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 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 or the ability to transfer files from an Internet connected computer to a computer not connected to the Internet. The files will be downloaded from the United States Coast Guard Navigation Center.

Capabilities covered

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

  • STK Pro
  • Coverage

Problem statement

You are testing a new radar system that is designed to provide highly accurate location information for aircraft flying through the United States' airspace. To determine the fidelity of the new radar system, you want to compare an aircraft's Global Positioning System (GPS)-determined location information to the radar tracking information. You will be making a test flight, during which you will be monitoring the aircraft's location via GPS. To accurately test the radar system, you need to simulate the GPS positional navigational accuracy (PACC) over the contiguous United States and determine the Horizontal Dilution of Precision (HDOP) along the aircraft's flight route. Your test will take place today.

Solution

Use the Navigation Files Plugin to model the current GPS satellite constellation and the Coverage capability to determine PACC at the planned flight altitude of 20,000 feet over the contiguous United States during a 24-hour period. Next, measure the GPS HDOP at altitude over the route of the test flight, which will fly from Long Beach, California to Philadelphia, Pennsylvania.

What you will learn

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

  • How to download and use GPS almanac files in your analysis
  • How to download and use satellite outage files in your analysis
  • How to measure only the accuracy associated with the positional portion of the navigation solution
  • How to use the Coverage tool to perform single-object coverage analysis
  • How to measure the dilution of precision for the horizontal (latitude / longitude) components of the positional portion of the navigation solution

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

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

  1. Launch the STK application ().
  2. Click Create a Scenario when the Welcome to STK dialog box opens.
  3. Enter the following in the STK: New Scenario Wizard:
    4. Option Value
    Name PACC_HDOP
    Start Today
    Stop + 1 day
  4. Click OK to confirm your changes, close the STK: New Scenario Wizard and load your scenario.
  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 scenario!

Turning off streaming terrain

By default, the STK application connects to the Ansys Geospatial Data Cloud to distribute Earth terrain data for analysis and visualization. Turn off streaming terrain, as it is not required for your analysis.

  1. Right-click on PACC_HDOP () 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.

Preparing the 2D Graphics window for situational awareness

Before you begin to define your analysis area, clear all the background imagery that you don't need in the 2D Graphics window.

  1. Bring the 2D Graphics window to the front.
  2. Click Properties () in the 2D Window Defaults toolbar.
  3. Select the Imagery page when the Properties Browser opens.
  4. Clear the Show check box in the Background Image panel.
  5. Click OK to confirm your change and to close the Properties Browser.

Defining the analysis area

Use an Area Target object and the Select Countries and US States method to outline the contiguous United States in both the 2D and 3D Graphics windows so you can identify and focus on that area of the Earth.

  1. Bring the Insert STK Objects () tool to the front.
  2. Select Area Target () in the Select An Object To Be Inserted list.
  3. Choose Select Countries and US States () in the Select A Method list.
  4. Click Insert....
  5. Select United_States_of_America in the list of pre-built Area Target objects when the Select Countries and US States dialog box opens.
  6. Ensure the Primary Area Only option is selected in the Insert panel.

    7. Selecting Primary Areas Only during the creation process will ensure that only the contiguous United States is outlined. If you had selected All Areas, an area target representing each of the Hawaiian islands, Alaska and all its islands, etc. would have been imported.

  7. Click Insert.
  8. Click Close to close the Select Countries and US States dialog box.

Viewing the contiguous United States in the 2D Graphics window

Use the Zoom In function in the 2D Graphics window to outline the contiguous United States.

  1. Click Zoom In () in the 2D Window Defaults toolbar.
  2. Using your left mouse button, draw a box around the contiguous United States.

    3. 2D View of the contiguous United States

Viewing the contiguous United States in the 3D Graphics window

Use the Zoom In function in the 3D Graphics window to outline the contiguous United States.

