A geostationary transfer orbit (GTO) is an orbit around the Earth used as an intermediate stage between a low Earth orbit (LEO) and a geostationary orbit (GSO). It is an ellipse where the perigee is a point on the LEO and the apogee has the same distance from the Earth as the GSO.
A similar concept is a geosynchronous transfer orbit. Since a geostationary orbit is also a geosynchronous orbit, a geostationary transfer orbit can also be termed a geosynchronous transfer orbit. However a geosynchronous transfer orbit could alternatively refer to an orbit used to reach an arbitrary geosynchronous orbit. In the context of satellite launches the geostationary orbit is usually the one of interest.
After a typical launch the inclination of the LEO (the angle between the plane of the orbit and the plane of the equator) is determined by the latitude of the launch site and the direction of launch. The GTO inherits the same inclination. It requires less energy to adjust the inclination at the GSO distance than at LEO. The inclination must be reduced to zero to obtain a geostationary orbit.
A launch vehicle can move from LEO to GTO by firing a rocket at a tangent to the LEO to increase its velocity. Typically the upper stage of the vehicle has this function. Once in the GTO, it is usually the satellite itself that performs the conversion to geostationary orbit by firing a rocket at a tangent to the GTO at the apogee.
The spent upper stages of launch vehicles are left behind in the GTO.
