Measurement of soil moisture using microwave radiometer

Abstract

The science of microwaves owes its origin to the development of radar. Microwaves are part of electromagnetic spectrum. This field became vitally important as man reached out to space. Frequency range of these waves are from 3 GHz to 30 GHz. Microwaves have unique capabilities in remote sensing. The field of microwaves remote sensing has come to a stage of rapid growth. Microwaves can penetrate clouds and so the sensors can operate in all weather conditions. They are sensitive to the presence of moisture in the soil as well as in vegetation or any another material which absorbs moisture. Microwave sensors are of two types: active sensors and passive sensors. Passive sensors have been used for soil studies for the determination of moisture content, oceanographic application to determine winds over the ocean surface and water vapor content in atmosphere as well as liquid water content in clouds. Soils are composed of solids, liquids and gases mix in variable proportions. Soil texture depends upon the size of the particle and structure of soil depends on the way particles are arranged. Soil has physical as well as electrical properties. Colour, texture, grain soil etc. comprised the physical properties where the electrical properties include dielectric constant, conductivity and permeability. Dielectric constant is the primary electrical property which is used to estimate emissivity and brightness temperature of soil. Emissivity is an important parameter for microwave remote sensing, which provides information about soils. All substances at a finite absolute temperature radiate electromagnetic energy. Emissivity is the ratio of energy emitted by object to black body maintained at same physical temperature. Emissivity is a function of physical and electrical parameters of the object and electrical parameter of sensors. These are the moisture content in the object surface type (smooth or rough), dielectric constant (e), angle of incidence, polarization. Emissivity can be obtained from the measured dielectric constant (e) using the available models. The emissivity can be measured by instrument radiometer, which is highly sensitive receiver. Radiometers are passive microwave sensor, which collects the incoming radiations, amplify as well as process the signal, and gives the output, which is linearly related to incoming radiation collected by antenna. The electromagnetic radiations are measured by passive remote sensor in the form of brightness temperature. The radiometer system used here consists of LNBC, receiver and power meter. The LNBC (low noise block down converter) converts the signal to a lower frequency and sends them out to the cable connector, which is connected to satellite receiver via co-axial cable. LNBC have input frequency range 10.75 to 12.75 GHz and noise temperature of 35 degK. The receiver output is connected to microwave power meter. The power monitored is related to incoming EM radiations. The radiometer is calibrated using liquid nitrogen and sky as the targets. Then the EM radiations emitted by dry and wet soil are measured at different look angles (10deg to 60deg) with step of 5deg. The measured power is converted into brightness temperature and the brightness temperature is co-related to soil moisture. In the paper the relation between soil moisture and brightness, temperature is presented. This provides input for determination of soil moisture using passive sensors.