Position Accuracy in Mountainous Terrain

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.

Required product install: installation of the Navigation Files Plugin, which is included with STK UI Plugins on the Ansys STK Resource Utilities install media, is required to complete this tutorial. You can obtain the necessary installer by visiting https://support.agi.com/downloads or by contacting AGI Support.

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.

This tutorial requires an internet connection.

Capabilities covered

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

  • STK Pro
  • Coverage

Problem statement

Engineers and operators require a quick way to determine if local terrain is affecting GPS reception for a variety of purposes, such as location, navigation, tracking, mapping, and timing. In this scenario, engineers are performing a training exercise on mountainous terrain in the vicinity of Mount St. Helens. You need to determine the positional uncertainty of the GPS being used to pinpoint their location.

Solution

Use the STK Pro application and the Coverage capability to examine the accuracy of your navigation solution within a specified area based on satellite outages and GPS position uncertainty.

What you will learn

Upon completion of this tutorial, you will understand the following:

  • How to determine navigation accuracy
  • How to obtain a satellite outage file
  • How to use the GPS Satellite Outage tool
  • How to use the Navigation Files Plugin
  • How to use the Grid Inspector tool

Creating a new scenario

First, you must create a new STK scenario, then build from there.

  1. Launch the STK application ().
  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 NavAccuracy
    Start 10 Jan 2020 18:00:00.000 UTCG
    End + 1 day
  4. Click OK when you finish.
  5. Click Save () when the scenario loads.

    6. The STK application automatically creates a folder with the same name as your scenario for you.

  6. Verify the scenario name and location in the Save As dialog box.
  7. Click Save.

Save () often during this lesson!

Turning off streaming terrain

By turning off streaming terrain, you're simulating what you'd see in a setting that doesn't have an internet connection.

  1. Right-click on NavAccuracy () in the Object Browser.
  2. Select Properties () in the shortcut menu.
  3. Select the Basic - Terrain page when the Properties Browser opens.
  4. Clear the Use terrain server for analysis check box in the Terrain Server panel.
  5. Click OK to confirm your change and to close the Properties Browser.

Adding analytical and visual terrain

An STK terrain inlay (.pdtt) file can be used both for analysis and for visualization in the 3D Graphics window. Use a preinstalled terrain inlay file to your scenario using the Globe Manager.

  1. Bring the 3D Graphics window to the front.
  2. Click Globe Manager () on the 3D Graphics window's Globe Manager toolbar.
  3. Click Add Terrain/Imagery () on the Globe Manager Hierarchy toolbar when Globe Manager opens.
  4. Select Add Terrain/Imagery... () in the drop-down menu.
  5. Click the Path ellipsis () when the Globe Manager: Open Terrain and Imagery Data dialog box opens.
  6. Browse to the install directory at C:\Program Files\STK_ODTK 13\Data\Resources\stktraining\imagery when the Browse For Folder dialog box opens.
  7. Click OK to confirm your selection and to close the Browse For Folder dialog box.
  8. Select the StHelens_Training.pdtt check box.
  9. Click Add.
  10. Click Yes when prompted to use StHelens_Training.pdtt for analysis.

Viewing the inlaid terrain

View the terrain inlay in the 3D Graphics window.

  1. Bring the 3D Graphics window to the front.
  2. Right-click on StHelens_Training.pdtt () in the Globe Manager hierarchy.
  3. Select Zoom To () in the shortcut menu.
  4. Use your mouse to move around and zoom in and out to view the terrain in the 3D Graphics window.

Modeling the training area

Use an Area Target object to outline the training area. An Area Target object models a region on the surface of the central body.

Inserting an Area Target object

Use the Area Target Wizard to add an area target and define its boundaries.

  1. Go to the Insert STK Objects () tool.
  2. Select Area Target () in the Select An Object To Be Inserted list.
  3. Select Area Target Wizard () in the Select A Method list.
  4. Click Insert....
  5. Enter ExerciseArea in the Name field when the Area Target Wizard opens.
  6. Click Insert Point four times.
  7. Set the following in the Points panel in the order shown:
    8. Latitude Longitude
    46.00 deg -123.00 deg
    46.00 deg -122.00 deg
    47.00 deg -122.00 deg
    47.00 deg -123.00 deg
  8. Click OK to confirm your changes and to close the Area Target Wizard.

