Of considerable importance in the current era is the fact that satellite data are widely
available through the Internet.
Some satellites are in polar orbits completing an orbit roughly every hour and a
half at altitudes between 700 and 1,700 km. That is, they orbit south to north or
north to south passing near the poles. The Earth rotates underneath the satellites
so that each orbit brings the satellite over longitudes west of the previous orbit.
These are sun synchronous satellites in that they pass over the same latitudes at
the same time each day so that solar illumination differences are minimized. Their
relative low orbits make them relatively “inexpensive” to launch. Many weather
satellites and all the Earth resources satellites are in polar orbits. The longest
continuous Earth resources satellite series is the Landsat series first launched in
- These polar orbiters image large swaths of Earth in each scene. Current
scene sizes are about 170 km in latitudeby 183 km in longitude with spatial
resolutions of 30 m across in some of the wavelengths. Yet, a drawback is that
the current versions recapture the same scene area once every 16 days. This is
problematic if the goal is to do some monitoring of short-period events such as
floods or crop harvests. Another limitation is Landsat’s inability to sense the
surface throughclouds; thus some passes are useless because of cloud cover.
New American satellite series such as the Earth Observation System satellites
are able to cover the globe in a day or two with somewhat lesser resolution but
can image in three dozen different spectral bands. The new instruments allow
specializedworksuchasthree-dimensional profiling of atmospheric features.
Also, other countries (e.g., SPOT from France and DAICHI from Japan) have
launched their own Earth resources satellites.
Geosynchronous satellites are placed in much higher orbits and their orbital
speeds around Earth’s axis match the rotation of the Earth in a way such that they
remain over the same Earth location. Geosynchronous orbits are roughly
36,000 km above Earth’s surface and there is considerable expense in placing satel-
lites in such orbits and keeping their orbits synchronous. As a result, there are
considerably more polar-orbiting satellites than geosynchronous satellites. GOES
(Geostationary Environmental Satellite) is a long-running satellite program of the
U.S. National Oceanic and Atmospheric Administration that has two satellites
positioned to monitor the United States. GOES East is positioned over the equator at
75 °W; it views the eastern United States and can monitor Atlantichurricanethreats
to the western Atlantic Ocean basin. GOES West is positioned over the equator at
125 °W; it has a view overlapping that of GOES East and monitors the western
United States and themiddle latitude cyclonesapproaching from the Pacific
Ocean. Other countries have deployed geostationary satellites around the globe so
that virtually the entire Earth is under continuous monitoring (e.g., the European
Community’s METEOSAT, India’s INSAT and Japan’s GMS).
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