Principles of Microwave Remote Sensing

Principles of Microwave Remote Sensing 

Microwave remote sensing:-

using microwave radiation using wavelengths from about one centimeter to a few tens of centimeters enables observation in all weather conditions without any restriction by cloud or rain. This is an advantage that is not possible with the visible and/or infrared remote sensing. In addition, microwave remote sensing provides unique information on for example, sea wind and wave direction, which are derived from frequency characteristics, Doppler effect, polarization, back scattering etc. that cannot be observed by visible and infrared sensors. However, the need for sophisticated data analysis is the disadvantage in using microwave remote sensing.

There are two types of microwave remote sensing; active and passive.

The active type:-

receives the backscattering which is reflected from the transmitted microwave which is incident on the ground surface. Synthetic aperture radar (SAR), microwave scatterometers, radar altimeters etc. are active microwave sensors.

The passive type:-

receives the microwave radiation emitted from objects on the ground. The microwave radiometer is one of the passive microwave sensors. 

Radar is an acronym for Radio Detection and Ranging. It operates in part of the microwave region of the electromagnetic spectrum, specifically in the frequency interval from 40,000 to 300 megahertz (MHz). The latter frequency extends into the higher frequencies of the broadcast-radio region. Commonly used frequencies and their corresponding wavelengths are specified by a band nomenclature, as follows:

Ka Band  : Frequncy 40,000-26,000 MHz; Wavelength (0.8- 1.1 cm)

K Band  : 26,500-18,500 MHz; (1.1-1.7 cm)

Ku Band  : 18,500 – 12,500 MHz ( 1.7 – 2.4 cm)

X Band  : 12,500 8,000 MHz; (2.4-3.8 cm)

C Band  : 8,000-4,000 MHz; (3.8-7.5 cm)

L Band  : 2,000-1,000 MHz; (15.0-30.0 cm)

P Band  : 1,000-300 MHz; (30.0-100.0 cm)

wavelength x frequency = the speed of light.

By choosing the proper units, this equation can be derived:

Wavelength (in cm) = 30/frequency (in GHz or 109cycles per second).

Given a frequency of 20 GHz, what Wave Band would this radar pulse fall into? 

Microwave Radiation

The earth surface radiates a little microwave energy as well as visible and infrared because of thermal radiation. The thermal radiation of a black body depends on Plank's law in the visible and infrared region, while the thermal radiation in the microwave region is given by the Rayleigh Jeans radiation law .

Real objects, the so called gray bodies are not identical to a black body but have constant emissivity which is less than a blackbody . The brightness temperature TB is expressed as follows.

TB =    T

where T : physical temperature
    : emissivity (0 <   < 1)

Emissivity     of an object changes depending on the permittivity, surface roughness, frequency, polarization, incident angle, azimuth etc., which influence the brightness temperature

Generally the scattering coefficient, that is scattering area per unit area, is a function of incident angle and the scattering angle. However in the case of remote sensing, the scattering angle is identical to the incident angle because the receiving antenna of radar or scatterometer is located at the same place as the transmitting antenna. Therefore, in remote sensing only back- scattering may be taken into account.

Scattering area per a unit area       is called the backscattering coefficient.

The backscattering coefficient depends on the surface roughness and incident angle.

Surface scattering 

is defined as the scattering which takes place only on the border surface between two different but homogeneous media, from one of which electro-magnetic energy is incident on to the other. Scattering of microwave on the ground surface increases according to the increase of complex permittivity, and the direction of scattering depends on the surface roughnes.

Volume scattering is defined as the scattering occurring in a medium when electro-magnetic radiation transmits from one medium to another medium.

(a)scattering by widely distributed particles such as rain drops and

(b)scattering in uneven media with different permittivities.

Scattering by trees or branches, subsurface or soil layers, snow layers etc. are examples of volume scattering. Volume scattering can be observed if microwave radiation penetrates into a medium. The penetration depth   

is defined as the distance when the incident power attenuates to 1/e (exponential coefficient). The intensity of volume scattering is proportional to the discontinuous inductivity in a medium and the density of the heterogeneous medium. The scattering angle depends on surface roughness, average relative permittivity and wavelength. 

A typical radar system consists of the following components:

(1)a pulse generator that discharges timed pulses of    

(1)  microwave/radio energy

(2) a transmitter

(3) a duplexer

(4) a directional antenna that shapes and focuses each pulse into a stream

(5) returned pulses that the receive antenna picks up and sends to a receiver that converts (and amplifies) them into video signals

(6) a recording device which stores them digitally for later processing and/or produces a real time analog display on a cathode ray tube (CRT) or drives a moving light spot to record on film.