Orbital Satellite: 4) Non synchronous satellites have to be used when available,which may be little 15 minutes

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Christal Snow
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the speed of the satellite is depend upon its height above the earth.

1 Orbital Satellite: 1) Most of the satellites are orbital satellites and also called as Nonsynchronous Satellites. 2) Nonsynchronous Satellites are rotate around the earth in an elliptical or in circular pattern 3) In circular orbit the speed of rotation is constant for the satellites 4) For elliptical pattern the speed of the satellite is depend upon its height above the earth. 5) Speed of the satellite is greater when it is close to the earth than when it is farther away 1) If the satellite is orbiting in the same direction as Earth s rotation and at an angular velocity greater than that of Earth then the orbit is called prograde or posigrade 2) If the satellite is orbiting in the opposite direction as Earth s rotation or in the same direction at an angular velocity less than that of Earth, then the orbit is called retrograde 3) Non synchronous orbit always revolve around in prograde orbit.so it is continuously changing in respect to a fixed position on earth 4) Non synchronous satellites have to be used when available,which may be little 15 minutes 5) Advantage orbital satellites is propulsion rockets are not required to keep them in the orbit path 6) Expensive tracking equipment is needed to track the satellite is the main dis advantage

2 Satellite Elevation Categories: ORBITAL SATELLITES LOW EARTH ORBIT MEDIUM EARTH ORBIT GEOSYNCHRONOUS EARTH ORBIT LEO MEO GEO Frequency 1Ghz to 2.5 Ghz 1.2Ghz to 1.66Ghz Distance from Earth 480 Miles 6000 to Miles 2Ghz to 18Ghz to Miles Types of satellites Commercial Satellite for defense Commercial satellites and national use communication satellites Type of orbit Elliptical Elliptical Circular Advantages No path loss and less weight ) Geostationary satellites have an orbital time of approx. 24hrs.so it appears to be stationary and looks as fixed with respect to earth 2) Satellites between miles and miles said to be near synchronous orbit

3 3) If Satellite is slightly lower than miles from earth, the orbital time will be lower compared to the earth rotation period and move slowly around from west to east, so it is called as sub synchronous. 4) If Satellite is slightly higher than miles from earth the orbital will be longer compared to the earth rotation and it appear to have a reverse motion from east to west. Satellite Orbital Patterns: 1) The point in an orbit that is located farthest from earth is called Apogee. 2) The point in an orbit that is located closest from earth is called Perigee. 3) The line joining the Perigee and Apogee through center of earth is called Major axis. Sometimes called as line of apsides. 4) The line perpendicular to the major axis and halfway between the perigee and apogee is called minor axis. 5) Although we have numerous infinite numbers of orbits, only three of them is useful for communication purpose. 1) Inclined orbits 2) Polar orbits 3) Equatorial orbits 6) All satellite in an orbit forms a plane which passes through the center of gravity of earth called Geocenter. 7) The angle inclination is the angle between the earth equatorial s plane and the orbital plane of a satellite measured counterclockwise at the point in the orbit. 8) The point where a polar or inclined orbit crosses the equatorial plane travelling from south to north. This point is called ascending node.

4 9) The point where a polar or inclined orbit crosses the equatorial plane travelling from north to south. This point is called descending node. 10) The line joining the ascending and descending nodes through the center of earth is called line of nodes. 11) Inclined orbits are generally elliptical and vary between 0 and ) From Kepler s Law angular velocity of the satellite is slow at apogee. Therefore the satellite remains at longer period of time. 13) An equatorial orbit is when the satellite rotates in an orbit directly above the equator, usually in a circular path. Since the angle of inclination is 0 so there is no ascending order and descending order and hence no line of nodes. 14) All geosynchronous satellites are in equatorial orbits. 15) A polar orbit takes it over the north and south pole in an orbit perpendicular to the equatorial plane. 16) It follows a very low altitude path that is close to earth and passes over and very close to both the north and South Pole. 17) The angle of inclination of a polar orbit is nearly 90.A satellite also covers 100% of earth surface in a polar orbit 18) Satellite revolves around in a longitudinal orbit and earth rotate on its axis in latitudinal rotation. 19) Earth is not is a perfect sphere it bulges at the equator. The important reason is causing the elliptical orbit to rotate in a manner that causes the apogee and perigee to move around the earth. This phenomenon is called rotation of the line of apsides. 20) At a angle of inclination of 63.4 line of apsides is zero. 21) Another interesting type is molniya system of satellites, it is used in Russia for government communication purpose.

