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In the context of spaceflight, launch period is the collection of days, and launch window is the time period on a given day, during which a particular rocket must be launched in order to reach its intended target. If the rocket is not launched within a given window, it has to wait for the window on the next day of the period. Launch periods and launch windows are dependent on both the rocket's capability and the orbit to which it is going.
A launch period refers to the days that the rocket can launch to reach its intended orbit. A mission could have a period of 365 days in a year, a few weeks each month, a few weeks every 26 months (e.g. Mars launch periods), or a short period time that won't be repeated.
A launch window indicates the time frame on a given day within the launch period that the rocket can launch to reach its intended orbit. This can be as short as a second (referred to as an instantaneous window) or as long as the entire day. The launch window can straddle two calendar days (for example, starting at 11:46 p.m. and ending at 12:14 a.m.). Launch windows are rarely at exactly the same times each day. For operational reasons, the window almost always is limited to no more than a few hours.
Launch windows and launch periods are often used interchangeably in the public sphere, even within the same organization. The definitions given here are as used by launch directors and trajectory analysts at NASA and other space agencies.
Launch periods are often calculated from , which show the delta-v needed to achieve the mission plotted against the launch time.
For trips into largely arbitrary Geocentric orbit, no specific launch time is required. But if the spacecraft intends to space rendezvous with an object already in orbit, the launch must be carefully timed to occur around the times that the target vehicle's orbital plane intersects the launch site.
Earth observation satellites are often launched into sun-synchronous orbits which are Polar orbit. For these orbits, the launch window occurs at the time of day when the launch site location is aligned with the plane of the required orbit. To launch at another time would require an orbital plane change Orbital maneuver which would require a large amount of propellant.
For launches above low Earth orbit (LEO), the actual launch time can be somewhat flexible if a parking orbit is used, because the inclination and time the spacecraft initially spends in the parking orbit can be varied. See the launch window used by the Mars Global Surveyor spacecraft to the planet Mars at [1].
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 overconstrained. 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.
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