  1. Bring the 3D Graphics window to the front.
  2. Click and grab to turn the Earth until you can see the contiguous United States.
  3. Click Zoom In () in the 3D Window Defaults toolbar.
  4. Using your left mouse button, draw a box around the contiguous United States.

3D View of the contiguous United States

Clearing the Area Target's label

You don't need to display the Area Target's United_States_of_America label or the visible centroid. Clear those options by updating its 2D Graphics Attributes.

  1. Open United_States_of_America's () Properties ().
  2. Select the 2D Graphics - Attributes page when the Properties Browser opens.
  3. Clear the following check boxes in the Inheritable Settings panel:
    • Inherit from Scenario
    • Show Label
    • Show Centroid
  4. Click OK to confirm your changes and to close the Properties Browser.

Downloading the current satellite almanac file to propagate the GPS satellites

The GPS almanac is a set of data that every GPS satellite transmits, and it includes information about the state (health) of the entire GPS satellite constellation and coarse data on every satellite's orbit. The U.S. Coast Guard makes GPS almanacs in the System Effectiveness Model (SEM) format available on a routine basis — currently once per day. SEM almanac files are ASCII messages containing the almanac information and usually have a .al3 file extension. Note that the Coast Guard almanac data includes only ephemeris and clock data, and does not include covariance or the high-accuracy GPS to UTC time correlation.

  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 Almanacs (YUMA & SEM) section.
  4. Click the Current SEM Almanac - .al3 link to download the most recent almanac file, current_sem.al3.
  5. Keep your web browser open.

Copying the almanac file to your scenario folder

Copy the downloaded almanac file to your scenario folder.

  1. Go to the location of your downloaded SEM almanac in Windows File Explorer.
  2. Right-click on current_sem.al3.
  3. Select Copy in the shortcut menu.
  4. Select Documents in the navigation pane.
  5. Navigate to your scenario folder (for example, C:\Users\<username>\Documents\STK_ODTK 13\PACC_HDOP).
  6. Right-click in the content pane.
  7. Select Paste in the shortcut menu.

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_sem.al3 file in your custom location.

Downloading the current satellite outage file

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. 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.

  1. Return to the GPS NANUS, Almanacs, OPS Advisories, & SOF page at https://www.navcen.uscg.gov/gps-nanus-almanacs-opsadvisories-sof.
  2. Scroll down to the Satellite Outage File (SOF) section.
  3. Click the Current SOF - .sof link to download the most recent SOF file, current_sof.sof.
  4. Close your web browser.

Copying the outage file to your scenario folder

Copy the SOF to your 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\PACC_HDOP).
  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.

Inserting the GPS Constellation

Use the From GPS Almanac method to insert the GPS constellation from the almanac file you downloaded from the U.S. Coast Guard.

Inserting a new Satellite object

Insert the GPS constellation by inserting a Satellite object.

  1. Bring the Insert STK Objects tool () to the front.
  2. Insert a Satellite () object using the From GPS Almanac () method.

Using the From GPS Almanac method

The From GPS Almanac method allows you to specify almanac information, scenario information, and GPS satellite selection choices.

  1. Select the Automatic - File method in the Catalog Source panel when the Insert From GPS Almanac dialog box opens.

    2. This option will direct STK to reload the specified almanac file every time the satellite is propagated. When you load that scenario, it will repropagate the satellites based on the date. You will however need to ensure you update your almanac file accordingly when you reopen your scenario in the future.

  2. Click the ellipsis () next to the File field.
  3. Navigate to your scenario folder (for example, C:\Users\<username>\Documents\STK_ODTK 13\PACC_HDOP) when the Select GPS Almanac File dialog box opens.
  4. Select current_sem_al3.
  5. Click Open to confirm your selection and to close the Select GPS Almanac File dialog box.
  6. Click Select All.

