The Physics Of Satellites

Eric W. / Physics 336 / 7 April 1997
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[ Orbits ] [ The Ways Satellites Are Controlled ]
Satellites have literally been around since the dawn of the space age. Interest in satellites has been growing fast world wide. Businesses, government, universities and other organization around the world are starting their own satellite programs. Satellites are used for both exploration and communication.


Many types of orbits exists, but most artificial satellites travel in one of the three types: 1) Geosynchronous Orbit 2) Polar Orbit 3) Low Earth Orbit (LEO). "Satellite orbits have a variety of shapes. Some are circular, while others are highly elliptical. Orbits also vary in altitude. Some circular orbits, for example, are just above the atmosphere at an altitude of about 155 miles, while others are more than 20,000 miles above earth" (Oberright 2).

Leos are either elliptical or circular orbits at a height of less than 2,000 km above the surface of the earth. The orbit period at these altitudes varies between ninety minutes and two hours. A Geosynchronous Orbit is any type of orbit which produces a repeating ground track. A satellite in a geosynchronous orbit follows a circular over the equator at an altitude of 35,800 km completing one orbit every 24 hours, in the time that it takes the earth to rotate once. Moving in the same direction as the earth's rotation, the satellite remains in a fixed position over a point on the equator, thereby providing uninterrupted contact between ground stations in its line of sight. A Polar orbit is inclined at about 90 degrees to the equatorial plane, covering both poles. The orbit is fixed in space, and the Earth rotates underneath. Therefore, a single satellite in a Polar Orbit, provides in principle coverage to the entire globe, although there are long periods during which the satellites is out of view of a particular ground station.

The Ways Satellites Are Controlled

Most satellites operate under the direction of a control center that is located on the earth. Computers and human operators at the control center monitor the satellite's position, send instructions to its computers and retrieve information that the satellite has gathered. A satellite can not always receive constant direction from the control center. So it has to be able to act like an orbiting robot. It also has to be able to control its solar panels to them keep them pointed toward the sun and keep its antennas ready to receive commands. "Satellites in a high altitude, Geosynchronous orbit are always in contact with the earth. Ground stations can contact satellites in low orbits as often as 12 times a day, so they do not need constant direction (Oberright 3).