Satellites
Last updated: May 5, 2009.
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 Great Images in NASA.
How satellites are launched
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 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.
How communications satellites work
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
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.
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.
Further reading
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.
