Geostationary Satellites
A geostationary satellite orbits the Earth in an equatorial
orbit at an altitude where its period is equal to that of the Earth's
rotation (24 hours). The result is that the geostationary satellite
turns with the Earth and remains over the same fixed point of the
planet at all times. A geostationary orbit is usually circular with
an inclination of 0º.
The fixed nature of a geostationary satellite with respect to a given
point on the Earth makes them very useful for surveillance, communications
and broadcasting, and environmental monitoring. Satellite television
broadcasts make use of geostationary satellites, as do many of the
telecommunications companies around the world. The GOES meteorological
satellites operated by the U.S. provide constant satellite coverage
of the entire hemisphere from a geostationary orbit.
One limitation of a geostationary satellite is that the platform
is only useful from the equator up to a latitude of about 70 degrees
north and south of the equator. Therefore, to provide communications
or other satellite support to higher latitudes, either polar orbiter
or highly elliptical orbiters must be used.
Highly Elliptical orbiters
Highly Elliptical Orbits (HEO) satellites orbit the
Earth in an orbital plane with an inclination between 50 and 70º.
The period of a HEO satellite is approximately 12 hours and the shape
of the orbit is highly elliptical. During the HEO satellite's orbit,
it comes very close the planet for part of its orbit, causing its
velocity to increase. Then it travels very far away from the Earth
and its orbital velocity decreases. As a result it spends much of
its time in the portion of the orbit that is at a very high altitude.
A HEO satellite is placed in an orbit in such a way that it will
spend the greatest amount of time over a specific area of the planet.
Thus, if a HEO communications satellite is launched to provide communications
in the arctic region, its orbit would be configured so that is spends
the bulk of its time in orbit above these latitudes. This is especially
useful in providing communications and navigation services from a
satellite. One example of HEO satellites is the Global Positioning
System, which uses a fleet of HEO satellites to provide constant coverage
of the entire planet and which are capable of providing precise locational
data to any point on the surface of the Earth.
References
- NASA
Observatorium Education-Reference Module: Satellite Orbits
- http://observe.ivv.nasa.gov/nasa/education/reference/orbits/orbits.html
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