Modeling GPS Position Accuracy Using Coverage

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.

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
  • Coverage

Problem statement

The FAA is testing a new experimental radar system that is designed to provide highly accurate location of aircraft as they fly through United States airspace. To test the quality of this new system, you will be making a test flight from which you will be monitoring your aircraft's location via GPS. You will compare the GPS-determined location information to the radar tracking information to determine the fidelity of the new radar system. To accurately test the radar system, you need to know how accurate you can expect the GPS position information to be over the aircraft route.

Breaking it down

You have some information that may be helpful. Here’s what you know:

  1. A positional navigational accuracy (PACC) value of less than ten (10) meters will yield results good enough to use as a reference for your radar.
  2. You must measure GPS navigational accuracy over the continental United States at the altitude of your flight.
  3. You must also measure the GPS dilution of precision over the path of a test flight, which will go from Long Beach to Philadelphia at an altitude of 20,000 feet.

Solution

Using STK's Coverage capability, you will build a scenario to examine navigation accuracy over a large area, namely the entire airspace of the continental United States. You will determine if there are any areas at any times that would provide an unacceptable navigational accuracy value. If so, you would not be able to use your GPS-reported position to provide a meaningful baseline for a comparison of the radar system's tracking data. Then, you will examine the navigational accuracy values for a specific aircraft route.

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 scenario

First, you need to define the times during which the conditions that you set for your world, and the objects in your world, will be relevant. You are not concerned with examining the navigational accuracy over US airspace on a specific day at a specific time, but instead you are just doing some general analysis. Therefore, you can accept the STK default 24-hour time period for this example.

  1. Click Create a Scenario ().
  2. Enter the following in the New Scenario Wizard:
    3. Option Value
    Name Navigation
    Description How will I determine the accuracy of my in-flight navigation solution?
    Location C:\Users\<username>\Documents\STK 12\
    Start Leave the default start time as is.
    Stop Leave the default stop time as is.
  3. When you finish, click OK .
  4. When the scenario loads, click Save (). A folder with the same name as your scenario is created for you in the location specified above.
  5. Verify the scenario name and location and click Save .

Cleaning up

Before you begin to visually define your analysis area, remove all the stuff that you don’t want cluttering up the 2D Graphics window.

  1. Open the 2D Graphics window properties ().
  2. Go to the Imagery page.
  3. Clear the Background Image - Show check box.
  4. Click OK to apply the changes and dismiss the Properties Browser.

    5. 2D View: 2D Map with only borders and outlines

Defining the analysis area

Outline the United States on the map so that you can more easily identify and focus on that area of the map.

  1. Insert an Area Target () object using the Select Countries And US States () method.
  2. In the Select Countries And US States window, select United_States_of_America.
  3. Ensure the Primary Area Only option is selected.
  4. Click Insert .
  5. Click Close to dismiss the area target introduction dialog box.

    6. Selecting Primary Areas Only during the creation process will ensure that only the continental US is included. If you had selected All Areas, an area target representing each one of the Hawaiian islands, Alaska, etc. would have been imported.

Getting a better look

  1. Bring the 2D Graphics window to the front.
  2. Zoom In () around the continental United States ().

    3. The United States should now be clearly outlined in the 2D and 3D Graphics windows.

    2D View: Area target of the continental US

Removing the Area Target label

You don’t really need to label the area target or mark the center point of the continental United States for your analysis. Let’s remove the area target marker from United_States_of_America.

  1. Open United_States_of_America’s () properties ().
  2. Go to the 2D Graphics - Attributes page.
  3. Set the following:
    4. Option Action
    Inherit From Scenario Clear the check box
    Show Label Clear the check box
    Show Centroid Clear the check box
  4. Click OK .

Modeling ground locations

Take a look back at the list you made in the "Break it down" section. According to what you know, you need to model two locations: the origination and destination locations of your aircraft.

See if you can find a database entry for Philadelphia and Long Beach, and use them to model STK place objects representing the city from which your aircraft will depart (Long Beach) and the aircraft's destination location (Philadelphia).

  1. Insert a Place () object using the From City Database () method.
  2. Use the City database to insert the following cities:
    • Long Beach, California
    • Philadelphia, Pennsylvania
  3. When you finish, close the City Database Search tool.

    4. Your departure and destination locations should now be clearly visible on the map in the 2D Graphics window.

    2D View: Philadelphia and Long Beach on the map

Change your perspective

How does that look in 3D?