Making the area target visible

Earlier in the scenario you turned off steaming terrain. This affects the view of the Area Target object when using a local terrain file. You need to change your Scenario object's properties so you can view the area target on the terrain by turning the display of surface lines on.

  1. Open NavAccuracy's () Properties ().
  2. Select the 3D Graphics - Global Attributes page.
  3. Open the On Terrain drop-down list in the Surface Lines panel.
  4. Select On.
  5. Click OK to confirm your selection and to close the Properties Browser.

Decluttering labels

Objects located on the surface of the terrain could be covered by the terrain, which makes them unreadable. You can fix this by making a change to the 3D Graphics window's properties to enable Label Declutter.

  1. Bring the 3D Graphics window to the front.
  2. Click Properties () on the 3D Window Defaults toolbar.
  3. Select the Details page.
  4. Select the Enable check box in the Label Declutter panel.
  5. Click OK to confirm your selection and to close the Properties Browser.

Exercise Area

Loading the GPS constellation

Use the Insert STK Objects tool to load the GPS Constellation using orbital elements from GPS almanac files. The STK application creates a constellation that includes all of the satellites in the almanac. Keep in mind, you can also build Constellation objects yourself by loading a Constellation object using the Insert Default or Define Properties method and assigning the individual GPS satellites required for your analysis.

  1. Return to the Insert STK Objects () tool.
  2. Insert a Satellite () object using the Load GPS Constellation () method.

    3. Once loaded, you will see each individual Satellite () object and a Constellation () object containing all of the satellites.

  3. Select gps-23_svn60 () in the Object Browser.
  4. Click Delete () on the Object Browser toolbar.
  5. Click Delete n the Delete Object dialog box to confirm your deletion.

This satellite was decommissioned during the analysis period of the scenario.

Defining a coverage grid

The STK software's Coverage capability allows you to analyze the global or regional coverage provided by one or more assets (facilities, vehicles, sensors, etc.) while considering all accesses. To address area coverage capabilities, the Coverage capability provides you with two STK object classes: Coverage Definition objects and Figure of Merit objects. You will use these objects to analyze navigational accuracy.

Inserting a Coverage Definition object

Before running a coverage analysis, you must first insert a Coverage Definition object. A Coverage Definition object defines a coverage areas for analysis.

  1. Return to the Insert STK Objects () tool.
  2. Insert a Coverage Definition () object using the Insert Default () method.
  3. Right-click on CoverageDefinition1 () in the Object Browser.
  4. Select Rename in the shortcut menu.
  5. Rename CoverageDefinition1 () PosAccCov.

Choosing the Grid Area of Interest

Coverage analyses are based on the accessibility of assets (objects that provide coverage) and geographical areas. For analysis purposes, you can further refine the geographical areas of interest using regions and points. Points have specific geographical locations, and the STK application uses them in the computation of asset availability. Regions are closed boundaries that contain points. The STK application computes accessibility to a region based on accessibility to the points within that region. The combination of the geographical area, the regions within that area, and the points within each region is called the coverage grid. Define your coverage grid to include the whole of the exercise area by updating the Coverage Definition object's grid properties.

  1. Open PosAccCov's () Properties ().
  2. Select the Basic - Grid page when the Properties Browser opens.
  3. Open the Type drop-down list in the Grid Area of Interest panel.
  4. Select Custom Regions.
  5. Open the Area Of Interest drop-down list.
  6. Select Area Targets.
  7. Select ExerciseArea () in the Area Targets list.
  8. Move () ExerciseArea () to the Selected Regions list.

Defining the grid

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. You can determine the spacing between grid points using the Grid Definition options.

  1. Open the Lat/Lon drop-down list in the Grid Definition - Point Granularity panel.
  2. Select Distance.
  3. Enter 1 mi in the Distance field.
  4. Open the Altitude above WGS84 drop-down list in the Point Altitude panel.
  5. Select Altitude above Terrain.
  6. Leave the default point altitude set to 0 km above the terrain.
  7. Click OK to confirm your changes and to close the Properties Browser.

Constraining the coverage grid

Your coverage grid is situated in mountainous terrain. For your analysis to be accurate, each point in your coverage grid needs to take the local terrain into account when computing GPS navigation accuracy. Now that you have defined the grid area, you can specify an object class or a specific object to constrain the points within the grid.

