EUMETSAT satellites operate in two orbits — Meteosat in a geostationary orbit and Metop in a Lower Earth Orbit (LEO).
A satellite is in a geostationary orbit when it appears stationary when viewed from Earth. This can only occur when:
- The orbit is geosynchronous . A geosynchronous orbit has an orbital period matching the rotation rate of the Earth. This is a sidereal day, which is 23h 56m 4s in length, and represents the time taken for the Earth to rotate once about its polar axis relative to a distant fixed point. This is about four minutes shorter than the civil day length, which is relative to the Sun. A geostationary orbit is a special case of a geosynchronous orbit. The distance of a satellite in geosynchronous orbit is calculated from Kepler’s third law, which states that the average orbit radius squared, divided by the orbital period cubed, is constant. As a consequence, the orbital period increases with distance, but has a fixed value for a given distance. In the case of the Moon, for example, R = about 383,000 km and T = about 27.3 days. For a geosynchronous satellite whose period T equals one sidereal day, the equation produces a value R = 42,155 km. Subtracting the Earth's radius yields the average orbit height above the Earth’s surface as 35,786 km.
- The orbit is a circle. If the orbit is not a circle, the satellite does not move at constant velocity, it appears to oscillate east-and-west at a rate of two cycles per sidereal day.
- The orbit lies in the plane of the Earth’s equator. If the orbit does not lie in the equatorial plane, the satellite does not remain at a fixed point in the sky, it appears to oscillate north-and-south at a rate of one cycle per sidereal day.
Typical Parameter Values
- Height above equator 35,786 km
- Orbit radius 42,155 km
- Orbit circumference 264,869 km
- Arc length per degree 736 km
- Orbital velocity 11,066 km/h = 3.07 km/s
The theoretical coverage area of a geostationary satellite extends to an angle of 81° from the sub-satellite point (the point on the Earth’s surface directly beneath the satellite), corresponding to more than 40% of the Earth's surface. In practice, the useful coverage is less than this. If observed at latitude 81° the satellite would lie on the horizon, making communication difficult; a more realistic coverage value would be about 75°. For a weather satellite, the distorted perspective introduced by the Earth’s curvature limits the useful study of features to about 70° from the sub-satellite point, corresponding to about one third of the Earth's surface. For the quantitative derivation of meteorological products from Meteosat data, EUMETSAT imposes a further limit of 60°. Even so, it would need as least three geostationary satellites to provide coverage of all but the polar regions of the Earth.
Sometimes we need to correct or change the orbit of a satellite. Because of the uneven shape of the Earth and the gravitational influence of the Moon and Sun, the satellite does not stay precisely at its nominal location, causing two major effects:
Inclination — A gradual increase in satellite inclination, which affects the north-south position. The inclination of the satellite orbit is the small angle between its orbital plane and the equatorial plane of the Earth. This causes an apparent daily north-south motion of the satellite, centred over its nominal location. The maximum excursion north and south of the equator is the same as that of the inclination. While the inclination remains less than 0.3° no action is taken to control this small movement. However, during the lifetime of the satellite the inclination tends to increase, and at intervals it is necessary to perform a so-called 'north-south manoeuvre' to adjust the orbital plane of the satellite. North-south station-keeping is expensive because it uses a lot of fuel, so is often the limiting factor in the lifetime of the satellite. When the fuel is exhausted, the inclination increases continuously, at about 0.9° each year, and eventually the daily north-south movement makes reception of data difficult. The precise inclination limits for successful reception of data depend on the location and characteristics of the individual user stations.
