NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGYAmafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite Assignment 1 SATELLITE COMMUNICATIONSTagasa, Jerald A. September 05, 2011Communications 1/BSECE 41A1 Score: Engr. Grace Ramones Instructor
HISTORY OF SATELLITE COMMUNICATIONSThe first artificial satellite was placed in orbit by theRussians in 1957. That satellite, called Sputnik, signalled thebeginning of an era.The United States, who was behind the Russians, made an all-outeffort to catch up, and launched Scorein 1958. That was thefirst satellite with the primary purpose of communications.The first regular satellite communications service was used bythe Navy in 1960. The moon was used to bounce teletypewritersignals between Hawaii and Washington, D.C. During the early1960s, the Navy used the moon as a medium for passing messagesbetween ships at sea and shore stations. This method ofcommunications proved reliable when other methods failed.Military satellite communications technology was at a low leveluntil 1965. At that time high quality voice transmissions wereconducted between a satellite and two earth stations. That wasthe stepping stone to the Initial Defense CommunicationsSatellite Program (IDCSP), which will be covered later in thischapter.Experience with satellite communications has demonstrated thatsatellite systems can satisfy many military requirements. Theyare reliable, survivable, secure, and a cost effective method oftelecommunications. You can easily see that satellites are theideal, if not often the only, solution to problems ofcommunicating with highly mobile forces. Satellites, if properlyused, provide much needed options to large, fixed-groundinstallations.For the past fifty years, the Navy has used high-frequency (hf)transmissions as the principal method of sending messages. Inthe 1970s, the hf spectrum was overcrowded and "free"frequencies were at a premium. Hf jamming and electroniccountermeasures (ECM) techniques became highly sophisticatedduring that period. As a result the need for new and advancedlong-range transmission methods became apparent.Communications via satellite is a natural outgrowth of moderntechnology and of the continuing demand for greater capacity andhigher quality in communications.In the past, the various military branches have had theresources to support their communications needs. Predicted usage
indicates that large-scale improvements will have to be made tosatisfy future needs of the Department of Defense. These needswill require greater capacity for long-haul communications topreviously inaccessible areas. Satellite communications has themost promise for satisfying these future requirements.DEFENSE COMMUNICATIONS SATELLITE PROGRAM (DCSP)The Defense Communications Satellite Program (DCSP) wasinitiated by the Secretary of Defense in 1962. Phase I of theprogram was given the title Initial Defense CommunicationsSatellite Program (IDCSP). The first satellite launch occurredin June 1966 when seven experimental satellites were placed intoorbit. The final launch of this program consisted of eightsatellites and occurred in June 1968.DEFENSE SATELLITE COMMUNICATIONS SYSTEM (DSCS) PHASE IIThe Phase II Defense Satellite Communications System (DSCP PhaseII) has changed from an all-analog communications system to anall-digital communications system. The performance capabilityprovided by the Phase II DSCS is limited by equipmentavailability. Extensive digital traffic capability has becomecommon. You can credit this to the availability of digitalmodems (modulator/demodulator) and broadband equipment. Overallperformance of the Phase II DSCS is a great improvement over thecapabilities provided by Phase I DSCS. The Phase II satellitesprovide a great increase in effective radiated power and rfbandwidths. You will find these satellite configurations usewide coverage and narrow beam antennas. They provide anextensive range of communications services and capabilities.FUNDAMENTAL SATELLITE COMMUNICATIONS SYSTEMA satellite communications system uses satellites to relay radiotransmissions between earth terminals. The two types ofcommunications satellites you will study are ACTIVE and PASSIVE.A passive satellite only reflects received radio signals back toearth. An active satellite acts as a REPEATER; it amplifiessignals received and then retransmits them back to earth. Thisincreases signal strength at the receiving terminal to a higherlevel than would be available from a passive satellite.A typical operational link involves an active satellite and twoor more earth terminals. One station transmits to the satelliteon a frequency called the UP-LINK frequency. The satellite thenamplifies the signal, converts it to the DOWN-LINK frequency,
and transmits it back to earth. The signal is next picked up bythe receiving terminal. Figure 4-1 shows a satellite handlingseveral combinations of links simultaneously.Figure 4-1. - Satellite communications system.DESCRIPTION OF COMMUNICATIONS SATELLITE SYSTEMThe basic design of a satellite communications system depends toa great degree upon the characteristics of the orbit of thesatellite. In general terms, an orbit is either elliptical orcircular in shape. A special type of orbit is a SYNCHRONOUSORBIT. In this type you will find the period (time required forone revolution) of the orbit the same as that of the earth. Anorbit that is not synchronous is called ASYNCHRONOUS. A periodof orbit that approaches that of the earth is called NEARSYNCHRONOUS (subsynchronous). Orbits are discussed in moredetail later in this chapter.In addition to the fundamental components shown in figure 4-1,the design of the overall system determines the complexity ofthe various components and the manner in which the systemoperates. Current satellites are capable of handling manyteletypewriter (tty) and voice circuits at the same time.
Orbit DescriptionsOrbits generally are described according to the physical shapeof the orbit and the angle of inclination of the plane of theorbit. These terms are discussed in the following paragraphs:PHYSICAL SHAPE. - All satellites orbit the earth in ellipticalorbits. (A circle is a special case of an ellipse.) The shape ofthe orbit is determined by the initial launch parameters and thelater deployment techniques used.PERIGEE and APOGEE are two, of the three parameters used todescribe orbital data of a satellite. These are shown on figure4-2. Perigee is the point in the orbit nearest to the center ofthe earth. Apogee is the point in the orbit the greatestdistance from the center of the earth. Both distances areexpressed in nautical miles.Figure 4-2. - Elliptical satellite orbit.ANGLE OF INCLINATION. - The ANGLE OF INCLINATION (angle betweenthe equatorial plane of the earth and the orbital plane of thesatellite) is the third parameter used to describe the orbitdata of a satellite. Figure 4-3 depicts the angle of inclinationbetween the equatorial plane and the orbital plane. Mostsatellites orbit the earth in orbital planes that do notcoincide with the equatorial plane of the earth. A satelliteorbiting in any plane not identical with the equatorial plane isin an INCLINED ORBIT.Figure 4-3. - Inclined satellite orbit.
