Indian Journal of Radio & Space PhysicsVol. 16, April 1987, pp. 225-231

Noise Problem in the Design of Sodar System

S P SINGAL

National Physical Laboratory, New Delhi 110012

Received 19 November 1985; revised received 14 May 1986

The signal received in an acoustic sounding system after scattering from the inhomogeneities of temperature, wind velocityand humidity is generally weak compared with the prevalent ambient noise received along with the signal. It is due to theoperational frequency of the sounder lying essentially in the low audio range. The noise problem has been overcome' to acertain extent through appropriate design of the sodar antenna, use oflow noise electronic circuitry, higher transmitting powerand narrow band widths of the received signal. Design features of a monostatic sodar system fabricated at the NationalPhysical Laboratory, New Delhi, have been discussed, and the potential of the device to study the thermal boundary layer ofthe lower atmosphere has been brought out through a brief description of the salient features of the results obtained.

1 IntroductionGilman et al.' were the first to use acoustic waves to

probe the lower atomosphere, but it was Mcallister?who demonstrated the practical application of thetechnique, on which extensive work has subsequentlybeen carried out all over the world:'. In this technique,highly directional short bursts of sound energy areradiated into the atmosphere which after scatteringfrom atmospheric fluctuations of eddy sizes within theinertial subrange (0.1-10 m) are received back by areceiving antenna. Useful qualitative informationabout the ground-based thermal activity, nocturnalinversions and breaking waves can be immediatelyobtained by visual inspection of the facsimile chart,while quantitative information about the temperaturestructure and wind fields in the scattering volume canbe computed by measuring the amplitude andfrequency of the received signal.

to probe the thermal boundary layer, a monostaticsodar system has been set up at the National PhysicalLaboratory (NPL), New Delhi, through indigenousefforts in respect of design and fabrication. In thispaper, the design features of the sodar systemdeveloped and the associated problems due to ambientnoise have been discussed.

2 Sodar Design Considerations and Ambient NoiseThe received power after scattering in the acoustic

sounder is given by the following expression":

Cr A 0'(0)P,= P'2' ---W-. G.L.

where P, is the received power; P, the transmittedpower; A the collecting area of the receiving antenna; Cthe speed of sound waves; r the duration of the tone

burst; R the height range of the target volume; L theattenuation factor which includes transmit and receivetransducer efficiencies, atmospheric attenuation andantenna efficiency; G the antenna gain; and 0"(0) thescattering cross-section at an angle (J per unit volumeper unit distance per unit solid angle at frequency Fand range R, and is expressed as:

0'(0) = 0.03kl/3COSl0

rCl 0 Cl}x V.cos12+0.l3~ sin 0/2)-11/3

(MKS units)

where k is the wave number of the sound waves givenby 2n/A, A the wavelength of the sound waves, T theambient temperature, and c; and C;, are thetemperature and wind velocity structure parametersrespectively.

A look at the variation in the scattering cross-sectionwith angle (Fig. I) shows that (i)except for angles closeto back-scatter, i.e. between 171° and 180°, thescattered acoustic power due to wind fluctuationsdominates over the scattered power due totemperature fluctuations; (ii) there is no scattering at90°; (iii) only temperature fluctuations are responsiblefor back-scattering; and (iv)the scattering cross-sectionis generally weak (of the order of 10- 9 for back-scattering and an order higher for forward scattering).The following average values of the parameters (basedon the radiosonde data at Delhi) have been used for theplot of Fig. I: C;,= 8 x 10- 2m4/~s - 2; C? = 2x 10- 3K 1m - 2;3; T= 300 K; and C = 340 ms -I.

Fig. 1 shows that the received acoustic power is, ingeneral, very low. To design a sodar system capable ofoperation in the range of about a kilometer, it is thus

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INDIAN J RADIO & SPACE PHYS, VOL. 16, APRIL 1987

desirable to increase the received power as much aspossible. A look at the received power equation showsthat this can be achieved by increasing the transmittedacoustic power, duration of the tone burst, antennagain, antenna and transducer efficiencies, and area ofthe antenna. Along with all these factors, the acousticattenuation due to propagation of sound waves in theatmosphere should be made as low as possible.

