《电子工程师手册》学习资料（英文版）Satellites and Aerospace 74.pdf_大学文库

74 Satellites and Aerospace 74. 2 Satellite Applications 74.3 Satellite Functions 74.4 Satellite Orbits and Pointing angles 74.5 Communications Link 74.6 System Noise Temperature and G/T 74.7 74. 8 Interference 74.9 Some Particular Orbits 74.10 Access and Modulation Daniel E Difonzo 74.11 Frequency Allocations Planar Communications 74.12 Satellite Subsystems 74.13 Trends 74.1 Introduction The impact of satellites on world communications since commercial operations began in the mid-1960s is such that we now take for granted many services that were not available a few decades ago: worldwide TV, reliable communications with ships and aircraft, wide area data networks, communications to remote areas, direct Tv broadcast to homes, position determination, and earth observation(weather and mapping). New and proposed satellite services include global personal communications to hand-held portable telephones, and broadband voice,video, and data to and from small user terminals at customer premises around the world. Satellites function as line-of-sight microwave relays in orbits high above the earth which can see large areas f the earths surface. Because of this unique feature, satellites are particularly well suited to communications over wide coverage areas such as for broadcasting, mobile communications, and point-to-multipoint commu- ications Satellite systems can also provide cost-effective access for many locations where the high investment cost of terrestrial facilities might not be warranted 74.2 Satellite applications Figure 74.1 depicts several kinds of satellite links and orbits. The geostationary earth orbit(GEO)is in the equatorial plane at an altitude of 35, 786 km with a period of one sidereal day(23h 56m 4.09s). This orbit is sometimes called the Clarke orbit in honor of arthur c. clarke who first described its usefulness for commu- nications in 1945. GEO satellites appear to be almost stationary from the ground(subject to small perturbations and the earth antennas pointing to these satellites may need only limited or no tracking capabilit An orbit for which the highest altitude(apogee)is greater than gEO is sometimes referred to as high earth orbit(HEO). Low earth orbits(LEO) typically range from a few hundred km to about 2000 km. Medium earth orbits(MEO) are at intermediate altitudes. Circular MEO orbits, also called Intermediate Circular Orbits(ICO) c 2000 by CRC Press LLC

© 2000 by CRC Press LLC 74 Satellites and Aerospace 74.1 Introduction 74.2 Satellite Applications 74.3 Satellite Functions 74.4 Satellite Orbits and Pointing Angles 74.5 Communications Link 74.6 System Noise Temperature and G/T 74.7 Digital Links 74.8 Interference 74.9 Some Particular Orbits 74.10 Access and Modulation 74.11 Frequency Allocations 74.12 Satellite Subsystems 74.13 Trends 74.1 Introduction The impact of satellites on world communications since commercial operations began in the mid-1960s is such that we now take for granted many services that were not available a few decades ago: worldwide TV, reliable communications with ships and aircraft, wide area data networks, communications to remote areas, direct TV broadcast to homes, position determination, and earth observation (weather and mapping). New and proposed satellite services include global personal communications to hand-held portable telephones, and broadband voice, video, and data to and from small user terminals at customer premises around the world. Satellites function as line-of-sight microwave relays in orbits high above the earth which can see large areas of the earth’s surface. Because of this unique feature, satellites are particularly well suited to communications over wide coverage areas such as for broadcasting, mobile communications, and point-to-multipoint communications. Satellite systems can also provide cost-effective access for many locations where the high investment cost of terrestrial facilities might not be warranted. 74.2 Satellite Applications Figure 74.1 depicts several kinds of satellite links and orbits. The geostationary earth orbit (GEO) is in the equatorial plane at an altitude of 35,786 km with a period of one sidereal day (23h 56m 4.09s). This orbit is sometimes called the Clarke orbit in honor of Arthur C. Clarke who first described its usefulness for communications in 1945. GEO satellites appear to be almost stationary from the ground (subject to small perturbations) and the earth antennas pointing to these satellites may need only limited or no tracking capability. An orbit for which the highest altitude (apogee) is greater than GEO is sometimes referred to as high earth orbit (HEO). Low earth orbits (LEO) typically range from a few hundred km to about 2000 km. Medium earth orbits (MEO) are at intermediate altitudes. Circular MEO orbits, also called Intermediate Circular Orbits (ICO) Daniel F. DiFonzo Planar Communications Corporation

