The presented paper describes an instrument called Aurometer which was designed for both atmospheric visibility and rainfall rate measurement. The principle of operation of Aurometer is based on determining the amount of light scattered by particles in the air that passes through the sample volume. The achieved results show that simultaneous estimation of the atmospheric visibility and rainfall rate is possible

Czarnecki, Tomasz

Holejko, Krzysztof

Perlicki, Krzysztof

English

International Journal of Electronics and Telecommunications (Warsaw University of Technology)

INTL JOURNAL OF ELECTRONICS AND TELECOMMUNICATIONS, 2013, VOL. 59, NO. 2, PP. 109–112Manuscript received November 28, 2012; revised April, 2013. DOI: 10.2478/eletel-2013-0013Infrared Sensor to Predict Atmospheric Visibilityand Rainfall RateKrzysztof Holejko, Tomasz Czarnecki, and Krzysztof PerlickiAbstract—The presented paper describes an instrument calledAurometer which was designed for both atmospheric visibilityand rainfall rate measurement. The principle of operation ofAurometer is based on determining the amount of light scatteredby particles in the air that passes through the sample volume.The achieved results show that simultaneous estimation of theatmospheric visibility and rainfall rate is possible.Keywords—light scattering, atmospheric visibility, rainfall rate,rain gaugeI. INTRODUCTIONTHE CLEAR VISIBILITY of the environment is espe-cially important in all kinds of transport. Dense fog orrain developed along roadways may decrease the safety ofmotorists considerably, thus automated detectors and monitorsof the visibility, fog, rain, snow or dust are important. Suchdetectors are also useful for monitoring the air pollution inindustrial aerials [1]. Optical sensors such as transmitometersor scatterometers have been developed to measure some en-vironmental parameter. Transmitometers are reliable but arevery expensive yet, which explains why they are deployedat critical places like major airports. The reliability of scat-terometers is sometimes questionable but these instruments areless expensive than transmitometer [2], [3]. In turn real timequantitative measurement of rainfall rate is needed in manycivilian and military applications. Several real time rain gaugeswere proposed and tested [4], [5]. In this paper we proposeinstrument called Aurometer which was designed for bothatmospheric visibility and rainfall rate measurement. Principleof operation of presented instrument is based on determiningthe amount of light scattered by particles in the air whichpasses through the sample volume. The obtained results showthat simultaneous estimation of the atmospheric visibility andrainfall rate is possible.II. THEORETICAL BACKGROUNDThe principles of fog and rain detection are based on lightpropagation properties in the atmosphere, where absorption,scattering and reflection effects influence the transmitted beam.The power loss in a absorbing and scattering environment isproportional to the input power of the beam Pin and the pathThis paper includes the results of work carried out within the Project KBN –O/N517/624229/2010 supported during 2010-2013 by the Ministry of Sciencein Poland.K. Holejko, T. Czarnecki, and K. Perlicki are with Institute of Telecom-munications, Warsaw University of Technology, Nowowiejska 15/19, 00-665Warsaw, Poland (e-mail: perlicki@tele.pw.edu.pl).length of the transmission dR, which leads to the well knownformula for the attenuation α of the output power beam Pou:α =PouPin= e−bR, (1)where b is the extinction coefficient.According to Koschmider [6] the contrast k = (Lb−Lo)/Lbof dark objects seen against the horizon sky as a background inthe foggy atmosphere and determined by the image irradiancesof the object Lo and the background Lb, changes in the samemanner, as formula (1). The standard visibility Vn is definedas a distance R for the threshold of the human discriminationestimated for the contrast k = 0.02. Now a different valueof the threshold contrast is used mainly k = 0.05. So fork = exp(−bR) and k = 0.05 for R = Vn one obtainsb = 3/Vn and:Vn =3RlnPinPou=3Rln1k. (2)By means of formula (2) the standard visibility Vn can beestimated in two ways:1) By measuring the attenuation of transmitted beam alongthe known distance R, what is performed by means ofso called transmissometers, which give the most accurateresults of measurements.2) By measuring the contrast of a dark object located ata known distance R, for instance by means of the cameraTV.Another way of measuring the standard visibility isbased on measurement rather the lost of transmitted power,then the power attenuation itself. The lost of transmittedpower dP = Pin − Pou is caused mainly by scattering thebeam along the distance R. Such a whole scattered powerdP = Pin − Pou according to (1) and (2) is equal to:dP = Pin − Pou = Pin3RVn. (3)It is obviously difficult to collect and to measure thewhole scattered power in all space directions, but it is ratherpossible to measure only the part of the power Ps in a givendirection. Such a power is determined by so called scatteringfunction β(Θ), which determines the intensity of radiationI in the given direction Θ caused by a scattering volume dvilluminated by the power density E:I = Eβ(Θ)dv. (4)The function β(Θ) depends on the actual standard visibilityVn and the direction Θ. We have the following β(Θ) valuesfor visibilities Vn from 1 to 20 km [7]:110 K. HOLEJKO, T. CZARNECKI, K. PERLICKIRrr22÷øöçèæ=RrEEsEFig. 