Microwave propagation in dust storms: a review

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<ul><li><p>Microwave propagation in dust stormsa review</p><p>S.O. Bashirand N.J. McEwan</p><p>Indexing term: Radiowave propagation (microwave)</p><p>Abstract: A review is given of the literature covering the effects of dust storms on microwave links. Mostauthors have calculated the effects of absorption of energy, revealing that it is not very significant unless veryhigh suspended dust densities are assumed. A few papers indicate the possibility of more significant cross polar-isation. Air refractive effects associated with dust storms have been relatively neglected but are strongly indi-cated by observations.</p><p>1 IntroductionWith continued growth of satellite and terrestrial micro-wave systems and use of ever-higher frequencies, interesthas been expressed in the possible impact of dust stormson microwave link performance.</p><p>The intention of this survey is to summarise the presentknowledge of such effects. The possible impairments are:</p><p>(a) extinction of signal energy by flying dust particles(b) cross polarisation by flying particles(c) superrefraction, subrefraction and multipath effects</p><p>in some way associated with dust storms(d) effects of dust accretion on reflector antennas.</p><p>Most authors have concentrated on the first of theseeffects.</p><p>This paper simply reviews the existing microwave liter-ature on this subject. It is intended as the first of severalpapers in which we shall describe some further investiga-tions and conclude with an attempt to give the best practi-cal estimate of the severity of the effects that can beobtained without large-scale propagation measurements.Such experiments are unlikely to be performed in the nearfuture.</p><p>2 Direct and indirect methods</p><p>The only certain way of assessing the magnitudes and sta-tistical frequency of propagation impairments on radiolinks is by direct observation, i.e. by recording data on reallinks. Such a link may be an operational one or may be setup specially as a propagation experiment.</p><p>All other approaches to the problem are indirect andsemi-empirical. They are based on calculations but alwaysrely on some empirical inputs such as particle shapes, par-ticle dielectric constant, particle chemical composition etc.This is a familiar situation in connection with the micro-wave effects of rain.</p><p>2.1 Direct observationsWhereas hydrometeor effects have been extensively investi-gated by direct measurement, systematic observations ofdust storm effects on real links are very scarce. Only onestudy is known to the authors: Al-Hafid et al. [1, 2, 3]studied the direct influence of dust storms on the Nasiriya-Daraji (45 km) and Nasiriya-Sugal-Shiyki 11 GHz micro-wave links near Baghdad, Iraq. The authors concludedfrom these field measurements that dust storms attenuatedthe received signal to fade depths of typically 10 to 15 dBfor tens of minutes at a time. A 10 dB fade lasting 150</p><p>Paper 4573H, (Ell), received 12th August 1985The authors are with the School of Electrical &amp; Electronic Engineering, Universityof Bradford, Bradford, West Yorkshire BD7 1DP, United Kingdom</p><p>IEE PROCEEDINGS, Vol. 133, Pt. H, No. 3, JUNE 1986</p><p>minutes and a 26 dB fade lasting 40 minutes wereobserved. These experimental results showed much greaterattenuation than the authors predicted by an indirectmethod which was based on simple extinction, asdescribed later. The high attenuations are especially sur-prising as the optical visibilities quoted were in the range6-10 km, which appears to indicate very light dust stormsor dust haze [4].</p><p>2.2 Indirect methods: attenuationThe earliest work (1941) which discusses the microwavescattering properties of dust storms is believed to be thatof J.W. Ryde [5]. This report only deals with the radarreflectivity of dust storms, which is found to be negligibleexcept possibly for frequencies greater than 30 GHz andvery dense storms.</p><p>It is interesting to observe how the important issues inthe assessment of dust storm microwave effects have beenrecognised in this early work, and the uncertainties whichpervade it are made evident. Ryde draws the distinctionbetween the sandstorm, which contains particles mainlygreater than 75 micron radius and rises to at most 2 mabove the surface, and a dust storm containing small par-ticles and reaching much greater heights. Lacking a knowl-edge of precise particle sizes in the dust storm, Ryde givescalculations for specimen radii ranging from 1 micron to25 microns. To calculate a suspended mass density, Rydehas assumed that a 1 cm thick surface layer is simplyraised into a cloud of 300 m height, which implies a massdensity of about 7 x 10~5 gm cm"3. (Later work showsthis to be an extreme estimate of density). Finally, the dustparticle permittivity was taken to be that of quartz; theloss tangent was only known to be very small although itwas suggested that the presence of some iron in the dustmight raise it. Because of this uncertainty, Ryde has onlycalculated radar reflectivity and not attenuation.