Proceedings of International Conference on Microwave - 08

Measurement of Soil Moisture Using Microwave RadiometerO.P.N.Calla,* Dinesh Bohra,* Rajesh Vyas,* Dhawani Shankar Purohit,* Rakesh

Prasher,** Abhishek Loomba** and Naveen Kumar***Intemational center for radio science, Jodhpur - 342003, **Deptt. of Electronics, Jammu

University, Jammu

Abstract- The science of microwaves owes its origin to thedevelopment of radar. Microwaves are part ofelectromagnetic spectrum. This field became vitallyimportant as man reached out to space. Frequency rangeof these waves are from 3 GHz to 30 GHz. Microwaveshave unique capabilities in remote sensing. The field ofmicrowaves remote sensing has come to a stage of rapidgrowth. Microwaves can penetrate clouds and so thesensors can operate in all weather conditions. They aresensitive to the presence of moisture in the soil as well as invegetation or any another material which absorbsmoisture. Microwave sensors are of two types: activesensors and passive sensors. Passive sensors have beenused for soil studies for the determination of moisturecontent, oceanographic application to determine windsover the ocean surface and water vapor content inatmosphere as well as liquid water content in clouds. Soilsare composed of solids, liquids and gases mix in variableproportions. Soil texture depends upon the size of theparticle and structure of soil depends on the way particlesare arranged. Soil has physical as well as electricalproperties. Colour, texture, grain soil etc. comprised thephysical properties where the electrical properties includedielectric constant, conductivity and permeability.Dielectric constant is the primary electrical property whichis used to estimate emissivity and brightness temperatureof soil. Emissivity is an important parameter formicrowave remote sensing, which provides informationabout soils. All substances at a finite absolute temperatureradiate electromagnetic energy. Emissivity is the ratio ofenergy emitted by object to black body maintained at samephysical temperature. Emissivity is a function of physicaland electrical parameters of the object and electricalparameter of sensors. These are the moisture content inthe object surface type (smooth or rough), dielectricconstant (£), angle of incidence, polarization. Emissivitycan be obtained from the measured dielectric constant (£)

using the available models. The emissivity can bemeasured by instrument radiometer, which is highlysensitive receiver. Radiometers are passive microwavesensor, which collects the incoming radiations, amplify aswell as process the signal, and gives the output, which islinearly related to incoming radiation collected by antenna.The electromagnetic radiations are measured by passiveremote sensor in the form of brightness temperature. Theradiometer system used here consists of LNBC, receiverand power meter. The LNBC (low noise block downconverter) converts the signal to a lower frequency and

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sends them out to the cable connector, which is connectedto satellite receiver via co-axial cable. LNBC have inputfrequency range 10.75 to 12.75 GHz and noise temperatureof 35 oK. The receiver output is connected to microwavepower meter. The power monitored is related to incominge m radiations. The radiometer is calibrated using LiquidNitrogen and sky as the targets. Then the EM radiationsemitted by dry and wet soil are measured at different lookangles (10° to 60°) with step of 5°. The measured power isconverted into Brightness temperature and the Brightnesstemperature is co-related to soil moisture. In the paper therelation between soil moisture and brightness, temperatureis presented. This provides input for determination of soilmoisture using passive sensors.Index Terms: Soil, Soil moisture, Radiometer, LNBC,Emissivity,

I. INTRODUCTION

Microwaves are electromagnetic waves withwavelengths longer than those of terahertz (T Hz)frequencies, but relatively short for radio waves.Microwaves have wavelengths approximately in therange of 30 cm to 1 mm. The frequency range ofmicrowaves is from 3 GHz to 30 GHz. Microwaves alsoprovides information along with optical and infraredwave lengths. The frequency range from 30 GHz to 300GHz is known as millimeter and sub millimeter portionof electromagnetic spectrum .There are two types ofmicrowave remote sensors: Active sensors & Passivesensors. Microwaves have various advantageouscharacteristics, but the most important characteristics istheir penetration through clouds and rain and itsindependence on sun as source of illumination .Anotherreason for the use of microwaves is their behavior isexactly as the behavior of light. Microwaves are alsosensitive to the presence of moisture .They also provideinformation about the temporal and spatial variation ofatmospheric or surface and medium Parameters orproperties.

