A satellite can remain in the same orbit for a long period of time as the gravitational pull of the Earth counterbalances the centrifugal force. As the satellites are in orbit outside the atmosphere, there is no air resistance, and consequently, according to the law of inertia, the satellite's speed is constant, resulting in a stable orbit around the Earth for many years.
Gravitational pull diminishes as one travels further away from the Earth, whereas the centrifugal force increases as the orbital velocity is increased. A satellite in low orbit typically about 800 km from the Earth is exposed to an immense gravitational pull and has to move at considerable speed in order to generate a corresponding centrifugal force. There is a direct connection between the distance from the Earth and the orbital velocity of the satellite. At a distance of 36,000 km, the orbiting time is 24 hours, corresponding to the Earth's rotation time. At this distance, a satellite above the Equator will be stationary in relation to the Earth.
The Geostationary Orbit
The geostationary orbit of 36,000 km from the Earth's Equator is best known for its many satellites which are used for various forms of telecommunication, including television. Signals from these satellites can be sent all the way around the world. Telecommunication needs to 'see' their satellite all time. The satellites must therefore remain stationary with regards to the Earth's surface.
One disadvantage of geostationary orbits, however, is their great distance from the
Earth, which reduces the possible spatial resolution. There are a number of weather satellites evenly distributed in geostationary orbit all around the world in order to provide us with a global view.
Many satellites are equipped with passive sensor systems which are dependent on solar illumination and orbit around the Earth. As they measure the reflection of sunlight from the Earth, their orbits must be adjusted to the rhythm of day and night. It is important to be able to compare images recorded over a period of time. If they are to be comparable, the light conditions must be identical. The recordings must take place at the same local time of day so that the altitude of the Sun above the horizon is the same, and the plane of the satellite orbit must remain at a constant angle to the Sun's light. These prerequisites can be achieved by placing the satellite in a polar orbit.
While the satellite revolves on its orbit, the Earth rotates on its axis below. Every time the satellite makes a complete rotation, a new strip of the Earth's surface is scanned, and after a certain number of rotations the entire surface of the Earth will have been acquired. Some satellites scan a broad strip everytime and can, therefore, cover the entire Earth in a few rotations. In comparison, high resolution satellites scan only a narrow strip at a time, and take several days to cover the entire Earth.
Last update: 21 July 2010