Satellites .


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some theories about satellites
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Satellites .

  2. 2.        What is the satellite . Orbital mechanics . Launching ! Orbital altitude and velocity. Types of satellites. Types of orbits. Space junk .
  3. 3.  A satellite is a moon, planet or machine that orbits a planet or star. For example, Earth is a satellite because it orbits the sun. Likewise, the moon is a satellite because it orbits Earth. Usually, the word "satellite" refers to a machine that is launched into space and moves around Earth or another body in space. Earth and the moon are examples of natural satellites. Thousands of artificial, or man-made, satellites orbit Earth. Some take pictures of the planet that help meteorologists predict weather and track hurricanes. Some take pictures of other planets, the sun, black holes, dark matter or faraway galaxies. These pictures help scientists better understand the solar system and universe. Still other satellites are used mainly for communications, such as beaming TV signals and phone calls around the world. A group of more than 20 satellites make up the Global Positioning System, or GPS. If you have a GPS receiver, these satellites can help figure out your exact location.
  4. 4.  The bird's-eye view that satellites have allows them to see large areas of Earth at one time. This ability means satellites can collect more data, more quickly, than instruments on the ground. Satellites also can see into space better than telescopes at Earth's surface. That's because satellites fly above the clouds, dust and molecules in the atmosphere that can block the view from ground level. Before satellites, TV signals didn't go very far. TV signals only travel in straight lines. So they would quickly trail off into space instead of following Earth's curve. Sometimes mountains or tall buildings would block them. Phone calls to faraway places were also a problem. Setting up telephone wires over long distances or underwater is difficult and costs a lot. With satellites, TV signals and phone calls are sent upward to a satellite. Then, almost instantly, the satellite can send them back down to different locations on Earth.
  5. 5.  Satellites come in many shapes and sizes. But most have at least two parts in common -- an antenna and a power source. The antenna sends and receives information, often to and from Earth. The power source can be a solar panel or battery. Solar panels make power by turning sunlight into electricity. Many satellites carry cameras and scientific sensors. Sometimes these instruments point toward Earth to gather information about its land, air and water. Other times they face toward space to collect data from the solar system and universe.
  6. 6.    Through a lifelong study of the motions of bodies in the solar system, Johannes Kepler (1571-1630) was able to derive three basic laws known as Kepler's laws of planetary motion. Using the data compiled by his mentor Tycho Brahe (1546-1601), Kepler found the following regularities after years of laborious calculations: 1. All planets move in elliptical orbits with the sun at one focus. 2. A line joining any planet to the sun sweeps out equal areas in equal times. 3. The square of the period of any planet about the sun is proportional to the cube of the planet's mean distance from the sun. These laws can be deduced from Newton's laws of motion and law of universal gravitation. Indeed, Newton used Kepler's work as basic information in the formulation of his gravitational theory.
  7. 7.    After completion, a newly manufactured satellite is transferred to the launch site for final testing and fuelling, before being mated to the launch vehicle. The launch is a complex operation conducted in several stages, which differ according to the system used. One common method of placing satellites into geostationary orbit is based on the Hohmann transfer principle. Using this system the satellite is placed into a low earth orbit with an altitude of around 300 kilometers. Once in the correct position in this orbit rockets are fired to put the satellite into an elliptical orbit with the perigee at the low earth orbit and the apogee at the geostationary orbit. When the satellite reaches the final altitude the rocket or booster is again fired to retain it in the geostationary orbit with the correct velocity. Alternatively the satellite is launched directly into the elliptical transfer orbit, typically between 200km and several thousand kilometres from the Earth. Again when the satellite is at the required altitude the rockets are fired to transfer it into the required orbit with the correct velocity.
  8. 8.  Mission control monitors the launch and once the satellite has been released by the launch vehicle into geosynchronous orbit, the satellite operator takes over to acquire the telemetry signal; monitor proper deployment of the solar arrays; fire the necessary satellite motors to fly the satellite to its proper location and then, puts it through a series of in-orbit tests before validating the satellite and ground stations for commercial entry into service.