    7. All GPS satellites found in the almanac will appear on the left of the Insert from GPS Almanac dialog box.

  7. Select the Create Constellation from Selected Satellites check box in the Constellation panel.
  8. Enter GPSConstellation in the Name field.
  9. Click Insert to insert the current GPS satellites and to create the GPS Constellation.
  10. Click Close to close the Insert From GPS Almanac dialog box.

Clearing GPS satellites' orbit tracks

You don't need to visualize the orbits and ground tracks of the GPS constellation's satellites. Clear them by batch updating their 2D Graphics attributes.

  1. Multi-select all of the Satellite () objects in the Object Browser.
  2. Click Properties () on the Object Browser toolbar.
  3. Select the 2D Graphics - Attributes page when the Properties Browser opens.
  4. Clear the Inherit from Scenario check box in the Inheritable Settings panel.
  5. Clear the Show Orbit check box.
  6. Clear the Show Ground Track check box.
  7. Click OK to confirm your changes and to close the Properties Browser.

Loading the SOF into the GPS Satellite Outage tool

Use the GPS Satellite Outage tool to use the GPS satellite outage information from the SOF in your analysis.

  1. Select the View menu.
  2. Select Toolbars.
  3. Select Navigation Files Support in the Toolbars submenu.

    4. This will display the Navigation Files Support toolbar.

  4. Select GPSConstellation () in the Object Browser.
  5. Click Add GPS Satellite Outages () on the Navigation Files Support toolbar.
  6. Increase the size of the Add GPS Satellite Outages window when it opens, if needed, to see the full extent of the tool.
  7. Click the ellipsis () next to the Browse for a local Satellite Outage File (SOF) field in the Select GPS Satellite Outage Data panel.
  8. In the Open dialog box, browse to the location of the SOF file (for example, C:\Users\<username>\Documents\STK_ODTK 13\PACC_HDOP).
  9. Select current_sof.sof.
  10. 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 selected 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.

    No GPS Outages

    If an outage is found, the tool will list the interval of the outage. The identification of the object 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 Interval Constraint's Constraint Properties list, the outage interval from the SOF will automatically be loaded.

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

Defining the coverage area

The STK software's Coverage capability enables you to analyze the global or regional coverage provided by one or more assets (e.g. vehicles, facilities, sensors) while considering all access constraints. You want to assess coverage of the contiguous 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 object

The first thing you need to do is define the coverage area using a Coverage Definition object. Coverage Definition objects enable you to define and maintain an area of coverage, to define the STK objects providing coverage for the area, to define the time period of interest, and to calculate accesses to the region. Instead of entering the latitude and longitude bounds of the coverage region, you can use the boundaries of United_States_of_America Area Target object to define the boundaries of the coverage definition.

  1. Bring the Insert STK Objects () tool to the front.
  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 () ContUS_Cov.

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 contiguous United States by updating the Coverage Definition object's grid properties.

  1. Right-click on ContUS_Cov () in the Object Browser.
  2. Select Properties () in the shortcut menu.
  3. Select the Basic - Grid page when the Properties Browser opens.
  4. Open the Type drop-down list in the Grid Area of Interest panel.
  5. Select Custom Regions.
  6. Open the Area Of Interest drop-down list.
  7. Select Area Targets.
  8. Select United_States_of_America () in the Area Targets list.
  9. Move () United_States_of_America () to the Selected Regions list.

Defining the point granularity and altitude

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. For this simulation, you will analyze navigation accuracy every one degree at an altitude of 20,000 feet.

  1. Enter 1 deg in the Lat/Lon field in the Grid Definition - Point Granularity panel.
  2. Enter 20000 ft in the Altitude above WGS84 field in the Point Altitude panel.
  3. Click Apply to confirm your changes and to keep the Properties Browser open.

Assigning the coverage assets

The Coverage Definition Assets properties enable you to assign the STK objects used to provide coverage.

  1. Select the Basic - Assets page.
  2. Select GPSConstellation () in the Assets list.
  3. Click Assign.
  4. 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 the STK application computes coverage, you can 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 Apply to confirm your selection and to keep the Properties Browser open.

