What is Remote Sensing?

Remote sensing, in simplest words, means acquiring information about an object without touching the object itself. Conventionally, however remote sensing has come to imply that the sensor and the target are located remotely apart and the electromagnetic radiation serves as a link between the sensor and the object, the sun being the major source of energy illuminating the earth. The part of this energy is reflected, absorbed and transmitted by the surface. A sensor records the reflected energy.

Remote Sensing is performed using an instrument, often referred to as a sensor. Most of the remote sensing instruments record electromagnetic radiation (EMR) that travels at a velocity of 3x10⁸ ms^-1 from the source, directly through the vacuum of space or indirectly by reflection or reradiation to the sensor. The EMR represents an extremely efficient high-speed communication link between the sensor and the remotely located objects. Changes in the amount and the properties of the EMR become, upon detection by the sensor, a valuable source of data for interpreting important properties of the phenomena.

Photography is also Remote Sensing

Remote sensing can then be defined as-

The technique of acquiring information about an object by a recording device (sensor) that is not in physical contact with the object by measuring portion of reflected or emitted electromagnetic radiation from the earth’s surface.

Trivia-
["Your Eye is also a Remote Sensing Device".. never thought this right? So what about a mobile camera or DSLR?? ]

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Electromagnetic Spectrum

Electromagnetic spectrum (EMS) represents the continuum of electromagnetic radiation (EMR) arranged on the basis of wavelengths or frequency. Electromagnetic spectrum ranges from shorter wavelengths (gamma rays to x rays) to the longer wavelengths (microwave and radio waves). Most common remote sensing systems operate in one or several of the visible, infrared and microwave portions of the electromagnetic spectrum. Within the infrared portion of the spectrum it should be noted that only thermal infrared energy is directly related to the sensation of heat; not the near and mid infrared ones. Before discussing about EMS with reference to remote sensing it is important to understand it fully. 

Different radiations which constitute the EMS are as follows: 

Radio waves 

These are the longest wavelength (lowest frequency) radiations of the EMS. The wavelength of radio waves is more than 100 cm. These passes through Earth's atmosphere easily. Radio signals are used in radios, televisions, aircrafts, ship etc. These are also emitted by stars. 

Electromagnetic Spectrum 

Microwaves Their wavelength ranges between 1mm & 1m. RADAR (Radio Detection And Ranging) is the most common device used in Microwave Remote Sensing. Other applications are in cooking food (microwave oven), in broadcasting transmissions etc. 

Infrared Radiations 

When we feel hot it is because of infrared (IR) radiations. For common understanding we can call them as 'Heat'. The wavelength of IR is longer than visible light and shorter than microwaves approximately ranges between 1micron to 100 microns. These are very useful radiations for remote sensing. Thermal Imaging Systems detect objects by recording their temperature (infrared emissions). 

Visible Radiations 

As their name itself suggests these are the EMRs which are visible to our eyes in different colours. These ranges between 700nm to 400nm. Most of the remote sensing systems and camera records images in this range. 

Ultraviolet (UV) Radiations 

These radiations have wavelength shorter violet colour of visible light and longer than X-rays. UV radiations can be divided into near UV (400–200nm), far UV (200–10) and extreme UV 1–31 nm. 

X-rays 

These are very short length electromagnetic radiations wavelength in the approximate range from 0.01 to 10 nanometers. In EMS these falls between UV radiations and Gamma-rays. Mostly used in medical sciences. 

Gamma-rays Gamma-rays are the electromagnetic radiations with shortest wavelength in the range of the range of 10^-11 m to 10^-14 m. Their very high energy can cause serious damage to living cells.

Remote Sensing: An Overview

There are many interrelated process, which are involved in acquisition of remotely sensed images, hence an isolated focus on any single component produces a fragmented picture. It is necessary to identify these components to have proper knowledge of remote sensing.

There four essential components of it-

Physical Features

The first component includes the physical features, like buildings, vegetation, soil, water and rocks. Knowledge of the physical features resides within such specific disciplines as geology, forestry, soil science, geography, and urban planning.

Sensor Data

Sensor data are formed in the manner as a remote sensing device views the physical features by recording electromagnetic signals emitted or reflected from the landscape. The effective use of sensor of sensor data requires analysis and interpretation of to convert data into useful information.

Extracted Information

These interpretations create extracted information, which consists of transformations of sensor data designed to reveal specific kinds of information. A more realistic view demonstrates that the same sensor data can be examined from alternative perspectives to yield different interpretations. Therefore a single can be interpreted to provide information about, for instance, vegetation, soils, rocks, water, depending on the specific data, information required and the purpose of the analysis.

Application

The fourth component is the applications, in which the analyzed remote sensing data can be combined with other data to address a specific practical problem, such as land use planning, mineral exploration or vegetation mapping.