As few as 5-6 satellites placed at various longitudes around the Earth can provide global information but cannot see to the polar regions. The resolution in these measurements suffers from the great height at which observations must be made.
|At an altitude of 22,300 miles (35,890 kilometers), a satellite in a circular orbit in the equatorial plane moves around the Earth at the same speed that the Earth rotates. Because of this, it appears to remain suspended all the time over a fixed point on the Earth's surface.||Perfect for communications satellites because always in view of the ground station providing continuous TV and telecommunications services to customers. Also ideal for making uninterrupted observations of the weather or environmental conditions in a given area||GOES-East operated by NOAA (views North and South America) and GOES-West operated by NOAA (views the Pacific region to see weather approaching the US), Meteosat (views Europe and Africa), GMS (views Japan and Australia.|
This orbit allows the satellite to observe the entire Earth's surface as it rotates beneath it. Most desired orbits are between 700 and 800 km altitude with orbit periods between 98 and 102 minutes.
|If the orbit plane is inclined away from the true north-south direction, the asymmetric pull of the Earth's gravitational field causes the orbit to precess (move around the Earth in local time). The sun-synchronous orbit is a special case of near-Polar orbit with inclination of 98.7 degrees, that precesses at exactly the required rate (~ 1 degree per day) to remain in the same local time plane as the Earth rotates around the sun. In an inclined orbit the coverage is no longer truly global because the satellite does not travel over the polar region. However, in nearly polar orbits, the satellite's instruments can usually scan over the pole.||This orbit provides global daily coverage of the Earth with higher resolution than geostationary orbit. Even though satellites do not pass directly over the poles they come close enough that their instruments can scan over the polar region, providing truly global coverage. Remote sensing spacecraft (i.e., Landsat and SPOT place high resolution and multi-spectral coverage at higher priority than daily global coverage.||NOAA/POES and DMSP weather satellites, Landsat and SPOT (remote-sensing satellites)|