Passive sensors called Radiometers, detect theradiated energy in the microwave spectrum .Passivesensors are sensitive to the emmisivity and brightnesstemperature parameters of the soil. The scatteringcoefficient, emmisivity and brightness temperaturedepend on the dielectric constant of soil. Emissivity isthe ratio of energy emitted by an object as compared tothat emitted by a perfect black body maintained at samephysical temperature .A quantity related to the

Proceedings of International Conference on Microwave - 08

brightness is the brightness temperature which iscorresponding physical temperature of black body withthe same brightness; Emissivity is a function ofdielectric properties, surface roughness, physicaltemperature, frequency, angle of observation andpolarization etc.

II. SOIL & ITS ELECTRICAL PROPERTIES

Most of the earth's land surface is covered by thinlayer of soil ranging from few inches to few feet. Theword Soil, is derived from the Latin word "solum". Soilis a mixture of mineral and organic matter that iscapable of supporting plant life. Soil is composed ofsolid, liquid and gases mixed together in variousproportions and their relative amount depends onpacking of soil properties[1]. An ideal soil consists of40% solid matter (rocks and minerals), 10% organicmatters, 25% liquid (water and dissolved minerals) and25% gases.

The Electrical properties depend on the parameterssuch as emissivity (e), scattering coefficient(a) anddielectric constant (E). These parameters are influencedby the sensor parameters like frequency, angle ofincidence and polarization[3]. The emissivity of soil ismeasured by radiometer, the term "Radiometry" meansthe measurement of incoherent electromagneticradiation emitted from the target[4] .All substancesabove absolute temperature radiate electromagneticenergy. This electromagnetic radiation is measuredpassive remote sensor in the form of brightnesstemperature. The brightness temperature is related to thephysical temperature and relation is given in eqn (1)

e(9,<I> )=B(9,<I» /Bbb =T(9,<I» IT (1)where B(9,<I» is the brightness,T(9,<I» is the brightness temperature,T is the physical temperature ande(9,<I» is the emissivity.The microwave radiometers are highly sensitive

receivers. The performance of the microwaveradiometer is characterized by sensitivity or temperatureresolution[2] .The temperature sensitivity of theradiometer depends on the receiver noise figure, systembandwidth and integration time. The eqn (2) gives therelation between these parameters.

S = K (Ta + Tn) / .J~.rr (2)

Ta is antenna temperature,Tn is noise temperature,r is integration time.AI is the system BandwidthThe classification of microwave Radiometers are

depends on the stability and sensitivity. the radiometersare classified as :

Total power Radiometer ( TPR): In this outputvoltage is proportional to the input power. The output ofthis radiometer depends on noise temperature andreceiver gain. Schottky barrier diodes are good

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microwave detectors. If ultimate sensitivity is requiredthen TPR is used.

Dicke Radiometer (DR): In 1946, R. H. Dicke solvethe stability problems of radiometers, he measures thedifference between the antenna and referencetemperature (Tr). The output is proportional to thedifference between the two temperatures. Its sensitivityis half than TPR. In this tunnel diode detectors areemployed due to its low frequency behavior.

Noise Injection Radiometer (NIR): In this, theoutput is independent of gain and noise temperaturefluctuations. The NIR is a specialization of DickeRadiometer in which Ta-Tr=O condition is fulfilled byservo loop. The sensitivity of NIR is very close to DickeRadiometer. If the ultimate stability and accuracy isrequired, NIR is best.

III. EXPERIMENTAL SET UP

In this study we use a digital Satellite receiver as alow cost Radiometer and the slightly smooth surface ofthe soil is used as terrain. There are three majorcomponents used in the low cost radiometer, these are:Low noise block converter (LNBC), receiver and powermeter as shown in Fig 1 & 2. LNBC is connecting to aturntable having 0-180° elevation angle. The incomingsignal is fed to the receiver terminal LNB in and powermeter is connected to receiver terminal LNB out.

Fig. 2 Experimental setup of RadiometerThe low strength signals need to be immediately

amplified by a Low Noise Amplifier. The LNA actuallyconsists of 2 or 3 amplifying stages that boost the signalto a reasonable level. The amount of noise added byLNA to the received signal is indicated in terms of"noise temperature" in degrees Kelvin, as lower thenoise temperature better is the LNA's performance. Theblock diagram of LNBC is shown in Fig .3. Microwavesignals in the KU band would suffer very highattenuation if they were carried via coaxial cable. Toovercome this problem, the microwaves signals areconverted to a block of frequencies from 950 to 2150MHz. This range of frequencies is referred to as

Proceedings of International Conference on Microwave - 08

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%lIoIIture

intermediate frequencies. This function is carried by"block converter" located within the LNB. Acombination of LNA plus converter is referred to as anLNBC. The block converter uses the heterodyneprinciple for the conversion of KU band frequencies.Band pass filters are introduced before and after themixer. A filter is used to suppress all frequencycomponents except those required which is thedifference between the incoming microwave frequenciesand local oscillator frequencies.

