Satellites |
| Most people think of satellites as
flying east-west, circling the globe above the equator. While many
satellites do fly in such low equatorial orbits, there also are many
polar-orbiting satellites. which travel a north-south path across the poles.
Different satellites fly around Earth on different orbital paths. The choice of north-south or east-west route depends upon the job assigned to the satellite. Polar-orbiting satellites provide a more global view of Earth. Polar satellites look down on Earth's entire surface, passing above the North and South Poles several times a day. As the satellite loops around the globe, Earth seems to rotate under the orbit. Equatorial and polar satellites mostly fly at low altitudes between 100 and 1,000 miles. To an observer on the ground, they seem to climb up from the horizon, pass overhead, and fall below the horizon several times a day. Stationary satellites, on the other hand, are in very high equatorial orbits 22,300 miles above Earth. They are so far above the planet's surface they seem to us to be standing still overhead. ALTITUDES Polar Orbits Observation of the ground is improved if the surface always is illuminated at the same Sun angle when viewed from the satellite. Many weather, resource and reconnaissance satellites are in so-called Sun-synchronous polar orbits. Examples include LANDSAT, Nimbus, GEOS, TIROS, NOAA and Navy Oscar satellites. Geodetic-survey and navigation satellites usually are in almost-perfectly-circular polar orbits. The first polar-orbiting U.S. weather satellite was Nimbus-1 launched in 1964. The first U.S. Earth-resources observation satellite, Landsat-1, was sent to polar orbit in 1972. Stationary Satellites Stationary satellites orbit at altitudes around 22,300 miles. Back in 1945, science-fiction author Arthur C. Clarke imagined communications satellites in stationary orbits where they would travel around the world at the same speed the globe is spinning, making them hang stationary over one spot on Earth's surface. A satellite on its way to stationary orbit is fired to a special equatorial orbit known as a geostationary transfer orbit (GTO). In this highly-elliptical orbit, the satellite swings out 22,300 miles and back in to an altitude of 100 miles above Earth. At an assigned time and place, a "kick motor" attached to the satellite pushes it on out to a circular orbit at 22,300 miles altitude. The stationary-orbit region of space is referred to as the Clarke Belt. That name is in honor of Arthur C. Clarke, of course. Satellites in the Clarke Belt are said to be stationary, geostationary, synchronous or geosynchronous. A synchronous satellite has a high, fixed vantage point from which it can look down continuously on a large portion of Earth. That makes stationary satellites ideal for pinpoint broadcasting and for monitoring continent-wide weather patterns and environmental conditions. Clarke Belt Every nation on Earth has a set of positions assigned to it in the Clarke Belt. Scores of communications and weather satellites, operated by many countries and international organizations, occupy those assigned positions in the Clarke Belt. Many popular television satellites, sending programs to backyard dish antennas, are in stationary orbits. The U.S. communications satellite Syncom-3 was the first launched successfully to stationary orbit. That was in 1964. It made history that fall by sending TV pictures of the 1964 Olympic Games in Japan across the Pacific to the U.S. Other well-known stationary communications satellites have included Inmarsat and TDRS. North American cloud photos seen in television weathercasts usually have been made by American satellites known as Geostationary Operational Environmental Satellites (GOES). |