In the present design, a maximum acoustic power oflOW (130 dB) for a tone burst of lOOmsrepeated every4 s has been selected. The tone burst duration cannotbe made unusually long since the duration of thetransmitted pulse determines the range resolution(.t\h ~ Cr/2) and the receiver bandwidth (.t\B ~ 2/r).Resolution will become poor and the receiverbandwidth will become narrow which will make itdifficult to receive fully the scattered signal due toDoppler shift of the received signal. There are alsolimits- to the increase in antenna gain, antenna andtransducer efficiencies. and area of the antenna.

The factor which can be controlled to a large extentis the attenuation of the acoustic waves encounteredwhile propagating in the atmosphere. This factorinvolves the choice of the operational frequency of theacoustic waves, an important parameter to be selec-ted in the design of the sodar system. Data onacoustic absorption by Harris 7 coupled with the factthat absorption increases with frequency indicate that

10-5,....----------------,5

M 5e<t_ 2Ci) -7610

! 5

~ 2 ae(THI-~

(/)

g10-8

90 120 15030 60SCATTERING ANGLE (8). deg

Fig. I-Acoustic scattering scross-section asa function of angle

226

the most suitable operating frequency for sodar will be2-3 kHz. Lower frequencies cannot be used since theycan cause ground clutter and reflection from sideobjects due to the presence of strong side lobes alongwith the wide central lobe; while higher frequencies, onthe other hand, encounter more attenuation.

Environmental ambient noise presents a seriousproblem at lower audio frequencies since this noisemasks the weak scattered signals due to atmosphericinhomogeneities. The real problem lies in the design ofappropriate acoustic antenna which should offerrequisite gain but at the same time should suppressambient noise.

3 Design of the Sodar AntennaThe acoustic antenna in the sodar system serves as a

collimator for transmitting power into a narrow beamin the transmit mode and as a highly directionalmicrophone in the receive mode. Due to diffraction, theantenna always has sensitivity in directions other thanthe main lobe. The strength of these side lobes dependsupon the design of the antenna. To operate the sounderin noisy environments, it is always desirable tosuppress the antenna side lobes so that the randomnoise-receiving sensitivity of the antenna is reducedcompared to its signal receiving sensitivity.

Simmons et al.8 estimated the magnitude of thedesirable 90° side-lobe suppression for the acousticantenna, taking into account the prevailing ambientnoise, antenna transducer efficiency, and ambienttemperature of the environment. According to them,for antenna transducer efficiency of 25 mVIdeg/cm2 ata teinperature of 200e and signal to Johnson noiseratio of 15 dB, the required 90° side-lobe attenuationfactor of the acoustic antenna should be 97 dB belowthe main beam in order to operate it in the noisiest ofthe representative environments (noise around 110dB)and 53dB below the main beam in order to operate it inthe quiet residential localities (noise around 65 dB).The measured environmental noise at the NPL terrace(the surrounding comprises offices and residences) hasa maximum level of 70 dB. For the system describedhere, having about the same antenna transducerefficiency, an average ambient temperature of about200e and approximately the same signal to Johnsonnoise ratio, a 90° side-lobe suppression of 55-60 dB willbe required.

A 90° side-lobe rejection of 55-60 dB has beenachieved by using a parabolic dish as the acousticantenna surrounded by a partial sound absorbing andpartial sound insulating shield. An immobile acousticantenna (Fig. 2a) and a mobile acoustic antenna system(Fig. 2b) have been developed, and both are workingsatisfactorily. The paraboloid antenna ofthe immobilesystem is of 1.8m diameter aperture fitted with an

SINGAL: NOISE PROBLEM IN TIlE DESIGN OF SODAR SYSTEM

(a)

Fig. 2-A view of the paraboloid acoustic anten as surrounded by the acoustic shields [(a) immobile antenna system; and (b) mobileantenna system]

acoustic transducer (driver unit) and horn at its focus.It is surrounded by an acoustic screeen to give anadditional 90° side-lobe rejection of about 25-30 dB at2 kHz. The shield is octagonal in horizontal sectionwith its walls sloping out at 8 deg with the vertical suchthat the inside base diameter is 2 m, while the topopening is 3 m in diameter. Each panel of the slopingoctagonal assembly is a trapezoid (2.4 x 1.2x 0.9 m) of1.5 mm thick mild steel. The inside surfaces of thesetrapezoids have been lined with 3 mm thick tarfelt and5 em thick mineral wool. This arrangement has beenfound to work both as an effective insulator forexternal ambient noise as also as an effective absorberfor the transmitted side lobes. Rejection of 55-60 dB forthe 90° side lobe has been achieved with this system.This figure can be further increased if a thin sheet oflead is lined all' around the inner surface of the mildsteel sheet.

