Satellites

Last updated: June 11, 2006.

Satellites have revolutionized the way we look at the world and the way we send information around it. Like giant mirrors in space, they can be used to bounce television pictures, telephone calls, and Internet data from one part of Earth to another at the speed of light. Satellites are used for sensing information as well as communication. From planning military maneuvers during the Gulf War to helping ships and planes navigate around the planet, and from forecasting the weather to finding mineral deposits, information from distant satellites is truly invaluable.

Photo: The early NASA Echo II satellite, undergoing testing in 1964. Picture courtesy of Great Images in NASA.

What satellites do

Rockets and reusable spacecraft such as the Space Shuttle launch satellites into orbits high above Earth. Just as a string can keep a spinning rock turning in a circle about your head, so Earth's gravity provides a centripetal (center-seeking) force that keeps a satellite in orbit. Onboard gyroscopes stop satellites spinning off course due to variations in Earth's magnetic field; alternatively they are set spinning when they are released from the launch vehicle to achieve the same effect. Once in place, satellites are powered by large arrays of solar panels or even nuclear-powered electric motors.

Communications satellites maintain the same position above a fixed point on the Equator some 22,300 miles (35,900 km) from Earth. This is called a geostationary orbit and is similar to a geosynchronous orbit, in which satellites loop once or twice around a certain point on the Equator each day. Remote sensing satellites follow polar orbits. These are 155-621 miles (250-1000 km) high and loop over the north and south poles once each day.

Communication satellites receive information from transmitters on Earth (in an uplink) and beam it down to receivers elsewhere on the planet (in a downlink). Transmitters and receivers differ widely. Transcontinental telephone calls are sent and received by gigantic satellite dishes on opposite sides of the globe. At the other end of the scale, handheld electronic "compasses" called GPS (Global Positioning System) receivers pick up signals from 24 Navstar GPS navigational satellites, enabling travelers to pinpoint their position to within a few feet, anywhere on Earth.

Since Sputnik 1 was fired into space on October 4, 1957, several thousand satellites have been launched. Landsat 7 provides detailed pictures of Earth's surface and was used by the US to pinpoint Iraqi troop positions during the 1991 Gulf War. INMARSAT provides mobile telephone, fax, and computer data communication for ships, aircraft, and travelers on the move. 15 INTELSAT satellites provide telecommunications links to 150 nations around the globe.

Satellites have been sending pictures back to Earth since April 1960, when the Tiros 1 weather satellite first transmitted pictures of clouds taken from space. Satellites have two types of sensors. Passive sensors collect radiation (such as light) emitted from Earth, whereas active sensors fire out beams of radio waves and analyze the information reflected back from Earth's surface. A device called a thematic mapper splits the incoming radiation into seven bands and analyzes each one separately. For example, the band devoted to visible light can easily distinguish between uncultivated soil and dense forest, so is useful for agricultural mapping. Remote sensing satellites are so powerful that they can now send back images of individual houses and streets.

Further Reading

Favorite websites

• USGS Landsat: Explore satellite photos of Earth with the US Geological Survey's Landsat satellite.
• Space Imaging: Another good source of satellite photos of Earth.