Inter-satellite link Earth Orbit (HEO or GEO) LEO-GEO Direct Broadcast(DBS) Point-to-multipoint Links Low Earth Orbit Hand-held vsat Hub PSTN/PTT URE 74.1 Several types of satellite links. Illustrated are point-to-point, point-to-multipoint, VSAT, direct broadcast, have been proposed at an altitude of about 10, 400 km for global personal communications at frequencies designated for Mobile Satellite Services(MSS)Johannsen, 1995] EO systems for voice communications are called Big LEOs Constellations of so-called Little LEOs operating below 1 GHz and having only limited capacity have been proposed for low data rate non-voice services, such as paging and store and forward data for remote location and monitoring, for example, for freight containers and remote vehicles and personnel Kiesling, 1996 itially, satellites were used primarily for point-to-point traffic in the GEO fixed satellite service(FSS),e.g for telephony across the oceans and for point-to-multipoint TV distribution to cable head end stations. Large earth station antennas with high-gain narrow beams and high uplink powers were needed to compensate for limited satellite power. This type of system, exemplified by the early global network of the International Telecommunications Satellite Organization(INTELSAT)used Standard-A earth antennas with 30-m diameters. Since then, many other satellite organizations have been formed around the world to provide international, gional, and domestic services As satellites have grown in power and sophistication, the average size of the earth terminals has been reduced. High gain satellite antennas and relatively high power satellite transmitters have led to very small aperture earth terminals(VSAT) with diameters of less than 2 m, modest powers of less than 10 w[Gagliardi, 1991] and even smaller ultra-small aperture terminals( USAT) diameters typically less than 1 m As depicted in Fig. 74.1, VSAT erminals may be placed atop urban office buildings, permitting private networks of hundreds or thousands of terminals, which bypass terrestrial lines. VSATs are usually incorporated into star networks where the small rminals communicate through the satellite with a larger Hub terminal. The hub retransmits through the satellite to another small terminal. Such links require two hops with attendant time delays. with high gain satellite antennas and relatively narrow-band digital signals, direct single-hop mesh interconnections of VSATs may be used 74.3 Satellite Functions The traditional function of a satellite is that of a bent pipe quasilinear repeater in space. As shown in Fig. 74.2, uplink signals from earth terminals directed at the satellite are received by the satellites antennas, amplified, translated to a different downlink frequency band, channelized into transponder channels, further amplified to e 2000 by CRC Press LLC

© 2000 by CRC Press LLC have been proposed at an altitude of about 10,400 km for global personal communications at frequencies designated for Mobile Satellite Services (MSS) [Johannsen, 1995]. LEO systems for voice communications are called Big LEOs. Constellations of so-called Little LEOs operating below 1 GHz and having only limited capacity have been proposed for low data rate non-voice services, such as paging and store and forward data for remote location and monitoring, for example, for freight containers and remote vehicles and personnel [Kiesling, 1996]. Initially, satellites were used primarily for point-to-point traffic in the GEO fixed satellite service (FSS), e.g., for telephony across the oceans and for point-to-multipoint TV distribution to cable head end stations. Large earth station antennas with high-gain narrow beams and high uplink powers were needed to compensate for limited satellite power. This type of system, exemplified by the early global network of the International Telecommunications Satellite Organization (INTELSAT) used Standard-A earth antennas with 30-m diameters. Since then, many other satellite organizations have been formed around the world to provide international, regional, and domestic services. As satellites have grown in power and sophistication, the average size of the earth terminals has been reduced. High gain satellite antennas and relatively high power satellite transmitters have led to very small aperture earth terminals (VSAT) with diameters of less than 2 m, modest powers of less than 10 W [Gagliardi, 1991] and even smaller ultra-small aperture terminals (USAT) diameters typically less than 1 m. As depicted in Fig. 74.1, VSAT terminals may be placed atop urban office buildings, permitting private networks of hundreds or thousands of terminals, which bypass terrestrial lines. VSATs are usually incorporated into star networks where the small terminals communicate through the satellite with a larger Hub terminal. The hub retransmits through the satellite to another small terminal. Such links require two hops with attendant time delays. With high gain satellite antennas and relatively narrow-band digital signals, direct single-hop mesh interconnections of VSATs may be used. 74.3 Satellite Functions The traditional function of a satellite is that of a bent pipe quasilinear repeater in space. As shown in Fig. 74.2, uplink signals from earth terminals directed at the satellite are received by the satellite’s antennas, amplified, translated to a different downlink frequency band, channelized into transponder channels, further amplified to FIGURE 74.1 Several types of satellite links. Illustrated are point-to-point, point-to-multipoint, VSAT, direct broadcast, mobile, personal communications, and intersatellite links