</p><p>Ahmed [6], at the University of Newcastle upon Tyne,performed a series of 10 GHz measurements of thecomplex permittivity of both bulk and dispersed sand andclay. The main features of these experiments can be sum-marised as follows:</p><p>(a) The 10 GHz permittivities of low density dispersions(10~6 to 10"2 gm cm"3) of sand and clay dust in air weremeasured by observing their effect on the resonant fre-quency and Q-factor of a large open resonator. The mea-surement gives directly the apparent macroscopicrefractive index of the dispersed particles, equivalent to thespecific attenuation and phase shift that this particledensity would cause on a microwave link. However, atthese low densities, the permittivity of the solid materialcan also be inferred with little uncertainty because (i) theRayleigh scattering approximation (assuming sphericalparticles) works well for single-particle scattering at these</p><p>241</p></li><li><p>particle sizes, and (ii) the low density of the air dispersionmeans that 'multiple scattering' or interaction of particlesis negligible.</p><p>(b) Mean permittivities of compressed samples of thesame materials were measured by various techniques, e.g.by packing them into a short-circuited waveguide andmaking reflection measurements. The major problem inusing these results is that the samples are far from solidand a mixing formula must be used to infer the permit-tivity of the solid material from that of the air-mineralmixture. Unfortunately, no precise theory exists for suchmixing when the particles' spacing is comparable with orsmaller than their size. These theories usually providemaximum and minimum limits which the effective permit-tivity can reach. The non-precision is in the magnitude ofthe average electric field within the particles of the mixture.It is known that the effective permittivity of the mixture isrelated to the above electric field average. Uncertainty inits value is caused by the indefinite mutual interactionbetween the polarisation fields of the particles, which inturn depends upon their relative orientations if they are</p><p>non-spherical. Various formulas based on different theo-retical approximations have been proposed. An importantaim of Ahmed's work was to establish a workable formulaby comparing the compressed sample and open resonatormeasurements.</p><p>Ahmed applied several mixing formulas to the bulk(waveguide) measurements, where the fractional volume ofmineral in the sample ranged from 0.57 to 0.72, to infer thesolid particle permittivity.</p><p>Three mixing formulas, due to Bruggeman [7], Bottcher[8] and Looyenga [9], all gave a prediction of the permit-tivity of sand which agreed well with the already knownlow frequency permittivity for fused silica. No independentvalue for clay was known but the success achieved withsand implied that these formulas would be usable.</p><p>Ahmed also derived estimates for the solid permittivitiesof clay and sand by simply extrapolating to unity volumefraction on log-log plots of real refractive index and tan </p></li><li><p>measurements. His results are shown at line 1 of Table 1.Line 2 of the table shows alternative values he obtained bya mixing formula from the waveguide measurement.</p><p>Finally, the permittivities of the solid materials, derivedfrom the high density measurements, were used to predictthe low-density permittivities to be observed in the openresonator. Reasonable agreements were obtained for sand,but the predicted permittivities for clay were noticeablyless than those observed, especially for the imaginary part.</p><p>An interesting point is that the Bruggeman and Bott-cher formulas must be correct in the limit of a low densityof small spherical particles because they agree in this limitwith the results of the Rayleigh scattering approximation.This is not true of Looyenga's formula which, nevertheless,seems satisfactory at the high densities.</p><p>The open resonator results were also published byAhmed and Auchterlonie [10], giving the effective permit-tivities of the low density clay or sand in air. This paperhas been referenced by several later authors.</p><p>Finally, Ahmed, in his thesis [6] and a later paper [11],estimates the 10 GHz attenuation which could occur in areal dust storm. He derived the values: 0.1 dB/km forsand; 0.4 dB/km for clay (applicable to a true dust storm),these values being based on a density of 10 ~5 gm cm~3which he considered an upper limit. The Newcastle workrepresents a pioneering attack on the dust problem and isincomplete mainly in not fully investigating the effect ofwater uptake and chemical composition. (The sand andclay samples used were British).</p><p>Another early attempt to predict attenuation was givenby Ghobrial et al. [12]. At this stage assumptions had tobe made about the permittivity, which was taken to be3.7 +7I.O at its most lossy, and the suspended dust den-sities. 106 particles m~3 were assumed. However, a mea-sured particle size distribution for a Khartoum sand stormwas presented, showing an upper limit radius of 150microns. Below this limit the distribution of radius a couldbe roughly approximated as exponential</p><p>N(a) = ea0</p><p>-a/ao</p><p>with a mean radius a0 of about 8 microns. The worst cal-culated X-band attenuation was about 10"3 dB/km.