Raising the grid points visually in the 3D Graphics window

You can visualize the grid points and coverage at 20,000 feet in the 3D Graphics window by updating the Coverage Definition object's 3D Graphics Attributes. Even if you don't raise the grid points visually, analytically you'll still be analyzing coverage at 20,000 feet.

  1. Select the 3D Graphics - Attributes page.
  2. Select the Show at Altitude check box in the Fill Options panel.
  3. Click OK to accept your changes and to close the Properties Browser.

Viewing the coverage grid in the 2D and 3D Graphics windows

Place the 2D and 3D Graphics windows side-by-side.

  1. Select the Window menu in the Menu Bar.
  2. Select Tile Vertically in the Window menu.

coverage grid in the 2D and 3D graphics windows

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 ContUS_Cov () in the Object Browser.
  2. Select the CoverageDefinition menu in the Menu Bar.
  3. Select Compute Accesses in CoverageDefinition menu.

Improving the view in the 2D and 3D Graphics windows

Clear the grid points from the 2D and 3D Graphics windows. This is purely aesthetic and isn't required for analysis.

  1. Open ContUS_Cov's () Properties ().
  2. Select the 2D Graphics - Attributes page when the Properties Browser opens.
  3. Clear the Show Points check box in the Grid panel.
  4. Click OK to confirm your change and to close the Properties Browser.

Measuring the accuracy of a navigation solution with a Figure of Merit

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 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 accuracy you are analyzing does not apply to any GPS devices. You are measuring the expected accuracy at 20,000 feet. A standard applied by the U.S. Government is less than or equal to 2.0 meters.

You can evaluate navigation accuracy by attaching a Figure of Merit object to the Coverage Definition object of interest. The Navigation Accuracy Figure of Merit type 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 ContUS_Cov Coverage Definition object.

  1. Bring the Insert STK Objects () tool to the front.
  2. Insert a Figure Of Merit () object using the Insert Default () method.
  3. Select ContUS_Cov () 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 () PACC.

Selecting a definition type

Select Navigation Accuracy as the definition type.

  1. Open PACC's () Properties ().
  2. Select the Basic - Definition page when the Properties Browser opens.
  3. Open the Type drop-down list.
  4. Select Navigation Accuracy.

Choosing a static definition of navigation accuracy

You need to set the method for computing the static definition for navigation accuracy using the options in the Compute field. The reported values depend on the specific number selected and the allowed number of assets.

  1. Open the Compute drop-down list.
  2. Select Maximum.

This will compute maximum uncertainty at each point over the entire coverage interval.

Selecting a specific navigation accuracy measure

Navigation accuracy can be calculated in a number of ways, depending on your task. You will use PACC (position accuracy). PACC measures only the accuracy associated with the positional portion of the navigation solution.

  1. Open the Method drop-down list.
  2. Select PACC.

Choosing the allowed number of assets

Although four satellites are needed for the navigation solution, additional satellites can be used to improve the accuracy of the solution.

  1. Open the Type drop-down list to view the various options.
  2. Leave the default type of Over Determined.

The Over Determined option computes the navigation accuracy 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.

Specifying a time step

In the Time Step field, enter the value to be used during the sampling of the dynamic definition for use in the static definition.

  1. Enter 60 sec in the Time Step field.
  2. Click Apply to confirm your changes and to keep the Properties Browser open.

Generating a Grid Stats Over Time report

You want to visualize the quality of coverage in the 2D and 3D Graphics windows. To accurately display contour levels for figures of merit in the 2D and 3D Graphics windows, you should know the approximate range of values for the current Figure Of Merit. You can use the Report & Graph Manager for the Figure Of Merit to identify the widest range of values that were computed. Generate a Grid Stats Over Time report, which 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 on PACC () in the Object Browser.
  2. Select Report & Graph Manager... () in the shortcut menu.
  3. Select the Grid Stats Over Time () report in the Installed Styles () folder of the Styles panel when the Report & Graph Manager opens.
  4. Click Generate....
  5. Scroll through the report to see the data.

