In space mission design, a geostationary transfer orbit (GTO) or geosynchronous transfer orbit is a highly elliptical type of geocentric orbit, usually with a perigee as low as low Earth orbit (LEO) and an apogee as high as geostationary orbit (GEO). Satellites that are destined for geosynchronous orbit (GSO) or GEO are often put into a GTO as an intermediate step for reaching their final orbit. Manufacturers of launch vehicles often advertise the amount of payload the vehicle can put into GTO.
Geostationary and geosynchronous orbits are very desirable for many communication and Earth observation satellites. However, the delta-v, and therefore financial, cost to send a spacecraft to such orbits is very high due to their high orbital radius. A GTO is an intermediary orbit used to make this process more efficient. Satellite operators often use a high-thrust, low-efficiency launch vehicle to put their satellite into GTO, and then, after detaching the launch vehicle, use low-thrust, high-efficiency thrusters onboard the satellite itself to circularise its orbit (to GEO). This mission architecture is useful because it minimises the mass that the spacecraft must push to GEO, allows for maximally efficient circularisation burns taking advantage of the Oberth effect, and allows the spent launch vehicle to deorbit primarily through aerobraking due to its low perigee, minimising its orbital lifetime.
GTO is a highly elliptical Earth orbit with an apogee of 42,164 km (26,199 mi), or a height of 35,786 km (22,236 mi) above sea level, which corresponds to the geostationary altitude. The period of a standard geosynchronous transfer orbit is about 10.5 hours. The argument of perigee is such that apogee occurs on or near the equator. Perigee can be anywhere above the atmosphere, but is usually restricted to a few hundred kilometres above the Earth's surface to reduce launcher delta-V (
) requirements and to limit the orbital lifetime of the spent booster so as to curtail space junk.
If using low-thrust engines such as electric propulsion to get from the transfer orbit to geostationary orbit, the transfer orbit can be supersynchronous (having an apogee above the final geosynchronous orbit). However, this method takes much longer to achieve due to the low thrust injected into the orbit.
The typical launch vehicle injects the satellite to a supersynchronous orbit having the apogee above 42,164 km. The satellite's low-thrust engines are thrusted continuously around the geostationary transfer orbits. The thrust direction and magnitude are usually determined to optimize the transfer time and/or duration while satisfying the mission constraints. The out-of-plane component of thrust is used to reduce the initial inclination set by the initial transfer orbit, while the in-plane component simultaneously raises the perigee and lowers the apogee of the intermediate geostationary transfer orbit. In case of using the Hohmann transfer orbit, only a few days are required to reach the geosynchronous orbit. By using low-thrust engines or electrical propulsion, months are required until the satellite reaches its final orbit.
The orbital inclination of a GTO is the angle between the orbit plane and the Earth's equatorial plane. It is determined by the latitude of the launch site and the launch azimuth (direction). The inclination and eccentricity must both be reduced to zero to obtain a geostationary orbit. If only the eccentricity of the orbit is reduced to zero, the result may be a geosynchronous orbit but will not be geostationary. Because the
required for a plane change is proportional to the instantaneous velocity, the inclination and eccentricity are usually changed together in a single maneuver at apogee, where velocity is lowest.
for an inclination change at either the ascending or descending node of the orbit is calculated as follows:
{\displaystyle \Delta V=2V\sin {\frac {\Delta i}{2}}.}
For a typical GTO with a semi-major axis of 24,582 km, perigee velocity is 9.88 km/s and apogee velocity is 1.64 km/s, clearly making the inclination change far less costly at apogee. In practice, the inclination change is combined with the orbital circularization (or "apogee kick") burn to reduce the total
for the two maneuvers. The combined
is the vector sum of the inclination change
, and as the sum of the lengths of two sides of a triangle will always exceed the remaining side's length, total
in a combined maneuver will always be less than in two maneuvers. The combined
{\displaystyle \Delta V={\sqrt {V_{t,a}^{2}+V_{\text{GEO}}^{2}-2V_{t,a}V_{\text{GEO}}\cos \Delta i}},}
is the velocity magnitude at the apogee of the transfer orbit and
{\displaystyle V_{\text{GEO}}}