5 22) It is synchronous orbit, its perigee is 400km and apogee is 40,000km.the size of the ellipse was chosen to make its period exactly one-half a sidereal day. Geosynchronous satellites: 1) These satellite orbit having same angular velocity as earth, hence it is appeared to be fixed above one spot on earth s surface. 2) Since it seems to be fixed no special antenna tracking system is needed, it provide reliable communication to 40% of the earth surface. 3) Satellite is in orbit because of the balance between the centrifugal force and earth gravitational pull. If velocity of the satellite is high it will break out from the orbit, if velocity is low it suffering the earth gravitational pull. 4) Geosynchronous orbits are circular, so the speed of the rotation will be constant throughout the period. 5) There is only one geosynchronous orbit earth orbit; however it is occupied by so many satellites. 6) The excursion above and below the equator would build up at a rate of between 0.6 to 0.9 per year.in addition to that geosynchronous satellites in an elliptical orbit also rift in an east or west direction as viewed from earth. This process is called station keeping. 7) There are several requirements for satellites 1) It must have 0 angle of inclination 2) It should orbiting around same direction as earth s rotation with the same angular velocity. 8) The semi major axis is α=a (p) 2/3 = ( ) (0.9972) 2/3 = 42164km. 9) Earth equatorial radius is 6378km. so height (h) above mean sea level of a satellite is H= = 35768km.

6 10) Circumference (c) of a geosynchronous orbit is c = 2π(42164) = km 11) Therefore the velocity (v) of a geosynchronous satellite is v =(264790/24)=11033km/hr. 12) The round trip propagation delay between satellite and earth station located directly below it is t = ( d/c) =2(35768)/3*10 5 km/s = 238ms. Clarke Orbit: 1) Geosynchronous earth orbit is some time called as Clarke orbit or Clerk belt, 1) Be located directly above the equator 2) Travel in the same direction as earth s rotation at 6480mph. 3) Have an altitude of miles above earth 4) Complete one revolution in 24 hours. Advantages: 1) Expensive equipment is not required for geosynchronous satellite 2) They are available to all earth stations within their shadow 100% of the time. 3) There is no need for switch from one satellite to another satellite as they orbit overhead. 4) The effects of Doppler shift are negligible.

7 Disadvantages: 1) It needs sophisticated and heavy propulsion devices to keep them in a fixed orbit. 2) High altitude satellites introduce much longer delay. The delay between two earth stations is 500ms to 600ms 3) It requires high transmit power and more sensitive receivers because of longer distances and greater path losses. 4) High precision spaceman ship is required to put the satellite into the orbit and keep it there. Antenna look Angle: 1) To optimize the performance of the satellite, the direction of maximum gain of an earth station antenna must be pointed out directly at the satellite. 2) To determine that earth station antenna is aligned,two angles must be determined 1) Azimuth angle 2) Elevation angle 1) Geostationary satellite, the look angles of the earth station antenna need to be adjusted for once as the satellite will remain in a given position permanently, except for occasional minor variations. 2) The satellite location is specified with latitude and longitude similar to the way the location of a point on earth. 3) Since satellite is revolving around many miles above the earth, it has no latitude or longitude, therefore its location is identified by a point on the surface of earth directly below the satellite.it is called sub satellite point and for geo satellites it should fall on equator.so they all have 0 latitude. 4) Hence it will be given in degrees longitude east or west of the Greenwich meridian

Angle of Elevation: 1) It is the vertical angle formed between the direction of travel of an electromagnetic wave radiated from an earth station antenna pointing directly toward a satellite and the

8 Angle of Elevation: 1) It is the vertical angle formed between the direction of travel of an electromagnetic wave radiated from an earth station antenna pointing directly toward a satellite and the horizontal plane. 2) Smaller the elevation angle greater the distance a propagated wave must pass through earth s atmosphere. Due to absorption in atmosphere, and noise the signal wave may deteriorate and may not provide acceptable transmission quality. So 5 is considered as minimum acceptable angle of elevation. Azimuth Angle: 1) It is defined as the horizontal pointing angle of an earth station antenna.it is usually measured in clockwise direction in degrees from true north. 2) Angle of elevation and azimuth angle both depend on the latitude of the earth station and longitude of both earth station and satellite.

9 Satellite system link models: 1) A satellite system consist of three basic sections 1) An uplink 2) Transponder 3) Downlink Uplink model 1) The primary component within the uplink section of a satellite system is the earth station transmitter. 2) It consists of an IF modulator, an IF-to-RF microwave up converter, a high power amplifier and some means of band limiting the final output spectrum. Modulator BPF Mixer BPF HPA MW Generator 3) The above diagram is the transmitter in the earth station, the modulator converts the data signal to either FM or PSK or QAM signal in if frequency range. 4) Mixer converts the IF to RF frequencies; HPA provides adequate gain and output power to propagate the signal to the satellite transponder. HPA are normally klystron and travelling wave tube.

