SATELLITE COMMUNICATION AND IT'S APPLICATION IN GPS

Editor's Notes

• #5: The dot pattern shows that as the planet is closest the sun, the planet is moving fastest and as the planet is farthest from the sun, it is moving slowest. Nonetheless, the imaginary line joining the center of the planet to the center of the sun sweeps out the same amount of area in each equal interval of time.
• #6: In 1903, Konstantin Tsiolkovsky (1857–1935) published Means of Reaction Devices (in Russian: ???????????? ??????? ??????????? ??????????? ?????????), which is the first academic treatise on the use of rocketry to launch spacecraft. He calculated the orbital speed required for a minimal orbit around the Earth at 8 km/s, and that a multi-stage rocket fueled by liquid propellants could be used to achieve this. He proposed the use of liquid hydrogen and liquid oxygen, though other combinations can be used.In 1928 Slovenian Herman Potocnik (1892–1929) published his sole book, The Problem of Space Travel — The Rocket Motor (German: Das Problem der Befahrung des Weltraums — der Raketen-Motor), a plan for a breakthrough into space and a permanent human presence there. He conceived of a space station in detail and calculated its geostationary orbit. He described the use of orbiting spacecraft for detailed peaceful and military observation of the ground and described how the special conditions of space could be useful for scientific experiments. The book described geostationary satellites (first put forward by Tsiolkovsky) and discussed communication between them and the ground using radio, but fell short of the idea of using satellites for mass broadcasting and as telecommunications relays.In a 1945 Wireless World article the English science fiction writer Arthur C. Clarke (1917–2008) described in detail the possible use of communications satellites for mass communications.[3] Clarke examined the logistics of satellite launch, possible orbits and other aspects of the creation of a network of world-circling satellites, pointing to the benefits of high-speed global communications. He also suggested that three geostationary satellites would provide coverage over the entire planet.
• #10: A low Earth orbit (LEO) is generally defined as an orbit below an altitude of approximately 2,000 kilometers (1,200 mi). Given the rapid orbital decay of objects below approximately 200 kilometers (120 mi), the commonly accepted definition for LEO is between 160 kilometers (99 mi) and 2,000 kilometers (1,200 mi) above the Earth's surface.[1][2] With the exception of the lunar flights of the Apollo program, all human spaceflights have taken place in LEO (or were suborbital). The altitude record for a human spaceflight in LEO was Gemini 11 with an apogee of 1,374.1 kilometers (853.8 mi). All manned space stations to date, as well as the majority of artificial satellites, have been in LEOOrbit period ranges from 1.5 to 2 hours.Diameter of coverage is about 8000 km.Round-trip signal propagation delay less than 20 ms.Maximum satellite visible time up to 20 min.Atmospheric drag results in orbital deterioration.
• #11: Medium Earth orbit (MEO), sometimes called intermediate circular orbit (ICO), is the region of space around the Earth above low Earth orbit (altitude of 2,000 kilometres (1,243 mi)) and below geostationary orbit (altitude of 35,786 kilometres (22,236 mi)).[3]The most common use for satellites in this region is for navigation, communication, and geodetic/space environment science.[3] The most common altitude is approximately 20,200 kilometres (12,552 mi)), which yields an orbital period of 12 hours, as used, for example, by the Global Positioning System(GPS).[3] Communications satellites that cover the North and South Pole are also put in MEO.[4]
• #12: A geostationary orbit, or Geostationary Earth Orbit (GEO), is a circular orbit 35,786 kilometres (22,236 mi) above the Earth's equator and following the direction of the Earth's rotation. An object in such an orbit has an orbital period equal to the Earth's rotational period (one sidereal day), and thus appears motionless, at a fixed position in the sky, to ground observers. Communications satellites and weather satellites are often given geostationary orbits, so that the satellite antennas that communicate with them do not have to move to track them, but can be pointed permanently at the position in the sky where they stay. A geostationary orbit is a particular type of geosynchronous orbit.A geostationary orbit can only be achieved at an altitude very close to 35,786 km (22,236 mi), and directly above the Equator. This equates to an orbital velocity of 3.07 km/s (1.91 mi/s) or a period of 1,436 minutes, which equates to almost exactly one sidereal day or 23.934461223 hours. This ensures that the satellite is locked to the Earth's rotational period and has a stationary footprint on the ground. All geostationary satellites have to be located on this ring.Satellites in geostationary orbits are far enough away from Earth that communication latency becomes significant — about a quarter of a second for a trip from one ground-based transmitter to the satellite and back to another ground-based transmitter; close to half a second for a round-trip communication from one Earth station to another and then back to the first.
• #13: . In satellite communications it is the process of transferring satellite control responsibility from one earth station to another without loss or interruption of service.
• #14: Molniya orbit is a type of highly elliptical orbit with an inclination of 63.4 degrees, an argument of perigee of −90 degrees and an orbital period of one half of a sidereal day. Molniya orbits are named after a series ofSoviet/Russian Molniya (Russian: "Lightning") communications satellites which have been using this type of orbit since the mid-1960s.A satellite in a highly eccentric orbit spends most of its time in the neighborhood of apogee which for a Molniya orbit is over the northern hemisphere, the sub-satellite point at apogee having a latitude of 63.4 degrees North. As the apogee altitude is as high as 40,000 km, it will therefore, for a considerable period around apogee, have an excellent visibility from the Northern hemisphere, from Russia but also from northern Europe, Greenland and Canada.Much of the area of the former Soviet Union, and Russia in particular, is located at high latitudes. To broadcast to these latitudes from a geostationary orbit would require considerable power due to the low elevation angles. A satellite in a Molniya orbit is better suited to communications in these regions because it looks directly down on them.An additional advantage is that considerably less launch energy is needed to place a spacecraft into a Molniya orbit than into a geostationary orbit. Disadvantages are that, as opposed to a spacecraft in a geostationary orbit, the ground station needs a steerable antenna to track the spacecraft and that the spacecraft will pass the Van Allen belt four times per day.It is necessary to have at least three spacecraft if permanent high elevation coverage is needed for a large area like the whole of Russia where some parts are as far south as 45° N. If three spacecraft are used, each spacecraft is active for periods of eight hours per orbit centered at apogee