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PRINCIPLES AND CONCEPTS OF REMOTE SENSING

  • - P. Kesava Rao, R R Hermon and V Madhava Rao
    INTRODUCTION

    Remote sensing means acquiring information about a phenomenon object or surface while at a distance from it. This name is attributed to recent technology in which satellites and spacecraft are used for collecting information about the earth's surface. This was an outcome of developments in various technological fields from 1960 onward. The concept is illustrated in the following figure.


    Stages in Remote Sensing

    data_collection_by_remote_sensing.jpg Emission of electromagnetic radiation, or EMR (sun/self- emission)
    Transmission of energy from the source to the surface of the earth, as well as absorption and scattering
    Interaction of EMR with the earth's surface: reflection and emission
    Transmission of energy from the surface to the remote sensor
    Sensor data output
    Data transmission, processing and analysis

    What we see?

    At temperature above absolute zero, all objects radiate electromagnetic energy by virtue of their atomic and molecular oscillations. The total amount of emitted radiation increases with the body's absolute temperature and peaks at progressively shorter wavelengths. The sun, being a major source of energy, radiation and illumination, having a sharp power peak around 0.5 μm, allows capturing reflected light with a camera.

    Everything in nature has its own unique signature of reflected, emitted and absorbed radiation. These spectral characteristics, if ingeniously exploited, can be used to distinguish one thing from another or to obtain information about shape, size and other physical and chemical properties. In so far as we know the spectral characteristics, we can pick an appropriate detector to make the desired measurement, remembering that for a given collector's field of view we get our greatest spatial resolution where wavelengths are shortest and energies greatest, and that these energies decrease at longer wavelengths and distances.

    ELECTROMAGNETIC RADIATION AND SPECTRUM:

    Electro magnetic radiation is energy that propagates in wave form at a velocity of C = 3 x 10 10 cm/sec. The parameters that characterize a wave motion are wavelength (λ), frequency (ν) and velocity (C). The relationship between the above is C = νλ. The following figure illustrates spectral bands used in remote sensing. bands_remote_sensing.jpg


    Interaction of EMR with the Earth's Surface

    Radiation from the sun, when incident upon the earth's surface, is reflected, transmitted or absorbed and emitted by the surface. The electro magnetic radiation, on interaction, experiences a number of changes in magnitude, direction, wavelength, polarization and phase. These changes are detected by the remote sensor and enable the interpreter to obtain useful information about the object of interest. The remotely sensed data contain both spatial information (size, shape and orientation) and spectral information (tone, color and spectral signature).

    The (visible and infra-red) wavelengths from 0.3 μm to 16 μm can be divided into three regions. The spectral band from 0.3 μm to 3 μm is known as the reflective region. In this band, the radiation sensed by the sensor is that due to the sun, reflected by the earth's surface. The band corresponding to the atmospheric window between 8 μm and 14 μm is known as the thermal infra-red band. The energy available in this band for remote sensing is due to thermal emission from the earth's surface. Both reflection and self-emission are important in the intermediate band from 3 μm to 5.5 μm.

    In the microwave region of the spectrum, active sensors called RADAR are used to collect information about an object. The EMR produced by the radar is transmitted to the earth's surface and the EMR reflected (back-scattered) from the surface is recorded and analysed. The microwave region can also be monitored with passive sensors, called microwave radiometers, which record the radiation emitted by the terrain in the microwave region.

    Of all the interactions in the reflective region, surface reflections are the most useful and revealing for remote sensing applications. Depending upon whether the surface is smooth or rough, the reflection is specular or diffuse. Surface roughness is a function of the wavelength of incident radiation. According to the rayleigh criterion, if surface height variations are less than λ/8, the surface is considered to be smooth, otherwise, it is rough. Hence, the wavelength of the incident EMR determines the surface roughness. For radiowaves, rocky terrain appears smooth to incident EMR, whereas in the optical band, fine sand appears rough. If the surface is flat and smooth, specular reflections occur, which follow the law of reflection (the angle of incidence equals the angle of reflection). Specular reflections are undesirable for remote sensing because they produce solar glint or glare. Water and certain man-made features reflect specularly. A rough or diffuse surface reflects the incident EMR in all directions independent of the angle of incidence. Among the natural features, which display essentially diffuse reflection, are sand, tilled soils and certain types of vegetation. Mixed reflections occur most frequently in nature. In this case, a reflecting surface returns radiant energy both diffusely and specularly. Spectral reflectance, (ρ (λ)), is the ratio of reflected energy to incident energy as a function of wavelength. Various materials of the earth's surface have different spectral reflectance characteristics. Spectral reflectance is responsible for the color or tone in a photographic image of an object. Trees appear green because they reflect more of the green wavelength. The values of the spectral reflectances of objects averaged over different, well-defined wavelength intervals comprise the spectral signature of the objects or features by which they can be distinguished. To obtain the necessary ground truth for the interpretation of multispectral imagery, the spectral characteristics of various natural objects have been extensively measured and recorded. The following figure illustrates spectral signature of natural features.