The mobile acoustic antenna (Fig. 2b), which iscomparatively a light weight antenna system, offerspractically the same operational conditions asprovided by the immobile antenna system. Thisantenna consists of a paraboloid combined with aconical surface which serves as the shield. The systemhas been fabricated in fibreglass with layers of thin leadsheet on the inside and a mineral wool blanket of 5 ernthickness on the outside. The shield from inside hasbeen further covered by a layer of tarfelt to give it a softacoustic absorbing surface for the transmitted sidelobes. The lead sheet practically works as an effectivereflector for the ambient noise. At the focus of theparaboloid surface, an acoustic transducer matchedwith a suitable horn has been placed to act both as anacoustic transmitter and a receiver. The conical shieldis 1.5m high, and the paraboloid has an aperture of 1.2

m and the focus is at a height of 0.4 m from the apex ofthe paraboloid surface. The 90° side-lobe rejectionfrom this system has been found to be 55-60 dB at afrequency of 2 kHz.

4 System PerformanceThe schematic block diagram of the NPL sodar

system is shown in Fig. 3; the optimum operationalparameters are given in Table 1.The facsimile serves asthe central pivot in this system since it records thereceived signals to give continuous information of thethermal structure of the lower atmopshere and alsooffers synchronization to the whole system bygenerating a timing trigger.

Based on the design parameters, given in Table I,thereceived power equation can be written as:

P,:::::;3.35 x 1O-8~ W

The attenuation factor (L) in this received powerequation for the various ranges of propagation hasbeen estimated using an antenna efficiency factor of1.27x 10- 1 (considering tranducer efficiencyof 0.1 andeffective collecting area as half of the total area of theparaboloid) and acoustic attenuation 7 of 2.2 dB per100 m round trip at a frequency of 2.2 kHz,temperature of 20°C and relative humidity of 50 %.Acoustic attenuation has been estimated for only onevalue each of temperature and humidity for oneparticular height range, since its values for differenttemperatures and humidity during the various times ofthe day, as seen from Harris' curves 7, are not largelydifferent. The estimated values of the attenuationfactor and of the received power for different values ofheight range are given in Table 2.

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INDIAN J RADIO & SPACE PHYS, VOL. 16, APRIL 1987

ACOUSTICSHIELD

Fig. 3-Schematic block diagram of the NPL monostatic sodar system

Table I-Characteristics of NPL Monostatic Sodar System

Transmitted acoustical powerTransduction efficiencyPulse widthPulse repetition periodOperational rangeResolutionReceiver bandwidthFrequency of operationAcoustic velocityAmbient temperatureTransmit-receive antenna

lOW10%100 ms4 s700 m (approx.)17 m (approx.)50 Hz2200 Hz350 mls (average)300 K (average)Parabolic reflector surroun-ded by acoustic culT2.5 sq. mFraction of a microvoltFacsimile recorder

Receiver areaPreamplifier sensitivityRecorderThermal structure

parameter C} (average value)

The received power as a function of height range hasalso been plotted in Fig. 4, so that the performancecharacteristics of the sodar system could be assessedgraphically also. Evidently, the detection of thereceived power in terms of genuine signals will dependupon the ambient noise power received at the sametime by the acoustic antenna. The ambient noise powerat the site inside the shroud under normal weatherconditions has been measured to be 24 dB during daytime and 15 dB during night time in the bandwidth

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Table 2-Estimated Values of Attenuation Factor (L) andReceived Power (P,) for Various Height Ranges (R)

R,m L P" W

100 7.5xlO-2 6.25 X 10-13

150 6.5 x 10-2 2.43 X 10-13

200 5.6 x 10- 2 1.18 X 10-14

300 3.2 X 10-2 3.0 X 10-14

450 1.3x 10-2 5.38 x 10-15

600 6.5 X 10-3 1.5 X 10-15

900 1.3x 10-3 1.35 X 10-16

range of 1250-2500Hz. This noise power as received bythe sodar antenna through the narrow bandwidth filterof 50 Hz at the operating frequency of 2.2 kHz has beenestimated according to the relationship:

SL=SPL-1O log ~v

where SL is the received spectrum level, SPL the actualsound pressure level, and ~v the ratio of the octavebandwidth to the receiver bandwidth and has beenlimited by the effectivecollecting area of A2/4n (antennaeffect"), The values so obtained are 1.8 x 10- 15W

during day time and 2.3 x 1O-16W during night time.The received noise power for abnormal conditions ofstrong wind, dust storm, rain and aircraft passage, etc.over the sounder, will of course be higher than underthe normal conditions.

