geosynchronous / geostationary transfer orbit (GTO)
A geosynchronous / geostationary transfer orbit (GTO) is an elliptical orbit, with an apogee (high point) of 35,784 kilometers, a perigee (low point) of a few hundred km, and an inclination roughly equal to the latitude of the launch site, into which a spacecraft is initially placed before being transferred to a geosynchronous or geostationary orbit.
After attaining GTO, the spacecraft's apogee kick motor is fired to circularize the orbit and thereby achieve the desired final orbit. Typically, this burn will also reduce the orbital inclination to 0° so that the final orbit is not only geosynchronous but also geostationary. Because the greater the initial inclination, the greater the velocity change (delta v) needed to remove this inclination, it is important that launches of GSO satellites take place as close to the equator as possible. For example, in a Delta or Atlas launch from Cape Canaveral the transfer orbit is inclined at 28.5° and the required delta v increment at apogee is 1,831 m/s; for an Ariane launch from Guiana Space Centre the inclination is 7° and the delta v is 1,502 m/s; while for a Zenit flight from the Sea Launch platform on the equator the delta v is 1,478 m/s. By the rocket equation, assuming a (typical) specific impulse of 300 seconds, the fraction of the separated mass consumed by the propellant for the apogee maneuver is 46% from Cape Canaveral, 40% from Kourou, and 39% from the equator.
As a rough guide, the mass of a geostationary satellite at the start of its operational life (in GSO) is about half its initial on-orbit mass when separated from the launch vehicle (in GTO). Before carrying out the apogee maneuver, the spacecraft must be reoriented in the transfer orbit to face in the proper direction for the thrust. This reorientation is sometimes done by the launch vehicle at spacecraft separation; otherwise, it must be carried out in a separate maneuver by the spacecraft itself. In a launch from Cape Canaveral, the angle through which the satellite must be reoriented is about 132°.