To learn more about orbits and other topics in space flight, try JPL's Basics of Space Flight Learners' Workbook . A detailed technical treatment of orbital mechanics can be found at this company's site . In general, the higher the orbit, the longer the satellite can stay in orbit. At lower altitudes, a satellite runs into traces of Earth's atmosphere which creates drag. The drag causes the orbit to decay until the satellite falls back into the atmosphere and burns up.
Satellites usually start out in an orbit that is elliptical.
The ground control station controls small onboard rocket motors to provide correction. The goal is to get the orbit as circular as possible. By firing a rocket when the orbit is at the apogee of its orbit, its most distant point from Earth, and applying thrust in the direction of the flight path, the perigee , or lowest point from Earth, moves further out. The result is a more circular orbit. A launch window is a particular period of time in which it will be easier to place the satellite in the orbit necessary to perform its intended function.