Most communications and weather satellites that end up in geostationary orbit are not placed there directly by the rocket. They are released into a geostationary transfer orbit, or GTO, and finish the journey with their own propulsion. Understanding GTO helps operators compare launch offers, because two launches to "GTO" can leave a satellite with very different amounts of work still to do.
From GTO to GEO
Geostationary orbit lies about 35,786 km above the equator. A satellite there circles the Earth once per day and appears fixed in the sky. GTO is an elongated orbit whose lowest point is a few hundred kilometres above the Earth and whose highest point is close to geostationary altitude. At the highest point, the satellite fires its engine to raise the lowest point and turn the ellipse into a circle.
Why inclination matters
A geostationary orbit has an inclination of zero degrees. A rocket launched from a site far from the equator reaches a GTO with a higher inclination, and changing inclination is expensive in terms of propellant. This is one reason why launch sites close to the equator, such as Kourou in French Guiana, are well suited to geostationary missions.
Standard GTO, supersynchronous and subsynchronous
- Standard GTO: the apogee is close to geostationary altitude and the satellite performs the remaining manoeuvres.
- Supersynchronous transfer orbit: the apogee is well above geostationary altitude, which can make the inclination change cheaper for the satellite.
- Subsynchronous or reduced orbits: used when the rocket has less performance margin, leaving more work to the satellite.
- Direct GEO insertion: some heavy launchers can place a satellite directly into geostationary orbit, which removes most of the orbit raising.
Chemical or electric orbit raising
Chemical propulsion delivers high thrust, so orbit raising is comparatively quick. All-electric satellites use far less propellant but take much longer: Airbus reported that the all-electric EUTELSAT 172B needed four months in 2017, which it called the fastest electric orbit raising from transfer to geostationary orbit. A longer orbit raising delays the start of commercial service. The choice of transfer orbit and propulsion should therefore be planned together.
Frequently asked questions
What is a geostationary transfer orbit?
An elongated orbit whose lowest point is a few hundred kilometres above the Earth and whose highest point is close to geostationary altitude. At the highest point, the satellite fires its engine to circularise its orbit at 35,786 km.
Why are equatorial launch sites preferred for GEO missions?
A geostationary orbit has an inclination of zero degrees. Launching from near the equator, for example from Kourou in French Guiana, reduces the costly inclination change the satellite has to perform.
How long does it take to reach GEO from GTO?
It depends on the propulsion. Electric propulsion saves propellant but takes longer. Airbus reported that the all-electric EUTELSAT 172B completed its electric orbit raising to GEO in four months in 2017, which it called the fastest electric orbit raising from transfer to geostationary orbit, using almost six times less propellant mass than a satellite with chemical propulsion.
Related reading
- LEO, MEO and GEO Satellites Compared: Altitudes, Uses and Examples
- Electric vs Chemical Propulsion: How Satellites Move in Orbit
- Delta-v Explained: The Currency of Spaceflight
Sources
- ESA: Types of orbits
- NASA: Basics of Space Flight
- Airbus: Full electric EUTELSAT 172B reaches geostationary orbit in record time (2017)
Cover image: Illustration of a GOES weather satellite in geostationary orbit. Credit: NASA (MSFC-20180918-1801666), public domain.