You can constrain the grid using an azimuth-elevation (AzEl) mask to model the effects of the terrain. When computing an AzEl Mask from terrain, terrain blockage is only modeled up to the ground distance specified by the maximum range that was considered when generating the mask. The AzEl Mask constraint leverages a provided or computed AzEl Mask to determine visibility; the mask may be computed from terrain information to be representative of terrain-based visibility restrictions.

You can construct terrain-based AzEl masks by extending a number of rays in directions of constant azimuth outwards from the facility, place, or target location. Obstruction information is stored along each ray. During visibility computations, the STK software uses obstruction information from the two rays that bound the current direction of interest to compute an interpolated visibility metric.

Inserting a Place object for a constraint template

Create a Place object to model the AzElMask constraints, which you can then apply to all the points in the coverage grid.

  1. Return to the Insert STK Objects () tool.
  2. Insert a Place () object using the Define Properties () method.
  3. Select the Basic - Position page when the Properties Browser opens.
  4. Enter the following coordinates in the Position panel:
    5. Option Value
    Latitude 46.5 deg
    Longitude -122.5 deg

    These coordinates are close the centroid location of ExerciseArea.

  5. Click Apply to confirm your changes and to keep the Properties Browser open.

Using an azimuth-elevation mask for analysis

The AzElMask properties enable you to define an AzEl mask for a facility, place, or target.

  1. Select the Basic - AzElMask page.
  2. Set the following options:
    3. Option Value
    Use Terrain Data
    Max range to consider 160 km
    Use Mask for Access Constraint Selected

    Using Terrain Data automatically creates and stores an AzEl mask file, which is an ASCII text file that is formatted for compatibility with the STK software and ends in an .aem extension, into your scenario folder. Selecting Use Mask for Access Constraint enables the AzEl Mask constraint located on the Constraints - Active page. Using the AzElMask constraint constrains access to a 360-degree field of view around the object being constrained.

  3. Click OKto confirm your changes and to close the Properties Browser.
  4. Rename Place1 () ConstraintTemplate.
  5. Clear the check box for ConstraintTemplate () in the Object Browser.

You don't need to visualize ConstraintTemplate in the 2D or 3D Graphics windows. The location of the constraints template is unimportant, so long as it is somewhere on the terrain within the area target boundary. You are going to use it to constrain all the points in the coverage grid.

Applying the constraint to the coverage definition

You have a constraint source and you have a defined coverage area. You need to associate the constraint with the points in the grid to which the constraint will be applied.

  1. Open PosAccCov's () Properties ().
  2. Select the Basic - Grid page when the Properties Browser opens.
  3. Click Grid Constraint Options... in the Grid Definition panel.
  4. Open the Reference Constraint Class drop-down list in the Grid Point Access Options panel when the Grid Constraint Options dialog box opens.
  5. Select Place.
  6. Select the Use Object Instance check box.
  7. Select ConstraintTemplate in the list.
  8. Click OK to confirm your selection and to close the Grid Constraint Options dialog box.
  9. Click Apply to confirm your changes and to keep the Properties Browser open.

Using satellite outage files

You've defined and constrained the area within which you'd like to analyze coverage. The next step is to identify your assets. The GPSConstellation Constellation object is the asset with which you want to assess the quality of your coverage. Prior to assigning the GPSConstellation as your asset, however, you need to check for any outages. Satellite outage files (SOFs) provide information about GPS satellite outages. Taking these outages into account is crucial for obtaining an accurate navigation error prediction. Without considering outages, your navigation errors may be smaller than actually observed.

Connecting to the United States Coast Guard Navigation Center website

The U.S. Space Force produces a new SOF each time a new outage is completed, experienced or predicted. You can download the current SOF from U.S. Coast Guard Navigation Center. The SOF provides GPS outage information for historical, current and predicted outages. The SOF provides GPS outage information for historical, current and predicted outages. The historical outages go back to 1998.

  1. Open your preferred web browser.
  2. Navigate to the U.S. Coast Guard's Navigation Center's GPS NANUS, Almanacs, OPS Advisories, & SOF page at https://www.navcen.uscg.gov/gps-nanus-almanacs-opsadvisories-sof.
  3. Scroll down to the Satellite Outage File (SOF) section.
  4. Click the Current SOF - .sof link to download the most recent SOF file, current_sof.sof.
  5. Close your browser.