    6. To set up the contour levels, you need to 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.

  6. Close the report when you are finished viewing it.

Creating a custom report

You can modify the contents and format of an existing report style by changing its Content Properties. Because installed styles are locked and cannot be changed. duplicate the Grid Stats Over Time report so you can add the required summary information needed to configure the dynamic contours.

  1. Return to the Report & Graph Manager ().
  2. Select the Grid Stats Over Time () report in the Installed Styles () folder of the Styles panel.
  3. Click Duplicate Style () on the Styles toolbar.

Adding Summary Options

Set up the Summary Options to include the minimum and maximum value outputs. The min and max values are found after doing an iterative search using the report values as an initial sample set and then sub-sampling for the global min and max. The values are found to within a time tolerance of 5 msec.

  1. Select the Content page when the Properties Browser opens.
  2. Select Overall Value by Time-Minimum in the Report Contents list.
  3. Click Options....
  4. Select the Min check box in the Summary Options - Statistics panel when the Options dialog box opens.
  5. Click OK to confirm your selection and to close the Options dialog box.
  6. Select Overall Value by Time-Maximum in the Report Contents list.
  7. Click Options....
  8. Select the Max check box in the Summary Options - Statistics panel when the Options dialog box opens.
  9. Click OK to confirm you selection and to close the Options dialog box.
  10. Click OK to confirm your changes and to close the Properties Browser.

Renaming and generating the custom report

Although it's not required, it's a good idea to rename your custom report.

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

Viewing and understanding the data

The report gives you a lot of data. For now, you are interested in the minimum and maximum values for your 24-hour analysis period. Recall that a standard applied by the U.S. Government is less than or equal to 2.0 meters.

  1. Scroll to Statistics at the bottom of the report.
  2. Take note of the minimum - minimum (best value) and maximum - maximum (worst value) values which you will use to create contours in the 2D and 3D Graphics windows.

    3. minimum and maximum values

    Your dates and values will be different from the above image. Your values are based on your date, and any applicable outages for your date.

  3. Close the Custom Grid Stats Over Time report.
  4. Click Close to close the Report & Graph Manager.

Creating dynamic contours

You can control the display of coverage results with contour graphics depicting the various levels of coverage provided by the assigned assets over time. Display the quality of coverage based on the scenario time by using dynamic contours by updating the 2D Graphics- Animation properties.

  1. Return to PACC's () Properties ().
  2. Select the 2D Graphics - Animation page.
  3. Enter 25 in the % Translucency field in the Show Points As panel.

Defining the contour levels and colors

Define the contour levels and colors.

  1. Select the Show Contours option in the Display Metric panel.
  2. Click the Remove All in the Level Attributes panel.
  3. Enter the following values in the Level Adding panel:
    4. Option Value
    Add Method Start, Stop, Step
    Start Statistics - Min Minimum value (for example, 0.900 m)
    Stop Statistics - Max Maximum value (for example, 2.213 m)
    Step 0.1 m
  4. Click Add Levels.
  5. Enter the following values in the Level Attributes panel:
    6. Option Value
    Color Method Color Ramp
    Start Color Blue (best value)
    End Color Red (worst value)
  6. Select the Natural Neighbor option in the Contour Interpolation (points must be filled) panel.

    7. STK will interpolate the value of the Figure Of Merit for each screen pixel within the coverage area. It will use a natural neighbor algorithm based on the computed values of the Figure Of Merit at the grid points. If the Figure of Merit is discrete, STK will round interpolated values to the nearest integer.

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

Displaying the Contour Legends

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 PACC 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 Navigation Accuracy (Meters) in the Title field in the Text Options panel.
  6. Enter 2 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 PACC window.
  10. Click OK to close the Properties Browser.

