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Satellites

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A satellite is something that orbits a planet.

Natural satellites are called 'moons'. Some planets have several natural satellites - others have none. We have one!

Artificial satellites are launched into orbit of a planet by us. Look at the links below to find out moore about them.

Uses of Satellites


Satellites are launched into space to do a specific job. The type of satellite that is launched to monitor cloud patterns for a weather station will be different than a satellite launched to send television signals for Sky TV. The satellite has to be designed specifically to fulfill its function.

Here are examples of nine different types of satellites:

Astronomy satellite-
e.g. Hubble Space Telescope
A telescope orbiting the Earth. An astronomy satellite's vision is not clouded by the gases that make up the Earth's atmosphere, so it gives clearer pictures than telescopes on Earth.
Astronomy satellites study stellar phenomenona like black holes, quasars, and distant galaxies. These are not to be confused with space exploration satellites, which also study these phenomena - see below .
Astronomy satellites have many different applications:
  • they can be used to make star maps
  • they can be used to study mysterious phenomena such as black holes and quasars
  • they can be used to take pictures of the planets in the solar system
  • they can be used to make maps of different planetary surfaces
Atmospheric Studies satellites-
e.g. Polar
A type of scientific satellite that studies the Earth's atmosphere. They were some of the very first satellites launched into space
Communications satellites-
e.g. Anik E

A type of satellite used for communications on Earth by allowing radio, television, and telephone transmissions to be sent live anywhere in the world.
Before satellites, transmissions were difficult or impossible at long distances. The signals, which travel in straight lines, could not bend around the round Earth to reach a destination far away. They had to be reflected off layers in the atmosphere and mountain ranges etc. caused shadow areas.
Because communications satellites are in orbit, the signals can be sent instantaneously into space and then redirected to another satellite or directly to their destination.
Navigation satellites-
e.g. Navstar
A type of satellite that gives ships and aircraft their coordinate positions on the Earth.
Navigation satellites were developed in the 1950s, and they rely on the doppler effect to calculate the position of vessels emitting a radio signal. Navigation satellites are also widely used by the military.
Reconaissance or spy satellites-
e.g. Kennan, Big Bird, Lacrosse
Reconnaissance satellites are used to spy on other countries. They provide intelligence information on the military activities of foreign countries.
These satellites can even detect missile launches or nuclear explosions in space.
Reconnaissance satellites can pick up and record radio and radar transmissions while passing over a country and they can be used as an orbital weapon by placing warheads on a low orbit satellite to be launched at a ground target.
Remote Sensing satellites-
e.g. Radarsat
Remote sensing is observing and measuring our environment from a distance. Remote sensing satellites are usually put into space to monitor resources that are important for humans. For example, remote sensing satellites might track animal migration, locate mineral deposits, watch agricultural crops for weather damage, or see how fast the forests are being cut down.
All of these things can be done best from space because a satellite in orbit can normally take photographs of large expanses of land all over the world. Since these satellites are able to take photographs and observe areas all over the globe, the satellite is able to monitor areas in which the climate is very harsh, or which are nearly impossible tor reach by land.
Search and Rescue satellites-
e.g. Cospas-Sarsat
Search and rescue satellites are designed to provide a way for vessels at sea and in the air to communicate from remote areas. These satellites can detect and locate emergency beacons carried by ships, aircrafts, or individuals in remote or dangerous places.
Space Exploration satellites-
e.g. Galileo
Space exploration satellites are not really satellites at all; they are actually space probes. A satellite is defined as something that is orbiting something else (usually a planet), but space probes don't do that - instead they travel deep into the solar system. However, they are similar to orbiting satellites in design and function.

On their journeys, space probes send back detailed pictures and other data of faraway planets and other stellar phenomena. Space exploration satellites are responsible for many of astronomy's most important achievements. Jupiter's rings, for example, were discovered by a space exploration satellite.
Space exploration satellites must be built to last because it takes so long for the satellites to reach their destinations. Space exploration satellites are different from astronomy satellites (see above) because they do not operate from Earth orbit; they are actually sent out into deep space on their own.
Weather satellites-
e.g. Meteosat
  • Communications Satellites


    Communications satellites allow radio, television, and telephone transmissions to be sent live anywhere in the world. Before satellites, transmissions were difficult or impossible at long distances. The signals, which travel in straight lines, could not bend around the round Earth to reach a destination far away and so had to be 'bounced' from station to station off the atmosphere (see diagram above). This caused loss of signal and a time delay in transmission.
    Because satellites are in orbit, the signals can be sent instantaneously into space and then redirected to another satellite or directly to their destination. The microwave frequency that is used allow the signals to pass easily through the atmosphere without much loss of signal.
    The satellite can either have a passive role in communications by simply bouncing signals from the Earth back to another location on the Earth; or take a more active role by using electronic devices called transponders that receive, amplify (increase the amplitude - strength of the signal), and re-broadcast the signals to the Earth.
    Communications satellites are usually in geostationary orbit but they can also be in highly elliptical orbits.
    This type of orbit is roughly egg-shaped - the satellite's velocity changes depending on where it is in its orbital path.
    When the satellite is in the part of its orbit that's close to the Earth, it moves faster because the Earth's gravitational pull is stronger.
    This means that a communications satellite can be over the region of the Earth that it is communicating with for the long part of its orbit but itt will only be out of contact with that region when it quickly zips close by the Earth.