olid State Power fier(SSPA) 少( LNa Low Noise Amplifier Multi-Beam MUX Multiplexer L O. Local Oscillator Antenna(s) BFN Beam Forming Network FIGURE.2 A satellite repeater receives uplink signals(U), translates them to a downlink frequency band(D), channel- zes, amplifies to high power, and retransmits to earth. Multiple beams allow reuse of the available band Interference(dashed lines)can limit performance. Downconversion may also occur after the input multiplexers. Several intermediate frequencies and downconversion may be used. relatively high power, and retransmitted toward the earth. Transponder channels are generally rather broad (e.g, bandwidths from 24 MHz to more than 100 MHz)and each may contain many individual or user channels The functional diagram in Fig. 74.2 is appropriate to a satellite using frequency-division duplex(fDD), which refers to the fact that the satellites use separate frequency bands for the uplink and downlink and where both links operate simultaneously. This diagram also illustrates a particular multiple access technique, known as frequency-division multiple access(FDMA), which has been prevalent in mature satellite systems. Multiple access, to be discussed later, allows many different user signals to utilize the satellite's resources of power and bandwidth without interfering with each other. Multiple access systems segregate users by frequency division(FDMA)where each user is assigned a specific frequency channel, space-division multiple access( SDMA) by frequency reuse, that is by reusing the same frequencies on multiple spatially isolated beams, time-division multiple access(TDMA) where each user signal occupies an entire allocated frequency band but for only part of the time, polarization-division(PD)where frequencies may be reused on spatially overlapping but orthogonally olarized beams, and code-division multiple access( CDMA)where different users occupy the same frequency band but use spread spectrum signals that contain orthogonal signaling codes [ Sklar, 1988; Richharia, 1995 Frequency modulation(FM) has been the most widely used modulation. However, advances in digital voice and video compression have led to the widespread use of digital modulation methods such as quadrature phase shift keying(QPSK) and quadrature amplitude modulation(QAM)(Sklar, 1988] Newer satellite architectures incorporate digital modulations and on-board demodulation of the uplink signals to baseband bits, subsequent switching and assignment of the baseband signals to an appropriate downlink antenna beam, and re-modulation of the clean baseband signals prior to downlink transmission These regenerative repeaters or onboard processors permit flexible routing of the user signals and can improve the overall communications link by separating the uplink noise from that of the downlink. The baseband signals may be those of individual users or they may represent frequency-division multiplexed( FDM)or time-division multiplexed(TDM) signals from many users. Examples include the NASA Advanced Communications Technology Satellite(ACTS)and the Iridium system built by Motorola for Iridium LLC. The ACTS is an FDD satellite system operating in the Ka-bands with uplink frequencies from 29.1 to 30.0 GHz and downlink frequencies from 19.2 to 20.1 GHz. It is intended to demonstrate technologies for future broadband voice, video, and data services applicable to the emerging concepts of the Global Information Infrastructure(GII)and National Information Infrastructure(NII)[Ged c 2000 by CRC Press LLC

© 2000 by CRC Press LLC relatively high power, and retransmitted toward the earth. Transponder channels are generally rather broad (e.g., bandwidths from 24 MHz to more than 100 MHz) and each may contain many individual or user channels. The functional diagram in Fig. 74.2 is appropriate to a satellite using frequency-division duplex (FDD), which refers to the fact that the satellites use separate frequency bands for the uplink and downlink and where both links operate simultaneously. This diagram also illustrates a particular multiple access technique, known as frequency-division multiple access (FDMA), which has been prevalent in mature satellite systems. Multiple access, to be discussed later, allows many different user signals to utilize the satellite’s resources of power and bandwidth without interfering with each other. Multiple access systems segregate users by frequency division (FDMA) where each user is assigned a specific frequency channel, space-division multiple access (SDMA) by frequency reuse, that is by reusing the same frequencies on