Viewing the contours in the 2D Graphics window

You've set your color ramp such that blue will represent a low PACC (which is preferable) and red will represent a high PACC.

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

    2. Dynamic contours for positional Navigation accuracy

    Your contours will look different from the above image.

  2. Click Start () in the animation toolbar to animate the scenario.
  3. Click Reset () when finished.
  4. Clear the check box next to PACC () in the Object Browser to remove the graphics.

Building the test flight

The mission aircraft is flying from Long Beach, California, to Philadelphia, Pennsylvania. You can use Place objects as waypoints for the mission.

Inserting the first waypoint for the mission aircraft

Insert a Place object to represent Long Beach.

  1. Bring the Insert STK Objects () tool to the front.
  2. Insert a Place () object using the From City Database () method.
  3. Enter Long Beach in the Name field when the Search Standard Object Data dialog box opens.
  4. Click Search.
  5. Select Long Beach - California in the Results list.
  6. Click Insert.

Inserting the second waypoint

Insert a second Place object to represent Philadelphia.

  1. Enter Philadelphia in the Name field.
  2. Click Search.
  3. Select Philadelphia - Pennsylvania in the Results list.
  4. Click Insert.
  5. Click Close to close the Search Standard Object Data dialog box.

Inserting an Aircraft object

An Aircraft object models the properties and behavior of a vehicle that travels in a great arc route, generally above the surface of the earth.

  1. Go to the Insert STK Objects () tool.
  2. Insert an Aircraft () object using the Insert Default () method.
  3. Rename Aircraft1 () Test_Flight.

Defining Test_Flight's flight route

Use the Great Arc propagator to create the flight route from Long Beach to Philadelphia.

  1. Open Test_Flight's () Properties ().
  2. Select the Basic - Route page when the Properties Browser opens.
  3. Look in the Propagator field. GreatArc is the default propagator.
  4. Bring the 2D Graphics window to the front.
  5. Click Maximize () in the upper right corner of the 2D Graphics window.

    6. This will make it easier to see the entirety of the route.

  6. Click on the Long_Beach marker ().

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

  7. Click on the Philadelphia marker ().
  8. Return to Test_Flight's () Properties ().

Changing the waypoint altitude

You want to analyze Test_Flight at an altitude of 20,000 feet

  1. Click Set All....
  2. Select the Altitude check box when the Set All Grid Values dialog box opens.
  3. Enter 20000 ft in the Altitude field.
  4. Click OK to confirm your change and to close the Set All Grid Values dialog box.
  5. Click OK to confirm your changes and to close the Properties Browser.

Viewing the flight route in the 2D and 3D Graphics windows

You can view Test_Flight's route in the 2D and 3D Graphics windows.

  1. Click Restore the window to normal size () in the upper right corner of the 2D Graphics window.

    2. Flight route in the 2D and 3D graphics windows

  2. Click Start () in the Animation toolbar to animate the scenario.
  3. Click Reset () when finished.

Measuring HDOP with single-object coverage

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

Perform a single-object coverage analysis on the aircraft to determine the Horizontal Dilution of Precision (HDOP) along the flight route from Long Beach to Philadelphia. HDOP is very important in aviation. HDOP measures the quality of GPS positioning along the horizontal plane. This ensures proper spacing between aircraft at the same altitude.

Defining single-object coverage

Start by opening the Coverage tool.

  1. Right-click on Test_Flight () in the Object Browser.
  2. Select Coverage... () in the shortcut menu.

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. The STK application uses assets to calculate whether coverage to the object is possible. If you define a figure of merit for the coverage, the STK application measures coverage to the object from the assets by the figure of merit values.