  1. Bring the 3D Graphics window to the front.
  2. Right-click the United_States_of_America () area target.
  3. Select Zoom To to reposition the view so that United_States_of_America () is the focal point in the 3D Graphics window.

    4. When you set an area target as the focal point in the 3D Graphics window, STK sets the centroid of the area target as the focal point. You will have to zoom out to see the entire area target.

  4. Use your mouse to zoom out until you can see the entire continental United States.

Using streaming terrain for visualization

By default, streaming terrain is active in the Globe Manager and visible on the globe if you have an internet connection. If this is the case, make the following changes:

  1. Open the 3D Graphics window Properties ().
  2. Go to the Details page.
  3. Select the Enable check box in the Label Declutter section.
  4. Click OK .
  5. Open Navigation's () properties ().
  6. Go to the 3D Graphics - Global Attributes page.
  7. In the Surface Lines - On Terrain field, select When Terrain Server is on.
  8. Click OK .

3D View: Philadelphia and Long Beach on the globe

Now, both of your visualization windows are focused on your defined region of interest.

Modeling an Aircraft

You are flying across the continental United States. Your aircraft takes off from Long Beach and flies to Philadelphia. You have modeled these two cities, and now you can model your aircraft and its route.

  1. Insert an Aircraft () object using the Insert Default () method.
  2. Rename it Test_Flight.

Defining Test_Flight’s route

Now you can model Test_Flight’s flight from Long Beach to Philadelphia.

  1. Bring the 2D Graphics window to the front.
  2. Open Test_Flight's () properties ().
  3. With the Basic - Route page open, return to the 2D Graphics window and orient the Properties Browser and the 2D Graphics window so you can see both at the same time.
  4. Click Long Beach on the 2D Graphics window.

    5. Be careful! Each click on the 2D Graphics window is recognized and added as an aircraft route waypoint.

  5. Return to Test_Flight's () properties ().
  6. Set the Altitude to 20000 ft.
  7. Click Philadelphia on the 2D Graphics window.
  8. Click OK .

Getting a better look

  1. Position the 2D and 3D Graphics windows so that you can clearly see them both.
  2. Reset () the animation.
  3. Play () the animation.
  4. Reset () the animation when done.

    5. You can watch Test_Flight fly its route equally well in either 2D or 3D.

2D View: Test Flight’s route

3D View: Test Flight’s route

Modeling the GPS Network

You also need to model the GPS satellite constellation.

  1. Insert a Satellite () object using the Load GPS Constellation () method.

    2. If you do not have an internet connection, you can download the current TLE file. Open the Scenario's Properties and select the Basic - Database page. Or you can download it from AGI's website. You can then insert the GPS database using the From TLE File method.

  2. Bring the 3D Graphics window to the front.
  3. Click the Home View button () on the 3D Graphics toolbar to restore the default Earth-centered view.
  4. Use the mouse to zoom out until you can see the satellites orbit around the globe.

    5. 3D View: GPS satellite orbital tracks

By default, the GPSConstellation Orbit tracks are disabled.

Measuring navigation accuracy

You need to assess the accuracy with which you will be able to determine your current position while en route from Long Beach to Philadelphia. You want to assess coverage, or your ability to “see”, over the appropriate number of defined assets throughout the duration of Test_Flight’s flight.

To do this, you will assess coverage of the continental United States based on the boundaries of the area target that is being used to outline that area. You will need to specify the region being examined (United_States_of_America), how each grid point should be treated, and what assets will be used to examine the region (GPSConstellation).

Inserting a Coverage Definition

The first thing you need to do is define the coverage area. Instead of entering the latitude and longitude bounds of the coverage region, you can have STK use the boundaries of your area target to define the boundaries of the coverage definition.

  1. Insert a Coverage Definition object () using the Insert Default method.

    2. If the Coverage Definition object does not appear in the Insert Objects tool, click Edit Preferences... and add it. You can also add the Figure of Merit object to the Coverage Definition, since you will use that too.

  2. Rename the Coverage Definition object to "ContUS_Cov".
  3. Open ContUS_Cov's () properties (), which by default opens the Basic - Grid page.
  4. Change the Grid Area of Interest - Type to Custom Regions.
  5. Select Area Targets in the drop-down menu below Custom Regions.
  6. Select United_States_of_America in the Area Targets list.
  7. Move () United_States_of_America to the Selected Regions list.
  8. Change the Point Granularity field to Lat/Lon and 1 deg in the Grid Definition panel.
  9. Click Apply to accept the changes and keep the Properties Browser open.

