Launch Window Calculator


Requirements and Setup
The Launch Window Calculator (LWC) is a utility created using STK's programming interface. You can use this tool to create an Astrogator satellite that, starting from a given launch pad, achieves a defined orbit by using a Launch segment and a Propagate segment. The LWC requires Integration, Astrogator,  and Analysis Workbench licenses, and you must have an STK 11 scenario open before launching it. Unzip the LWC_Plugin folder, then put the LWC folder and LWC_Plugin.xml into C:\ProgramData\AGI\STK 11 (x64)\Plugins. For 32 bit STK put the LWC folder and LWC_Plugin.xml into C:\ProgramData\AGI\STK 11\Plugins and update the CodeBase location in the LWC_Plugin.xml. Upon opening STK the LWC should now be accessible as a toolbar and as a Scenario Plugin.

If you do not see the LWC, check the Message Viewer and see if you get an Exception: "Could not load file or assembly 'file:///C:\ProgramData\AGI\STK 11 (x64)\Plugins\LWC\LaunchWindowCalculatorPlugin.dll' or one of its dependencies. If you get this exception, go back to the LWC folder and right click the LaunchWindowCalculatorPlugin.dll and open its properties. Unblock the file, this will be a check box or a button in the bottom of the General tab. Open STK again and the LWC should be available.
Running the Launch Window Calculator

First, the LWC needs a reference orbit to target. The reference orbit is circular by default, and you can define its size and orientation in space using properties displayed on the left side of the LWC window.

Next, select a facility from the Launch Site drop-down list; the list is comprised of all of the facility objects contained in the current STK scenario and can be updated by selecting Update Facility List from the File menu.
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Once you have defined the starting conditions, click the Configure Geometry button to calculate the launch windows that fall within the scenario time span. The tool defines a launch window to be a period of time when a spacecraft can be launched directly into the specified orbit from the selected launch site. The launch window calculation always depends on the chosen geometry; in a nominal 24-hour period there will be two (or just one, when the target orbit inclination is equal to the launch pad latitude) opportunities:

- the ascending node opportunity, which happens when the orbit plane intersects the launch pad position near the orbit ascending node
- the descending node opportunity, which happens when the orbit plane intersects the launch pad position near the orbit descending node

Select which opportunity that you want to use in the Launch section of the window; the epoch of currently selected opportunity is displayed. AGI recommends that you use a 24-hour scenario for the calculation, since such a duration contains no more than two launch opportunities. If the scenario is longer, only the latest ascending and descending opportunities will be available in the tool.
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At this point, the Launch Window Calculator possesses all of the necessary geometry data to easily detect and recognize the launch opportunities. You can set the relevant animation time by clicking the Change Anim Timebutton.
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Finally, click the Create Satellite button to generate the Astrogator satellite in the current scenario; the satellite object will be named, "Launch". A Mission Control Sequence is generated for the satellite, with the following segments:
1. Launch segment inside a Target Sequence with 3 Differential Correctors:
     1) Burnout Velocity and Altitude to correct for eccentricity and Semimajor Axis;
     2) Burnout Azimuth correction for inclination;
     3) Launch Epoch for RAAN correction.

2. Propagate segment with a duration of approximately one orbit.
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Most of the time, the Differential Correctors will converge, since the LWC provides good first guesses about burnout azimuth/velocity and launch epoch to the MCS. If the DCs are not converging, try manually running the MCS to check each step individually. By default, the launch time of the flight is set to 600 seconds and the downrange distance is set to 3,500 km. If necessary, you can change those values after the first run.
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Enjoy your launches!
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