Copying the outage file to your scenario folder

Copy the SOF to you scenario folder.

  1. Navigate to the location of the downloaded current_sof.sof file in Windows File Explorer.
  2. Copy current_sof.sof.
  3. Select Documents in the navigation pane.
  4. Navigate to your scenario folder (for example, C:\Users\<username>\Documents\STK_ODTK 13\TLE_Almanac_Files).
  5. Paste current_sof.sof in your scenario folder.
  6. Close Windows File Explorer.

If you chose a non-default folder in which to store your scenarios during the STK application install, you will need to place the current_sof.sof file in your custom location.

Using the GPS Satellite Outage tool

Use the GPS Satellite Outage tool, which is included with the Navigation Files Plugin, to use the GPS satellite outage information from the SOF in your analysis.

  1. Return to the STK application.
  2. Right-click on GPSConstellation () in the Object Browser.
  3. Select Constellation Plugins in the shortcut menu.
  4. Select Add GPS Satellite Outages in the Constellation Plugins submenu.
  5. Increase the size of the GPS Satellite Outage tool window when it opens.

    6. This will allow you to see the full extent of the tool.

  6. Click the ellipsis () next to the Or browse for a local Satellite Outage File (SOF) field in the Select GPS Satellite Outage Data panel.
  7. In the Open dialog box, browse to the location of the SOF file (for example, C:\Users\<username>\Documents\STK_ODTK 13\TLE_Almanac_Files).
  8. Select current_sof.sof.
  9. Click Open.

A message noting "Satellite Outage Data Loaded" will appear on the bottom of the Select GPS Satellite Outage Data panel.

Applying the outages to the GPS Constellation

With your SOF file loaded, apply the outage to the GPS constellation.

  1. Ensure GPSConstellation is selecting in the Apply to Which GPS Constellation? list.
  2. Click Apply.
  3. Read the message that satellite outage data has been updated in the Update Complete dialog box.
  4. Click OK to close the message.
  5. Review the outage information in the Outage Results Applied panel.

    6. The outages for your scenario are determined by your scenario's time frame. Once applied, the text box at the bottom of the tool will list any outages for your scenario. If an outage is found, the tool will list the interval of the outage. The identification of the GPS satellite is based on the PRN assigned to it, which is matched to the space vehicle identifier (SVID) value in the SOF.

    GPS Outage found

    If an outage is reported, the STK application will remove the satellite from your analysis by creating a temporal constraint. You can open the satellite's properties and go to the Constraints - Active page. An Intervals constraint will already exist in the Active Constraints list. In the Intervals constraints Constraint Properties list, the outage interval from the SOF will automatically be loaded.

  6. Click OK to close the GPS Satellite Outage tool.

Assigning coverage assets

Now that you've added the SOF to the GPS Constellation use it as the asset providing coverage in your coverage definition. The Coverage Definition Assets properties enable you to assign the STK objects used to provide coverage.

  1. Return to PosAccCov's () Properties ().
  2. Select the Basic - Assets page.
  3. Select GPSConstellation () in the Assets list.
  4. Click Assign.
  5. Click Apply to confirm your selection and to keep the Properties Browser open.

Turning off the automatic re-computation of accesses

The STK application 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 The Coverage Definition's object's Advanced properties allow you to adjust the manner in which access information is stored and computed.

  1. Select the Basic - Advanced page.
  2. Clear the Automatically Recompute Accesses check box in the Access panel.
  3. Click OK to confirm your change 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 PosAccCov () in the Object Browser.
  2. Select the CoverageDefinition menu in the Menu Bar.
  3. Select Compute Accesses in Parallel in the CoverageDefinition menu.

Since you have a large amount of grid points and you're using a terrain constraint, parallel computing will speed up your analysis. You can view the progress bar in the lower-right corner of STK. Be patient. This could take a few minutes depending on your computer's specifications.

Measuring the accuracy of a navigation solution

Navigation accuracy measures the uncertainty of a navigation solution based on one-way range measurements from a set of transmitters. Most often, the transmitters are those on board Global Positioning System (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.