Getting a better look

  1. Bring the 2D or 3D Graphics window to the front.

    2. 2D View: Coverage grid over continental United States

  2. Answer the following question:
    • Is the coverage grid limited to the boundaries of the United States area target?

Defining the altitude of the grid points

For your analysis, you want to assess the quality of the navigation solution for an airborne vehicle (Test_Flight) within the continental United States. That means the points for which you want to calculate access will not be on the surface of the Earth. Currently, the coverage grid is on the surface of the Earth. How will you compute coverage to grid points at the same altitude as Test_Flight?

You can associate the properties and constraints of an airborne vehicle, such as Test_Flight, with points in the grid just as you would if you wanted to associate the properties and impose the constraints of a ground-based object, such as a facility, to points on the surface of the Earth. You can associate Test_Flight with the coverage grid, and have points in the grid defined according to Test_Flight’s altitude.

The altitude entered is the altitude used for the coverage definition grid points, not the altitude of the template object. Using a template object would apply any constraints or properties, excluding positional information to each grid point. Since Test_Flight does not have any constraints, you do not need to use this option.

  1. Return to the Basic - Grid page of ContUS_Cov's () properties ().
  2. In the Point Altitude section, select Altitude Above WGS84 from the drop-down menu.
  3. Set the Altitude to 20000 ft.
  4. Click Apply to apply the changes.
  5. Select the 3D Graphics - Attributes page.
  6. In the Fill Options section, select the Show at Altitude check box.
  7. Click Apply to apply changes and keep the Properties Browser open.

Assigning assets

The GPS satellites are the assets with which you want to assess the quality of your coverage. Assign them now.

  1. Select the Basic - Assets page.
  2. In the Assets field, select the GPSConstellation () constellation.
  3. Click Assign .
  4. Click Apply to accept the changes and keep the Properties Browser open.

Turning off automatic access calculations

In an effort to manage your resources more efficiently, don't let STK recompute access automatically every time you make a change. To turn that off, tell STK when to compute accesses.

  1. Select the Basic - Advanced page.
  2. Clear the Automatically Recompute Accesses check box.
  3. Click Apply to accept the changes and keep the Properties Browser open.

Computing coverage!

Now that you specified the Coverage Definition object, you can compute access to points in the grid.

  1. Bring the 2D Graphics window to the front.
  2. Right-click ContUS_Cov () in the Object Browser.
  3. Select the Coverage Definition menu item.
  4. Click Compute Accesses.

    5. For large-scale calculations, consider computing the accesses for coverage in parallel using multiprocessing. You can do this using multiple cores on a local machine or by taking advantage of cluster configurations, depending upon your machine configuration. For more information on machine configuration, installation of the Parallel Extension, licensing, and more, please see the STK Parallel Cluster Overview.

    You can watch the status of the calculations in the Status Bar in the bottom right of the STK GUI.

  5. When the coverage computations complete, return to ContUS_Cov's () properties ().
  6. Go to the 2D Graphics - Attributes page.
  7. Clear the Show Points check box.
  8. Click OK .

Assessing navigation accuracy

Navigation accuracy measures the uncertainty of a navigation solution based on one-way measurements from a set of transmitters. Most often, the transmitters are those on-board Global Positioning Systems (GPS) satellites. If four or more of these satellites are in view of a ground receiver, a navigation solution consisting of the position of the receiver and the offset between the receiver clock and the GPS clock can be computed.

The Navigation Figure of Merit considers the effect of the number of measurements of those satellites visible at each moment in time and the geometry of the transmitters, and you can specify the uncertainty in the one-way range measurements as a constant value or as a function of the elevation angle of each transmitter.

When using Navigation Accuracy to measure the quality of coverage, you need to specify the following settings on the Figure Of Merit Basic - Definition page:

  1. Method: Choose the Navigation Accuracy type you want STK to measure.
  2. Type: Specify the maximum number of assets that STK can use to produce navigation solutions.
  3. Compute: Choose the method for computing the static value of Navigation Accuracy over the entire coverage interval.
  4. Time Step: Specify the step size for STK to use when computing the static value of Navigation Accuracy across the coverage interval.
  5. Uncertainties... : Choose the method for computing the range uncertainty for each Coverage asset.