  9. 9.  Most satellites are launched into space on rockets. A satellite orbits Earth when its speed is balanced by the pull of Earth's gravity. Without this balance, the satellite would fly in a straight line off into space or fall back to Earth. Satellites orbit Earth at different heights, different speeds and along different paths. The two most common types of orbit are "geostationary" (jee-oh-STAY-shun-air-ee) and "polar." A geostationary satellite travels from west to east over the equator. It moves in the same direction and at the same rate Earth is spinning. From Earth, a geostationary satellite looks like it is standing still since it is always above the same location. Polar-orbiting satellites travel in a north-south direction from pole to pole. As Earth spins underneath, these satellites can scan the entire globe, one strip at a time.
  10. 10.        SPUTNIK, OCT. 4, 1957 Because of Soviet government secrecy at the time, no photographs were taken of this famous launch. Sputnik was a 23-inch (58-centimeter), 184-pound (83-kilogram) metal ball. Although it was a remarkable achievement, Sputnik's contents seem meager by today's standards: Thermometer Battery Radio transmitter - changed the tone of its beeps to match temperature changes Nitrogen gas - pressurized the interior of the satellite On the outside of Sputnik, four whip antennas transmitted on short-wave frequencies above and below what is today's Citizens Band (27 MHz)
  11. 11.         Weather satellites Communications satellites Broadcast satellites Scientific satellites Navigational satellites Rescue satellites Earth observation satellites Military satellites
  12. 12.   Geostationary A satellite in a geostationary orbit appears to be in a fixed position to an earth-based observer. A geostationary satellite revolves around the earth at a constant speed once per day over the equator. The geostationary orbit is useful for communications applications because ground based antennas, which must be directed toward the satellite, can operate effectively without the need for expensive equipment to track the satellite’s motion.
  13. 13.   LEO is typically a circular orbit about 400 to 900 kilometres above the earth’s surface and, correspondingly, has a much shorter period (time to revolve around the earth) of about 90 minutes. Because of their low altitude, these satellites are only visible from within a small area (about 1000 km radius) beneath the satellite as it passes overhead. In addition, satellites in low earth orbit change their position relative to the ground position quickly. So even for local applications, a large number of satellites are needed if the mission requires uninterrupted connectivity. For this reason, LEO satellites are often part of a group of satellites working in concert otherwise known as a satellite constellation. Low earth orbiting satellites are less expensive to launch into orbit than geostationary satellites and, due to proximity to the ground, do not require as high a signal strength.
  14. 14.   Molniya orbits can be an appealing alternative. The Molniya orbit is highly inclined, guaranteeing good elevation over selected positions during the northern portion of the orbit. (Elevation is the extent of the satellite’s position above the horizon. Thus, a satellite at the horizon has zero elevation and a satellite directly overhead has elevation of 90 degrees).The Molniya orbit is designed so that the satellite spends the great majority of its time over the far northern latitudes, during which its ground footprint moves only slightly. Its period is one half day, so that the satellite is available for operation over the targeted region for eight hours every second revolution. In this way a constellation of three Molniya satellites (plus in-orbit spares) can provide uninterrupted coverage. Molniya satellites are typically used for telephony and TV services over Russia. Another application is to use them for mobile radio systems (even at lower latitudes) since cars travelling through urban areas need access to satellites at high elevation in order to secure good connectivity, e.g. in the presence of tall buildings
  15. 15.  MEO is the region of space around the Earth above low Earth orbit and below geostationary orbit . The most common use for satellites in this region is for navigation, such as the GPS (with an altitude of 20,200 kilometres), Glonass (with an altitude of 19,100 kilometres) and Galileo (with an altitude of 23,222 kilometres) constellations. Communications satellites that cover the North and South Pole are also put in MEO. The orbital periods of MEO satellites range from about 2 to 24 hours. Telstar, one of the first and most famous experimental satellites, orbits in MEO.