The inclination of the orbit determines the area covered by thepath of the satellite. As shown in figure 4-4, the greater theinclination, the greater the amount of surface area covered bythe satellite.Figure 4-4. - Effect of orbit plane inclination on satellitecoverage.SPECIAL TYPES OF INCLINED ORBITS. - A satellite orbiting in aplane that coincides with the equatorial plane of the earth isin an EQUATORIAL ORBIT. A satellite orbiting in an inclined
orbit with an angle of inclination of 90 degrees or near 90degrees is in a POLAR ORBIT.SPECIAL TYPES OF CIRCULAR ORBITS. - We stated previously that acircular orbit is a special type of elliptical orbit. You shouldrealize a circular orbit is one in which the major and minoraxis distances are equal or approximately equal. Mean heightabove earth, instead of perigee and apogee, is used indescribing a circular orbit. While we are discussing circularorbits, you should look at some of the terms mentioned earlierin this chapter. A satellite in a circular orbit at a height ofapproximately 19,300 nautical miles above the earth is in asynchronous orbit. At this altitude the period of rotation ofthe satellite is 24 hours, the same as the rotation period ofthe earth. In other words, the orbit of the satellite is in syncwith the rotational motion of the earth. Although inclined andpolar synchronous orbits are possible, the term synchronoususually refers to a synchronous equatorial orbit. In this typeof orbit, satellites appear to hover motionlessly in the sky.Figure 4-5 shows how one of these satellites can providecoverage to almost half the surface of the earth.Figure 4-5. - Illumination from a synchronous satellite.Three of these satellites can provide coverage over most of theearth (except for the extreme north and south polar regions). Apolar projection of the global coverage of a three-satellitesystem is shown in figure 4-6.Figure 4-6. - Worldwide synchronous satellite system viewed fromabove the North Pole.
A satellite in a circular orbit at other than 19,300 nauticalmiles above the earth is in a near-synchronous orbit. If theorbit is lower than 19,300 nautical miles, the period of orbitof the satellite is less than the period of orbit of the earth.The satellite then appears to be moving slowly around the earthfrom west to east. (This type of orbit is also calledsubsynchronous.) If the orbit is higher than 19,300 nauticalmiles, the period of orbit of the satellite is greater than theperiod of orbit of the earth. The satellite then appears to bemoving slowly around the earth from east to west. Althoughinclined and polar near-synchronous orbits are possible, nearsynchronous implies an equatorial orbit.A satellite in a circular orbit from approximately 2,000 milesto 12,000 miles above the earth is considered to be in a MEDIUMALTITUDE ORBIT. The period of a medium altitude satellite isconsiderably less than that of the earth. When you look at thisaltitude satellite, it appears to move rather quickly across thesky from west to east.
SUMMARYA PASSIVE SATELLITE is one that reflects radio signals back toearth.An ACTIVE SATELLITE is one that amplifies the received signaland retransmits it back to earth.REPEATER is another name for an active satellite.The UP LINK is the frequency used to transmit a signal fromearth to a satellite.The DOWN LINK is the frequency used to transmit an amplifiedsignal from the satellite back to earth.A SYNCHRONOUS ORBIT is one in which the satellite moves orrotates at the same speed as the earth.An ASYNCHRONOUS ORBIT is one where the satellite does not rotateor move at the same speed as the earth.A NEAR SYNCHRONOUS ORBIT is one in which the satellite rotatesclose to but not exactly at the same speed as the earth.PERIGEE is the point in the orbit of a satellite closest to theearth.APOGEE is the point in the orbit of a satellite the greatestdistance from the earth.The ANGLE OF INCLINATION is the angular difference between theequatorial plane of the earth and the plane of orbit of thesatellite.INCLINED ORBITS are orbits where there is some amount ofinclination. These include equatorial and polar orbits.An EQUATORIAL ORBIT is an orbit that occurs when the plane of asatellite coincides with the plane of the earth at the equator.A POLAR ORBIT is an orbit that has an angle of inclination of ornear 90 degrees.A MEDIUM ALTITUDE ORBIT is an orbit from 2,000 to 12,000 milesabove the earth. The rotation rate of the earth and satellite
are quite different, and the satellite moves quickly across thesky.An ECLIPSE is when the satellite is not in view or in directline of sight with the sun. This happens when the earth isbetween them.An EPHEMERIS is a table showing the precalculated position of asatellite at any given time.PROGRAMMED TRACKING uses known satellite orbital parameters togenerate antenna pointing angles.AUTOMATIC TRACKING is done by the equipment comparing thedirection of the antenna axis and the direction of the receivedsignal.HAND OVER is the period of time for one earth terminal to yieldcontrol to another as a satellite moves out of its area ofcoverage.SATELLITE-SUN CONJUNCTION is when the satellite and sun areclose together and the noise from the sun prevents or hamperscommunications.A SATELLITE ECLIPSE is an eclipse where the rays of the sundont reach the satellite. This prevents recharging of the solarcells of the satellite and decreases the power to thetransmitter.The ZONE OF MUTUAL VISIBILITY is where the satellite can be seenby both the up- and down-link earth terminals.