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Introduction of Remote Sensing

Definition History of Remote Sensing Basic components of Remote sensing Electromagnetic Remote Sensing

Process Electromagnetic spectrum Passive and active remote sensing

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Different types of Resolution Spatial Resolution Spectral Resolution Radiometric Resolution Temporal Resolution

Characteristics of various sensors Satellites: IRS Fundamentals of Image Processing

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Remote sensing is the science of obtaining information about an object, area or phenomenon through the analysis of data acquired by a device that is not in contact with the object, area or phenomenon under investigation

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DATA ACQUISITION (a) the energy source; (b) propagation of energy through the atmosphere; (c) energy interaction with the earth features;(d) Retransmission of energy through the atmosphere; (e) Sensing System: airborne and /or space borne sensors; (f)Sensing product: generation of sensor data in pictorial and/or numerical form. DATA ANALYSIS(g) Interpretation and analysis: examining the data using various viewing and

interpretation devices; (h) Information Product : data presentation in the form of maps, tables or reports; (i) Users: Information presented to usersData usage in decision making process.

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•Electromagnetic Radiation•Two characteristics of electromagnetic Radiation

•wavelength • frequency.

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:

3 10 /

cwherewavelengthof light

c speed of light m sfrequency of light

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The Electromagnetic Spectrum

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EM spectrum used in remote sensing lie along a continuum characteristised by magnitude changes of many powers of 10

EM spectrum in logarithmic plot Visible portion of such plot extremely

small, since the spectral sensitivity of human eye extends only from 0.4 to 0.7 µm

Primary color BGR B - 0.4 to 0.5 µm; G -0.5 to 0.6 µm; R 0.6

to 0.7 µm9

IR waves Near IR: 0.7 to 1.5 µm Mid IR: 1.3 to 3 µm Thermal IR: Beyond 3 to 14 µm

Only Thermal IR energy is directly related to sensation of heat.

Most common sensing system operate in one or several of the visible, IR or microwave portion s of the spectrum

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Three basic laws governing the Remote sensing process Planck law: E= hγ Stefan -Boltzmann law: M= ξρ T4

: M= ρ T4 [for perfectly black body, ξ=1 ]

Wein’s displacement law: λm= A/T

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Where, E=energy of a quantum h=planck’s constant γ= frequency of light, Hertz or sec-1

M= Total radiant exitance from the surface of material, W/m2

σ= Stefan- Boltzmann constant,5.6697× 10-8 Wm-2K-4

ξ= Emissivity T= absolute temperature of emitting material λm= wavelength of maximum spectral radiant exitance,

µm A= 2898 µm K,

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Energy interactions in the atmosphere

The atmosphere is not completely transparent.

The outgoing spectral radiance of objects at the earth’s surface is modified before it reaches the sensor.

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Atmospheric window

The atmospheric gasses, carbon dioxide, ozone and water vapour, causes reflection, scattering, and attenuation of energy, so that only a few ‘windows’ are available through which part of all the pertinent radiation is transmitted.

One important atmospheric window exists in the visible spectral region (0.4 - 0.7 µm).

There are three windows in the infra–red region (3.0 – 4.5 µm, 8.5 – 14.0 µm and 17. – 19.0 µm ).

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Ex. Assume the speed of light to be 3x108 m/s. If the frequency of an electromagnetic wave is 500,000 GHz (GHz =gigahertz = 109 m/s), what is the wavelength of that radiation? Express your answer in micrometres (μm).

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Energy interactions with earth surface features

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The proportion of energy reflected, absorbed and transmitted will vary for at different wavelength even for a given feature type.

Thus, two features may be indistinguishable in one spectral range and be different in another wavelength band.

The reflectance characteristics of earth surface features may be quantified by measuring the portion of incident energy that is reflected.

This is measured as a function of wavelength, and is called Spectral Reflectance, R.

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A graph of a spectral reflectance of an object as a function of wavelength is termed as Spectral Reflectance Curve

ER(λ) Energy of wavelength reflected from the objectR= = × 100EI(λ) Energy of wavelength incident on the object

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Spatial Resolution Spatial resolution may be defined as

the minimum distance between two objects that a sensor can record distinctly...[but] it is the format of the sensor system that determines how spatial resolution is measured'.

Spectral Resolution- It is the width of the spectral bands in which the image is taken

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Radiometric Resolution- It is the capability to differentiate the spectral reflectance/emittance between various targets.