</p><p>The paper by Al-Hafid et al. [2] in addition to describ-ing fades on an operational link, also describes some mea-surements of particle permittivity made using an openresonator. Their measured loss tangent of 0.00024 is sur-prisingly low (Table 1, line 4) even though the particlesused are described as 'sand' which is expected to be lesslossy than dust. In consequence the predicted attenuationsare much lower still than Ahmed's.</p><p>A significant advance is made in the paper by Chu [13]which makes clear the connection between optical visibil-ity and dust density. Using the Rayleigh approximation forsmall particles, and noting that scatter cross sections arealso negligible, Chu points out that the microwave absorp-tion is proportional to</p><p>1.N(a)a3 dawhere a denotes particle radius, and N(a) is the number ofparticles per m3 per unit radius range.</p><p>Thus attenuation is proportional to the suspendedmass, as all dusts have roughly the same density. Opticalextinction is derived as proportional to</p><p>I.N(a)a2 da</p><p>i.e. it is a cross-sectional area effect. Thus for monodispersedust, the attenuation is directly proportional to particlesize for a given visibility. Chu presents specimen calcu-lations of 11 GHz attenuation per km against optical visi-bility for monodisperse particles of radii 0.01 and 0.1 mmand for specimen solid permittivities of 2.5 + jO.025 and10 + jOA (Table 1).</p><p>The lower value is a quoted one for dry soil and mustbe an underestimate because soil is an air-mineral mixture.The upper value appears to be the approximate upperlimit of quoted values for likely (solid) minerals.</p><p>The assumed loss tangent of 0.01 is a similar upper limitfor dry minerals, i.e. desert conditions are assumed for thestorm.</p><p>Chu also used his assumed permittivities, with Rayleighscattering, to predict the apparent permittivity of a low-density air-dust mixture, as would be observed in an openresonator. A direct comparison with Ahmed and Auchter-lonie's results [10] then showed fair agreement except thatthe loss tangent of clay dust was greatly under-estimated.A similar discrepancy was already evident in Ahmed's owninternal comparisons, as noted before. Chu commentedthat the water content of Ahmed's dust was not known.</p><p>The largest specific attenuation, based on the lower losstangent, predicted by Chu was 0.03 dB/km for 0.1 mmradius particles and 100 m visibility. (Following Middleton[14], the visibility distance was taken to correspond to15 dB of optical extinction.) Chu commented on the pos-sible effect of antenna height in reducing the attenuation.</p><p>At this stage of development it was evident that therewas still uncertainty about the permittivities of dust stormparticles, and particularly the effect of moisture content. In1980 Ghobrial [15] described measurements on three dustsamples collected in Khartoum. A conventional resonantcavity method was used, giving permittivities for com-pacted dust which were extrapolated to the solid materialusing Mandel's mixing formula [16]. Table 1 shows hisresults. It is interesting that 4.3% of water can raise theloss tangent to 0.068, but it is also noteworthy that thedehydrated material had a loss tangent of 0.039 which issubstantially larger than Chu assumed.</p><p>Another 1980 paper by Ghobrial [17] gives permit-tivities, obtained by the same methods, for other samples(Table 1). Here the effect of water is not discussed, but par-ticle size distributions are given for four of the samples. Itwas again found that no particles had radii greater than0.15 mm and that typically 70% of particles (by number)had radii between 50 and 0.5 microns. The calculationsshowed that 108 particles m" 3 could produce 1 dB/km ofabsorption at X-band but this very high density was notrelated to a visibility.</p><p>The 1982 paper by Ansari and Evans [18] is notable inextending calculations of attenuation up to 37 GHz. Acareful attempt was made to deduce from the literature thenecessary particle permittivities over a wide range of fre-quency and water contents. It was necessary for them touse published bulk permittivities for soils, mostly mea-sured with a view to remote sensing of the earth's surface.This can be criticised in that they have not scaled thesepermittivities of air-water-particle mixtures, to the permit-tivity of the particle material, and so will have underesti-mated the permittivities. However, some useful generalconclusions are drawn:</p><p>(a) Both real and imaginary parts of the bulk permit-tivity of dry soils are nearly frequency independent.</p><p>(b) Moisture content dominates the imaginary partincreasing it by an amount which has a complex frequencydependence, but which increases between 1 and 24 GHz.</p><p>IEE PROCEEDINGS, Vol. 133, Pt. H, No. 3, JUNE 1986 243</p></li><li><p>(c) Moisture also increases the real part by an amountwhich is constant up to about 8 GHz and then decreasesup to 24 GHz.</p><p>(d) Cihlar and Ulaby [19] are quoted to the effect th...</p></li></ul>

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