Propagation Effects On Satellite Systems

CHAPTER 6PROPAGATION EFFECTS ON MOBILE-SATELLITE SYSTEMS6.1 GROUND WAVES AND EFFECTS OF TERRAINPrevious chapter have dealt largely with atmospheric effectson radio-wave propagation, the exception being Sec. 5.3 whichconsiders effects of vegetation. Terrestrial telecommunicationlinks and earth-space transmissions, especially at small elevationangles or between satellites and mobile systems, however, may alsobe influenced by the electrical properties of the Earth’s surface andby features of terrain: This section deals with ground waves andobstruction or shadowing by terrain or structures. Section 6.2treats the physical phenomena of specular reflection and diffusescatter by the Earth’s surface, and Sec. 6.3 relates thesephenomena to system design considerations. Sections 6.4, 6.5, and6.6 give attention to land-mobile, marine-mobile, and aeronauticalmobile systems, and the final Sec. 6.7 describes the GlobalPositioning System (GPS). All of the major propagation effects onsatellite mobile systems (not only the effects of terrain) are givenat least brief mention in Sees. 6.4-6.7.6.1.1. Ground WavesOne means by which radio waves propagate from one locationto another is by groud waves. In analyzing propagation near theEarth’s surface, what are referred to as ground waves are oftenseparated into s ace waves and surface waves. A space waveconsists of the direct wave from transmitter to receiver and thereflected wave, if any, that reaches the receiver after reflectionfrom the Earth’s surface. It is the surface wave that is moststrongly affected by the electrical properties of the Earth. Theattenuation of the surface wave is high and surface wave propagationis limited to short distances for high frequencies. The surface waveis the principal component of the ground wave for frequencies of afew MHz, is of secondary importance at VHF (30-300 MHz), andcan be neglected for frequencies greater than 300 MHz (Burlington,1977).An approximate expression for the attenuation or loss factor L sfor a surface wave is6-1

mLs -121 - j2nd/A (sin 0 t Z)(6.1)wherePz ( K - j - COS26 ) “2o(6.2)for horizontal polarization and(K-j - COS26 ) 1’ 2oz .K - j (U/W o),(6.3)for vertical polarization.Ls has a maximum value o! mity. Theexpression is most accurate for L 0.1 and w hin 2 dB inamplitude in any case but is in error in phase by 180 deg as Lsapproaches 1 (Burlington, t 977). In the above expressions O/WCcan be replaced by its approximate equivalent 60 uA.Th:conductivity o is in mhos/m, 0 is the elevation angle, w 2rf where f is frequency, E. is the electric permittivity of emptyspace (8.854 x 10-12 F/m), and K is relative dielectric constant. Ifusing 60 uA, A is in m. Surface waves are most important atfrequencies below the 100 MHz lower limit of this handbook and ina region within a few wavelengths of the ground. They can ben e g l e c t e d i n m o s t applications of m i c r o w a v e mobilecommunications (Jakes, 1974, where the micmwave range istreated as from about 450 MHz to 10 or 20 GHz). A more thoroughtreatment of surface waves can be found in Jordan and Balmain(1968). Ground-wave propagation at frequencies from 10 kHz to 30MHz is treated in CCIR Recommendation 368-5 (CCIR, 1986a).6.1.2 Effects of ObstructionsObstructions along a path in the form of hills and buildingsintroduce loss with respect to free-space propagation, and the lossvaries with time because tropospheric refraction varies with time.6-2i.r!m

Ii.For considering the effect of obstructions, the concept of Fresnelzones is useful. To intrmduce this topic consider Fig. 6.1 whichshows tWO Daths TPR and TSR between ‘a transmitter T-and receiverR. TPR ii a direct path, and TSR is longer than TPR. If TSR TPR A/2 where A “is wavelength, the figion within the radius r(in the plane perpendicular to TR), at a distance dT from T and dRfrom R, is defined as the first Fresnel zone. The particular valueof r in this case is the fi t Fresnel zone radius and is desi natedas F1. The concept can be extended to the case that TSR ?PR nA/2, for which the corresponding Fresnel zone dius can bedesignated as Fn.The significance of the fi t Fresnel zone is that.all the elements of radiation passing through this zone havecomponents of electric field intensity that add constructivelyRadiation passing through the second Fresnel zone (values of rbetween FI and Fz), however, interfe s destructively with radiationassing the fimt Fresnel zone, that passing through the thirdresnel zone adds constructively with that in the first zone butmakes a smaller contribution, etc. The princess can be unde toodin terms of Huygen’s principle which states that every elementarya a of a wavefront can be regarded as a source of secondaryspherical waves. When r is small compared to dT and dR, it can bedetermined thatF(6.2) .where d dT dR and all kngt are in meters Or t tF1 17.3J“#Rm(6.3)fdif distances are in km, f is measured in GHz, and F1 is in meters.For the situation that dT is app ximately eq@l to d the expressionfor F1 co sponding to Eq. (6.2) is(6-3

