NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite Assignment 1 SATELLITE COMMUNICATIONSOlaño, Reymart September 05, 2011Communications 1/BSECE 41A1 Score: Engr. Grace Ramones Instructor
INTRODUCTION TO SATELLITE TELECOMMUNICATIONA satellite communications (sometimes abbreviated to Comsat) is an artificial satellite stationed in space forthe purposes of telecommunications using radio at microwave frequencies. Most communications satellitesuse geosynchronous orbits or near geostationary orbits, although some recent systems use low Earth-orbiting satellites.Communications satellites provide a technology that is complementary to that of fibrotic submarinecommunication cables. Unlike fiber optic communication, satellite communication has a propagation delay(also called a path delay) of at least 270milliseconds, which is the time it takes the radio signal to travel35,800 km from earth to a satellite and then back to earth. Satellite Internet connections average a 600-800millisecond delay, about ten times than that of a terrestrial Internet link. This delay is a challenge todeployment of Virtual private networks over satellite internet connections.MAIN COMPONENTS:Satellite communications are comprised of two basic elements:1. THE SATELLITE - The satellite is also known as the space segment. It is composed of the following separateunits; i.e.a) The satellite and telemetry controlsb) The transponder
The transponder comprised of the receiving antenna to catch-up signals from the ground station, a broadband receiver, an input multiplexer and a frequency converter that is used to reroute the received signalsthrough a high powered amplifier for downlink.The main function of satellite is to reflect signals. In case of a telecom satellite, the primary role is to pick upsignals from a ground station, which is located, a considerable away from the first. This relay action can betwo way, as in the case of a long distance phone call. Another use of satellite is the television broadcasts.Number of programs are first up-linked and then down-linked over wide region. The customer havingappropriate devices can receive and watch the programs. One of the modern uses of satellite is gettinginformation along with image (commonly known as space/satellite image) of any desired location on earth.2. THE GROUND STATION - This is called the earth segment. A base band processor, an up-converter, highPowered amplifier and a parabolic dish antenna is involved to transmit the terrestrial data to an orbitingsatellite. In the case of downlink, the ultimate reverse operation is being down and up-linked signals arerecaptured through parabolic antenna.
DIFFERENT UTILITIES OF SATELLITE COMMUNICATION1. TRADITIONAL TELECOMMUNICATIONS - People have been facing problems in case of long distancetelephone network; there has-been a need to connect the telecommunications network of one country toanother. Submarine cables played a major role to achieve the goal. Still some crucial and critical factorsremained unachieved to connect transoceanic points, geographically remote areas or poor countries thathave little communications infrastructure.2. CELLULAR - Various schemes are invented to make the satellites feasible to increase the bandwidthavailable to ground based cellular networks. Every cell in a cellular network consists affixed range ofchannels which consist of either frequencies, as in the case of FDMA systems, or time slots, as in the case ofTDMA. A particular cell that can only operate within those channels allocated to it, overloading can occur.With the help of satellite which operate at frequency outside those of the cell, extra channel can beprovided on demand to an overloaded cell. Any of the overloaded cells can easily use these newly satellite-transmitted signals. In this case the cell is not bound by the bandwidth restrictions as compare to theprevious scenario. In other words the satellite provides its own bandwidth for a network of cells that can beused by any cell (that need it) without being bound by terrestrial bandwidth and location restrictions.3. TELEVISION SIGNALS - Satellites have been used since 1960 to transmit broadcast television signalsbetween the network hubs of television companies and their network members. Sometime, a whole setoffprograms is transmitted at once and recorded at the affiliate, and then broadcast to the local populaceaccording to the appropriate time. In the 1970’s it became possible for private individuals to download thesame signals that the network and cable companies were transmitting, using C-band reception dishes. Thisfree viewing of the corporate contents by individuals let to scrambling and subsequent resale of thedescrambling codes to individual customers, which started the direct-to-home industry. The direct-to-homeindustry has gathered even greater response since the introduction of digital direct broadcast service.4. MARINE COMMUNICATIONS - In the marine community, satellite communication systems such asImmarsat provide good communication links to ships at sea. These links use a VSAT type device to connectto geosynchronous satellites, which in turn links the ship to a land based point having respectivetelecommunications system.5.SPACE BOURNE LAND MOBILE - Along the same lines as the marine based service, there are VSAT deviceswhich can be used to establish communication links even from the world’s most remote regions. Thesesdevices can be hand-held or fit into a briefcase or might be bigger. Digital data at 64KISDN is available withsome (Immarsat).6.SATELLITE MESSAGING FOR COMMERCIAL JETS - Another service provided by geosynchronous satellite isthe ability for passenger on an airborne aircraft to connect directly to a land based telecom network.7.GLOBAL POSITIONING SERVICES - Another VSAT oriented service, in which a small apparatus containingthe ability to determine navigational coordinates by calculating a triangulating or the signals from multiplegeosynchronous satellites.