multiple spatially isolated beams, time-division multiple access (TDMA) where each user signal occupies an entire allocated frequency band but for only part of the time, polarization-division (PD) where frequencies may be reused on spatially overlapping but orthogonally polarized beams, and code-division multiple access (CDMA) where different users occupy the same frequency band but use spread spectrum signals that contain orthogonal signaling codes [Sklar, 1988; Richharia, 1995]. Frequency modulation (FM) has been the most widely used modulation. However, advances in digital voice and video compression have led to the widespread use of digital modulation methods such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM) [Sklar, 1988]. Newer satellite architectures incorporate digital modulations and on-board demodulation of the uplink signals to baseband bits, subsequent switching and assignment of the baseband signals to an appropriate downlink antenna beam, and re-modulation of the clean baseband signals prior to downlink transmission. These regenerative repeaters or onboard processors permit flexible routing of the user signals and can improve the overall communications link by separating the uplink noise from that of the downlink. The baseband signals may be those of individual users or they may represent frequency-division multiplexed (FDM) or time-division multiplexed (TDM) signals from many users. Examples include the NASA Advanced Communications Technology Satellite (ACTS) and the Iridium® system built by Motorola for Iridium LLC. The ACTS is an FDD satellite system operating in the Ka-bands with uplink frequencies from 29.1 to 30.0 GHz and downlink frequencies from 19.2 to 20.1 GHz. It is intended to demonstrate technologies for future broadband voice, video, and data services applicable to the emerging concepts of the Global Information Infrastructure (GII) and National Information Infrastructure (NII) [Gedney, 1996]. FIGURE 74.2 A satellite repeater receives uplink signals (U), translates them to a downlink frequency band (D), channelizes, amplifies to high power, and retransmits to earth. Multiple beams allow reuse of the available band. Interference (dashed lines) can limit performance. Downconversion may also occur after the input multiplexers. Several intermediate frequencies and downconversions may be used

Proposed Ka-band satellite systems that would operate at the 20-and 30-GHz bands may incorporate inter- satellite links at Ka-band or even at 60 GHz. These systems are intended to provide broadband voice, video, and data services for the GIl. Systems have been proposed for operation at GEO and LEO The Iridium satellites operate at LEO (altitude =780 km) with time-division duplex(TDD), using the same enable communications directly to and from small handheld portable telephones at any time and anywhere in the world. Other PCS satellite systems will operate at 1.6 GHz for the uplink and 2.5 GHz for the downlink (e.g, FCC filings for Globalstar and Odyssey) High-power direct broadcast satellites(DBS)or direct-to-home(DTH) satellites are operating at Ku-band. In the U.S., satellites operating in the broadcast satellite service(BSS)with downlink frequencies of 12.2 to 12.7 GHz, deliver Tv directly to home receivers having parabolic dish antennas as small as 46 cm(18 in. )in diameter. DBS with digital modulation and compressed video is providing more than 150 National Television Systems Committee(NTSC)TV channels from a single orbital location having an allocation of 32 transponder channels, each with 24-MHz bandwidth. DBS is seen as an attractive medium for delivery of high-definition TV (HDTV) to a large number of homes. Other systems using analog FM are operational in Europe and Japan. In the U.S., DTH is also provided by satellites in the FSS frequency bands of 11.7 to 12.2 GHz. These are constrained by regulation to operate at lower downlink power and, therefore, require receiving dishes of about 1-m diameter Digital radio broadcast(DRB) from high power GEO satellites has been proposed for direct broadcast of igitally compressed D quality audio to mobile and fixed users in the 2310-2360 MHz bands. [ Briskman Mobile satellite services(MSS)operating at L-band around 1.6 GHz have revolutionized communications ith ships and aircraft, which would normally be out of reliable communications range of terrestrial radio signals. The International Maritime Satellite Organization(INMARSAT) operates the dominant system of this Links between LEO satellites (or the NASA Shuttle), and GEO satellites are used for data relay, for examp via the NASA tracking and data relay satellite system(TDRSS). Some systems will use intersatellite links(ISL) to improve the interconnectivity of a wide-area network. ISL systems would typically operate at frequencies such as 23 GHz, 60 GHz, or even use optical links 74. 