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Communications satellite

Satellites

by Chris Woodford. Last updated: September 22, 2013.

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 wars 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, until recently, 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.

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.

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).

Satellite orbits

Satellites follow three different types of orbits around Earth: geostationary, geosynchronous, and polar. 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 much closer to Earth (just 250–1000 km or 155–621 miles above our planet) and pass over the north and south poles once each day.

How do communications satellites work?

What do they do?

Communications 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.

Satellite communication across Earth using an uplink and downlink

Artwork: 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.

What's inside them?

Labeled parts of a typical communications satellite.

These are amazingly complex and expensive machines with tons of electronic bits and pieces jammed into them, but let's not get too bogged down in the details: the basic idea is very simple. In this outside view of a typical satellite, from a patent filed in 1968 by German engineer Hans Sass (US Patent: #3,559,919: Active communication satellite), you can see all the main bits and it's easy to figure out what they do.

I've left the original numbers on the diagram and I won't bother to label them all, since some are obvious and some are duplicates of others. The most interesting bits are the fold-out solar panels that power the satellite, the sending and receiving antennas that collect signals coming up from Earth and send them back down, and the motors and engines that keep the satellite in exactly the right position at all times:

4: Large parabolic dish antenna for sending/receiving signals. (Orange)

5: Small parabolic dish antenna for sending/receiving signals. (Orange)

6: Lower solar "battery" of four solar panels. (Red)

7: Upper solar "battery" of four more solar panels. (Red)

8: Supports fold out the lower solar panels once the satellite is in orbit. (Gray-brown)

9: Supports fold out the upper solar panels. (Gray-brown)

10: Main satellite rocket motor. (Light blue)

11, 12, 15, 17: Small control engines keep the satellite in its precision position, spin, and orbit. (Green)

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.

Who invented satellites?

The Echo communications satellite pictured at NASA's Langley Research Center, 1960.

The idea of using a satellite as a mirror in space—to bounce signals from one side of Earth to the other—was "launched" in 1945 by science fiction author Arthur C. Clarke (1917–2008), who wrote two hugely influential articles setting out his plan in detail (one was unpublished, the other published as "Extra-Terrestrial Relays: Can Rocket Stations Give World-Wide Radio Coverage?" in Wireless World, October 1945). His proposal was to place three satellites in a geosynchronous orbit 35,000km (23,000 miles) above Earth, spaced out evenly to cover about a third of the planet each: one would cover Africa and Europe, a second would cover China and Asia, and a third would be dedicated to the Americas. Although Clarke didn't patent the geostationary communications satellite, he is generally credited with its invention, even though other space pioneers (notably German wartime pioneer Herman Oberth) had proposed similar ideas years before.

It took another decade for Clarke's bold plan to move toward reality. First, satellites themselves had to be proved viable; that happened with the launch of the Russian Sputnik 1 in October 1957. Three years later, when the Echo communications satellite was launched, engineers successfully demonstrated that radio telecommunications signals could be relayed into space and back, just as Clarke had predicted. Telstar, the first communications satellite, was launched in July 1962 and immediately revolutionized transatlantic telecommunications. During the mid-1960s, 11 nations came together to form INTELSAT (International Telecommunications Satellite Consortium), which launched the world's first commercial communications satellite INTELSAT 1 ("Early Bird"), in geosychronous orbit, in April 1965. This modest little space machine was a tiny electronic miracle: weighing just 35kg (76 lb), it could transmit 240 telephone simultaneous calls or a single black-and-white TV channel.

Photo: Echoes of history: Designed by NASA, the Echo communications satellite was a giant mylar balloon some 30m (100ft) in diameter designed to sit in space and bounce signals back like a mirror. You can see how big it is from the size of the car and people at the bottom, which I've colored red to help you pick them out. Picture courtesy of Great Images in NASA.

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Text copyright © Chris Woodford 2000, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2000) Satellies. Retrieved from http://www.explainthatstuff.com/satellites.html. [Accessed (Insert date here)]

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