10 Transponder: 1) It consists of a BPF, low noise amplifier, frequency translator, allow level power amplifier and an output band pass filter. 2) The fig. bellow shows a simplified transponder diagram. 3) It is a RF to RF repeater, other configurations are IF and baseband repeaters similar to those used in microwave repeaters. 4) From the output of LNA the freq. translator which converts the high band uplink frequency to the low band down link frequency. 5) The low level power amplifier normally a travelling wave tube which will boost the power and send the data to earth station, each RF satellite channel requires a separate transponders. Downlink Model: 1) It consists of BPF, an LNA, and an RF-IF down converter; the figure below shows a typical earth station receiver. 2) The BPF limits the input noise power to the LNA. The LNA is a high sensitive, low noise device, such as a tunnel diode amplifier or a parametric amplifier.

3) The RF-IF down converter is a mixer/band pass filter combination that converts the received RF signal to an IF frequency BPF LNA Mixer BPF Demodula tor Cross Links: MW Generator 1) Occasionally,

11 3) The RF-IF down converter is a mixer/band pass filter combination that converts the received RF signal to an IF frequency BPF LNA Mixer BPF Demodula tor Cross Links: MW Generator 1) Occasionally, there is an application where it is necessary to communicate between satellites. This is done by satellite cross links are intersatellite links. 2) A disadvantage of using an ISL is that both the transmitter and the receiver are space bound 3) Here both the transmitter output power and input sensitivity are limited. Satellite system Link Equations: 1) The error performance of a digital satellite system is quite predictable. 2) When evaluating the performance of a satellite sytem,the uplink and downlink parameters are first considered separately, then the overall performance is determined in appropriate manner.

12 3) For a satellite radio the original or demodulated signal is digital in nature. And Rf portion of the radio is analog,that is FSK,PSK,QAM or some of the microwave carrier signal riding on a microwave carrier. Link Equations: 1) These equations separately analyze the uplink and down link sections 2) It considers only the ideal gains and losses, effects of the thermal noise associated with earth station transmitter, earth station receiver and the satellite transponder. Uplink Equation: (C/N 0 ) = (A t P in (L p L u )A r ) / (KT e ) = (A t P in (L p L u ))*G / KT e Where L p and L u are the additional uplink and downlink atmospheric losses, respectively. 3) The uplink and downlink signals must pass through earth s atmosphere, where they are partially absorbed by the moisture, oxygen, and particulates in the air. 4) Depending on the elevation angle Rf signal travels through the atmosphere varies from on earth station to another Advantages of Fiber Optic Communication: 1) Fiber cables are very thin, smaller and weigh lesser than coaxial/metallic cable 2) Fiber optic system use light frequencies as carrier.so it is having larger band width 3) It requires less space and easy to install without worrying about volatile liquids and gases. 4) It is immune to static interference and electrical noise sources like electrical motors. In fiber there is no magnetic induction between fiber cables and there is no possibility of cross talk

13 5) Metallic cables form capacitance between them and inductance along their conductors which act like low pass filter and limit the bandwidth of the system, but fiber optic is free from all. 6) It is virtually impossible to tap the information from a fiber cable. 7) It can operate at higher temperature and less affected by corrosive liquids and gases 8) It is cheaper comparing to other cables. 9) It is lost longer and are more reliable than metallic conductors because it is having higher tolerance to change in environmental conditions. Loses in optical fiber communication: 1) Power loss in an optical fiber cable is probably the most important characteristic of the cable. Power loss is often called attenuation and results in an reduction in the power of the light wave as it travels down the cable. 2) A db = 10log(p out / p in ) where A db is total reduction in power level,attenuation(unitless) p out is cable output power(watts) p in is cable input power 3) In general multi-mode fibers have more attenuation than single-mode cables because of increased scattering of light wave produced from the dopants in the glass. 4) A 13db cable loss reduces the output power to 50% of the input power. Attenuation of light is depending on the wavelength. 5) Three wavelengths 0.85microns- 20% loss per kilometer,1.3microns and 1.55microns- 5% loss per kilometer. 6) Attenuation of optical power is expressed in db. 7) P = p t * 10 -Al/10 where p= measured power level(watts) p t =transmitted power level (watts) A = cable power loss(db/km) l = cable length (km) 8) Likewise in db p(dbm) = p in (dbm) Al(dbm) where p = measured power level (dbm) p in = transmit power (dbm) Al = cable power loss, attenuation (db)

14 9) Transmission losses are one of the most important characteristics of fiber loss,the predominant losses in the optical fiber are 1) Absorption loss 2) Material or Rayleigh, scattering losses 3) Chromatic, or wavelength, dispersion 4) Radiation losses 5) Modal dispersion 6) Coupling losses

Advanced Electronic Communication Systems. Lecture 4. Satellite Orbits (2) Dr.Eng. Basem ElHalawany

Advanced Electronic Communication Systems Lecture 4 Satellite Orbits (2) Dr.Eng. Basem ElHalawany Orbital (nonsynchronous) Satellites (cont.) Posigrade orbit or Prograde: If the satellite is orbiting in