Aircraft Properties | Ground Vehicle Properties | Satellite Properties | Ship Properties

Great Arc Propagator

The Great Arc Propagator defines vehicles that follow a point-by-point path over or below the surface of the Earth at a given altitude or depth.

Start and Stop Times
Defining a Great Arc Route
Using Terrain Data

Start and Stop Times

The Start Time defaults to the scenario start time and you can reset it. The Stop Time field is read-only and is based on the last waypoint for the vehicle. If you change the Start Time after you enter waypoints, all time entries in the waypoint table update automatically. Use the current scenario time period by turning on Use Scenario Analysis Period.

Defining a Great Arc Route

To specify the points along the route of a great arc vehicle, select GreatArc as the Propagator, and select one of the following to define the waypoints:

Import a Great Arc file. The waypoints table and altitude reference values will be automatically updated with information from the file.
Manually enter the parameters of each waypoint in the waypoints table.

A Great Arc path, which lies in a plane that intersects the center of the Earth, connects the waypoints.

Importing a Great Arc File

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

Adding a New Waypoint

To add a new waypoint to the route:

Click Insert Point to add a new line of data.
Select Update Map Graphics to display the ground track for the route in the 2D Graphics window as new waypoints are entered and calculated.
Use the Route Calculation Method to select the manner in which the route is calculated.

Route calculation method

Method	Description
Specify Rate/Acc	Calculates route based on Rate and Acceleration values.
Specify Time	Calculates route based on the Time entered for each waypoint.
Smooth Rate	Calculates the appropriate acceleration or deceleration if different speeds are entered at successive waypoints.

Define or edit the parameters for each waypoint in the waypoints table.

One row of values describes a single waypoint in the route of the vehicle. The values that you provide will depend on the Route Calculation option selected above. When specifying points using the Smooth Rate method, the accel and time will not be editable. The table below outlines the required parameters for each calculation method.

Great arc waypoint options

Option	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.	Specify Rate/Acc and Smooth Rate Only
Accel	The rate of increase (if positive) or decrease (if negative) in the velocity of the vehicle.	Specify Rate/Acc Only
Time	The time at which the vehicle is at the waypoint.	Specify Time Only
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 can also use the mouse to enter waypoints directly in the 2D Graphics window. Simply click anywhere in the 2D Graphics window to add new waypoints (latitude/longitude values) in the waypoint table. However, you must use the Route page to enter altitude/depth, rate, and turn radius values. When you use the mouse to define a great arc route, it is recommended that you specify the altitude/depth and rate data for the first point before you create the second point, so that the initial altitude/depth and route information become the default for all additional points.

To change an existing waypoint with this method, select Clicking on map changes current point. Select the point to be changed in the waypoints list, and then click the desired location in the 2D Graphics window.

For more information on setting object properties using the mouse, see Mouse Shortcuts.

Note: You do not need to click OK or Apply after you have entered waypoints in the 2D Graphics window. You will need to click OK or Apply if you have made any other changes on the Basic Route Properties page.

Modifying Waypoints

To copy a waypoint, select a waypoint in the waypoints table and click Copy Point. You can use this feature as a shortcut for adding new and slightly modified waypoints to the route.

To delete an existing waypoint, select the point of interest in the waypoints table and click Delete Point.

Specifying 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 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 sampling points along a vehicle 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 greatArc 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.

Arc Granularity

Specifies the arc granularity, which is the frequency of interpolated points. A low number will make the arc look better, but draw slower. Valid for WGS84 and MSL.