Geostationary transfer orbit
A geosynchronous or geostationary transfer orbit (also Geotransferorbit ; abbreviation GTO from English geosynchronous / geostationary transfer orbit ) is an earth orbit on which satellites are launched by launchers in order to be finally positioned on a geosynchronous or geostationary orbit (GEO). The path maneuver required for this is preceded by an exact path determination .
The GTO has the shape of an elongated ellipse ; one of their focal points is the center of the earth. The point furthest from the earth - the apogee - is mostly near the geostationary orbit at 35,786 km above the equator . However, the path speed there is still too low for the desired circular path and the path inclination (inclination) is usually too great.
Typically, a rocket releases the satellite at (or near) the closest point to earth (the perigee ) of the elliptical orbit. The orbit time on a typical GTO (250 × 36,000 km) is approx. 10.5 hours, so that the height of the geostationary orbit is passed for the first time after a little more than 5 hours.
Special procedures of some launch vehicles
Some launch vehicles first fly to a low parking orbit and from there usually start a Hohmann transfer , the elliptical part of which is the geostationary transfer orbit.
Some launchers like the Ariane However, the satellites take you directly to the geostationary transfer orbit, including from an equatorial location, as Kourou , also a / s is needed of just 9.8 kilometers.
Some launch vehicles that launch at a very high latitude , e.g. B. the Russian Proton , control a GTO + ("plus") or a super-synchronous transfer orbit with a very high apogee. Because of the low orbital speed there, they can reduce the high inclination with less expenditure of energy ( bi-elliptical transfer ). For this purpose, when crossing the equatorial plane , the missile receives a transverse acceleration in its orbital plane, which deflects its inclination to zero.
Thrusters for transition to geostationary orbit
To switch from the elliptical GTO to the circular GEO, a speed of almost 1.5 km / s in the apogee of the GTO is necessary. Some launch vehicles do this with their senior level. However, this remains in the vicinity of the GEO or has to be "disposed of" in a cemetery orbit .
Satellites that make the switch with a solid propulsion engine as an apogee engine often remain connected to it. The ignition can already happen after half a revolution or even after a few earth orbits in the GTO, to z. B. to check the satellite technically.
A liquid fuel apogee motor can be ignited multiple times, each time at apogee, to gradually raise the perigee. This has the advantage that the structural mass of the engine can be reduced in favor of the payload. A division of the drive power between the upper stage and the apogee engine would be technically possible, but this is unusual.
Ion thrusters , the even lower propulsion power of which is supplied by the solar modules, are particularly suitable for the orbit corrections that are repeatedly required during the life of the satellite. In order to be able to use this engine to raise the perigee as well, the bi-elliptical transfer via a GTO + achieved with chemical propulsion is also used here.
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- ↑ a b Bernd Leitenberger: Orbits and orbits of satellites , accessed: August 28, 2012 (calculated with the computer on the page)
- ^ B. Stanek: Raumfahrtlexikon , Hallwag Verlag, Bern (1983), pp. 304–305, ISBN 3-444-10288-7