4 Satellite Orbits and Pointing angles Reliable communication to and from a satellite requires a knowledge of its position and velocity relative to a location on the earth. Details of the relevant astrodynamic formulas for satellite orbits are given in griffin and French [1991], Morgan and Gordon [1989],and Chobotov [1991]. Launch vehicles needed to deliver the satellites to their intended orbits are described in Sakowitz [1991] A satellite, having mass m, in orbit around the earth, having mass M, traverses an elliptical path such that the centrifugal force due to its acceleration is balanced by the earths gravitational attraction, leading to the quation of motion for two bodies d- where r is the radius vector joining the earth's center and the satellite and u =g(m+ M)=GM =398,600.5 km/s is the product of the gravitational constant and the mass of the earth. Because m < Me, the center of rotation of the two bodies may be taken as the earths center, which is at one of the focal points of the orbit ellipse. Figure 74.3 depicts the orbital elements for a geocentric right-handed coordinate system where the x axis points to the first point of Aries, that is, the fixed position against the stars where the sun's apparent path round the earth crosses the earths equatorial plane while traveling from the southern toward the northern e 2000 by CRC Press LLC

© 2000 by CRC Press LLC Proposed Ka-band satellite systems that would operate at the 20- and 30-GHz bands may incorporate intersatellite links at Ka-band or even at 60 GHz. These systems are intended to provide broadband voice, video, and data services for the GII. Systems have been proposed for operation at GEO and LEO. The Iridium satellites operate at LEO (altitude = 780 km) with time-division duplex (TDD), using the same 1.6-GHz L-band frequencies for transmission and reception but only receiving or transmitting for somewhat less than half the time each. Iridium uses 66 LEO satellites for personal communications systems (PCS) to enable communications directly to and from small handheld portable telephones at any time and anywhere in the world. Other PCS satellite systems will operate at 1.6 GHz for the uplink and 2.5 GHz for the downlink (e.g., FCC filings for Globalstar and Odyssey). High-power direct broadcast satellites (DBS) or direct-to-home (DTH) satellites are operating at Ku-band. In the U.S., satellites operating in the broadcast satellite service (BSS) with downlink frequencies of 12.2 to 12.7 GHz, deliver TV directly to home receivers having parabolic dish antennas as small as 46 cm (18 in.) in diameter. DBS with digital modulation and compressed video is providing more than 150 National Television Systems Committee (NTSC) TV channels from a single orbital location having an allocation of 32 transponder channels, each with 24-MHz bandwidth. DBS is seen as an attractive medium for delivery of high-definition TV (HDTV) to a large number of homes. Other systems using analog FM are operational in Europe and Japan. In the U.S., DTH is also provided by satellites in the FSS frequency bands of 11.7 to 12.2 GHz. These are constrained by regulation to operate at lower downlink power and, therefore, require receiving dishes of about 1-m diameter. Digital radio broadcast (DRB) from high power GEO satellites has been proposed for direct broadcast of digitally compressed near-CD quality audio to mobile and fixed users in the 2310-2360 MHz bands. [Briskman, 1996]. Mobile satellite services (MSS) operating at L-band around 1.6 GHz have revolutionized communications with ships and aircraft, which would normally be out of reliable communications range of terrestrial radio signals. The International Maritime Satellite Organization (INMARSAT) operates the dominant system of this type. Links between LEO satellites (or the NASA Shuttle), and GEO satellites are used for data relay, for example, via the NASA tracking and data relay satellite system (TDRSS). Some systems will use intersatellite links (ISL) to improve the interconnectivity of a wide-area network. ISL systems would typically operate at frequencies such as 23 GHz, 60 GHz, or even use optical links. 74.4 Satellite Orbits and Pointing Angles Reliable communication to and from a satellite requires a knowledge of its position and velocity relative to a location on the earth. Details of the relevant astrodynamic formulas for satellite orbits are given in Griffin and French [1991], Morgan and Gordon [1989], and Chobotov [1991]. Launch vehicles needed to deliver the satellites to their intended orbits are described in Isakowitz [1991]. A satellite, having mass m, in orbit around the earth, having mass Me, traverses an elliptical path such that the centrifugal force due to its acceleration is balanced by the earth’s gravitational attraction, leading to the equation of motion for two bodies: (74.1) where r is the radius vector joining the earth’s center and the satellite and m = G (m + Me) ª GMe = 398,600.5 km3 /s2 is the product of the gravitational constant and the mass of the earth. Because m << Me, the center of rotation of the two bodies may be taken as the earth’s center, which is at one of the focal points of the orbit ellipse. Figure 74.3 depicts the orbital elements for a geocentric right-handed coordinate system where the x axis points to the first point of Aries, that is, the fixed position against the stars where the sun’s apparent path around the earth crosses the earth’s equatorial plane while traveling from the southern toward the northern d dt r 2 2 3 0 r + r m =

