus on the BER column.
  3. Click Refresh (F5) ().

    4. BER is a function of Eb/No, and you can improve it through adjustments in the receiver or transmitter, including reductions in Data Rate.

Disabling the constraint

Disable the BER constraint and refresh the Link Budget - Detailed report.

  1. Return to DLXmtr's () properties ().
  2. Select the Constraints - Active page.
  3. Clear the Enable Bit Error Rate check box in the Active Constraints list.
  4. Click OK.
  5. Return to the Link Budget - Detailed report and click Refresh ().
  6. Close the Link Budget - Detailed report, the Report & Graph Manager and the Access Tool.

Removing Accesses

Remove all accesses.

  1. Open the Analysis menu.
  2. Select Remove All Accesses.

Using communication constraints in Coverage

You can use the Coverage Definition () object to analyze a portion of a link budget (for example, C/N) over a wide area. Communication constraints will affect the coverage.

Inserting an Area Target

The Area Target () object models a region on the surface of the central body. Insert an Area Target to model the United States. This will be used to define the coverage grid.

  1. Insert an Area Target () object using the Select Countries and US States () method.
  2. Select United_States_of_America in the Select Countries And US States dialog box.
  3. Click Insert.
  4. Click Close to close the Select Countries And US States dialog box.

Defining the Coverage Definition

The Coverage Definition object defines a coverage area for analysis. Create a Coverage Definition over the United States, use the receiver to constrain the grid, then assign the transmitter as the asset.

Inserting a Coverage Definition object

Insert a Coverage Definition () object.

  1. Insert a Coverage Definition () object using the Insert Default () method.
  2. Rename CoveragDefintion1 () to AccessContraintsCOV.

Defining the coverage grid

Use the Area Target () to define the coverage grid. Coverage analyses are based on the accessibility of assets (objects that provide coverage) and geographical areas. For analysis purposes, you can refine the geographical areas of interest further using regions and points. The statistical data computed during a coverage analysis is based on a set of locations, or points, which span the specified grid area of interest.

  1. Open AccessConstraintsCOV's () properties ().
  2. Select the Basic – Grid page.
  3. Open the Type: shortcut menu in the Grid Area of Interest frame.
  4. Select Custom Regions.
  5. Open the Area Of Interest shortcut menu.
  6. Select Area Targets.
  7. Move () United_States_of_America () from the Area Target list to the Selected Regions list.
  8. Enter 1 deg in the Lat/Lon field in the Point Granularity frame.
  9. Click Apply.

Specifying grid constraints

Set the grid constraints by specifying the Receiver () object for the points within the grid. Once you have defined the grid area, you can specify an object class or a specific object for the points within the grid. You can use the object to associate three types of information with the grid points: access constraints, basic object properties, and the shape of the ellipsoidal obstruction surface used by the Line Of Sight constraint.

  1. Click Grid Constraints Options... in the Grid Definition frame.
  2. Select Receiver for the Reference Constraint Class.
  3. Select Offutt_AFB_SATCOM_Terminal/DLRcvr in the object list.
  4. Click OK to accept the changes and to close the Grid Constraints Options dialog box.
  5. Click Apply.

Assigning coverage assets

Assign the Transmitter () object as the Assets. Assets properties allow you to specify the STK objects used to provide coverage.

  1. Select the Basic - Assets page.
  2. Expand () CommSat () in the Assets list.
  3. Expand TgtOffutt ().
  4. Select DLXmtr ().
  5. Click Assign.
  6. Click Apply.

Turning off automatically recompute accesses

STK automatically recomputes accesses every time you update an object on which the coverage definition depends (such as an asset). If you want control as to when STK computes coverage, you need to turn this off.

  1. Select the Basic - Advanced page.
  2. Clear Automatically Recompute Accesses.
  3. Click Apply.

Clearing the grid points

Clear the grid points from the 2D and 3D graphics windows.

  1. Select the 2D Graphics - Attributes page.
  2. Clear Show Points in the Grid frame.
  3. Click OK to accept the changes and to close the Properties Browser.

Using the Compute Accesses tool

The ultimate goal of coverage is to analyze accesses to an area by using assigned assets and applying necessary limitations upon those accesses. Compute coverage with the Compute Accesses tool.

  1. Select AccessConstraintsCOV () in the Object Browser.
  2. Select the CoverageDefinition menu item.
  3. Select Compute Accesses in the shortcut menu.