Figures of Merit can exhibit two types of behavior: dynamic and static. You specify the dynamic definition of Navigation Accuracy through items 1, 2, and 5; STK then computes the corresponding value for each grid point at the current time. You specify the static definition of dilution of precision through items 3, 4, and 5; STK then computes it via sampling of the dynamic definition. While it is important to mention both, in this exercise you will employ the dynamic behavior to show changing PACC graphics for the coast-to-coast aircraft flight.

Choosing a method

STK can calculate Navigation Accuracy in a number of ways, depending on your task. The following table lists the methods available to you.

Method Description
GACC (Geometric Accuracy) Measures the accuracy of the entire navigation solution. GACC combines the accuracy of the position and clock-related components of the navigation solution.
PACC/PACC (3) (Position Accuracy) Measures only the accuracy associated with the positional portion of the navigation solution.
HACC/HACC (3) (Horizontal Accuracy) Measures the accuracy for the horizontal (latitude/longitude) components of the positional portion of the navigation solution.
VACC/VACC (3) (Vertical Accuracy) Measures the accuracy for the vertical (altitude) components of the positional portion of the navigation solution.
EACC/EACC (3)* (East Accuracy) Measures the accuracy in the Eastern component of the positional portion of the navigation solution.
NACC/NACC (3)* (North Accuracy) Measures the accuracy in the Northern component of the positional portion of the navigation solution.
TACC (Time Accuracy) Measures the accuracy of the time portion of the navigation solution.

*If you select PACC(3), HACC(3), or VACC(3), STK computes the accuracy value even if only three (3) navigation sources are available. It does this by ignoring the clock component of the navigation solution. If four (4) or more sources are available, STK includes the clock component.

The accuracy measure you choose affects the dynamic and static definition of the figure of merit.

Choosing a Type

Although you only need four satellites for the navigation solution, you can use additional satellites to improve the accuracy of the solution. The following table describes the options in the Type field.

Option Value
Over Determined Computes the NavAcc based on all of the currently available assets. If you select this method, you need to involve a minimum of three assets in the navigation solution. If you compute a navigation accuracy based on only three assets, you will be presented with answers to a subset of main options: PACC, HACC, and VACC. Also, note that a navigation accuracy with three assets assumes no uncertainty in time.
Best Four Computes the NavAcc based on the set of four satellites that yields the minimum GACC.
Best N Computes the NavAcc based on the specified number of satellites that yields the minimum GACC. If you select this method, you also need to specify a value for Best N.
Best Four Acc Computes the NavAcc based on the set of four satellites that yields the minimum geometric uncertainty.
Best N Acc Computes the NavAcc based on the set of the specified number of satellites that yields the minimum geometric uncertainty. If you select this method, you also need to specify a value for Best N.

The asset selection strategy you choose affects the dynamic and static definition of the figure of merit.

Choosing the Compute method

You also need to set the method for computing the static definition for navigation accuracy. Use the options in the Compute field, as described in the following table.

The reported values depend on the specific number selected and the allowed number of assets.

Option Value
Minimum Calculates the minimum uncertainty at each point over the entire coverage interval.
Maximum Calculates the maximum uncertainty at each point over the entire coverage interval.
Average Computes Navigational Accuracy based on the specified number of satellites that yields the minimum GACC. If you select this method, you also need to specify a value for Best N.
Percent Below The value of the uncertainty is less than the computed value X% of the time, where X is a Percent Level that you specify.

This option only affects the static definition of the figure of merit.

Specifying Time Step

In the Time Step field, enter the value for STK to use when computing the static value of Navigation Accuracy across the coverage interval.

Define the quality of coverage

You have defined the boundaries of the coverage area, set the resolution or location of the grid points used to fill the bounded area, specified an altitude for the grid, assigned assets, and adjusted the display of coverage graphics. Now it’s time to determine the quality of that coverage.

  1. Insert a Figure of Merit () object using the Insert Default method.
  2. Select ContUS_Cov () in the Select Object dialog box.
  3. Rename the figure of merit PACC.
  4. On the Basic - Definition page, make the following changes: 
    5. Option Value
    Type Navigation Accuracy
    Compute Maximum
    Method PACC
    Type Over Determined
    Time Step 60 sec
  5. Click OK .

Generating a Grid Stats Over Time report

In this lesson, a value of zero (0) is our best value and a value over ten (10) is considered unacceptable. The Grid Stats Over Time report summarizes the minimum, maximum, and average of the figure of merit's dynamic value over the entire grid as a function of time.

