Fundamental of Satellite
The orbit of a spacecraft around the Earth can be in the shape of a circle
or an ellipse. An artificial satellite in a circular orbit travels at
a constant speed. The higher the altitude, however, the lower the speed
relative to the surface of the Earth. Maintaining an altitude of 35,800 km
(22,300 mi) over the equator, a satellite is geostationary. It moves in
geosynchronous orbit, at exactly the same speed as the Earth, so that it
remains in a fixed position over some particular spot on the equator. Most
communications satellites are placed in such orbits.
In an elliptical orbit, the
speed varies and is greatest at perigee (minimum altitude) and least at
apogee (maximum altitude). Elliptical orbits can lie in any plane that
passes through the Earth's centre. A polar orbit lies in a plane passing
through the North and South Poles-that is, it passes through the axis of
rotation of the Earth. An equatorial orbit is one that lies in a plane
passing through the equator. The angle between the orbital plane and the
equatorial plane is called the inclination of the orbit.
The Earth rotates once every 24
hours under a satellite in a polar orbit. A polar-orbit weather satellite,
carrying television and infrared cameras, can thus observe meteorological
conditions over the entire globe from pole to pole in a single day. An orbit
at another inclination covers a smaller portion of the Earth, omitting areas
around the poles.
As long as the orbit of an object keeps it in the vacuum of space, the
object will continue to orbit without propulsive power because no frictional
force slows it down. If part or all of the orbit passes through the
atmosphere of the Earth, however, the body is slowed by aerodynamic friction
with the air. This causes the orbit to decay gradually to lower and lower
altitudes until the object has fully re-entered the atmosphere and burns up,
like a meteor.