Geostationary transfer orbit
Encyclopedia
A geosynchronous transfer orbit or geostationary transfer orbit (GTO) is a Hohmann transfer orbit
used to reach geosynchronous
or geostationary orbit
.
It is a highly elliptical
Earth orbit
with apogee at about 35,700 km, geostationary (GEO) altitude, and an argument of perigee such that apogee occurs on or near the equator. Perigee can be anywhere above the atmosphere, but is usually limited to only a few hundred km to reduce launcher delta-v
(
V) requirements and to limit the orbital lifetime of the spent booster.
The 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 is a geosynchronous orbit. Because the
V 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. The required 
V for an inclination change at either the ascending or descending node
of the orbit is calculated as follows:
For a typical GTO with a semimajor 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, so additional
V is required.
Even at apogee, the fuel needed to reduce inclination to zero can be significant, giving equatorial launch sites a substantial advantage over those at higher latitudes. Kennedy Space Center
is at 28.5 degrees north, the Guiana Space Centre, the Ariane launch facility, is at 5 degrees north
latitude and Sea Launch
launches from a floating platform directly on the equator in the Pacific Ocean
. All have a significant advantage over Russia's high latitude launch sites.
Expendable
launchers generally reach GTO directly, but a spacecraft already in a low Earth orbit (LEO
) can enter GTO by firing a rocket
along its orbital direction to increase its velocity. This is done when a geostationary spacecraft is launched from the space shuttle
; a "perigee kick motor" attached to the spacecraft ignites after the shuttle has released it and withdrawn to a safe distance.
Although some launchers can take their payloads all the way to geostationary orbit, most end their missions by releasing their payloads into GTO. The spacecraft and its operator are then responsible for the maneuver into the final geostationary orbit. The five hour coast to first apogee can be longer than the launcher's battery lifetime, and the maneuver is sometimes performed at a later apogee. The solar power available on the spacecraft supports the mission after launcher separation. Also, many launchers now carry several satellites in each launch to reduce overall costs, and this practice simplifies the mission when the payloads may be destined for different orbital positions.
Because of this practice launcher capacity is usually quoted as separated spacecraft mass to GTO, and this number will be higher than the payload that could be delivered directly into GEO.
For example, the capacity (separated spacecraft mass) of the Delta IV
Heavy:
If the manoeuver from GTO to GEO is to be performed with a single impulse, as with a single solid rocket motor, apogee must occur at an equatorial crossing. This implies an argument
of perigee of either 0 or 180 degrees. Because the argument of perigee is slowly perturbed
by the oblateness of the Earth, it is usually biased at launch so that it reaches the desired value at the appropriate time. (If the GTO inclination is zero, as with Sea Launch, then this does not apply.)
The preceding discussion has primarily focused on the case where the transfer between LEO and GEO is done with a single intermediate transfer orbit. More complicated trajectories are sometimes used. For example, the Proton M uses a set of four intermediate orbits, requiring five rocket firings, to place a satellite into GEO from the high-inclination site of Baikonur Cosmodrome
, in Kazakhstan
.
Hohmann transfer orbit
In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits, typically both in the same plane....
used to reach geosynchronous
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...
or geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
.
It is a highly elliptical
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...
Earth orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
with apogee at about 35,700 km, geostationary (GEO) altitude, and an argument of perigee such that apogee occurs on or near the equator. Perigee can be anywhere above the atmosphere, but is usually limited to only a few hundred km to reduce launcher delta-v
Delta-v
In astrodynamics a Δv or delta-v is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver....
(
V) requirements and to limit the orbital lifetime of the spent booster.The inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
of a GTO is the angle between the orbit plane and the Earth's equatorial plane
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
. It is determined by the latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
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
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
of the orbit is reduced to zero, the result is a geosynchronous orbit. Because the
V 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. The required V for an inclination change at either the ascending or descending nodeOrbital node
An orbital node is one of the two points where an orbit crosses a plane of reference to which it is inclined. An orbit which is contained in the plane of reference has no nodes.-Planes of reference:...
of the orbit is calculated as follows:
For a typical GTO with a semimajor axis of 24,582 km, perigee
Perigee
Perigee is the point at which an object makes its closest approach to the Earth.. Often the term is used in a broader sense to define the point in an orbit where the orbiting body is closest to the body it orbits. The opposite is the apogee, the farthest or highest point.The Greek prefix "peri"...