  1. Right-click PACC () in the Object Browser.
  2. Select Report & Graph Manager... ().
  3. Select the following:
    4. Option Value
    Object Type Figure of Merit
    Object (Below Object Type) CoverageDefinition/ContUS_Cov/PACC
    Show Reports On
    Show Graphs Off
    Style Installed Styles - Grid Stats Over Time
    Generate as Report/Graph
  4. Click Generate... .

    5. To set up the dynamic contours, you need to identify the widest range of values that were computed. In other words, identify the smallest value from the Min column and the largest value from the Max column. There is an easier way to do this rather than scrolling.

  5. Close the report when you are finished viewing.

Creating a custom report

Next, duplicate the report style used in the previous section and add the required summary information needed to configure the dynamic contours.

  1. Return to the Report & Graph Manager ().
  2. Right-click the Report Style - Grid Stats Over Time.
  3. Click Properties ().

Setting the minimum

Set up the minimum value output.

  1. In the Report Contents list, select Overall Value by Time - Minimum .
  2. Click Options... .
  3. In the Summary Options-Statistics section, select the Min check box.

    4. This will display the Minimum value of the Minimum column of FOM data.

  4. Click OK .

Setting the maximum

Set up the maximum value output.

  1. Select Overall Value by Time - Maximum.
  2. Click the Options... button.
  3. In the Summary Options-Statistics section, select the Max check box.

    4. This will display the Maximum value of the Maximum column of FOM data.

  4. Click OK .
  5. Click OK to accept the property changes.
  6. Click OK  to close the warning message.

Running the report

Now that you have set up the minimum and maximum reporting, run the report.

  1. Expand the MyStyles directory in the Styles list.
  2. Rename the new report "My Grid Stats Over Time."
  3. Select the My Grid Stats Over Time report.
  4. Click Generate... .

    5. Take a look at the new report. You have all of the FOM values listed over time, as well as the summary information. Make a note of the Global Statistics - Min Minimum and Max Maximum values for the ranges for the FOM.

  5. Leave the My Grid Stats Over Time report open, but close the Report & Graph Manager ().

Using dynamic contours

Display the quality of coverage based on the scenario time using dynamic contours.

  1. Open PACC’s () properties ().
  2. Go to the 2D Graphics - Animation page.
  3. Verify that the Show Animation Graphics check box is selected.
  4. Select the Filled Area option.
  5. Set the % Translucency to 15.
  6. Click Apply to accept the changes and keep the Properties Browser open.

Defining the contours

Next, let's define the contour definitions.

  1. In the Display Metric option, select the Show Contours option.
  2. Click the Remove All button in the Level Attributes section.
  3. Enter the following values in the Level Adding area:
    4. Option Value
    Add Method Start, Stop, Step
    Start Integer below Global Statistics - Min Minimum report value
    Stop Integer above Global Statistics - Max Maximums report value
    Step 0.2
  4. Click Add Levels .
  5. Enter the following values in the Level Attributes area:
    6. Option Value
    Color Method Color Ramp
    Start Color Blue
    End Color Red
    Natural Neighbor On
    Sampling Medium Sampling
  6. Click Apply to accept the changes and keep the Properties Browser open.

Displaying the Legend window

Next, let's set up the legend window.

  1. Click Legend... .
  2. Move the Legend window to the upper left side of STK.
  3. Click Layout... .
  4. In the 3D Graphics Window section, select the Show at Pixel Location check box.
  5. Click OK to dismiss the Figure of Merit Legend Layout dialog box.
  6. Resize the legend window, if required, to enable you to see all the legend values.
  7. On the Properties Browser window, click OK .

Getting a better look

You’ve set your color ramp such that blue will represent a low PACC (preferred) and red will represent a high PACC. Now see what PACC values look like over the continental United States.

  1. Bring the 2D Graphics window to the front.
  2. Zoom In on the United_States_of_America () area target.
  3. Reset () the animation.
  4. Play () the animation.

    5. 2D View: Dynamic contours for positional NavAcc

  5. When you are finished, close the Legend window.
  6. Reset () the scenario.
  7. Clear the check box next to PACC () in the Object Browser to remove the graphics.

