Wednesday, October 22, 2008


All satellite are held in balance between the inertia due to their motion and the centripetal force of the gravity. This is called orbital satellites.

There are several different types of satellite orbits for space systems in orbit around the Earth. The types vary in altitude, real or apparent path with respect to the earth, and duration of each orbit. Altitude further divides into the lowest perigee and highest apogee in the orbit.

A low perigee is valuable for missions in which sensors need to be close to the earth, or where low-power communications are being sent to the satellite. Satellites rotate around the Earth in an elliptical pattern or circular pattern. In circular pattern, apogee equal to perigee or rotation is constant. But in the elliptical pattern, apogee not equal to perigee and the speed depends on the height of the satellite to Earth. When it close to Earth, the speed is greater than it is farther away.


A polar orbit is an orbit in which a satellite passes above or nearly above both poles of the body (usually a planet such as the Earth, but possibly another body such as the sun) being orbited on each revolution. The Satellite rotates over the North and South Poles, which is an orbit perpendicular to equatorial plane. It therefore has an inclination of (or very close to) 90 degree to the equator.

A Polar orbit is a particular type of Low Earth Orbit. A satellite in a polar orbit will pass over the equator at a different longitude on each of its orbits. Polar orbits are often used for earth-mapping, earth observation and reconnaissance satellites, as well as some weather satellites. Not generally use for communication.

A satellite can hover over one polar area a large part of the time, albeit at a large distance, using a polar highly elliptical orbit with its apogee above that area. This is the principle behind a Molniya orbit.

Polar orbiting:

Polar Operational Environmental Orbits(POES)

The POES satellite system offers the advantage of daily global coverage, by making nearly polar orbits roughly 14.1 times daily. As we know, the polar orbiting nature of the POES series satellites, these satellites are able to collect global data on a daily basis for a variety of land, ocean, and atmospheric applications.

Example of Near-Polar orbit:

The ground track of a polar orbiting satellite is displaced to the west after each orbital period, due to the rotation of the Earth. This displacement of longitude is a function of the orbital period (often less than 2 hours for low altitude orbits).

Map of the ground path of one revolution of a typical near-polar orbiting satellite.

The orbit of a near polar satellite as viewed from a point rotating with the Earth.


An equatorial orbit generally known as geosynchronous orbit. An orbit around the Earth with an orbital period matching the Earth's sidereal rotation period. Synchronization means that for an observer at a fixed location on Earth, a satellite in a geosynchronous orbit returns to exactly the same place in the sky at exactly the same time each day.

In principle, any orbit with a period equal to the Earth's rotational period is technically geosynchronous, however, the term is almost always used to refer to the special case of a geosynchronous orbit that is circular (or nearly circular) and at zero (or nearly zero) inclination, that is, directly above the equator.

Geosynchronous Equatorial Orbit:

Geostationary orbit.

Sometimes called a geostationary orbit. A circular geosynchronous orbit in the plane of the Earth's equator has a radius of approximately 42,164 km (from the center of the Earth). A satellite in such an orbit is at an altitude of approximately 35,786 kilometers above mean sea level and has 24 hour orbit period.

It will maintain the same position relative to the Earth's surface. A perfect stable geostationary orbit is an ideal that can only be approximate. This is because, the satellite will drift out of this orbit (because of perturbations such as the solar wind, radiation pressure, variations in the Earth's gravitational field, and the gravitational effect of the Moon and Sun), and thrusters are used to maintain the orbit in a process known as station-keeping. The satellite visible from 1/3 Earth’s surface, so three satellite are needed for full coverage of the Earth.

Geostationary satellite.

Geostationary satellite at the geosynchronous orbit.