This information is adapted and expanded from "Looking at
Earth From Space", a Teacher's Guide with Activities for Earth and Space
Science, 1994, available from NASA Educator
Satellites can operate in several types of Earth orbit.
The most common orbits for environmental satellites are geostationary and
polar, but some instruments also fly in inclined orbits. Other types of
orbits are possible, such as the Molniya orbits commonly used for Soviet
A geostationary (GEO=geosynchronous) orbit is one in which
the satellite is always in the same position with respect to the rotating
Earth. The satellite orbits at an elevation of approximately 35,790 km
because that produces an orbital period (time for one orbit) equal to the
period of rotation of the Earth (23 hrs, 56 mins, 4.09 secs). By orbiting
at the same rate, in the same direction as Earth, the satellite appears
stationary (synchronous with respect to the rotation of the Earth).
Geostationary satellites provide a "big picture" view,
enabling coverage of weather events. This is especially useful for monitoring
severe local storms and tropical cyclones.
Because a geostationary orbit must be in the same plane
as the Earth's rotation, that is the equatorial plane, it provides distorted
images of the polar regions with poor spatial resolution.
Polar-orbiting satellites provide a more global view of Earth,
circling at near-polar inclination (the angle between the equatorial plane
and the satellite orbital plane -- a true polar orbit has an inclination
of 90 degrees). Orbiting at an altitude of 700 to 800 km, these satellites
cover best the parts of the world most difficult to cover in situ (on site).
For example, McMurdo, Antartica, can be seen on 11-12 of the 14 daily NOAA
These satellites operate in a sun-synchronous orbit. The
satellite passes the equator and each latitude at the same local solar
time each day, meaning the satellite passes overhead at essentially the
same solar time throughout all seasons of the year. This feature enables
regular data collection at consistent times as well as long-term comparisons.
The orbital plane of a sun-synchronous orbit must also rotate approximately
one degree per day to keep pace with the Earth's surface.
The Terra/Aqua satellites are polar orbiting satellites.
How can there be more than one overpass in a single day?
Inclined orbits fall between those above. They have an inclination
between 0 degrees (equatorial orbit) and 90 degrees (polar orbit). These
orbits may be determined by the region on Earth that is of most interest
(i.e., an instrument to study the tropics may be best put on a low inclination
satellite), or by the latitude of the launch site. The orbital altitude
of these satellites is generally on the order of a few hundred km, so the
orbital period is on the order of a few hours. These satellites are not
sun-synchronous, however, so they will view a place on Earth at varying
times. You can find several satellite tracking tools
which will show you what various orbits look like.