  • Geostationary Orbit Satellites
  • From Earth a geostationary or geosynchronous satellite appears to stay still, always above the same point on the Earth's surface. It is sychronised with the rotation of the Earth. This is because, orbiting very high above the Earth, at an altitude of 35,800 kilometres, geostationary satellites orbit the Earth once every 24 hours. This orbit allows the satellite to monitor the same region of the Earth all of the time.

    The area to which it can transmit is called a satellite's footprint. This footprint can be very large indeed - for example, many Canadian communications satellites have a footprint which covers most of Canada.

    Geostationary satellites usually measure in "real time", meaning they transmit photographs to the receiving system on the ground as soon as the camera takes the picture. A series of photographs from these satellites can be displayed in sequence to produce a movie showing cloud movement.

    This allows weather forecasters to watch the progress of large weather systems such as fronts, storms, and hurricanes. Forecasters can also find out the wind direction and speed by monitoring cloud movement. geostationary satellites are therefore used as weather satellites.

    They are also used as communications satellites.


  • Geosynchronous Orbit Satellites
  • From Earth a geostationary or geosynchronous satellite appears to stay still, always above the same point on the Earth's surface. It is sychronised with the rotation of the Earth. This is because, orbiting very high above the Earth, at an altitude of 35,800 kilometres, geostationary satellites orbit the Earth once every 24 hours. This orbit allows the satellite to monitor the same region of the Earth all of the time.

    The area to which it can transmit is called a satellite's footprint. This footprint can be very large indeed - for example, many Canadian communications satellites have a footprint which covers most of Canada.

    Geostationary satellites usually measure in "real time", meaning they transmit photographs to the receiving system on the ground as soon as the camera takes the picture. A series of photographs from these satellites can be displayed in sequence to produce a movie showing cloud movement.

    This allows weather forecasters to watch the progress of large weather systems such as fronts, storms, and hurricanes. Forecasters can also find out the wind direction and speed by monitoring cloud movement. geostationary satellites are therefore used as weather satellites.

    They are also used as communications satellites.


  • Navigation Satellites
    In the middle of the ocean or out of sight of landmarks, you can't find out your position accurately just by looking out the window. The idea of using satellites for navigation began with the launch of Sputnik 1 on October 4, 1957.
    Scientists at the John Hopkins University's Applied Physics Laboratory monitored that satellite. They noticed that when the transmitted radio frequency was plotted on a graph, a pattern developed. This pattern was recognizable as the doppler effect. The doppler effect is an apparent change of radio frequency as something that emits a signal in the form of waves passes by. You will experience hearing a change in pitch when a siren goes past you at the roadside. This is the doppler effect. The lowering of the perceived frequency has the police car passes you is illustrated on the right.
    Since the satellite was emitting a signal, scientists were able to show that the doppler curve described the orbit of the satellite.
    Today, most navigation systems use time and distance to determine location. they then use a computer system to analyse the data and pinpoint the position on a map graphic. This is far simpler than reading star positions and plotting points manually on a chart!
    Given the velocity and the time required for a radio signal to be transmitted between two points, the distance between the two points can be worked out using the equation: velocity = distance travelled divided by time taken.
    The calculation must be done precisely, and the clocks in the satellite and in the ground-based receiver must be telling exactly the same time - they must be synchronized. If they are, the time it takes for a signal to travel can be measured and then multiplied by the exact speed of light to obtain the distance between the two positions.
    'Satnav' devices are now the 'must have' accessory for drivers. the technology has become much cheaper as it has been mass produced - economy of scale comes into play!

  • Polar Orbit Satellites
  • Polar orbiting satellites orbit in a path that closely follows the Earth's meridian lines, passing over the north and south poles once each revolution.

    As the Earth rotates to the east beneath the satellite, each pass of the satellite monitors a narrow area running from north to south, to the west of the previous pass. These 'strips' can be pieced together to produce a picture of a larger area.

    Polar satellites circle at a much lower altitude than geostationary satellites - at about 850 km. This means that polar satellites can photograph closer up than the high altitude geostationary satellites.

    Polar satellites, therefore, provide more detailed information about violent storms and cloud systems.



  • Weather Satellites
  • Weather satellite technology and communications satellite technology allows you to find out the weather anywhere in the world any time of the day. Television stations and websites carry weather information all day long. Meteorologists use weather satellites images for many purposes:

    • Radiation measurements from the earth's surface and atmosphere give information on heat energy released from the Earth and the Earth's atmosphere.
    • Fishermen can find out valuable information about the temperature of the seas.
    • Satellites monitor the amount of snow, ice flow in the Arctic and Antarctic, and the depth of the ocean.
    • Infrared sensors on satellites examine crop conditions, areas of deforestation and regions of drought.
    • Some satellites have a water vapour sensor that can measure how much water vapouris in the atmosphere.
    • Satellites can detect volcanic eruptions and monitor the motion of ash clouds.
    • Satellites monitor freezing air movement allowing weather forecasters to warn growers of coming low temperature spells.
    • Satellites receive environmental information from remote data collection platforms on the surface of the Earth. These include transmitters in floating buoys, gauges of river levels and conditions, automatic weather stations, stations that measure earthquake and tidal wave conditions, and ships. This information, sent to the satellite from the ground, is then relayed from the satellite to a central receiving station back on Earth.
  • There are two basic types of weather satellites: those in geostationary orbit and those inpolar orbit.

     

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