Satellites

by Chris Woodford. Last updated: July 4, 2011.

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. Let's take a closer look at satellites and find out how they work!

Photo: A typical communications satellite from the 1980s. The blue squares are solar panels that provide power. The white circles are the sending and receiving antennas. Picture courtesy of NASA Glenn Research Center (NASA-GRC).

How satellites are launched

The Space Shuttle launching a communications satellite from its payload bay.

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.

Photo: The Space Shuttle launches a communications satellite from its payload bay in 1984 by spinning it gyroscopically. You can see Earth to the left. Picture courtesy of NASA Johnson Space Center (NASA-JSC).

Communications satellites

Communications satellites maintain the same position above a fixed point on the Equator some 35,900 km (22,300 miles) 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 250-1000 km (155-621 miles) 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 dish antennas 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.

How communications satellites work

Satellite communication across Earth using an uplink and downlink

Communications satellites bounce signals from one side of Earth to the other, a bit like giant mirrors in space:

A ground-based satellite transmitter dish (red) beams a signal to the satellite's receiving dish (yellow). The satellite boosts the signal and sends it back down to Earth from its transmitter dish (red) to a receiving dish somewhere else on Earth (yellow). Since the whole process happens using radio waves, which travel at the speed of light, a "satellite relay" of this kind usually takes no more than a few seconds, at most. The various transmitters and receivers on the satellite and on Earth are examples of antennas.

Other kinds of satellites

Landsat satellite photo of Havana, Cuba

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—just as you can see by zooming in on a Google map of your street!

Photo: Satellite photography has revolutionized map-making. This is Havana, Cuba photographed by the Landsat satellite. Picture courtesy of NASA Landsat program.

Launched in 1993, NASA's Advanced Communications Technology Satellite (ACTS) is an experiment designed to test out future methods of satellite communications. Unlike a traditional satellite, which broadcasts over a wide area, ACTS can transmit information on demand to much smaller regions using "spot beams." It is the first satellite to carry all-digital information at the same rates as fiber-optic transmissions on Earth, which makes it cheaper and improves the quality of communication.