1. Scattering from a rain drop, ρ is reflection coefficient, r is dropradius.a) in forward direction (direction of transmitted beam):β(0◦) = 8− 0.18 [km]−1b) in perpendicular direction:β(90◦) = 0.05− 0.006 [km]−1c) in back direction:β(180◦) = 0.07− 0.007 [km]−1d) in inclination of 40◦:β(40◦) = 0.5− 0.03 [km]−1As is clearly seen from the above relationships the mainpart of the beam is scattered in forward direction especiallyfor lower visibilities, while scattering for high visibility hasrather spherical character. The scattered power density Esis proportional to the size of the drops for larger scatteringparticles (like rain drops). This effect is shown in Fig. 1.III. AUROMETERThe principle of operation of Aurometer is based on themeasurement of scattered light power in the inclination of 40degrees. The Aurometer construction is presented in Fig. 2. Inturn its block diagram is shown in Fig. 3.Our Aurometer consists of two parts: transmitting head andreceiving head. Transmitting head emits IR radiation fromLED1 with the wavelength of 875 nm and stabilized opticalpower. The radiation is concentrated in nearly parallel beamwith the diameter of D = 25 mm and mean power density ofE = 0.6 mW/cm2. Optical radiation is modulated in squaremanner with the frequency of F = 2725 Hz.Transmitting beam intersects with the receiving beam of thesame diameter. Both beams are inclined each other to about40◦ in order to avoid the direct coupling between the receiverFig. 2. Aurometer construction.TransmitterSelector& ControlGeneratorMicroprocessorRs232ControlS&HA/DAlarmD/ ILED1LED2PHD1PHD2Drop det.TemperatureFig. 3. Block diagram of the Aurometer; LED1, LED2 – light diodes, PHD1,PHD2 – photodiodes, Drop det. – drops sensor.and the transmitter. The two beams form the common spacevolume dv ≈ 14 cm3, with the center separated from thereceiving lens of R = 30 cm.Optical power entered on the receiving photodiode may beestimated by means of the formula (4). It should be noted thatthe power received by the lens with the diameter D from thesource with intensity of radiation I is given:Pr =piD24IR2=piD2Eβ(Θ)4R2dv. (5)The calculations for D = 35 mm show that the input powerentering the photodetector for visibilities from 1 to 20 km arein a range of Pr = (10.7− 179.0) pW. The receiver containssilicon photodiode, selective low noise amplifier, homodynemixer and a narrow band amplifier for amplifying slowlyvariable visibility signals and pulsed signals caused by thescattering from rain drops or snow flakes. The rain dropshaving the diameters from 0.7 mm (drizzle) to 3.0 mm (downpour) fall with velocities from 1 m/s to 10 m/s. Thereforethe crossing times of the vertical cross section of the spacevolume dv (Fig. 3) are in a range of 35.0 ms – 3.5 ms. Thereceived pulse amplitude is proportional to the surface of thedrops cross section (Fig. 3). Because of this the rainfall rateRrain [mm/h] can be estimated summing up the volume of Nwater drops in the given time and dividing it by the horizontalcross section A of the volume dv:Rrain =N∑143pir3nA. (6)In our instrument the rain signals are measured by summingup the slope of raising rain signals in a standard interval oftime.To reduce the noise and possible cross couplings effect thereceiver output signal of the receiver is measured twice. Firstwhen optical transmitter is switched on, and later when it isswitched out. The output signal of the receiver is a differenceof both signals. Next this signal is converted to digital formand send to the microprocessor. These both procedures areperformed by the block named S&H, A/D (Fig. 3).Some additional blocks are used for testing and controllingthe instrument (Fig. 3). First of all the clearness of lensesare tested in order to check whether they are not covered bysnow or other obstacles. For that purpose the reflection fromINFRARED SENSOR TO PREDICT ATMOSPHERIC VISIBILITY AND RAINFALL RATE 111Fig. 4. Estimation of the contrast for a specific foggy scenery.the transmitting lens is measured in the transmitter by thephotodiode PHD1. Then the additional LED2 is switched onand the reflection from the receiving lens is measured. Alsothe additional temperature sensor and drops sensor are appliedto recognize the precipitation character.Microprocessor performs the function of controlling thesystem operation, testing the electronic system, calculation ofoutput signals and generation of alarm signals. In particularmicroprocessor:1 Generates