If'output950­1950MHz

Inputpower12-

Fig. 3 Block diagram ofblocK converterThis amplified signal is then filtered and fed to the

output via DC blocking capacitor. The capacitor allowsthe signal to pass through but steers the power comingfrom the satellite receiver to the LNB power supply. TheLNB is remotely powered from receiver the samecoaxial cable that carries the IF signal from the receiveralso carries DC supply from receiver to LNB.

The power meter shows the power emitted from thematerial. The power meter has frequency range 10-18MHz with a sensor's Sensitivity of -70 dB to +20 dB.

IV. CALIBRATION

Calibration of Radio meter is done through a LiquidNitrogen; Fig. 4 shows a Calibration graph betweenbrightness temperature and received power, this isobtained from repeated measurement of received powerfrom liquid nitrogen, sky and lime surface. Fig. 5 showsa practical setup of the Radiometer calibration. Theliquid Nitrogen is contained in an insulated metalcontainer and LNBC is placed over the fumes emittedby the liquid nitrogen & make sure that the uniformtemperature is distributed around LNBC. The LNBC isalso used to measure the Sky temperature. Thetemperature range of the simple setup is quite limitedbut useful especially for calibration and stability of theradiometer.

Bnghtness Temp vIs Power

o+----------,----,--------r-----,----r--------r--...,.----,

05

Fig. 4 Calibration Graph for Brightness temperature(OK)vis Power(1lW)

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Fig. 5 Calibration with liquid nitrogen

V. RESULTS & CONCLUSION

In this paper, an attempt is has been made to studythe variation of Brightness temp. at different angles ofincidence with different moisture content for bothhorizontal & vertical polarization.

Fig 6 shows the variations between brightnesstemperatures with angle of incidence for vertical andhorizontal polarizations at 25.4 %moisture content.

At moisture content 25.4% the maximum value ofBrightness temperature fu;:-horizontal & verticalpolarization is observed at angles 25° & 75°respectively. For horizontal polarization its range variesfrom 114.0 - 101.67 OK and for vertical polarization itsrange varies from 135.76- 119.55°K.

MoIlture=25.4%

~150

'" . ....•...•.~ ...•...•...•...•...•......•..,!100

,;:; 50 ---HPol••• VPol

0+---....----....-----.-----...,.---...,.--25

AnglllnDegree

Fig. 6 Variations in brightness temperature withincidence angles for both polarizations at 25.4%

MoistureBrightness Temp vIs Moisture

.•.......... - -......

~ 00 I V-p~ I,;:; ••• H·Pol

40 +-----.------,-----.------,.------r--__a

Fig. 7 Variations in brightness temperature with differentmoisture contents for both polarizations

Fig. 7 shows the variations in brightnesstemperature with percentage moisture contents forvertical and horizontal polarization and suggests that, as

Proceedings of International Conference on Microwave - 08

the moisture content increases, the value of brightnesstemperature reduces.

The value ofbrightness temperature increases as thereceived power increases. It is also concluded that, thevalue of brightness temperature for vertical polarizationis higher as compared to the horizontal polarization.

VI. REFERENCES

[I]. Fawwaz T Ulaby, Richard K Moore and Adrian KFung, "Microwave remote sensing active andpassive"vol.l(1981), vo1.2 (1982) Addison ­Wesley publishing company inc. vo1.3(1986)Artech house inc.

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[2]. Neils Skou, "Microwave Radiometer System:Design and Analysis", Artech House.

[3]. Calla OPN, Deka.B., "Study of emissivity of dryand wet loamy sand soil at microwave frequency,Indian Journal ofRadio & Space Physics, Vo1.29,June2000, pp.140-145.

[4]. O.P.N.Calla, Rajesh Vyas, Dinesh Bohra, AmitArora, HStudy ofEmission Behavior ofDry and WetSoil At Microwave Frequency" Presented atInternational conf. on communication andElectronics(ICECC-08) at university of Rajashahi,Bangladesh.