SINGAL: NOISE PROBLEM IN TIlE DESIGN OF SODAR SYSTEM

-to!5 10

1000J

iI

II :II800 l-

I

~I1

•II I~I

~ ... ~

.1loll

col

;:)1I !!I0,I

I 01eJ ••

Ili~lol'

zi~I RAIN a AIRCRAFT

1&1400 .101~ ZSI?c51o I NOISE

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II 111111' II-IT

---IS-M-I.-I.-II10 2 !510 2 !510 2 !5102 !510 2 !510 2 !5102

RECEIVED ACOUSTIC POWER (Pr),W

Fig. 4-Estimated received acoustic power for the NPL sodar system as a function of height (The operating range limitations due to thevarious environmental factors are also shown)

It is seen from Fig. 4 that ambient noise limits theoperating range of the sounder. The sounder canoperate up to the heights of 800 m during night timeand up to about 600 m during day time. Noise due toheavy winds lowers the range of the sounder to about150 m; while noise due to storm, rain, and aircraft,completely masks the received acoustic power.

5 Boundary Layer Studies Using SocIarVarious types of structures observed on sodarduring

the course ofthe present study are given in Fig. 5.Thesestructures can be classified into thermal plumes, risinglayer with thermal plumes below it, surface-based layerwith flat or spiky (short or tall) top, and stratifiedlayered structure with or without the medium slowoscillations superimposed over it (stable atmosphere).Thermal plumes represent the presence of convectivestructure in the boundary layer (unstable conditions)and are known as thermal echoes. They give a measureof the active mixing height of the boundary layerduring day time. During the night, layered structuresare formed (stable weather conditions). They are calledshear echoes. Layered structures may be surface-basedor elevated or both. The morning rising layer above thethermal plumes gives information about fumigationconditions. The surface-based layer with a flat topformed under slight or no wind conditions indicatesthe stable layer produced by radiation inversion.Spikes above the layer show influence of surface winds(turbulence), with depth of the stable layer increasingdue to mechanical mixing. Stratified layered structureis presumably powered by wind shear.

To represent the various characteristic features ofthe sodar echograms, monthly variations of height ofthermal plumes measured during 1200-1400 hrs havebeen studied. It has been observed that the maximum

observed height of thermal plumes on sodar is 500 m,

the maximum in their mean height (around 350 m)occurs in the month of October, while the minimum(around 220 m) occurs in January. It has, however,been observed that the height ofthe unstable boundarylayer (mixing depth) at Delhi calculated fromHolzworth model using morning radiosonde data 9 isvery much larger than that determined by sodaroAronlO in his studies, however, has shown that mixingheight as determined by the conventional me­thodsll•12 gives an inconsistent indicator of -airpollution concentration and that temporal and spatialvariations in mixing height are much more thangenerally estimated by the conventional methods. Asemphasized by AronIo, a better method fordetermining mixing height· needs to be developed.From tht' present studies with the sodar, it is felt thatheight ofthermal plumes seen on sodar may be a betterindicator of mixing height during day time.

Monthly variations of the height of the surface­based layer structures measured during 0300-0500 hrs,having flat top, short spikes at the top, and tall spikes atthe.top, have also been studied. It has been observedfrom these studies that the maximum observed heightof the nocturnal boundary layer is 300 m for flat topstructures, 350 m for short spikes at the top, and 550 mfor tall spikes at the top. It has also been observed thatthe maximum mean height of nocturnal boundarylayer for flat top structures is 175m and occurs duringJune, for short spike structures it is 230 m and occursduring August, while for tall spike structures it is 250 mand occurs during July. Further, the nocturnalboundary layer is generally higher in summer than inwinter.