You can evaluate navigation accuracy by attaching a Figure of Merit object to the coverage definition of interest. A Navigation Accuracy Figure of Merit considers the effect of the number of measurements (of those satellites visible at each moment in time), the geometry of the transmitters and the uncertainty in the one-way range measurements in the computation of the uncertainty in the navigation solution. The uncertainty in the one-way range measurements may be specified as a constant value or as a function of the elevation angle on a transmitter basis.

Inserting a Figure of Merit object

Attach a Figure of Merit object to the PosAccCov Coverage Definition.

  1. Return to the Insert STK Objects () tool.
  2. Insert a Figure Of Merit () object using the Insert Default () method.
  3. Select PosAccCov () when the Select Object dialog box opens.
  4. Click OK to confirm your selection and to close the Select Object dialog box.
  5. Rename FigureOfMerit1 () NavAcc.

Measuring the accuracy of a navigation solution

To evaluate coverage quality, you will first need to set basic parameters that determine the way in which quality is computed. This involves choosing the method for evaluating the quality of coverage provided, setting measurement options and identifying the criterion needed to achieve satisfactory coverage.

  1. Open NavAcc's () Properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.
  3. Open the Type drop-down list in the Definition panel.
  4. Select Navigation Accuracy.
  5. Set the following Navigation Accuracy options:
    6. Option Value
    Compute Maximum
    Method PACC
    Type Over Determined
    Time Step 60 sec
  6. Click Apply to confirm your changes and to keep the Properties Browser open.

Viewing navigation uncertainties

Uncertainty exists in any navigation solution. You cannot eliminate uncertainty; you can, however, account for it.

  1. Click Uncertainties... in the Definition panel.
  2. View the Asset Range Uncertainty and Receiver Range Uncertainty for your GPS satellites when the Figure of Merit NA Uncertainties Model dialog box opens.

    3. The STK application defaults the Asset and Receiver Range Uncertainties to one meter and zero meters for each satellite.

  3. Click Cancelto close the Figure of Merit NA Uncertainties Model dialog box.
  4. Click OK to close the Properties Browser.

Checking asset range uncertainty

Navigation uncertainties are directly related to your GPS receiver's navigation accuracy. Using Prediction Support Files (PSFs), you can model your navigation accuracy with higher fidelity. PSFs provide statistical error data for both GPS ephemeris and satellite clock errors. Knowing these uncertainties helps provide a better prediction of your GPS receiver's accuracy. The STK application has the ability to use this data from the Navigation Accuracy Figure of Merit properties page. This plugin helps fill that data in, and uses the best data for the scenario time period. Additionally, this plugin can scale the data to achieve a desired level of confidence in the results.

Opening the Navigation Files Plugin

The GPS constellation experiences errors in its ephemeris (position in space) and clock data that translate into positioning errors in GPS receivers.

  1. Right-click on NavAcc () in the Object Browser.
  2. Select FigureOfMerit Plugins in the shortcut menu.
  3. Select Add Navigation Uncertainties in the FigureOfMerit submenu.

You can also access the Navigation Files Plugin and the GPS Satellite Outage tool through the Navigation Files Support toolbar (). You can enable the toolbar by selecting the Toolbars submenu in the View menu, then selecting Navigation Files Support.

Using the Navigation Files Plugin

Use the Navigation Files Plugin to model the actual uncertainties for the analysis time period of this scenario. There is a Prediction Support File (PSF) located in the STK application install that you can use for this scenario.

  1. Increase the size of the Add Navigation Uncertainties window when it opens.
  2. Click the ellipsis () beside the Browse for a local Prediction Support File (PSF) field in the GPS Satellites panel.
  3. Browse to C:\Program Files\AGI\STK_ODTK 13\Data\Resources\stktraining\samples.
  4. Select uncertainties.psf.
  5. Click Open.

    6. Note that a message reading "Uncertainty Data Loaded" has appeared in the GPS Satellites panel.

  6. Ensure the PosAccCov/NavAcc FOM check box is selected in the Apply to Which Navigation FOMs? panel.
  7. Click Apply.

    8. An Asset Anomalies message appears, noting that "Assets with these PRNS are not in the PSF File and have not been updated: 4." The message refers to gps-04_svn74. This is the satellite with the reported outage.