Performing single-object coverage

Now, you need to perform a single-object coverage analysis on the test flight to determine the PACC value along the actual path from Long Beach to Philadelphia. To evaluate the quality of coverage to a single object in STK, you can use STK's Coverage capability. This capability is available for all types of vehicles, facilities, targets, and sensors. Use the single-object coverage method to determine coverage if you are interested in coverage of a small number of well-defined locations or along the trajectory of a moving object.

The main differences between normal and single-object coverage are:

  • You can use single-object coverage to analyze objects with time-dependent positions.
  • You can only analyze one figure of merit at a time using single-object coverage.

To determine the accuracy of the GPS position throughout Test_Flight’s flight, you need to assign the GPS satellites as assets for that object. To assess the quality of that coverage, in this instance, you would choose a Navigation Accuracy figure of merit type and then review the value of the PACC as Test_Flight flies from Long Beach to Philadelphia.

Assigning assets to a single object

To determine coverage for a single object, you must assign one or several assets for use in coverage computations. For single-object coverage, it is important to have a clear understanding of the asset time periods. Construct the time period for the coverage analysis as the interval of the reference object time period and the asset time period. STK uses assets to calculate whether coverage to the object is possible. If you define a figure of merit for the coverage, STK measures coverage to the object from the assets by the figure of merit values.

  1. Right-click Test_Flight () in the Object Browser.
  2. Select Coverage... ().
  3. Select GPSConstellation from the Assets list.
  4. Click Assign .
  5. Click Compute .

Defining a single-object Figure Of Merit

Position Accuracy (PACC) is a navigation method that measures only the accuracy associated with the positional portion of the navigation solution. In your case, the portion you are interested in is the flight path for Test_Flight.

  1. In the Figure of Merit section, click Define... .
  2. Set the following:
    3. Option Value
    Type Navigation Accuracy
    Compute Maximum
    Method PACC
    Type Over Determined
    Time Step 60 sec
  3. Click OK to return to the Coverage Tool ().
  4. In the Figure of Merit section, take note of the Value field value. This is the maximum value along Test_Flight's route.

Reporting PACC

The FOM Value report shows the PACC value in meters at one-minute intervals for the entire coverage interval. Create that now.

  1. In the Reports section, click FOM Value... .
    • Does the data reflect what you saw when you computed PACC over the continental US airspace?
    • Is it better or worse?
  2. Close the FOM Value report window.

Graphing PACC

You can also graph the FOM value over time.

  1. In the Graphs section, click FOM Value... .

    The graph shows how Test_Flight’s dilution of precision changes as it travels.

  2. Note the minimum and maximum FOM values.
  3. Leave the FOM Value graph open.

Setting up single-object contour graphics

Now, visualize the quality of coverage as Test_Flight flies from Long Beach to Philadelphia.

  1. Return to the Coverage Tool ().
  2. In the Graphics section, click Contours... .
  3. Set the following options:
    4. Option Action
    Show Marker Animation Highlight Select the check box.
    Show FOM Graphics on Vehicle Track Select the check box.

    Both of these options will allow the coverage to appear in the Graphics windows.

  4. In the Level Attributes section, click Remove All .
  5. Enter the following values in the Level Adding area:
    6. Option Value
    Add Method Start, Stop, Step
    Start Integer below minimum FOM graph value
    Stop Integer above maximum FOM graph value
    Step 0.2
  6. Click Add.
  7. Enter the following values in the Level Attributes section:
    8. Option Value
    Use Color Ramp Selected
    Select ramp colors Blue in the first field, Red in the second field
    Line Width Thickest
  8. Click OK to return to the Coverage Tool ().

Getting a better look

  1. Bring the 2D Graphics window to the front.
  2. Reset () the animation.

    3. Test_Flight’s route is colored to reflect the navigational accuracy at points along the flight.

  3. Play () the animation.

    4. As Test_Flight travels along its path, its label color will also reflect the navigational accuracy at that point in time.

    2D View: Contour graphics for Test Flight’s PACC

    Your colors may be different from this view.

Changing your perspective

  1. Bring the 3D Graphics window to the front.
  2. Zoom To Test_Flight ().
  3. Play () the animation.

    4. Test_Flight’s object label and flight path also change color based on the PACC values in 3D.

    3D View: Contour graphics for Test Flight’s PACC

Deactivating assets

You just received a Notice of Advisory to Navstar Users (NANU) stating that GPS_23_SVN76 has been taken offline for some maintenance work. How will that impact your PACC values? Will you have to recreate your STK scenario? Fortunately, coverage objects enable you to keep an asset assigned but switch its status to inactive. This will essentially remove it from calculations. With GPS_23_SVN76 no longer in the picture, will you still meet your design criteria of never exceeding a PACC value of ten (10) meters?

