When performing a launch windows analysis, in most cases the Launch segment is sufficient to solve for the launch epoch. But what if you have ECF ephemeris provided by a launch vehicle manufacturer and want to use the precomputed trajectory?
Using the Scripting Tool in Astrogator and an auxiliary object, you can have Astrogator target the start time of the ECF trajectory, mimicking the Launch segment but using your ephemeris instead. The steps in this process are:
1. Create a new vehicle and use the StkExternal propagator. Turn on the option to "Override the times contained in the file". Select the ephemeris file containing an ECF trajectory.
2. Use a Follow segment and set the new vehicle to the reference vehicle. Use "Join at Beginning of Leader's Ephemeris" as the joining condition. If you wish to use the entire ephemeris, use "Separate at End of Leader's Ephemeris" as the separation condition.
3. Move the Follow segment into a Target Sequence and go to the Scripting Tool embedded in the profile you wish to use (Differential Corrector, Optimizer, etc.). Create a parameter which will act as the control for the profile. Inside the script, get access to the Object Model and set the "Time of first ephemeris point" to the parameter value.
4. Configure the profile for any other controls and results you wish to achieve.
Now when the target sequence profile runs, the start time of the reference vehicle will be changed as the profile attempts to find the value of the parameter which meets the desired constraints/results. Because the Astrogator satellite is following and joining at the beginning of the reference vehicle you are also moving the start of the follow segment indirectly.