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, so additional
V is required.Even at apogee, the fuel needed to reduce inclination to zero can be significant, giving equatorial launch sites a substantial advantage over those at higher latitudes. Kennedy Space Center
Kennedy Space Center
The John F. Kennedy Space Center is the NASA installation that has been the launch site for every United States human space flight since 1968. Although such flights are currently on hiatus, KSC continues to manage and operate unmanned rocket launch facilities for America's civilian space program...
is at 28.5 degrees north, the Guiana Space Centre, the Ariane launch facility, is at 5 degrees north
5th parallel north
The 5th parallel north is a circle of latitude that is 5 degrees north of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Southeast Asia, the Pacific Ocean and South America....
latitude and Sea Launch
Sea Launch
Sea Launch is a spacecraft launch service that uses a mobile sea platform for equatorial launches of commercial payloads on specialized Zenit 3SL rockets...
launches from a floating platform directly on the equator in the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
. All have a significant advantage over Russia's high latitude launch sites.
Expendable
Expendable launch system
An expendable launch system is a launch system that uses an expendable launch vehicle to carry a payload into space. The vehicles used in expendable launch systems are designed to be used only once , and their components are not recovered for re-use after launch...
launchers generally reach GTO directly, but a spacecraft already in a low Earth orbit (LEO
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
) can enter GTO by firing a rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...
along its orbital direction to increase its velocity. This is done when a geostationary spacecraft is launched from the space shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
; a "perigee kick motor" attached to the spacecraft ignites after the shuttle has released it and withdrawn to a safe distance.
Although some launchers can take their payloads all the way to geostationary orbit, most end their missions by releasing their payloads into GTO. The spacecraft and its operator are then responsible for the maneuver into the final geostationary orbit. The five hour coast to first apogee can be longer than the launcher's battery lifetime, and the maneuver is sometimes performed at a later apogee. The solar power available on the spacecraft supports the mission after launcher separation. Also, many launchers now carry several satellites in each launch to reduce overall costs, and this practice simplifies the mission when the payloads may be destined for different orbital positions.
Because of this practice launcher capacity is usually quoted as separated spacecraft mass to GTO, and this number will be higher than the payload that could be delivered directly into GEO.
For example, the capacity (separated spacecraft mass) of the Delta IV
Delta IV rocket
Delta IV is an active expendable launch system in the Delta rocket family. Delta IV uses rockets designed by Boeing's Integrated Defense Systems division and built in the United Launch Alliance facility in Decatur, Alabama. Final assembly is completed at the launch site by ULA...
Heavy:
- GTO 12,757 kg (185 km x 35,786 km at 27.0 deg inclination), theoretically more than any other currently available launch vehicle (has not flown with such a payload yet)
- GEO 6,276 kg
If the manoeuver from GTO to GEO is to be performed with a single impulse, as with a single solid rocket motor, apogee must occur at an equatorial crossing. This implies an argument
of perigee of either 0 or 180 degrees. Because the argument of perigee is slowly perturbed
by the oblateness of the Earth, it is usually biased at launch so that it reaches the desired value at the appropriate time. (If the GTO inclination is zero, as with Sea Launch, then this does not apply.)
The preceding discussion has primarily focused on the case where the transfer between LEO and GEO is done with a single intermediate transfer orbit. More complicated trajectories are sometimes used. For example, the Proton M uses a set of four intermediate orbits, requiring five rocket firings, to place a satellite into GEO from the high-inclination site of Baikonur Cosmodrome
Baikonur Cosmodrome
The Baikonur Cosmodrome , also called Tyuratam, is the world's first and largest operational space launch facility. It is located in the desert steppe of Kazakhstan, about east of the Aral Sea, north of the Syr Darya river, near Tyuratam railway station, at 90 meters above sea level...
, in Kazakhstan
Kazakhstan
Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...
.