© 2000 by CRC Press LLC hemisphere at the vernal equinox. The z axis points to the north and the y axis is in the equatorial plane and points to the winter solstice. The elements shown are longitude or right ascension of the ascending node W measured in the equatorial plane, the orbit’s inclination angle i relative to the equatorial plane; the ellipse semimajor axis length a, the ellipse eccentricity e, the argument (angle) of perigee w, measured in the orbit plane from the ascending node to the satellite’s closest approach to the earth; and the true anomaly (angle) in the orbit plane from the perigee to the satellite n. The mean anomaly M is the angle from perigee that would be traversed by a satellite moving at its mean angular velocity n. Given an initial value Mo, usually taken as 0 for a particular epoch (time) at perigee, the mean anomaly at time t is M = Mo + n(t – to ), where n = . The eccentric anomaly E may then be found from Kepler’s transcendental equation M = E – e sinE which must be solved numerically by, for example, guessing an initial value for E and using a root finding method. For small eccentricities, the series approximation E ª M + e sinM + (e2 /2)sin2M + (e3 /8)(3sin3M – sinM) yields good accuracy [Morgan and Gordon, 1989, p. 806]. Other useful quantities include the orbit radius, r, the period, P, of the orbit, [i.e., for n(t – to ) = 2p], the velocity, V, and the radial velocity, Vr : (74.2) (74.3) (74.4) (74.5) Figure 74.4 depicts quantities useful for communications links in the plane formed by the satellite, a point on the earth’s surface and the earth’s center. Shown to approximate scale for comparison are satellites at altitudes representing LEO, MEO, and GEO orbits. For a satellite at altitude h, and for the earth’s radius at the equator re = 6378.14 km, the slant range rs, elevation angle to the satellite from the local horizon el, and the satellite’s nadir angle q, are related by simple FIGURE 74.3 Orbital elements. m a3 § r = = a(1 e cos E) P = 2 a m 3 p V r a 2 2 1 = m - Ê Ë Á ˆ ¯ ˜ V e a E a e E r = (m ) ( - ) 1 2 1 sin cos

FIGURE 74.5 Quantities for a satellite RF link. P= transmit power(dBW). G= antenna gain(dBi )C= received carrier ive loss(dB). r,= slant range(m). fa d= downlink. e= earth. s= satellite (c EIRP-10 4 +2286-10log{4x/22)-A+T-B where the subscripts in Eq (741la) refer to transmit(n) and receive (r). Lower case terms are the actual quantities in watts, meters, etc and the capitalized terms in Eq (74 11b) correspond to the decibel(dB)versions of the parenthesized quantities in Eq (74 11a). For example, EIRP=P+ G=10llogp+ 101logg decibels relative to I w(dBW)and the expression(C/N) should be interpreted as 10logc-10logn. The uplink and downlink equations have identical form with the appropriate quantities substituted in Eq (74. 11). The relevant quantities are described below The ratio of received carrier power to noise power c/n, and its corresponding decibel value(C/N)=10log(d/n) dB is the primary measure of link quality. The product of transmit power P(W) and the transmit antenna gain gp, or equivalently, P, (dBW)+ G.[(dBi), that is, gain expressed in decibels relative to an isotropic antenna is called the equivalent isotropically radiated power(EIRP) and its unit is dBw because the antenna gain is dimensionless. The antenna gain is that in the direction of the link, i. e, it is not necessarily the antennas peak gain. The received thermal noise power is n= kTB W where k=1.38x 10-2J/K is Boltzmann's constant and 10 log (k)=-2286 dBW/K/Hz. T is the system noise temperature in kelvins(K)and B is the bandwidth in dB Hz. Then, G-101logT dB/K is a figure of merit for the receiving system. It is usually written as G/T and read as "gee over tee". The antenna gain and the noise temperature must be defined at the same reference point, e.g at the receivers input port or at the antenna terminals The spreading factor 4Ir, is independent of frequency and depends only on the slant range distance r,. The gain of an antenna with an effective aperture area of 1 m2 is 10log (4T/2), where the wavelength 2=c/f f is the frequency in Hz, and c=2.9979 x 10 m/s is the velocity of light. The dB sum of the spreading factor and ne gain of a l-m2 antenna is the frequency-dependent "path loss". " A"is the signal attenuation due to dissipative losses in the propagation medium. B is the bandwidth in dB Hz, i.e., B=10 log(b) where b is the bandwidth in hz The polarization mismatch factor between the incident wave and the receive antenna, is given by r= 10logp where spsl. This factor may be obtained from the voltage axial ratio of the incident wave fw, the voltage axial ratio of the receive antennas polarization response r,, and the difference in tilt angles of the wave and antenna polarization ellipses At=t-t, as follows 4r r+ (7412) where the axial ratios are each signed quantities, having a positive sign for right-hand sense and a negative sign for left-hand sense. Therefore, if the wave and antenna are cross-polarized(have opposite senses), the sign of c 2000 by CRC Press LLC