  1. Select GPSConstellation () in the Assets list when the Coverage tool opens.
  2. Click Assign.
  3. Click Compute.

Defining a DOP Figure Of Merit

Dilution of Precision (DOP) measures the relative degradation or reduction in the certainty of a navigation solution based on one-way range measurements from a set of transmitters, like those on board GPS satellites. A DOP figure of merit measures the effects of the number of measurements and the geometry of the transmitters on the uncertainty in the navigation solution. The reported value can be multiplied by the uncertainty in the range measurements (assumed to be the same for all transmitters) to provide the uncertainty in the navigation solution. HDOP measures the dilution of precision for the horizontal (latitude/longitude) components of the positional portion of the navigation solution.

  1. Click Define... in the Figure of Merit panel.
  2. Set the following options when the Specify Figure of Merit dialog box opens:
    3. Option Value
    Type Dilution Of Precision
    Compute Maximum
    Method HDOP
    Type Over Determined
    Time Step 60 sec
  3. Click OK to confirm your changes and to close the Specify Figure of Merit dialog box.
  4. Note of the number in the Value field. This is the maximum HDOP value along Test_Flight's route.
  5. Click Apply to confirm your changes and to keep the Coverage tool open.

Graphing the HDOP FOM value

Creating a FOM Value graph of Test_Flight gives you a great snapshot of the HDOP values during the mission. The ideal value for HDOP is below 2, which indicates excellent accuracy. Values between 2 and 5 are considered good. Values above 5 are considered less reliable.

  1. Click FOM Value... in the Graphs panel.
  2. View the data in the graph.

    3. HDOP fom value graph

    Your graph will be different from the image shown above. In the above image, the HDOP values all fall below a value of 2, which indicates excellent accuracy.

  3. Place your cursor on the smallest value in the graph.
  4. Take note of the minimum value out to the hundredth and round down (for example, if your value is 0.64, use 0.63).
  5. Place your cursor on the largest value in the graph.
  6. Take note of the maximum value out to the hundredth and round up (for example, if your value is 0.87, use 0.88).

    7. You will use these values to create color contours along Test_Flight's route.

  7. Close the FOM Value graph.

Creating the contour graphics

Visualize the quality of coverage as Test_Flight flies from Long Beach to Philadelphia.

  1. Return to the Coverage tool ().
  2. Click Contours... in the Graphics panel.
  3. Click Remove All in the Level Attributes panel when the Contours... dialog box opens.
  4. Enter the following in the Level Adding panel:
    5. Option Value
    Start Rounded-down minimum FOM graph value
    Stop Rounded-up maximum FOM graph value
    Step 0.05
  5. Click Add.
  6. Enter the following values in the Level Attributes panel:
    7. Option Value
    Use Color Ramp Selected
    Select ramp colors Blue in the first field, Red in the second field
    Line Width Thickest
  7. Click Apply to confirm your changes and to keep the Contours... dialog box open.

Displaying the Legend window

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

  1. Click Legend....
  2. Click Layout... when the Static Legend for ObjectFOM 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 HDOP in the Title field in the Text Options panel.
  6. Enter 2 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 Static Legend for ObjectFOM window.

Viewing the flight route in the 2D Graphics window

Close out your open tools so can view the flight route in the 2D Graphics window.

  1. Click OK to close the Contours... dialog box.
  2. Click OK to close the Coverage tool.
  3. Bring the 2D Graphics window to the front.

    4. 2d graphics window flight route contours

    Your contour colors will be different from the above image.

  4. Click Start () in the Animation toolbar to animate the scenario.

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

  5. Click Reset () 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

This tutorial showed you how to determine GPS position accuracy (PACC) over the contiguous United States and horizontal dilution of precision (HDOP) along a single flight route. You downloaded the current SEM Almanac and satellite outage files from the US Coast Guard Navigation Center. You inserted the GPS constellation from the almanac file. Using the Navigation Files plugin and the GPS Satellite Outage tool, you determined any GPS outages and applied them to your analysis. Using an Area Target object to outline the contiguous United States, you determined PACC at an altitude of 20,000 feet. Next, you created an Aircraft object ,which flew from Long Beach to Philadelphia at an altitude of 20,000 feet. Using the coverage tool, you determined HDOP along the flight route.