SATELLITE

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A satellite is a body that revolves around a larger one. The moon is a natural satellite of earth. Artificial satellites are made by humans and help us in various fields. They are evidence of the enormous progress made by humans. Almost every country has launched one. Even the sun has nine of it's own satellites known as planets. Some of the jobs in which artificial satellites are applied include meteorology, astronomy, geology, communication and military, navigational needs. Highly powered rockets are used to push these satellites into orbit. Sometimes they are carried in shuttle orbiters and then released in space. Then tiny thrusters put the satellite in it's desired location. An orbit is a regular path on which satellites keep moving continuously while revolving around a larger body. This constant moving balances the satellite in orbit with centrifugal force. The speed of a satellite must match the pull of gravity. The closer a satellite moves to earth, the faster it goes and the less time it takes to complete one orbit. There are several different kinds of orbits. POLAR ORBIT is one, which revolves around earth over the north and South Pole. At an altitude of 540-km satellites here circle the earth 18 times a day. They always pass the equator at the same time on earth. Some satellites are place in the GEOSTATIONARY ORBIT. This is located about 22300 km above the equator Satellites in this orbit move with the same speed as the earths rotation and make one orbit every 24 hours. There are also elliptical orbits. Mostly used by environmental satellites, this orbit swings away from earth at a point called the apogee and comes close to earth at another low point called the pedigree. There is also an orbit called the sun synchronous orbit.

When in orbit, a satellite would gain 1% speed at a loss of 2% height. The distance to be traveled is also reduced by 2% while the orbital time is reduced by 3%. Velocity increases at the loss of altitude. The formula V2 = 2 times g times h, signifies this. Here V is velocity; G is acceleration due to gravity and his orbital altitude. If these factors are not achieved while switching between orbits, the craft may fall to earth and burn up in the atmosphere. To place a satellite in orbit, it is first injected into a low circular orbit. Then it is pushed into the pedigree of an elliptical or transfer orbit that intersects into the final required orbit. At the apogee of this elliptical orbit, the apogee kick motors are used to take it into the final orbit. These motors are actually small thrusters, which use hot or cold gas. Hot gas thrusters produce more power as they derive energy from a chemical reaction. REM thrusters make easy position changes for satellites. They are also Resisto jet thrusters, Microwave electro thermal thrusters, Multipropellant resistojets and others. The amount and size of thrusters are chosen according to their missions so they do not run out of gas. Hydrazine and hydrogen peroxide are commonly used fuels in thrusters. Hydrazine freezes near zero degree and thus requires heaters to warm it up. A satellite also needs stabilization during orbital placement to prevent misalignment or tumbling. Altitude control systems help to properly align satellites. Stabilization is needed for certain reasons such as,
1) Payloads like telescopes or cameras are needed to be properly positioned to observe certain celestial objects or to watch earth, etc.
2) If the satellites are not stabilized, they have to use special omni antennas to send signals. This creates a need for extra transmitting power since only small portions of the signals reach earth. Though with ACS stabilization, antennas can perfectly be pointed towards earth and the amount of power needed can be decreased.
3) In space, satellites movements are often disturbed by solar wind, atmospheric drag, radiation pressure, magnetic fields, gravitational fields, micrometeorites and movement of machinery inside the satellite itself. There are usually three types of stabilizations used -Gravity gradient, spin stabilization and three axis stabilizations.

Satellite coverage over earth depends upon the increase and decrease in altitude. The higher the altitude the less satellites required to give coverage on earth. Though it would take extra launch capability to place it in high orbits. Strong signals will be needed to travel long distances. Following are a few altitude control systems:
Earth sensors
Horizon crossing indicator
Colonical earth sensors
Bore sight lim sensors
Sun sensor
Star scanner

Various commands are used to operate and maintain satellites. To give proper commands, information on the satellites condition and position is needed. The telemetry, tracking and command systems handle these requirements. They are classified into a system called TTC. Telemetry is the way of making various measurements of a certain object (in this case a satellite) and transmitting them to another distant place. Various sensors on a satellite gather information about the satellites. Then they are put into a desired form, modulated and transmitted down to an earth station. Local environment sensors measure voltage, current speed, temperature pressure, vibrations, etc. These sensors are often self-generating or may depend upon circuits for voltage supply. Another group of sensors called scientific measurement sensors detect things happening away from the satellite in another region of space. Some of these sensors include high energy particle sensors , magnetic field sensors, electric field sensors, plasma sensors , micrometeorite sensors. The satellites, are controlled with two groups of commands: · Value commands -they are digital words of various lengths. · Pulse commands -these are mainly ON/OFF relay commands.

Tracking is the means of observing and collecting data to find the moving path of a certain object. Radio, radar, optical tracking and infrared are common techniques applied in tracking. Optical tracking works on objects within visibility while the radar system uses transmitter and receiver to track quiet objects. Various kinds of antennas are used for transmitting and receiving radio signals. Some antennas with reflectors strengthen signals.

The paroboloidal reflector, cassegrain aerials and horn reflectors are amongst the many antennas used on satellites. The paroboloidal reflector offers high performance and works with a wide range of frequencies. A satellite requires thermal control systems to maintain proper temperature for all components aboard the satellite. Radiators are commonly used to transfer extra heat from the satellite into space. Heat pipes conduct heat from components to the radiators. These radiators are coated with special materials with high thermal conductivity. This is the amount of heat transmitted per unit time, per unit cross section, per unit temperature gradient. Any other surfaces of satellites not used as radiators are covered with multiplayer thermal insulation blankets, such as low conductivity foam. Special heat absorbers are also used to increase temperature inside a satellite. Satellites also use heat sinks which posses large thermal mass and capacity. These are placed in contact with equipment whose temperatures are to be controlled. When heat is generated in the components, it gets conducted into the heat sinks. Satellites also have refrigeration systems along with heaters.

Power is needed to run a satellite. This power comes from two main source solar cells and storage batteries. Solar cells are made out of silicon crystals. When solar light hits the surfaces of the cells small holes are created which move swiftly around the cell creating electrical charges. These cells also recharge the storage batteries. Radiation can damage solar cells by trapping electrons. Particles can penetrate far into the base of the cells and cut down it's lifetime and power output. Though latest inventions of lithium doping of solar cells have helped to repair solar cells.

A satellite has two types of batteries -primary and secondary. These cells come in use when the sun is not available such as an eclipse. These batteries are also needed when a satellite comes out of an eclipse. At this time because the solar cells are cooled, they produce extra power. This power can only be utilized if it can be stored. For example -NI-CD, AH NICD, NI-H2, AG H2, are a few of the batteries used. Nuclear power packs are also used though they can leak.

Various satellite tests are done to check it's performances. A few are given below:

Alignment verification tests Acceleration tests Shock tests Model engineering tests Vibration tests Mass properties measurement tests Inertia tests Appendage tests Antenna tests Electromagnetic tests Thermal vacuum tests Solar simulation tests Corona checks Leak checks, etc.