© 2000 by CRC Press LLC (74.11b) where the subscripts in Eq. (74.11a) refer to transmit (t) and receive (r). Lower case terms are the actual quantities in watts, meters, etc. and the capitalized terms in Eq. (74.11b) correspond to the decibel (dB) versions of the parenthesized quantities in Eq. (74.11a). For example, EIRP = P + G = 101logp + 101logg decibels relative to 1 W (dBW) and the expression (C/N) should be interpreted as 10logc – 10logn. The uplink and downlink equations have identical form with the appropriate quantities substituted in Eq. (74.11). The relevant quantities are described below. The ratio of received carrier power to noise power c/n, and its corresponding decibel value (C/N) = 10log(c/n) dB is the primary measure of link quality. The product of transmit power pt (W) and the transmit antenna gain gt , or equivalently, Pt (dBW) + Gt [(dBi), that is, gain expressed in decibels relative to an isotropic antenna] is called the equivalent isotropically radiated power (EIRP) and its unit is dBW because the antenna gain is dimensionless. The antenna gain is that in the direction of the link, i.e., it is not necessarily the antenna’s peak gain. The received thermal noise power is n = kTB W where k = 1.38 ¥ 10–23 J/K is Boltzmann’s constant and 10 log(k) = –228.6 dBW/K/Hz. T is the system noise temperature in kelvins (K) and B is the bandwidth in dB Hz. Then, G – 101logT dB/K is a figure of merit for the receiving system. It is usually written as G/T and read as “gee over tee”. The antenna gain and the noise temperature must be defined at the same reference point, e.g., at the receiver’s input port or at the antenna terminals. The spreading factor 4prs 2 is independent of frequency and depends only on the slant range distance rs . The gain of an antenna with an effective aperture area of 1 m2 is 10log(4p/l2 ), where the wavelength l = c/f, f is the frequency in Hz, and c = 2.9979 ¥ 108 m/s is the velocity of light. The dB sum of the spreading factor and the gain of a 1-m2 antenna is the frequency-dependent “path loss”. “A” is the signal attenuation due to dissipative losses in the propagation medium. B is the bandwidth in dB Hz, i.e., B = 10 log(b) where b is the bandwidth in Hz. The polarization mismatch factor between the incident wave and the receive antenna, is given by G = 10logr where 0 £ r £ 1. This factor may be obtained from the voltage axial ratio of the incident wave rw , the voltage axial ratio of the receive antenna’s polarization response ra , and the difference in tilt angles of the wave and antenna polarization ellipses Dt = tw – ta , as follows (74.12) where the axial ratios are each signed quantities, having a positive sign for right-hand sense and a negative sign for left-hand sense. Therefore, if the wave and antenna are cross-polarized (have opposite senses), the sign of FIGURE 74.5 Quantities for a satellite RF link. P = transmit power (dBW). G = antenna gain (dBi.) C = received carrier power (dBW). T = noise temperature (K). L = dissipative loss (dB). rs = slant range (m). f = frequency (Hz). u = uplink. d = downlink. e = earth. s = satellite. C N EIRP r G T A B ( ) s r = - ( ) + - ( ) + - ( ) - +- 10 4 10 228 6 10 4 2 2 log log . log p pl G r t = + + - ( )( - ) ( ) ( + )( + ) 1 2 4 1 12 21 1 2 2 2 2 rr r r r r wa w a w a cos D

Antenna gai Taclear Tram(a-D)a+Tl. 290(1-1) clea /1e+ Train (a-1)(al)+T, +290( -1)/16 FIGURE 74.6 Tandem connection of antenna, loss elements such as waveguide, and receiver front end. The noise temp re depends on the reference plane but GT is the same for both points shown. is negative. The axial ratio in g r. The polarization coupling is maximum when the wave and antenna are copolarized have identical axial ratios, and their polarization ellipses are aligned (At 0). It is minimum when the axial ratios are identical, the senses are opposite, and the tilt angles differ by 90o. 74.6 System Noise Temperature and G/T The system noise temperature, T, incorporates contributions to the noise power radiated into the receiving antenna from the sky, ground, and galaxy, as well as the noise temperature due to circuit and propagation losses, and the noise figure of the receiver. The clear sky antenna temperature for a directive earth antenna lepends on the elevation angle since the antennas sidelobes will receive a small fraction of the thermal noise power radiated by the earth which has a noise temperature Tarth= 290K. At 11 GHz, the clear sky antenna oise temperature, Tdeur, ranges from 5 to 10 K at zenith(el= 90%)to more than 50 K at el= 5[ Pratt and As shown in Fig. 74.6, the system noise temperature is developed from the standard formula for the equivalent (rain)loss of A=10log(a)dB, circuit losses between the aperture and receiver of