Temporal Resolution- It explains how often sensor records imagery of a particular area [which means frequency of repetitive coverage]

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RADAR

SLAR Side looking Air Borne Radar

SAR Synthetic aperture Radar

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Radar is essentially a ranging or distance measuring device. It consists fundamentally of a transmitter, a receiver, an antenna, and an electronics system to process and record the data.

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By measuring the time delay between the transmission of a pulse and the reception of the backscattered "echo" from different targets, their distance from the radar and thus their location can be determined.

As the sensor platform moves forward, recording and processing of the backscattered signals builds up a two-dimensional image of the surface.

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2ctSR

Slant range [direct distance between transmitter and object]c= speed of lightT= time between pulse transmission and echo reception

SR

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A radar system’s look angle is the angle from nadir point to a point of intersect on the ground

Slant range resolution, Ground range resolution, Rr

Azimuth resolution, Ra

SR

aR GR

2cosrd

cR

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2cosrd

cR

AL

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Problems Ex . A given SLAR system transmits pulses over a duration of

Find the range resolution of the system at a depression angle 450

Ans: 21m Ex. A given SLAR system has a 1.8 mrad antenna beamwidth.

Determine the azimuthal resolution of the system at ranges of and 12 km

Ans: 10.8 m & 21.6 m Ex. A given SLAR system has a 2 mrad antenna beamwidth and

wavelength of the transmitted pulse is 5 cm. Determine the length of the antenna

Ans: 25 m

0.1 sec.

AL

aR GR

2cosrd

cR

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The deficiencies of brute force operation are overcome in synthetic aperture radar (SAR) systems.

Having short physical antenna through modified data recording and processing techniques, but synthesize the effect of a very long antenna.

The result of this mode of operation is a narrow effective antenna beamwidth, even at far ranges, without requiring physically long antenna or a short operating wavelength.

At the detailed level, the operation of SAR systems is quite complex

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Fig. Concept of an array of real antenna positions forming a Synthetic Aperture Radar

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Operate on the principle of using the sensor motion along track to transform a single physically short antenna into an array of such antennas that can be linked together mathematically as part the data recording and processing procedures

The "real" antenna is shown in several successive positions along the flight line.

These successive positions are treated mathematically as if they are simply sucessive elements of a single long synthetic antenna

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Fig. Variation of spatial resolution of a) SLAR b) SAR

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Glossary Albedo : Ratio of the amount of electromagnetic

energy (solar radiation) reflected by a surface to the amount of energy incident upon the surface.

ASTER - Advanced Spaceborne Thermal

Emission and Reflection Radiometer.

AVHRR - Advanced very high-resolution radiometer.

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AVIRIS - Airborne visible-infrared imaging spectrometer.

Band - Broadcasting frequency within given limits. A subdivision within an electromagnetic region.

Bandwidth- The total range of frequency required to pass a specific modulated (spectral resolution) signal without distortion or loss of data. The wavelength interval recorded by a detector.

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ETM+ - Enhanced Thematic Mapper Plus EM - Electromagnetic GPS - Global Positioning System GIS - Global Information System / Geographic

Information System IFOV - Instantaneous field of view: the solid

angle through which a detector is sensitive to radiation. In a scanning system, the solid angle subtended by

the detector when the scanning motion is stopped.

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IKONOS - A high-resolution earth observation satellite launched in 1999, which occupies a 682-km sun synchronous orbit and employs linear array technology

collecting data in four multispectral bands at a nominal resolution of 4 m, as well as a 1-m-resolution panchromatic band.

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Landsat: A series of unmanned NASA earth resource satellites that acquire multispectral images in the visible and IR bands.

Remote sensing of energy naturally reflected or radiated from the terrain.

Radiation: Act of giving off electromagnetic energy.

RGB [ Red, Green, and Blue]—the colors used in constructing visible and false color image representations.

MIR : Mid Infrared

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Spatial Resolution The ability to distinguish between closely spaced objects on an image. Commonly expressed as the most closely spaced

line-pairs per unit distance distinguishable. Spectral Reflectance : Reflectance of

electromagnetic energy at specified wavelength intervals.

Spectral Resolution Range of wavelengths recorded by a detector. (bandwidth)

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NAD - North Atlantic Datum NDVI - Normalized Difference

Vegetation Index NIR - Near Infrared

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SWIR : Short Wave Infrared TM : Thematic Mapper UTM : Universal Transverse Mercator VI Vegetation Index NIR : Near Infrared

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