Creating a Figure of Merit

STK enables you to specify the method by which the quality of coverage is measured using a Figure Of Merit () object.

Inserting a Figure of Merit

Insert a Figure of Merit () object.

  1. Insert a Figure Of Merit () object using the Insert Default () method.
  2. Select AccessConstraintsCOV () in the Select Object dialog box.
  3. Click OK.
  4. Rename CoveragDefintion1 () to AccessContraintsFOM.

Measuring access constraints

Access constraints measure the value of various constraint parameters used to define visibility within STK.

  1. Open AccessConstraintsFOM's () properties ().
  2. Select the Basic - Definition page.
  3. Select Access Constraint for the Type.
  4. Select C/N for the Constraints.

    5. Default Compute is set to Average. In this case, average is good when creating static contours for a finite time period.

  5. Select the Enable check box in the Satisfaction frame.
  6. Click Apply.

    7. Be patient. This can take a couple of minutes to compute.

Generating a Grid Stats report

The Grid Stats report summarizes the minimum, maximum, and average static value for the Figure Of Merit over the entire grid.

  1. Right-click AccessConstraintsFOM () in the Object Browser.
  2. Select Report & Graph Manager... ().
  3. Select the Grid Stats report () in the Installed Styles list in the Report & Graph Manager.
  4. Click Generate....
  5. Note the Maximum value for the Grid Stats once STK generates the report.

    6. You will use the value to create contours on your 2D and 3D Graphics windows.

Defining static graphics for the Figure Of Merit

Define static graphics for the Figure Of Merit on the 2D Graphics - Static page.

  1. Return to AccessConstraintsFOM's () properties ().
  2. Select the 2D Graphics - Static page.
  3. Enter 20 in the % Translucency: field in the Show Points As frame.
  4. Select the Show Contours option in the Display Metric frame.
  5. Enter the following in the Level Adding frame:
    6. Option Value
    Start Level 0 dB
    Stop Level Round down the maximum integer from the Grid Stats report (e.g. 13 dB)
    Step 1 dB
  6. Click Add Levels.
  7. Select red as the Start Color.
  8. Select blue as the End Color.

    9. Points with no coverage will be red and any points at or above your highest Level Attribute value will be blue.

  9. Select the Natural Neighbor option in the Contour Interpolation (points must be filled) frame.

    10. Color is applied smoothly over all points in the grid to differentiate contour levels.

  10. Click Apply

Setting the 2D Graphics window legend

Once you have set the contours for coverage, you can set the display of the contour key, or legend.

  1. Click Legend... in the Level Attributes frame.
  2. Click OK to close AccessConstraintsFOM's () properties ().
  3. Click Layout... in the Static Legend for Cov_Time dialog box.
  4. Set the following on the Figure of Merit Legend Layout dialog box:
    5. Option Value
    2D Graphics Window - Show at Pixel Location on
    3D Graphics Window - Show at Pixel Location on
    Text Options - Title C/N (dB)
    Text Options - Number Of Decimal Digits 0
    Range Color Options - Color Square Width (pixels) 50
  5. Click OK to close the Figure of Merit Legend Layout dialog box.
  6. Close () the Static Legend for Cov_Time dialog box.
  7. Bring the 2D or 3D Graphics windows to the front.

    C/N Contours

Save your work

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

Summary

The purpose of the tutorial was to show you how using communication constraints affect what you see in a link budget report. Once you set up your communication system and create a link budget, based on your needs in the system, you can set constraints that add or remove accesses in the link budget. For instance, when looking at carrier-to-noise (C/N) ratios, if your system only works with a C/N of 5 dB or higher, you can go into the receiver's properties and set a minimum C/N of 5 dB constraint. When refreshing the report, you are left with accesses that work. This comes in handy when scheduling your communications or just giving you an overall situational awareness of your system. You also looked at tweaking things like gain or modulation and how they affect your link budget. Finally, you looked at wide area coverage and how to apply a constraint across your grid to provide you with a big picture of your system.

On your own

Using the steps you learned in this tutorial, you can continue to try various constraints in DLRcvr () and apply them to the Link Budget - Detailed report.

  • On the System Noise Temperature tab, reduce LNA Noise Figure to one (1) dB.
  • On the Model Specs tab, add (Right Hand or Left Hand) Circular Polarization. You must also set the polarization on the transmitter to the same type.
  • On the Additional Gains and Losses tab, add a Pre‐Receive gain of one (1) dB.
  • On the Model Specs tab, increase the Rain Model Outage Percent to 0.030.