Great Arc Propagator

The Great Arc Propagator defines the route of a vehicle that follows a point-by-point path along, over, or below the surface of the Earth at a given altitude or depth. A great arc path, which lies in a plane that intersects the center of the Earth, connects the waypoints.

There are two general methods you can use to define a great arc route:

Importing a Great Arc File

Click the Import from File... button to load waypoint data from an external Great Arc file (without using the StkExternal propagator).

Manually Defining a Route

To create a great arc route for the vehicle:

  1. Define the Start Time.
  2. Select a Route Calculation Method.
  3. Define the Altitude Reference.
  4. Define Waypoints.

Start Time

The Start Time defaults to the scenario start time and you can reset it. The Stop Time field is read-only and is defined by the last waypoint for the vehicle. If you change the Start Time after you enter waypoints, all time entries in the waypoint table will be updated automatically.

Click the button or right-click on the start or stop times to interact with the object time options.

Route Calculation Method

Use the Route Calculation Method drop-down menu to select the manner in which the route is calculated between each waypoint:

  • Specify Rate/Acc - uses the Speed and Acceleration properties of each waypoint.
  • Specify Time - uses the Time properties of each waypoint.
  • Smooth Rate - uses the Speed property of each waypoint.

Altitude Reference

Ground vehicles, aircraft, and ships can reference their altitude from mean sea level, terrain data, or WGS84.

To have the route of a ground vehicle follow the terrain, set Altitude Reference to Terrain, Interp Method to Terrain Height, and the Altitude of all waypoints to 0.

Altitude Reference options for great arc vehicles

Option Value
Altitude Reference Altitude can be referenced to:
  • MSL - Mean Sea Level
  • WGS84 - The central body's reference ellipsoid
  • Terrain - Waypoint altitudes can be referenced to the terrain under the vehicle's route if terrain sources are being streamed or are loaded into the scenario. The highest resolution terrain which overlaps the area of the vehicle route is used. Selecting terrain increases the time it takes to compute the vehicle route.

When a Great Arc vehicle uses an altitude reference of MSL, the altitude interpolator used by the Great Arc propagator will be set to MSL height, an interpolator that linearly interpolates using an MSL reference rather than the ellipsoid reference. For arcs at constant MSL altitude, all interpolated points along that arc will also be at that same MSL altitude.

The terrain data files available in this scenario for visualization and analysis are listed on the Terrain properties page.
Granularity Granularity defines the distance between interpolation points for computing the vehicle's altitude along the route.
Interp Method Interp Method sets the terrain interpolation method to be used to define the height of the vehicle with respect to terrain data.
  • Ellipsoid Height - interpolate using the Great Arc interpolator without considering terrain at all.
  • Terrain Height - assigns a height above terrain by a linear interpolation between the heights above terrain at the waypoints.
Terrain Resolution Displays the resolution of the terrain data used when referencing waypoint altitude to terrain. This value describes the distance between sampling points for the terrain data being used. Except in very rare circumstances, the Granularity should never be set to a smaller value than the Terrain Resolution since this will result in the altitude being interpolated without valid reference points.

Waypoints

The waypoints that comprise the great arc route are contained in a table that displays each point, along with all of its properties, in sequence; the table can be used to directly edit those properties. One row of values describes a single waypoint in the route of the vehicle.

Select Update Map Graphics to display the ground track for the route in the 2D Graphics window as new waypoints are entered and calculated.

Adding a New Waypoint

There are two methods you can use to add a new waypoint to the route:

  1. Click Insert Point to add a new line of data to the waypoint table after the currently selected row. The new waypoint's data will be a copy of the waypoint before it, if one exists. You can also use the Copy Point button to insert a copy of the currently selected row.
  2. Click anywhere in the 2D Graphics window to add a new waypoint after the currently selected row. The new waypoint's latitude and longitude will be defined by the map point that you selected while the rest of the its properties will be copied from the row above.
    • If you use the 2D Graphics window method to define a great arc route, AGI recommends that you specify the altitude/depth, turn radius, and calculation method data for the first point before you create a second point, so that the initial altitude/depth and route data become the default for all additional waypoints.
    • For more information on setting object properties using the mouse, see Mouse Shortcuts.

Modifying a Waypoint

There are two methods you can use to modify an individual waypoint in the route:

  1. Click on the property of the waypoint that you want to modify in the table and enter a new value directly into the cell.
  2. Use the 2D Graphics window:
    1. Select Clicking on map changes current point (beneath the waypoints table).
    2. Select the table row of the waypoint that you want to change.
    3. Click anywhere in the 2D Graphics window to redefine the latitude and longitude of the waypoint to the selected position.

The properties that you can define for a waypoint depend on the Route Calculation option that you have selected. The following table describes the properties that are used to define a great arc route waypoint. The Calculation Method column denotes which Route Calculation Methods allow you to edit that property.

Great Arc Waypoint Properties

Property Description Calculation Method
Latitude The latitude of the waypoint. All
Longitude The longitude of the waypoint. All
Altitude/Depth The altitude or depth of the waypoint. Changes in altitude or depth take effect linearly between two waypoints. All
Speed The velocity of the vehicle from the current waypoint to the next. A change in speed occurs immediately at the waypoint. Smooth Rate,
Specify Rate/Acc
Accel The rate of increase (if positive) or decrease (if negative) in the velocity of the vehicle. Specify Rate/Acc
Time The time at which the vehicle is at the waypoint. Specify Time
Turn Radius The curvature of the arc between the current waypoint and the next waypoint. A smaller turn radius produces a sharper curve in the arc. All

You do not need to click OK or Apply to save waypoints that you have inserted using the 2D Graphics window. You will, however, need to click OK or Apply to save any other changes that you have made on the Basic Route Properties page, including waypoints that you manually inserted.

Modifying All Waypoints

Click Set All... to open the Set All Grid Values tool, which enables you to modify the Altitude, Speed, Acceleration, and Turn Radius properties for all waypoints that are currently defined in the table. The properties that can be modified using this tool will be constrained with respect to the currently selected Route Calculation Method.

To use the tool, select each of the properties that you want to modify, enter the values that you want to set, and then click OK to implement the changes and close the window.

Removing Waypoints

To delete an existing waypoint, select the table row of the waypoint and click Delete Point.

Arc Granularity

If you have specified an EGS84 or MSL altitude reference for the route, you can use the Arc Granularity field to specify the frequency of interpolated route points between the waypoints that you have specified. The lower the value, the smoother the route that STK will produce, at the expense of a longer compute time.

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