Height, thickness, duration, time of occurrence andmonthly distribution of stratified layers have also beenstudied. It has been observed that the stratified layerslie mostly in the height range of 100-300 m above the

229

600

400

600

400

200

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600

400

E200'"l-

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400

200

o

600

400

200o

600

400

200

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INDIAN J RADIO & SPACIi PHYS. VOL. 16, APRIL 1987

(0)

200

o LL -L ~~------~~~~~-0700 0800 0900

4 MAR.,83, , r. ~t

(b)

1200 1300 140031 JAN.,83

o1700 1900 2100

29 JAN.,832300

1ST(d)

2300 00003 FEB.783

,ff•,

2200

2200

0530 0630 07302 FEB" 83

0830 1ST

Fig. 5-Sodat echograms displaying different thermal echoes and shear echoes [(a) Morning rising inversion layer with formation of thermalplumes under it; (b) Day time thermal plumes; (c) Flat top surface-based layer; (d) Short spiky top surface-based layer; (e) Tall spiky surface-

based layer; and (I) Stratified layer structure]

230

SINQAL, NOISE PROBIEMIN 1HE DESIGN OF SODARSYSTEM .. mm -- II

surface level, although they do occur sometimesthroughout the whole range of the equipment. Theirthickness is mostly in the range 25-75 m; their durationis mostly of the order of 2 hr, although they have alsobeen observed to exist for as long as 10 hr. Theoccurrence probability is maximum during the earlymorning hours (diurnal variations) and duringpremonsoon months (seasonal variations).

The undulating or quasi-periodic structures havebeen found to have periods of 1-15min and amplitudesaround 100m peak to peak. They can be grouped intooscillations of the nocturnal radiation inversion, anascending inversion or a front as reported byEymard 13. Their shapes and time periods correspondto characteristics of gravity waves14•

Studies thus made from the sodar echograms givecomprehensive information about the thermalstructure of the boundary layer. The data, whenmodelled properly, can be used to map height andwidth of the radio ducts in the lower atmosphere, todetermine the mixing height and stability of theatmospheric boundary layer, to study meso-scalephenomena, and to map the turbulent boundary layer.

The data have also applications in microwavecommunication15, in monitoring air pollution16, andin civil aviation 17. It is reported elsewhere18, that sodarcan be an effectivetool to nowcast (forecast for the nextfew hours) stability, mixing height of the boundarylayer, and meso-scale phenomena for which purposesodar systems have been installed at the Micro­Meteorological Laboratories (BARC), Tarapur, andthe Central Board of Prevention and Control of WaterPollution and Air Pollution, Delhi.

AcknowledgementThe author is thankful to Dr A P Mitra for his keen

interest in the project of acoustic sounding and for

extending facilities to do the above work. The author isalso thankful to the India Meteorological Department,New Delhi, for loaning him the radiosonde data andother help as and when needed.

References

I Gilman G W, Coxhead H B & Willis F N, J Acoust Sac Am(USA), 18 (1946) 274.

2 McAllister L G.] Atmos & Terr Phys (GB), 30 (1968) 1439.

3 Brown E H & Hall F F (Jr), Rev Geophys Space Phys (USA), 16(1978) 47.

4 Neff W D, Quantitative evaluation of acoustic echoes from theplanetary boundary layer, Tech Rept ERL 322 WPL 38NOAAjERL (Wave Propagation Laboratory, Boulder,Colorado, USA), 1975.

5 Beran D W, Remote sensing wind and wind shear system, TechRept No FAA-RD-74-3 NOAAjERL (Wave PropagationLaboratory, Boulder, Colorado, USA), 1974.

6 Little C G, Proc IEEE (USA), 57 (1969) 571.7 Harris C M,J Acoust Sac Am (USA), 40 (1969) 571.8 Simmons W R, Wescott J W & Hall F F (Jr), Acoustic echo

sounding as related to air pollution in urban environments,NOAA Tech Rept ERL 216-WPL 17, NOAA/ERL (WavePropagation Laboratory, Boulder. Colorado USA), 1971.

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lO Aron R, Atmos Environ (GB), 17 (1983) 2193.II Holzworth G C,] Appl Meteorol (USA), I (1962) 366.12 Holzworth G C, Mon Weather Rev (USA), 92 (1964) 235.

13 Eymard L, Gravity waves in the planetary boundary layer­Experimental study through acoustic sounding, Tech. Note,CRPE/54, CNRS, 92131, Issy Les Monlineaux, France,1978.

14 Beer Tom, Atmospheric waves (Adam Hilger, London), 1974.

15· Singal S P, Gera B S & Ghosh A B,J Sci & Ind Res (India), 40(1981) 765.

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