1.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. The internationalregions 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-stationaryorbit above the particular Ocean, where they provide a transoceanic telecommunication route. In additionto providing trans oceanic routes, the INTELSAT satellites are used for domestic services within any givencountry and regional services between countries. Two such services are vista for telephony and Intelnetfor data exchange.DOMSAT Domestic satellites are used to provide various telecommunication services, such as voice, data, andvideo transmission (T.V channels), with in a country. Satellite cell phones allow global travelers and thosein remote areas to avoid landlines and terrestrial cell phone services entirely. Satellite cell phones relayyour call to a satellite and down through a hub to the end user. This means that most of the earthsgeographical area is now accessible by a satellite cell phone! Third party providers of satellite cell includeSatcom Global, Roadpost Satcom, Online Satellite 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 aircrafts indistress .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 the planetsaround the sun. Johannes Kepler (1571 -1630) was derived empirically three laws describing planetarymotion . In 1665, Newton was able to derive Keplers laws from his own laws of mechanics and theory ofgravitation.Keplers laws: Keplers first law: Keplers first law states that the path followed by the satellite around the ptimarywill be an ellipse Keplers second law: Keplers second law states that for equal time intervals, the satellite will sweepareas in its orbital plane. Keplers third law: Keplers third law states that the square of periodic time of orbit is proportional tothe 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 to north. Descending node. The point where the orbit crosses the equatorial plane going from north to south. Line of nodes. The line line joining the ascending and descending nodes through the center of the 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 earths rotation. Retrograte orbit. An orbit in which the satellite moves in a direction counter to the earths rotation. Argument of perigee. The angle from ascending node to perigee, measured in the orbital plane at theearths center in direction of satellite motion. Mean anomaly. Mean anomaly M gives an average value of the angular position of the satellite withreference to the perigee True anomaly. The true anomaly is the angle from perigee to the satellite position, measured at theearths center. this gives the true angular position of the satellite in the orbit as a function of time.4.Satellite system A satellite communication system can be broadly divided into two segments, a ground segment and aspace-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. Individualcells can generate small amounts of power, and therefore array of cells in series-parallel connection arerequired . Cylindrical solar arrays are used with spinning satellites, (The gyroscopic effect of the spin is used formechanical orientational stability) Thus the array are only partially in sunshine at any given time. Another type of solar panel is the rectangular array or solar sail. solar sail must be folded during thelaunch phase and extended when in geo-stationary orbit. Since the full component of solar cells areexposed to sun light ,and since the Sail rotate to track, the sun , they capable of greater power outputthan 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 a satelliteis there for the purpose of controlling its attitude. Attitude control is necessary, for example, to ensure
that directional antennas point in the proper directions. In the case of earth environmental satellites theearth-sensing instrument must cover the required regions of the earth, which also requires attitudecontrol. A number of forces, referred to as disturbance forces can alter attitude, some examples being thegravitational forces of earth and moon, solar radiation, and meteorite impacts.Station keeping: A satellite that is normally in geo-stationary will also drift in latitude, the main perturbing forces beingthe gravitational pull of the sun and the moon . the force cause the inclination to change at the rate ofabout 0.85 deg./year. if left uncorrected, the drift would result in a cycle change in the inclination going0 to 14.67deg in 26.6 years and back to zero , when the cycle is repeated. To prevent the shift ininclination from exceeding specified limits, jets may be pulled at the appropriate time to return theinclination to zero. Counteracting jets must be pulsed when the inclination is at zero to halt that change ininclination.Thermal control: Satellites are subject to large thermal gradients, receiving the sun radiation on one side while the otherside faces into space. In addition, thermal radiation from the earth, and the earths abedo, which is thefraction on the radiation falling on the earth which is reflected can be sight for low altitude, earth-orbitingsatellites, although it is negligible for geo-stationary satellites. Equipment in the satellite also generatesheat which has to be removed. the most important consideration is that the satellites equipment shouldoperate as near as possible in a stable temperature environment. various steps are taken to achieve this.Thermal blankets and shields may be used to provide insulation. radiation mirrors are often used toremove heat from communication payload. These mirrored drums surrounded the communicationequipment shelves 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 the heartsmay be switched on to make reduction that occurs when transponders are switched off.TT&C subsystemTelemetry systemThe telemetry, tracking, and command (TT&C) subsystem performs several routine functions abroad aspacecraft. the telemetry or "telemetering" function could be interpreted as "measurement at a distance".specifically, it refers to the over all operation of generating an electrical signal proportional to the quantity
being measured, and encoding and transmitting this to a distant station, which for satellite is one of theearth stations, which for the satellite is one of the earth stations. Data that are transmitted as telemetrysignals include attribute information such as obtained from sun earth sensors; environmental informationsuch as magnetic field intensity and direction; the frequency of meteorite impact and so on ;andspacecraft information such as temperatures and power supply voltages, and stored fuel pressure.Command systemsCommand system receives instructions from ground system of satellite and decodes the instruction andsends commends to other systems as per the instruction.Tracking;Tracking of the satellite is accomplished by having the satellite is accomplished by having the satellitetransmit beacon signals which are received at the TT&C earth stations. Tracking is obviously importantduring the transmitter and drift orbital phases of the satellite launch. When on-station, a geo-stationarysatellite will tend to shifted as a result of the various distributing forces, as described previously .Therefore it is necessary to be able to track the satellites movements and send correction signals asrequired. Satellite range is also required for time to time. This can be determined by measurement ofpropagation delay of signals specially transmitted for ranging purposes.Transponders:A transponder is the series of interconnected units which forms a single communication channel betweenthe receive and transmit antennas in a communication satellite. Some of the units utilized by atransponder in a given channel may be common to a number of transponders. Thus, although referencemay be made to specific transponder, this must be thought of as an equipment channel rather than singleitem of equipment.Transponder consist of wideband receivers, input de-multiplexer, power amplifier components.Antenna sub system:The Antennas carried abroad a satellite provide the dual functions of receiving the up link and transmittingthe down link signals. They range from dipole-type antennas, where omni directional characteristics arerequired, to the highly directional antennas required for telecommunications purposes and TV relay andbroadcasting.