13-05-2014, 04:40 PM
REMOTE SENSING A Seminar Report
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ABSTRACT
Remote sensing is a technique to observe the earth surface or the
atmosphere from out of space using satellites (space borne) or from the air using aircrafts (airborne). Remote sensing uses a part or several parts of the electromagnetic spectrum. It records the electromagnetic energy reflected or emitted by the earth’s surface. The amount of radiation from an object (called radiance) is influenced by both the properties of the object and the radiation hitting the object (irradiance). The human eyes register the solar light reflected by these objects and our brains interpret the colours, the grey tones and intensity variations. In remote sensing various kinds of tools and devices are used to make electromagnetic radiation outside this range from 400 to 700 nm visible to the human eye, especially the near infrared, middle-infrared, thermal-infrared and microwaves.
INTRODUCTION
Remote sensing (RS), also called earth observation, refers to obtaining information about objects or areas at the Earth’s surface without beinging direct contact with the object or area. Humans accomplish this task with aid of eyes or by the sense of smell or hearing; so, remote sensing is day-today business for people. Reading the newspaper, watching cars driving in front
of you are all remote sensing activities. Most sensing devices record information about an object by measuring an object’s transmission of electromagnetic energy from reflecting and radiating surfaces. Remote sensing techniques allow taking images of the earth surface in various wavelength region of the electromagnetic spectrum (EMS). One of the major characteristics of a remotely sensed image is the wavelength region it represents in the EMS. Some of the images represent reflected solar radiation in the visible and the near infrared regions of the electromagnetic spectrum, others are the measurements of the energy emitted by the earth surface itself i.e. in the thermal infrared wavelength region. The energy measured in the microwave region is the measure of relative return from the earth’s surface, where the energy is transmitted from the vehicle itself. This is known as active remote sensing, since the energy source is provided by the remote sensing platform. Whereas the systems where the remote sensing measurements depend upon the external energy source, such as sun are referred to as passive remote sensing systems.
Electromagnetic spectrum:
The electromagnetic (EM) spectrum describes a continuous spectrum of energy. This energy is caused by the movement of electrical charges. Visible light, or the light that our eyes can detect, is just a small portion of the EM spectrum. Using specialized sensors, we can look into the realm of the invisible and see a greater portion of the EM spectrum than is possible with the human eye. Almost all EM energy surrounding us is produced by the Sun. Sunlight that travels through space and shines on the Earth is termed "solar radiation." This energy can be absorbed, transmitted (passed through), scattered, or reflected by particles in the atmosphere or objects on Earth.
Atmospheric interaction:
The energy from the source passes through vacuum and then the Earth’s atmosphere before reaching the target on Earth. The electromagnetic energy in transition is referred to as electromagnetic radiation (referred to as radiation here). During this passage the radiation interacts with the Earth’s atmosphere. Part of the radiation gets absorbed, part of it gets scattered, and a part of it gets transmitted all the way to the target. After hitting the target, this radiation again passes through the Earth’s atmosphere before it reaches the satellite sensor. Therefore, the atmospheric interactions take place twice in this process.
Principles of remote sensing:
Remote sensing has been defined in many ways. It can be thought of as including traditional aerial photography, geophysical measurements such as surveys of the earth’s gravity and magnetic fields and even seismic sonar surveys. However, in a modern context, the term remote sensing usually implies digital measurements of electromagnetic energy often for wavelengths that are not visible to the human eye. The basic principles of Remote Sensing may be listed as below:
1. Electromagnetic energy has been classified by wavelength and arranged to form the electromagnetic spectrum.
2. As electromagnetic energy interacts with the atmosphere and the surface of the Earth, the most important concept to remember is the conservation of energy (i.e., the total energy is constant).
3. As electromagnetic waves travel, they encounter objects (discontinuities in velocity) that reflect some energy like a mirror and transmit some energy after changing the travel path.
4. The distance (d) an electromagnetic wave travels in a certain time (t) depends on the velocity of the material (v) through which the wave is travelling; d = vt.
5. The velocity ©, frequency (f), and wavelength (l) of an electromagnetic wave are related by the equation: c = fl.
6. The analogy of a rock dropped into a pond can be drawn as an example to define wave front.
7. It is quite appropriate to look at the amplitude of an electromagnetic wave and think of it as a measure of the energy in that wave.
8. Electromagnetic waves lose energy (amplitude) as they travel because of several phenomena.
satellites acquire images:
Satellite sensors record the intensity of electromagnetic radiation (sunlight) reflected from the earth at different wavelengths. Energy that is not reflected by an object is absorbed. Each object has its own unique 'spectrum .
Remote sensing relies on the fact that particular features of the landscape such as bush, crop, salt-affected land and water reflect light differently in different wavelengths. Grass looks green, for example, because it reflects green light and absorbs other visible wavelengths
The spectrum also shows that grass reflects even more strongly in the infrared part of the spectrum. While this can't be detected by the human eye, it can be detected by an infrared sensor.