the modulation signal of the frequencyF = 2725 Hz.2 Generates the signal to control the measuring processconsisting of three modes: measurement of visibility(10 s), measurement of noise and cross couplings (4 s),measurement of lens reflections (1 s).3 Generates of test signals for testing the proper operationof the instrument. This procedure is repeated after eachswitching and resetting mode of the instrument.4 Receives commands from the central computer of thenet in which Aurometer is installed and elaborates ofrequired control signals.5 Switches on and off heating power for lenses in a casewhen the lens glass is bedimmed.6 Performs suitable calculations such as mean values ofvisibility and precipitation within 1 minute and 1 hour.7 Discriminates the character of precipitation basing ontemperature data and drop sensor data.8 Generates the output data via the RS 232 link and viathe digital – current converter.9 Generates threshold alarm signals.IV. EXPERIMENTAL RESULTSThe atmospheric visibility and rainfall rates were measuredunder different environment conditions. The measurementprocess took place: on open terrace of the six stage building,airstrip, meadow, forest. Below the atmospheric visibility andrainfall rates measurement results are presented.Fig. 5. Aurometer indications in different visibility conditions.A. Atmospheric VisibilityThe Aurometer atmospheric visibility was estimated bymeans Eq. (3). The instrument visibility indications werecompared to the visual observations of different buildingssituated around. The visual observations were made by TVcamera. The irradiance of camera TV picture was analyzed andthe distribution of the irradiance along any selected profileswas presented on the same picture (Fig. 4). Further to obtaineddistribution, the contrast of dark objects of known distanceR was estimated. Thereafter, on the basis of Eq. (2) the visualrange Vn was calculated. Figure 4 shows a dark buildingsituated on a distance of 1200 m. The actual so called standardvisibility was equal to Vn = 2.7 km.Figure 5 presents the Aurometer indications obtained fordifferent visibility conditions. The atmospheric visibility wasin a range of 200 m to 2700 m. Duration of single measure-ment series was 6 hours.B. Rainfall RateThe rainfall indication of Aurometer were compared witha real rainfall rate (Fig. 6).Fig. 6. Aurometer indications of rainfall rate and rain gauge results (rainfallrate).112 K. HOLEJKO, T. CZARNECKI, K. PERLICKITABLE IRAINFALL RATE VALUES FOR VARIOUS TYPES OF PRECIPITATION<0.25 0.25-0.75 0.75-2.5 2.5-12.5 12.5-30 30-100mm/h mm/h mm/h mm/h mm/h mm/hdrizzle light middle shower downpour rain-stormrain rain of rainThe Aurometer rainfall rate was calculated by means ofEq. (6). The real rainfall rate was achieved using simple raingauge (udometer). Figure 6 shows good agreement betweenpresented results.Additionally, we can estimate type of precipitation based onAurometer rainfall rate measurement.Table I provides rainfall rate values for various types ofprecipitation.V. CONCLUSIONPresented instrument is part of environment measurementsystem for assessment of weather i.e. rainfall rate, atmosphericvisibility, atmospheric phenomenon (fog, rain, snow). Thepreliminary measurements prove the possibility of simultane-ous estimation of the atmospheric visibility and rainfall rate,which is important in various means of transportation andnavigation. The atmospheric visible measurements show thatour instrument estimates visibility to 2700 m. We can observeda good agreement between Aurometer indications of rainfallrate and results which were achieved using rain gauge.REFERENCES[1] K. Holejko and R. Nowak, “Fog detection and visibility measurementsusing forward scattering radiation,” Opto-Electronics Review, vol. 2, pp.107–110, 1997.[2] R. Babari, N. Hautiere, and J. Papelard, “Computer Vision for the RemoteSensing of Atmospheric Visibility,” in IEEE International Conference onComputer Vision Workshops, Barcelona, Spain, 2011, pp. 219–226.[3] R. Siikamaki, “New product family of Present Weather Detectors andVisibility Sensors,” Vaisala News, vol. 164, pp. 32–33, 2004.[4] M. Kemp, A. Kruger, and J. Niemeier, “Design considerations and signalprocessing for a microwave rain gauge sensor,” in IEEE InternationalConference on Electro/Information Technology, Indianapolis, USA, 2012,pp. 1–4.[5] L. Kazovsky, “An Optical Rain Gauge Based on Forward Scattering,”IEEE Transactions on Geoscience and Remote Sensing, vol. 2, pp. 124–131, 1985.[6] W. Middleton, Vision through the atmosphere. Toronto: University ofToronto Press, 1952.[7] H. Voigt, “Visibility Measurements Using Backscattered Light,” Journalof the Atmospheric Sciences, vol. 5, pp. 912–918, 1968.

Infrared Sensor to Predict Atmospheric Visibility and Rainfall Rate

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Infrared Sensor to Predict Atmospheric Visibility and Rainfall Rate

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