  8. Click OK to confirm and to close the Asset Anomalies dialog box.
  9. Click OK to close Update Complete dialog box.
  10. Click OK to close the Add Navigation Uncertainties window.

Viewing the updated uncertainties

You can view the updated uncertainties that you applied to your figure of merit.

  1. Open NavAcc's () Properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.
  3. Click Uncertainties... in the Definition Panel.
  4. Look in the Asset Range Uncertainty panel in the Figure of Merit NA Uncertainties Model dialog box.

    5. You will notice the constant value for each satellite has been updated with values from the PSF file.

  5. Look in the Receiver Range Uncertainty panel.

    6. The values have likewise been adjusted.

  6. Click Cancel to close the Figure of Merit NA Uncertainties Model dialog box.
  7. Click Cancel to close the Properties Browser without making any changes.

Assessing position accuracy with a custom report

You can assess the position accuracy by customizing an installed report style in the Report & Graph Manager.

Duplicating the installed report style

A 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. This is a good basis for the data you're seeking.

  1. Right-click on NavAcc () in the Object Browser.
  2. Select Report & Graph Manager... () in the shortcut menu.
  3. Right-click on the Grid Stats Over Time () report in the Installed Styles () folder of the Styles panel when the Report & Graph Manager opens.
  4. Select Duplicate () in the shortcut menu.

Setting the minimum summary option

Summary options allow you to refine a report to make it easier to find minimum and maximum values for selected data provider elements. Start by setting the minimum summary option.

  1. Select the Content page when the Properties Browser opens.
  2. In the Report Contents list, select Overall Value by Time - Minimum.
  3. Click Options....
  4. In the Summary Options - Statistics panel, select the Min check box when the Options dialog box opens.

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

  5. Click OK to confirm your change and to close the Options dialog box.

Setting the maximum summary option

Now, set up the maximum summary option.

  1. Select Overall Value by Time - Maximum.
  2. Click Options....
  3. In the Summary Options-Statistics panel, select the Max check box when the Options dialog box opens.

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

  4. Click OK to confirm your change and to close the Options dialog box.
  5. Click OK to confirm your selections and to close the Properties Browser.

Generating the custom report

Rename and generate your custom report.

  1. Right-click on Grid Stats Over Time () in the My Styles () folder.
  2. Select Rename in the shortcut menu.
  3. Rename Grid Stats Over Time () My Grid Stats Over Time.
  4. Click Generate....

    5. Be patient. The report could take a couple of minutes to generate.

  5. Scroll through the report.
  6. Look at the different sections.

    7. First is the Coverage Properties section, which simply shows values from the Coverage Definition object. Next is the FOM properties section, which shows the asset range uncertainties for each GPS satellite and the minimum, maximum, and average uncertainties in meters over time.

    Grid Stats Over Time

  7. Scroll to the bottom of the report.

    8. You will see the min minimum, max maximum, and average uncertainties in meters.

    Global Statistics

    The min and max values can be useful when deciding on contour levels for visualizing data.

  8. Close the report and the Report & Graph Manager.

Visualizing the navigation uncertainties

You can set the display of coverage results in the 2D and 3D Graphics windows, with animated contour graphics depicting the various levels of navigation accuracy provided by the GPS constellation over the course of the 24-hour analysis period.

Specifying contour graphics

Specify how levels of coverage quality display in both the 2D and 3D Graphics windows using contour graphics. Contour levels represent the gradations in coverage quality and can be displayed for both static and animation values of the figure of merit. Since these values change over the 24-hour analysis period, you'll focus on animation graphics.

  1. Open NavAcc's () Properties ().
  2. Select the 2D Graphics - Animation page when the Properties Browser opens.
  3. Enter 30 in the Filled Area - % Translucency field in the Show Points As panel.
  4. Select the Show Contours option in the Display Metric panel.
  5. Click Remove Allin the Level Attributes panel.
  6. Enter the following options in the Level Adding panel:
    7. Option Value
    Level Adding - Start 0 m
    Level Adding - Stop 10 m
    Level Adding - Step 1 m

    For the purposes of this exercise, any range uncertainty greater than 10 meters will be considered unacceptable.