    EFFECTS OF ATMOSPHERE

    The sun is the source of radiation, and electromagnetic radiation (EMR) from the sun that is reflected by the earth and detected by the satellite or aircraft-borne sensor must pass through the atmosphere twice. Once on its journey from the sun to the earth and after being reflected by the surface of the earth back to the sensor. Interactions of the direct solar radiation and reflected radiation from the target with the atmospheric constituents interfere with the process of remote sensing and is called as "Atmospheric Effects".

    The interaction of EMR with the atmosphere is important to remote sensing for two main reasons. First, information carried by EMR reflected/emitted by the earth's surface is modified while traversing through the atmosphere. Second, the interaction of EMR with the atmosphere can be used to obtain useful information about the atmosphere itself.

    The atmospheric constituents scatter and absorb the radiation modulating the radiation reflected from the target by attenuating it, changing it's spatial distribution and introducing into field of view radiation from sunlight scattered in the atmosphere and some of the energy reflected from nearby ground area. Both scattering and absorption vary in their effect from one part of the spectrum to the other.


    Atmospheric scattering

    Spectral_reflectance_of_vegetation.jpg
    Scattering is the redirection of EMR by particles suspended in the atmosphere or by large molecules of atmospheric gases. Scattering not only reduces the image contrast but also changes the spectral signature of ground objects as seen by the sensor. The amount of scattering depends upon the size of the particles, their abundance, the wavelength of radiation, depth of the atmosphere through which the energy is travelling and the concentration of the particles. The concentration of particulate matter varies both in time and over season. Thus the effects of scattering will be uneven spatially and will vary from time to time.

    Theoretically scattering can be divided into three categories depending upon the wavelength of radiation being scattered and the size of the particles causing the scattering. The three different types of scattering from particles of different sizes are summarized below: wavelength_table.jpg

    Rayleigh scattering

    Rayleigh scattering appears when the radiation wavelength is much larger than the particle size, for example scattering of visible light (0.4 μm - 0.76 μm) by pure gas molecules (10 -4 λm) in a clear atmosphere. Rayleigh scattering causes the sky to appear blue. The scattering coefficient is proportional to the inverse fourth power of wavelength. Radiation in shorter blue wavelengths is scattered toward the ground much more strongly than radiation in the red wavelength region. Due to Rayleigh scattering multispectral data from the blue portion of the spectrum is of relatively limited usefulness.

    It is observed that there is strong scattering in the forward as well as backward directions. The strong backward scattering is responsible for the appearance of hot spots in aerial photographs taken in hazy atmosphere by wide angle cameras at times when the direction of solar radiation falls within the field of view of the sensor.

    Mie scattering

    Mie scattering occurs when radiation wavelength is comparable to the size of the scattering particles. In remote sensing Mie scattering usually manifest itself as a general deterioration of multispectral images across optical spectrum under conditions of heavy atmospheric haze. Depending upon the particle size relative to the wavelength, Mie scattering may fall any where between λ-4 and λ-0. The incident light is scattered mainly in the forward direction.

    Non selective scattering

    Non selective scattering usually occurs when the particle size is much larger than the radiation wavelength. Scattering does not depend on the wavelength of radiation. This type of scattering usually occurs when the atmosphere is heavily dust laden and results in a severe attenuation of the received data. There is a uniform attenuation at all wavelength. The whitish appearance of the sky under haze condition is due to non selective scattering.

    Occurrence of this scattering mechanism gives a clue to the existence of large particulate matter in the atmosphere above the scene of interest which itself is a useful data. Using negative blue filters can eliminate the effects of the Rayleigh component of scattering. However, using haze filters cannot eliminate the effects of heavy haze, when all the wavelengths are scattered uniformly. The effects of haze are less pronounced in the thermal infrared region. Microwave radiation is completely immune to haze and can even penetrate clouds.