e d u s p a c e

Remote Sensing Principles | Satellite Orbits

	Satellite Orbits [ ]
	Introduction
	The Geostationary Orbit
	Sunsynchronous Orbits
	Introduction
	A satellite can remain in the same orbit for a long period of time as the gravitational pull of the Earth provides a balance to the centrifugal force. As the satellites are in orbit outside the atmosphere there is no air resistance, and therefore, according to the law of inertia, the speed of the satellite is constant resulting in a stable orbit around the Earth for many years. Gravitational pull diminishes as one travels further from the Earth, whereas the centrifugal force increases as the orbital velocity is increased. Therefore, a satellite in low orbit, typically about 800km from the Earth is exposed to an immense gravitational pull and has to move at considerable speed to generate a corresponding centrifugal force. Therefore, there is a direct connection between the distance to the Earth and the orbital velocity of the satellite. At a distance of 36000km, 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
Meteosat and other satellites in geostationary orbit watch the weather conditions all over the globe except at the North and South Poles.	Geostationary orbits of 36000km from the Earth's equator are best known for the many satellites used for various forms of telecommunication, including television. Signals from these satellites can be sent all the way round the world. Telecommunication needs to "see" their saellite all time and hence it must remain stationary in the same positions relative to the Earth's surface. A stationary satellite provides the advantage for remote sensing that it always views the Earth from the same perspective, which means that it can record the same image at brief intervals. This arrangement is particularly useful for observations of weather conditions. One disadvantage of geostationary orbits is the great distance to the Earth, which reduces the achievable spatial resolution. There are a number of weather satellites evenly distributed in geostationary orbit all around the world to provide a global view.
	Solar Synchronous Orbits
Solar synchronous satellite orbit.	Many satellites are equipped with passive sensor systems which are dependent on solar illumination and therefore the satellites 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 light from the sun. These prerequisites can be achieved by placing the satellite in a polar orbit. While the satellite is revolving in 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 every time and can, therefore, cover the entire Earth in a few rotations; whereas high resolution satellites scanning only a narrow strip take several days to complete coverage of the Earth.
	See also the following animation: Area Coverage of Landsat satellite in polar orbit More information about satellites orbits can also be found in the View Graphs (filtered by "orbit") Interact with various satellite orbits on the following website: www.mcasco.com/p1aso.html (Home page: www.mcasco.com/)