Remote sensing - historical development, principle and process of remote sensing

 Introduction to remote sensing-

Evelyn Pruitt, a geographer with the U.S. Office of Naval Research, was the first to coin the term 'remote sensing’.

Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object. The term is applied especially to acquiring information about the Earth and other Planets . 

Remote sensing depends upon measuring some kind of energy that is emitted , transmitted, or reflected from an object in order to determine certain physical properties of the object.

     

In other words remote sensing is the art and science of deriving information about an object from measurements made at a distance from the object without coming in physical contact with it.

 there are many types of remote sensors . But these are chiefly divided in two categories-

1) active remote sensors – active remote sensors emit energy and then detect the emitted energy that gets reflected back off an object.

 Ex- photographic camera with flash, radar , laser, sonar etc.

2) passive remote sensors – the human eye and ear are passive remote sensors. Passive remote sensors do not EMIT energy but simply detect energy that is reflected back to the sensing device. The eye detects energy invisible portion of electromagnetic spectrum while ear detect sound and sound energy.

Remote sensing deals with the inventory ,monitoring and assessment of natural resources.

It has emerged as the major source of digital data for gis analysis .

This significance of remote sensing data is synoptic Vies , low relative cost , accuracy , receptivity.

It is widely used in underground water exploration , mineral exploration end of course mapping the remote and inaccesible areas.

Remote sensing is based on sensing electromagnetic energy emitted or reflected from the Earth surface and detected at some altitude above the ground.

Principle of remote sensing

 A) ENERGY SOURCE OR ILLUMINATION

 B) RADIATION AND THE ATMOSPHERE

 C) TERRAIN CONDITION

 D) RECORDING OF ENERGY BY THE SENSOR 

 E) TRANSMISSION, RECEPTION AND PROCESSING

 F) INTERPRETATION AND ANALYSIS

 G) APPLICATION

The base for most remotes sensing system is of measuring the varyng energy level of a single entity. Variations in energies are related to wavelength or frequency.

Electromagnetic radiation that varies from high to low energy levels comprises the electromagnetic spectrum.

Electromagnetic radiation-

Energy is capacity to do work. Energy can take many form such as light heat or sound and can be transmitted between objects through three process conduction convection and radiation. Radiation does not require physical contact or existence of a liquid or gas.

Environmental remote sensing system focus on electromagnetic energy which is dynamic form of energy caused by acceleration of an electric charge.

Each Photon consist of an electrical field which varies in magnitude and direction is perpendicular to the direction of the radiation, and a magnetic field oriented at right angles to the electric field.

Both these fields travel at the speed of light which is 3* 10⁸ m /s .

Two characteristic of electromagnetic radiation are particularly important for understanding remote sensing these are wavelength and frequency.

 c=λ⋅ν , where c is the speed of light, λ is the wavelength, and ν is the frequency.

Electromagnetic spectrum-

Depending on the physical and chemical properties, every object reflects emits or scatters a portion of electromagnetic energy which is unique radiation property over the electromagnetic spectrum. Electromagnetic spectrum is the ordering of radiation according to wavelength frequency or energy and use to identify and classify objects.

Source - 

The objects having a temperature of more than absolute zero radiate energy. The sun which is the earth’s only external form of heat m.Ed solar radiation mainly in the form of short wave visible and ultraviolet energy. It must be made clear at the stage that the type of light produced by an object will depend on its temperature

RADIATION AND THE ATMOSPHERE- 

All of the solar radiation passes through space to reach the top of earth atmosphere but not all reaches the earth at surface. The atmosphere scatters ,absorbs and reflects a portion of incoming solar radiation. The earth scatters absorbs and reflects the solar radiation that gets transmitted through the atmosphere.

Finally the atmosphere scatters absorbs and reflects the electromagnetic radiation that is reflected off the Earth surface back towards the sensor.

SCATTERING-

Scattering is the process by which small particle suspended in a medium of different index of refraction diffuse a portion of incident radiation in all direction.

Interaction of EMR with Earth surface -

On average 51% of the incoming solar radiation reaches the Earth surface. Of this 4% is reflected back into atmosphere and 47% is absorbed by the Earth surface to be radiated in the form of thermal infrared radiation. In general surfaces that are good reflectors are poor absorbers. Thus some surfaces or objects will reflect much of the incoming radiation while other surfaces will absorb most of the incoming radiation. The reflectivity of a surface can be measured for a specific wavelength or for the entire electromagnetic spectrum.

The ALBEDO often object is its reflectance aggregated over a broader segment of the electromagnetic spectrum eg: over visible portion of spectrum.

The higher the albedo the more reflective the surface and brighter the surface will appear in remotely sense imagery.

5) recording of energy by sensors - After the energy has been scattered by or emitted through the target We require a sensor to collect And record the electromagnetic spectrum. Sensors are devices made to measure photons. 

Two types – Active and passive sensors

6) Transmission, reception and processing –

 The energy recorded by the sensor has to be transmitted , Often in electronic form , To a receiving and processing station where data Are processed into an image ( digital) 

Interpretation and analysis –

The interpretation Of processed image is done , Visually or digital to extract information about the target which was illuminated.

8) Application- 

The final element of remote sensing is achieved when we apply the knowledge that we have gained by the remote sensing in problem solving, studies , policy making or assist a particular problem .

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