Editing Launch window (section)

==Instantaneous launch window==

Achieving the correct orbit requires the '''[[right ascension]] of the [[Longitude of the ascending node|ascending node]]''' (RAAN). RAAN is set by varying a launch time, waiting for the earth to rotate until it is in the correct position. For missions with very specific orbits, such as 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 a [[launch 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 [[Overdetermined system|overconstrained]]. An instantaneous launch window allows the RAAN be the uncontrolled variable. While some spacecraft, such as the [[Centaur (rocket stage)|Centaur upper stage]], can steer and adjust its RAAN after launch,<ref>{{cite web | title = How Close Atlas V Came to Failure in This Week's Cygnus Launch |url = https://spaceflight101.com/cygnus-oa6/by-the-numbers-how-close-atlas-v-came-to-failure-in-this-weeks-cygnus-launch/ | website = Spaceflight101 | first = Patrick | last = Blau | date =  March 27, 2016 | access-date = February 12, 2021}}</ref> choosing an instantaneous launch window allows the RAAN to be pre-determined for the spacecraft's guidance system.