GEOSTATIONARY SATELLITES
Geostationary satellites enable a quasi-continuous time sampling over certain regions on Earth. These satellites are geosynchronous, meaning their orbits keep them synchronized with Earth's rotation, i.e., they take 24 hours to complete one orbit.
Fig. 1. The position of a geosynchronous satellite at twelve hour intervals.
Figure courtesy of the National Space Agency of Japan (NASDA)
When these satellites orbit above the equator, with zero inclination, they are also geostationary (fixed) relative to a point on the equator, so that they observe the Earth without any significant relative motion.
Fig. 2. Position of a geostationary satellite with respect to the equator.
There is only one orbit in which a satellite can be geostationary. To have a 24 hour orbital period, they must keep an orbital altitude of 35,780 km (22,234 mi, or about 5.61 Earth radii), which sets their speed at 3.07 km/s (6,868 mph). An equatorial point travels underneath at a speed of about 0.465 km/s (1,040 mph). At this distance, and with a wide field of view (FOV), they see the Earth as a full disk, but the area covered is less than a hemisphere, being about 1/4th of the planetary surface. This results in a much wider field of view than is possible for polar orbiting satellites, however, the large distance from earth causes geostationary satellites to have much poorer spatial resolution than polar orbiting satellites.
Fig. 3. Comparison of the orbit altitudes and fields of view of geostationary
satellites and polar orbiting satellites.
Advantages of a geostationary orbits include:
(1) large spatial coverage (five geostationary satellites are enough to cover all of the non-polar regions of the Earth).
(2) permanent visibility of the satellite allowing continuous telecommunications and high rate of repetition for observations (near continuous time sampling - 30 min and 15 min for Meteosat, a few minutes for GOES).
(3) one ground segment is enough for the satellite monitoring.
Limitations of geostationary satellites include:
(1) polar regions are not observed.
(2) Not adequate for very high spatial resolution of the ground. For example, in visible and infrared wavelengths, the resolution could not reasonably be better than 1 km.
(3) active measurements are not feasible at such a distance from the Earth.
(4) some perturbations of the solar electricity power supply to the satellite occur during eclipse phenomena.
Geostationary satellites are useful for many applications:
Meteorology: real time operational surveys of the troposphere, cloud systems, sea and land surface temperatures; data acquisition and dissemination.
Telecommunications: world wide operational telecommunication systems for telephones, TV and digitized transmission lines.
Army: alarm systems - detection of rocket launches.