L dB, and receiver noise figure of F dB(corresponding to receiver noise temperature T, K). The system noise temperature referred to the antenna aperture is approximated by the following equation where T in= 280 K is a reasonable approximation for the physical temperature of the rain [Pratt and Bostian, 1986,P. 342 T= Talear Train (a-1)/a+T, l+290(4 (7413) The system noise temperature is defined at a specific reference point such as the antenna aperture or the receiver input. However, G/T is independent of the reference point when G correctly accounts for circuit losses The satellite's noise temperature is generally higher than an earth terminals under clear sky conditions because the satellite antenna sees a warm earth temperature of =150-300 K, depending on the proportion of clouds, oceans, and land in the satellite antennas beam, whereas a directive earth antenna generally sees cold sky and ne sidelobes generally receive only a small fraction of noise power from the warm earth. Furthermore, a satellite receiving system generally has a higher noise temperature due to circuit losses in the beam forming networks, protection circuitry, and extra components for redundancy. Figure 74.7 illustrates the link loss factors, maximum nadir angle, 0, earth central angle, Y, and earth-space time delay as a function of satellite altitude. The delay for a single hop between two earth locations includes the delays for the earth-space path, the space-earth path, and all circuit delays. The path losses are shown for several satellite frequencies in use. The variation in path loss and earth central angle is substantial. For example, L-band LEO personal communications systems to low-cost hand-held telephones with low gain (e.g,G e 2000 by CRC Press LLC

© 2000 by CRC Press LLC 4rw ra is negative. The axial ratio in dB is given as R = 10log*r* . The polarization coupling is maximum when the wave and antenna are copolarized, have identical axial ratios, and their polarization ellipses are aligned (Dt = 0). It is minimum when the axial ratios are identical, the senses are opposite, and the tilt angles differ by 90°. 74.6 System Noise Temperature and G/T The system noise temperature, T, incorporates contributions to the noise power radiated into the receiving antenna from the sky, ground, and galaxy, as well as the noise temperature due to circuit and propagation losses, and the noise figure of the receiver. The clear sky antenna temperature for a directive earth antenna depends on the elevation angle since the antenna’s sidelobes will receive a small fraction of the thermal noise power radiated by the earth which has a noise temperature Tearth ª 290K. At 11 GHz, the clear sky antenna noise temperature, Taclear , ranges from 5 to 10 K at zenith (el = 90°) to more than 50 K at el = 5° [Pratt and Bostian, 1986]. As shown in Fig. 74.6, the system noise temperature is developed from the standard formula for the equivalent temperature of tandem elements including the antenna in clear sky, propagation (rain) loss of A = 10log(a) dB, circuit losses between the aperture and receiver of Lc dB, and receiver noise figure of F dB (corresponding to receiver noise temperature Tr K). The system noise temperature referred to the antenna aperture is approximated by the following equation where Train ª 280 K is a reasonable approximation for the physical temperature of the rain [Pratt and Bostian, 1986, p. 342]: (74.13) The system noise temperature is defined at a specific reference point such as the antenna aperture or the receiver input. However, G/T is independent of the reference point when G correctly accounts for circuit losses. The satellite’s noise temperature is generally higher than an earth terminal’s under clear sky conditions because the satellite antenna sees a warm earth temperature of ª150–300 K, depending on the proportion of clouds, oceans, and land in the satellite antenna’s beam, whereas a directive earth antenna generally sees cold sky and the sidelobes generally receive only a small fraction of noise power from the warm earth. Furthermore, a satellite receiving system generally has a higher noise temperature due to circuit losses in the beam forming networks, protection circuitry, and extra components for redundancy. Figure 74.7 illustrates the link loss factors, maximum nadir angle, q, earth central angle, g, and earth-space time delay as a function of satellite altitude. The delay for a single hop between two earth locations includes the delays for the earth-space path, the space-earth path, and all circuit delays. The path losses are shown for several satellite frequencies in use. The variation in path loss and earth central angle is substantial. For example, L-band LEO personal communications systems to low-cost hand-held telephones with low gain (e.g., G ª FIGURE 74.6 Tandem connection of antenna, loss elements such as waveguide, and receiver front end. The noise temperature depends on the reference plane but G/T is the same for both points shown. T T T a a T l l aclear rain r c c = + ( - 1) + + 290( - 1)