FI ( dR) *iz(6.4)The value of Fn is related to that for F byFn 1/z F,(6.5)One might think that a satisfactory signal amplitude wouldresult on a telecommunications link as long as a direct line of sightfrom the transmitter to the receiver is provided, but considerationof Huygen’s principle suggests that having a direct line of sight maynot be sufficient. The analysis of the effect of an obstructionapproximating a knife edge is given in tex s on optics, for examplethat by Jenkins and White (1976), and in Jordan and Balmain(1968). The results are conveniently expressed in terms of theratio hc/F1 of path clearance hc to the first Fresnel zone radius Fl,as in Fig. 6,2. If the edge of the knife-edge obstruction is at thedirect line of sight, a loss of 6 dB is encountered. To avoidattenuation a clearance of about 0.6 F1 is required, Note thatFresnel zone analysis is in terms of field intensity. For zeroclearance the total field intensity at the receiver location is reducedto O.5 of the value for a completely unobstructed path, A reductionof field intensity to 0.5 corresponds to a reduction of power to 0.25and therefore to the loss of 6 dB, In analyses of diffraction aparameter v equal to 21’2 hc/F1 may be utilized and resulting valuesof attenuation may be plotted as a function. of v, The parameter v isused, for example, in CCIR Report 715-2 (CCIR, 1986b) and inJordan and Balmain (1968).The field intensity beyond an obstacle is dependent upon the formof the obstacle. The loss due to a knife-edge obstacle at grazingincidence is 6 dB, but the corres onding value for a smoothearth is about 20 dB (Burlington, 197 ). Formulas and nomogramsfor determining the loss due to diffraction by a smooth sphericalearth are given in CCIR Report 715-2. This same report discussespropagation over irregular terrain, and Hall (1979) treats thisdifficult topic. Multiple knife-edge diffraction is the subject of apaper by Deygout (1966). He finds which knife-ed e obstacle causesthe greatest loss and determines this loss. fhen locations and6-4

1sRTP‘T‘R.Figure 6.1. Geometry for consideration of Fresnel zones.-20IIIIIIII2461 1 1i)I I0;5I1II11.0IIIIiI,“III1.III III1.5111I2.06Figure 6.2. Attenuation due to knife-edge diffraction with relationto free space, as a function of hc/Ft nl’2 (Hall,1979).6-5

additional losses are calculated for the other knife-edge obstacles.Assis (1971), noting that the assumption of a knife edge often givesoverly optimistic results, employs the approach of Deygout butapplies it to the case of rounded obstacles. He provides a set ofcurves (Fig. 6.3) which give loss asa function of H/Fl, where H isthe height of the obstacle above a direct unobstructed path fromtransmitter to ceiver, and a parameter a whereIr(6.6)with A the wavelength, r the radius of curvature, and F1 the firstFresnel zone mdius.Note that the condition H/Ft -0.6corresponds to he/F, 0.6 and to free space propagation. AlsoH/F Oandcr O is h condition for the loss of 6 dB mentionedfor knife-edge diffracti and H/Ft 0 and CY 1.5 correspondsroughly to the loss of 20 dB mentioned earlier as well. Forpositive values of H/Ft, mm-esponding to obstructions extendingabove the lowest direct obstructed path, losses are shown toincrease above those for H/Fi.An alternative approach topropagation over irregular terrain utilizes an integral equationtheory (Ott, i971 ) instead of diffraction theory.It is possible for the signal beyond an obstacle, such as amountain, to be larger than if the obstacle was not present. Thiscondition occm due to diffraction alone in the case of a knife-edgeobstacle as in Fig. 6.2, where there is a direct line-of-sightpath and the obstacle is below the path. We consider now, however,the situation where there is no direct path, This is the case forwhich the term obstacle gain is normally applied. In this casemultipath propagation occurs as in Fig. 6.4, for example, wherefour paths exist between a tmnsmitter and receiver on the oppositesides of an obstacle. Obstacle gain depends upon the occurrence offavorable phase Aations between the signals arriving over thedifferent paths. It can be destroyed by meteorological variationsand thus may be subject to fading but can be used to advantage incertain circumstances (Kirby et al., 1955; Hall, 1979). The lossesassociated with the occurrence of obstacles on mobilecommunication systems am commonly referred to shadowing losses.6-6.P

0III10\20.304CPATH OBSTRUCTION H/RFigure 6.3. Attenuation due to diffraction over obstacles, withrelation to free s ace, as a function of the parametera and H/R H/ with H the height of the obstacleabove a direct unobstructed path (Assis, 1971).TFigure 6.4 Possible ray paths contrib ing to obstacle gain.6-7

6.2 SPECULAR REFLECTION AND DIFFUSE SCATTER6.2.1 IntroductionOn both earth-space and terrestrial line-of-sight paths, signalsmay reach a receiving antenna by a direct atmospheric path and byspecular reflection and diffuse scatter from the ground. In thefollowing Sec. 6.2.2, expressions are given for total signalamplitude as a function of elevation angle and antenna height for thecase of two equal sinusoidal signal components, one traveling overadirect atmospheric path and one experiencing specular reflectionfrom a flat, smooth, perfectly conducting surface. Reflectioncoefficients for specular flection from a flat, smooth earth havinga finite conductivity are given in Sec. 6.2.3, and surface roughnessis taken into account in Sec. 6.2.4. Diffuse scatter is discussed inSec. 6.2.5, and the facto affecting total signal amplitude aresummarized in Sec. 6.2.7.uw6.2.2 Multipath EffectsThe term multipath refe to a condition in which energyreaches the receiver of a telecommunications system by more thanone path. Multipath operation tends to be bdesim le, becausesignals arriving over the diffe nt paths tend to arrive with variablerelative phase, with the resdt that they alternately reinforce eachother and interfe destructively.The total signal is thencharacterized by fading involving Rpeated minima, and the dangerexists that the minima will fall below the acceptable signal level.The signals arriving over the different paths also have differenttime delays which can result in intersymbol interfe nce in di italsystems. ultipath mpagation may result from reflection Fmmland and water s Jaces and manmade structures.Multipathpropagation may also arise fmm atmospheric effects alone, in theabsence of flection fmm surface features,Reflections from a plane surface and the total electric fieldintensity which results when field intensities arriving over twopaths a summed can reconsidered with the aid of Fig. 6.5. Thefigure shows di ct and reflected rays reachin a receiver at aheight hR above a flat, smooth surface at h 0.% he transmitter is6-8\D.dPm