  7. Click Add Levels.
  8. Set the following in the Level Attributes panel:
    9. Option Value
    Color Method Color Ramp
    Start Color Blue
    End Color Red
  9. Select the Natural Neighbor option in Contour Interpolation (points must be filled) panel.
  10. Click Apply to confirm your changes and to keep the Properties Browser open.

Displaying the Legend window

Insert a legend display in the 2D and 3D Graphics windows.

  1. Click Legend... in the Level Attributes panel.
  2. Click Layout... when the Animation Legend for NavAcc window opens.
  3. Select the Show at Pixel Location check box in the 2D Graphics Window panel when the Figure of Merit Legend Layout dialog box opens.
  4. Select the Show at Pixel Location check box in the 3D Graphics Window panel.
  5. Enter Meters in the Title field in the Text Options panel.
  6. Enter 0 in the Number Of Decimal Digits field.
  7. Enter 50 in the Color Square Width (pixels) field in the Range Color Options panel.
  8. Click OK to confirm your changes and to close the Figure of Merit Legend Layout dialog box.
  9. Close the Animation Legend for NavAcc window.
  10. Click OK to close the Properties Browser.

Viewing the navigation uncertainties in the 3D Graphics window

During the 24 hour period, you can visualize, based on color contours, how your range uncertainties are changing. Any time an area turns red, you are at or above your cutoff uncertainty of 10 meters.

  1. Bring the 3D Graphics window to the front.
  2. Click Start () in the Animation toolbar to animate your scenario.

    3. Range Uncertainties

    You can see the range uncertainties change over time.

  3. Click Reset () when finished.

Inspecting regions and points with the Grid Inspector

If you know exactly where the test team is located, you can use the Grid Inspector tool, which enables you to focus more closely on a region or point within a coverage grid, to learn more about a specific location, furthering your analysis efforts.

Opening the Grid Inspector tool

Open the Grid Inspector with the NavAcc Figure of Merit object.

  1. Bring the 2D Graphics window to the front.
  2. Zoom In () to the exercise area.
  3. Clear the NavAcc () check box in the Object Browser.

    4. This enables you to see the grid points more easily inside the area target.

  4. Right-click on NavAcc () in the Object Browser.
  5. Select FigureOfMerit in the shortcut menu.
  6. Select Grid Inspector... () in the FigureOfMerit submenu.

The Grid Inspector tool will open.

Grid Inspector TOol

Using the Grid Inspector

For a Figure Of Merit object , the Grid Inspector provides detailed quality-related information.

  1. Click a grid point in the 2D Graphics window.
  2. Look at the information in the Messages panel.

Reviewing the Point Figure of Merit graph

There are several reports and graphs available for figures of merit through the Grid Inspector.

  1. Click Point FOM ... in the Graphs panel.

    2. A Point Figure of Merit graph provides a detailed summary of the dynamic value for the Figure Of Merit over time for the point selected in the Grid Inspector. This graph is very useful. Assume that 10 meters is your accuracy cutoff.

  2. Place your cursor at the top of any spikes in your graph.
  3. Read the date and time that the navigation accuracy spikes.
  4. Close the Point Figure of Merit graph when you are finished.
  5. If desired, try different points.
  6. Click Cancel to close the Grid Inspector tool when finished.

Saving your work

Clean up your workspace and save your work.

  1. Close any open reports, properties and tools.
  2. Save () your work.

Summary

You began by adding a local terrain inlay file (*.pdtt) to be used both analytically and visually. Next, you defined the area of interest (ExerciseArea) by inserting an Area Target object. After inserting the GPS satellite constellation, you created a Coverage Definition object that focused on ExerciseArea. Prior to assigning coverage assets, you download a satellite outage file and used the GPS Satellite Outage tool to create a temporal constraint on one of the GPS satellites. After assigning the GPS constellation as a coverage asset and computing coverage, you inserted a Figure of Merit object and set it up to report on Navigation Accuracy using the Position Accuracy figure of merit type. To add realism to the analysis, you downloaded the actual range uncertainty file for the analysis time period and used the Navigation Files plugin to quickly update the asset range and receiver range uncertainties. Next you created a custom report that added minimum and maximum global statistics to an existing report style. After adding contour colors specific to positional accuracy to both the 2D and 3D Graphics windows, you explored the function of the Grid Inspector tool.