Recomputing PACC for Test Flight

Since you’re already looking at the aircraft’s route, start with recomputing that.

  1. Return to the Coverage Tool ().
  2. Click the expand button () beside GPSConstellation to see the satellites.
  3. Select gps_23_svn76 from the list.
  4. Select Inactive from the status drop-down menu below the Assets list.
  5. Click Compute .
  6. In the Graphs section, click FOM Value... .
    • Do you still meet your PACC requirements for this flight?

Deactivating two or more satellites

Your maximum PACC has not decreased, though some of your values might have changed slightly. This is because GPS is such a robust constellation. To receive a noticeable increase in PACC values, exclude several GPS satellites from the analysis.

  1. Bring the Coverage Tool () to the front.
  2. Click the expand button () beside GPSConstellation.
  3. Select all individual GPS satellites numbered one through ten (1-10) from the list.
  4. Select Inactive from the status drop-down menu below the Assets list for each.
  5. Click Compute .
  6. Bring the FOM Value graph to the front.
  7. Refresh your graph.
    • Did your maximum PACC values increase noticeably?
    • Do you still meet your PACC requirements for this flight?

    If your FOM values have changed significantly, you may need to adjust your FOM contour values. Refer back to Setting up single-object contour graphics for instructions on how to do this.

  8. Close the FOM Value graph.
  9. Click OK to close to the Coverage Tool ().

Recomputing PACC for the Continental US

You have re-examined your PACC requirements for this flight, but what about the rest of the continental United States?

  1. Open ContUS_Cov’s () properties ().
  2. Select the Basic - Assets page.
  3. Click the expand button () beside GPSConstellation.
  4. Select GPS satellites one through ten (1-10) from the list.
  5. Select Inactive from the status drop-down menu beside the Assets list.
  6. Click OK .

Reporting Grid Stats Over Time

Run a new Grid Stats Over Time report and see if the minimum and maximum static values for the figure of merit over the entire grid are still within your target range (PACC < 10).

  1. Open the Report & Graph Manager ().
  2. Select the following:
    3. Option Value
    Object Type Figure of Merit
    Object (Below Object Type) PACC
    Show Reports On
    Show Graphs Off
    Style My Grid Stats Over Time
    Generate as Report/Graph
  3. Click Generate... .
    • Did your maximum PACC values increase noticeably?
    • Does the maximum static value for the figure of merit exceed ten (10) over some portion of the continental United States?
  4. Right-click the first time that the PACC maximum exceeds ten (10) in the My Grid Stats Over Time report.
  5. Open the Time context menu.
  6. Select Set Animation Time.
  7. Bring the 2D Graphics window to the front.
  8. Select the check box beside PACC in the Object Browser.

Adjusting the display of contours

Update the FOM contours so that you can see where or if the PACC values exceed ten (10), because you won’t want to fly your aircraft in that area.

  1. Open PACC’s () properties ().
  2. Select the 2D Graphics - Animation page.
  3. Return to the My Grid Stats Over Time report.
  4. Scroll to the bottom of the report and note the minimum value.
  5. Return to the PACC's () properties ().
  6. In the Level Attributes section, click Remove All .
  7. In the Level Adding section, define the following:
    8. Option Value
    Start value to an integer below minimum grid stats report value
    Stop 10 m
    Step .5 m
  8. Click Add Levels .
  9. Click Apply.

Displaying the Legend window

Next, display the legend window.

  1. Click Legend... .
  2. Click Layout... .
  3. In the 2D Graphics Window section, select the Show at Pixel Location check box.
  4. Click OK to dismiss the Figure of Merit Legend Layout window.
  5. Click OK .

Getting a better look

  1. Bring the 2D Graphics window to the front.

    2. The red on your map is where your position accuracy is over ten (10) meters, based on setting the animation graphics from your report.

  2. Reset () the animation.

Changing your perspective

  1. Bring the 3D Graphics window to the front.
  2. Zoom To United_States_of_America ().
  3. Use the mouse to zoom out until you can see the entire continental United States.
  4. Play () the animation.
  5. When you are finished, Reset () the scenario.

When you finish

  1. Close the My Grid Stats Over Time report.
  2. Close the Report & Graph Manager ().
  3. Save () your work.