NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite Assignment 1 SATELLITE COMMUNICATIONSAlido, Ronald C. September 05, 2011Communications 1/BSECE 41A1 Score: Engr. Grace Ramones Instructor
Satellite Communication1.Satellite systems:There are three different types of satellite systems. 1. International satellite communication system INTELSAT. 2. Domestic satellite system DOMSAT. 3. Search and rescue system SARSAT.INTELSAT: The INTELSAT Organization was established in 1964 to handle the myriad of technical andadministrative problems associated with a world wide telecommunication system. Theinternational regions served by INTELSAT are divided in to the Atlantic Ocean region (AOR),the Pacific Ocean Region (POR), and the Indian Ocean region (IOR). For each region ,satellites are positioned in geo-stationary orbit above the particular Ocean, where they provide atransoceanic telecommunication route. In addition to providing trans oceanic routes, theINTELSAT satellites are used for domestic services within any given country and regionalservices between countries. Two such services are vista for telephony and Intelnet for dataexchange.DOMSAT Domestic satellites are used to provide various telecommunication services, such as voice,data, and video transmission (T.V channels), with in a country. Satellite cell phones allow globaltravelers and those in remote areas to avoid landlines and terrestrial cell phone servicesentirely. Satellite cell phones relay your call to a satellite and down through a hub to the enduser. This means that most of the earths geographical area is now accessible by a satellite cellphone! Third party providers of satellite cell include Satcom Global, Roadpost Satcom, OnlineSatellite Communications, and others.SARSAT SARSAT is one type of Polar orbiting satellites. Polar-orbiting satellites orbit the earth in such a way as to cover the north and south polar-regions. Infinite number of polar polar satellite orbits are possible
Polar satellites are used to provide environmental data, and to help locate ships and aircraftsin distress .This service known as SARSAT, for search and rescue satellite. This figure shows polar satellite path and earth rotation2.Keplers Laws: Artificial satellites which orbit the earth follow the same laws that govern the motion of theplanets around the sun. Johannes Kepler (1571 -1630) was derived empirically three lawsdescribing planetary motion . In 1665, Newton was able to derive Keplers laws from his ownlaws of mechanics and theory of gravitation.Keplers laws: Keplers first law: Keplers first law states that the path followed by the satellite around theptimary will be an ellipse Keplers second law: Keplers second law states that for equal time intervals, the satellitewill sweep areas in its orbital plane. Keplers third law: Keplers third law states that the square of periodic time of orbit isproportional to the cube of the mean distance between the two bodies.3.Definitions And Related Terms Of Earth-Orbiting Satellites Apogee. The point farthest from earth. Perigee. The point of closest approach to earth. Line of apsides. The line joining the perigee and apogee through the center of the earth. Ascending node. The point where the orbit crosses the equatorial plane going from south tonorth.
Descending node. The point where the orbit crosses the equatorial plane going from north tosouth. Line of nodes. The line line joining the ascending and descending nodes through the center ofthe earth. Inclination. The angle between the orbital plane and the earths equatorial plane. Prograde orbit. An orbit in which the satellite moves in the same direction as the earthsrotation. Retrograte orbit. An orbit in which the satellite moves in a direction counter to the earthsrotation. Argument of perigee. The angle from ascending node to perigee, measured in the orbital planeat the earths center in direction of satellite motion. Mean anomaly. Mean anomaly M gives an average value of the angular position of the satellitewith reference to the perigee True anomaly. The true anomaly is the angle from perigee to the satellite position, measuredat the earths center. this gives the true angular position of the satellite in the orbit as a functionof time.4.Satellite system A satellite communication system can be broadly divided into two segments, a groundsegment and a space-segment. The space system includes Satellite. Satellite system consist of the following systems.Power supply: The primary electrical power for operating electronic equipment is obtained from solar cells.Individual cells can generate small amounts of power, and therefore array of cells in series-parallel connection are required .
Cylindrical solar arrays are used with spinning satellites, (The gyroscopic effect of the spin isused for mechanical orientational stability) Thus the array are only partially in sunshine at anygiven time. Another type of solar panel is the rectangular array or solar sail. solar sail must be foldedduring the launch phase and extended when in geo-stationary orbit. Since the full component ofsolar cells are exposed to sun light ,and since the Sail rotate to track, the sun , they capable ofgreater power output than cylindrical arrays having a comparable number of cells. To maintain service during an eclipse, storage batteries must be provided .Attitude control: The attitude of a satellite refers to its Orientation in space. Much of equipment carried abroad asatellite is there for the purpose of controlling its attitude. Attitude control is necessary, forexample, to ensure that directional antennas point in the proper directions. In the case of earthenvironmental satellites the earth-sensing instrument must cover the required regions of theearth, which also requires attitude control. A number of forces, referred to as disturbance forcescan alter attitude, some examples being the gravitational forces of earth and moon, solarradiation, and meteorite impacts.Station keeping: A satellite that is normally in geo-stationary will also drift in latitude, the main perturbing forcesbeing the gravitational pull of the sun and the moon . the force cause the inclination to changeat the rate of about 0.85 deg./year. if left uncorrected, the drift would result in a cycle change inthe inclination going 0 to 14.67deg in 26.6 years and back to zero , when the cycle is repeated.To prevent the shift in inclination from exceeding specified limits, jets may be pulled at theappropriate time to return the inclination to zero. Counteracting jets must be pulsed when theinclination is at zero to halt that change in inclination.Thermal control: Satellites are subject to large thermal gradients, receiving the sun radiation on one side whilethe other side faces into space. In addition, thermal radiation from the earth, and the earthsabedo, which is the fraction on the radiation falling on the earth which is reflected can be sightfor low altitude, earth-orbiting satellites, although it is negligible for geo-stationary satellites.
Equipment in the satellite also generates heat which has to be removed. the most importantconsideration is that the satellites equipment should operate as near as possible in a stabletemperature environment. various steps are taken to achieve this. Thermal blankets andshields may be used to provide insulation. radiation mirrors are often used to remove heat fromcommunication payload. These mirrored drums surrounded the communication equipmentshelves in each case and provide good radiation paths for the generated heat to escape in tosurround space. To maintain constant-temperature conditions, heaters may be switched on to make up for thehearts may be switched on to make reduction that occurs when transponders are switched off.