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[i]Achieving the correct orbit requires the Right Ascension of the Ascending Node (RAAN). RAAN is set by varying a launch time, waiting for Earth to rotate until it is in the correct position. For missions with very specific orbits, like rendezvousing with the International Space Station the launch window may be a single moment in time, known as an instantaneous launch window.
Trajectories are programmed into the vehicle prior to launch. The launch vehicle will have a target, and the guidance system will alter the steering commands to attempt to get to the final end state. At least one variable (apogee, perigee, inclination, etc.) must be left free to alter the values of the others, otherwise the dynamics would be over-constrained.
An instantaneous launch window allows the RAAN be the uncontrolled variable. While some spacecraft such as the Centaur upper stage can steer and adjust its RAAN after launch, choosing an instantaneous launch window allows the RAAN to be pre-determined for the spacecraft's guidance system.[/i]
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