Geo-Stationary and Polar Satellites

Geo-stationary orbits are circular orbits that are orientated in the plane of the earth's equator. In a geo-stationary orbit, the satellite appears stationary, i.e. in a fixed position, to an observer on earth.

More technically, a geo-stationary orbit is a circular pro-grade orbit in the equatorial plane with an orbital period equal to that of the earth; this is achieved with an orbital radius of 6.6107 (equatorial) earth radii, or an orbital height of 35786 km.

A satellite in a geo-stationary orbit will appear fixed above the surface of the earth, i.e. at a fixed latitude and longitude. In practice, the orbit has small non-zero values for inclination and eccentricity. The footprint, or service area, of a geo-stationary satellite covers almost 1/3 of the earth's surface (from about 75 degrees south to about 75 degrees north latitude), so that near-global coverage can be achieved with a minimum of three satellites in orbit.

By placing the satellite at an altitude where it's orbital period exactly matches the rotation of the earth (approximately 35,800 km), the satellite thus appears to 'hover' over one spot on the Earth's equator and thus appears to stay stationary over the same point.

A geo-stationary satellite completes one orbit revolution in circular orbit, round the Earth, every 24 hours. If the orbit is in the equatorial plane, and if rotation is in the same direction as the Earth, (rotating at the same angular velocity as the Earth) and it over-flies the same point on the globe permanently then the satellite is termed geo-stationary.

Geo-stationary satellites, however, do not see the poles at all yet geo-synchronous satellites do. In general, all geo-stationary orbits are geo-synchronous, but not all geo-synchronous orbits are geo-stationary.

Geo-synchronous means that a satellite makes one orbit every 24 hours so that it is 'synchronized' with the rotation period of the earth. As previously stated, this will happen when a satellite is in a circular orbit at a rough distance of 36,000 kilometers above the surface of the Earth, or roughly 42,000 kilometers from the center of the earth. However, to be a geo-stationary satellite, the geo-synchronous satellite must be in orbit in earth's equatorial plane - geo-stationary is a small subset of orbits that are geo-synchronous.

The orbital location of geo-stationary satellites is called the Clarke Belt in honor of Arthur C Clarke who first published the theory of locating geo-synchronous satellites in earth's equatorial plane for use in fixed communications purposes (Clarke, Arthur C., 1945, "Extra Terrestrial Relays", Wireless World.). For brevity, the term 'geo-stationary satellite' is often shortened to 'geo satellite'.
 Geo-stationary and Geo-synchronous Orbits
The National Aeronautics and Space Administration (NASA) have developed a new series of GOES and polar-orbiting satellites. The new GOES-I through M series provides higher spatial and temporal resolution images and full-time operational soundings (vertical temperature and moisture profiles of the atmosphere).

The newest polar-orbiting meteorological satellites provide improved atmospheric temperature and moisture data in all weather situations. This new technology will help provide the National Weather Service the most advanced weather forecast system in the world.

GOES satellites provide the kind of continuous monitoring necessary for intensive data analysis. They circle the Earth in a geo-synchronous orbit, which means they orbit the equatorial plane of the Earth at a speed matching the Earth's rotation. This allows them to hover continuously over one position on the surface.

The geo-synchronous plane is about 35,800 km (22,300 miles) above the Earth, high enough to allow the satellites a full-disc view of the Earth. Because they stay above a fixed spot on the surface, they provide a constant vigil for the atmospheric "triggers" for severe weather conditions such as tornadoes, flash floods, hailstorms, and hurricanes. When these conditions develop the GOES satellites are able to monitor storm development and track their movements.

GOES satellite imagery is also used to estimate rainfall during the thunderstorms and hurricanes for flash flood warnings, as well as estimates snowfall accumulations and overall extent of snow cover. Such data help meteorologists issue winter storm warnings and spring snowmelt advisories. Satellite sensors also detect ice fields and map the movements of sea and lake ice.

Primary instruments such as the Imager and the Sounder carry out the main mission. The imager is a multi-channel instrument that senses radiant energy and reflected solar energy from the Earth's surface and atmosphere. The Sounder provides data to determine the vertical temperature and moisture profile of the atmosphere, surface and cloud top temperatures, and ozone distribution.

Polar Orbits and Satellites

Polar Satellites
Polar orbits are LEO orbits. Their applications can be to view only the poles (to fill in gaps of geo coverage) or to view the same place on earth at the same time each 24hr day. By placing a satellite at an altitude of about 850 km, a polar orbit period of roughly 100 minutes can be achieved.

This will allow the earth to rotate beneath the satellite sufficiently that one polar satellite be used per application though for more continuous coverage, more than one polar orbiting satellite is employed. A special polar orbit that crosses the equator and each latitude at the same time each day is called a sun-synchronous orbit.

Polar satellites may carry sensors sensitive to both visible light and infrared (IR) radiation and can make measurements of temperature and humidity in the Earth's atmosphere, record surface ground and surface sea-water temperatures, and monitor cloud cover and water/ice boundaries. They may have the capability to receive, measure, process, and retransmit data from balloons, buoys, and remote automatic stations distributed around the globe.

These satellites may also carry search and rescue transponders to help locate downed airplanes or ships in distress. Polar orbiting satellites provide many services in communication and observation applications which geo satellites
are not capable of.

Polar Orbits
This illustration shows true relative distance from the Earth of geo-stationary and polar orbit satellites. From geo altitude, the entire Earth disk subtends an angle of 17.4 degrees in contrast to a typical polar orbiting satellite, which sees only a relatively small portion of the globe at any one time. By definition, a polar satellite has an inclination of 90 degrees to the equator.

The critical design goal is to place a polar satellite in an orbit that is low enough to allow a relatively short orbital period while at the same time its orbit altitude is sufficient to permit observation of a sufficiently wide path so that during a single orbit the Earth will rotate by less than the scan swath ability of the satellite instrumentation.

A polar orbit is fixed in space, and the earth rotates underneath; a polar orbit travels from north to South Pole. A typical polar satellite can cover the entire globe every 14 days and can 'see', as example, the entire east coast of the at one time, from southern Florida north to Hudson Bay, and from the Atlantic to west of the Great Lakes.
 POLAR ORBITING SATELLITE