Longitude — Satellite drift, which affects the east-west position. This orbital effect is caused by the uneven shape of the Earth, in particular the location of the deep oceans, which causes the gravitational field of the Earth to depart from a true spherical shape. The effect is as if the satellites were located on hills, which they may slide off, or in valleys, where they may remain stable. There are two stable locations in geostationary orbit, one over the Indian Ocean, the other over the eastern Pacific Ocean. Meteosat, at 0° longitude, is on the gravitational slope leading to this 'hole' and gradually drifts towards the east. The satellite is normally maintained within a defined box around its nominal location. When it reaches the eastern extremity of the permitted box, the satellite is moved back to the western extremity of the box, where the process starts again. This cycle repeats every few months (depending on the current size of the permitted box), but is not costly in terms of fuel use. While the satellite stays within this box it is compliant with system specification, and realignment of user antennas, due to satellite movement, is not necessary.
Executing a Manoeuvre
A manoeuvre is carried out by firing thrusters (small motors burning hydrazine fuel). Meteosat has a group of thrusters on its side, angled in different directions to allow thrust to be applied in the required direction. An orbital manoeuvre may involve applying thrust:
- perpendicular to the orbital plane — this is a north-south manoeuvre and is used to change the inclination of the orbit;
- in the orbital plane, either with or against the direction of orbital motion — this east-west manoeuvre speeds up or slows the satellite and shifts it into a higher or lower orbit. Two or more such manoeuvres would be used to change the longitude of the satellite or move it into a different orbit.
In the case of a spin-stabilised satellite such as Meteosat, an east-west manoeuvre is complicated because the thrusters are spinning with the satellite. The thrusters are designed to allow burns to be controlled to a fraction of a second. Meteosat spins at 100 revolutions per minute with its spin axis at right angles to its orbital plane. One revolution takes 0.6 seconds. The thrusters are fired over an angle of 60° centred on the required direction, i.e. for one-sixth of a revolution. The result is a staccato firing as the thrusters repeatedly burn for 0.1 seconds and switch off for 0.5 seconds, for the duration of the burn period. So a typical burn for placing a satellite into a graveyard orbit takes place over 20 minutes, but thrust is only applied for about three minutes.
Lower Earth Orbit (LEO)
Metop is in a sun-synchronous near-polar or Lower Earth Orbit (LEO) orbit, often shortened to 'polar orbit'.
At its simplest, a polar orbit is inclined at (or nearly at) 90° to the equatorial plane, so that a satellite is able to pass over both poles of the Earth. With the plane of the orbit lying north-south and almost fixed in space, the spinning Earth results in a polar-orbiting satellite sweeping over different swaths of ground with each orbit, eventually covering the entire globe.
The orbit is designed to ensure that the angle between the orbital plane and the Sun remains constant, resulting in consistent lighting conditions. This is achieved by a careful selection of orbital parameters to produce a precession of the orbit equal to the apparent motion of the Sun as seen from Earth orbit, i.e. about one degree eastward each day.
The satellite's orbital plane must be inclined away from a true north-south polar orbit. With an inclination of 98.7° to the equatorial plane, the asymmetric gravitational pull of the Earth causes the orbit to precess by the required amount. (Note that the satellite's motion is actually retrograde — it moves to the west, not the east.).
A key feature is that, in each half of this orbit, the satellite always crosses a particular line of latitude at the same local solar time. The angle of the sunlight (in the daytime half) is consistent, only varying slowly as the seasons change in the course of a year.
The altitude of a satellite in polar orbit is a compromise between different requirements:
- High ground resolution and a short orbital period for frequent coverage — these result from a low orbit.
- A swath of observation that is wide enough so that successive orbital swaths overlap. This ensures complete ground coverage, and is favoured by a higher orbit.
As a result, a typical polar satellite moves in a circular orbit with an altitude of about 850 km and a period of 100 minutes. The satellite scans a swath about 3000 km wide on the Earth's surface, which is also wide enough to cover the poles, despite the north-south orbital inclination of 8.7°. With these parameters, the satellite makes just over 14 orbits in a day, and every point on the Earth is covered at least twice.
The two types of weather satellite, polar and geostationary, are complementary. Each type has advantages and disadvantages, and an ideal observing system combines both.