Remote sensing is the science of obtaining information about objects or areas from a distance, typically from aircraft or satellites. They are used for resource management. They observe and collect Data through photography as they are fitted with various cameras.
A) Types of Remote Sensing Instruments
- They detect natural energy that is reflected or emitted from the observed scene. Passive instruments sense only radiation emitted by the object being viewed or reflected by the object from a source other than the instrument. Reflected sunlight is the most common external source of radiation sensed by passive instruments. Scientists use a variety of passive remote sensors.
Various Passive Remote Sensing Instruments
- Radiometer: An instrument that quantitatively measures the intensity of electromagnetic radiation in some band of wavelengths in the spectrum. Usually a radiometer is further identified by the portion of the spectrum it covers; for example, visible, infrared, or microwave.
- Imaging Radiometer: It includes a scanning capability to provide a two-dimensional array of pixels from which an image can be produced is called an imaging radiometer. Scanning can be performed mechanically or electronically by using an array of detectors.
- Spectrometer: A device designed to detect, measure and analyze the spectral content of the incident electromagnetic radiation is called a spectrometer. Conventional, imaging spectrometers use gratings or prisms to disperse the radiation for spectral discrimination.
- Spectroradiometer: A radiometer that can measure the intensity of radiation in multiple wavelength bands (i.e. multispectral). Often the bands are of a high spectral resolution- designed for the remote sensing of specific parameters such as sea surface temperature, cloud characteristics, ocean colour, vegetation, trace chemical species in the atmosphere, etc.
Active Remote Sensing Instruments
- They provide their own energy (electromagnetic radiation) to illuminate the object or scene they observe. They send a pulse of energy from the sensor to the object and then receive the radiation that is reflected from that object. Scientists use several types of active remote sensors:
- RADAR (Radio Detection and Ranging): A radar uses a transmitter operating at either radio or microwave frequencies to emit electromagnetic radiation and a directional antenna or receiver to measure the time of arrival of reflected radiation from distant objects. Distance to the object can be determined since electromagnetic radiation propagates at the speed of light.
- Scatter meter: A scatter meter is a high frequency microwave radar designed specifically to measure reflected radiation. Over ocean surfaces, measurements of reflected radiation in the microwave spectral region can be used to derive maps of surface wind speed and direction.
- LIDAR (Light Detection and Ranging): A Lidar uses a laser (light amplification by stimulated emission of radiation) to transmit a light pulse and a receiver with sensitive detectors to measure the reflected light. Distance to the object is determined by recording the time between the transmitted and reflected pulses and using the speed of light to calculate the distance travelled. Lidars can determine atmospheric profiles of aerosols, clouds, and other constituents of the atmosphere.
- Laser Altimeter: A laser altimeter uses a lidar to measure the height of the instrument platform above the surface. By independently knowing the height of the platform with respect to the mean Earth’s surface, the topography of the underlying surface can be determined.
B) Types of Remote Sensing
Satellite Remote Sensing
- The remote sensing satellites are equipped with sensors looking down to the earth. They are “the eyes in the sky” constantly observing the earth as they go round in orbits.
- In satellite remote sensing of the earth, the sensors are looking through a layer of atmosphere separating the sensors from the Earth’s surface being observed. So, the analysis of the effects of atmosphere on the electromagnetic radiation travelling from the earth to the sensor through the atmosphere provides vital inputs. The atmospheric constituents cause wavelength dependent absorption and scattering of radiation. These effects lead to the deterioration of quality of images.
- An important consequence of atmospheric absorption is that certain wavelength bands in the electromagnetic spectrum are strongly absorbed and effectively blocked by the atmosphere. The wavelength regions in the electromagnetic spectrum usable for remote sensing are determined by their ability to penetrate atmosphere. These regions are known as the atmospheric transmission windows.
- Remote sensing systems are often designed to operate within one or more of the atmospheric windows. These windows exist in the microwave region, some wavelength bands in the infrared, the entire visible region and part of the near ultraviolet regions. Although the atmosphere is practically transparent to x-rays and gamma rays, these radiations are not normally used in remote sensing of the earth.
Optical and Infrared Remote Sensing
- In optical remote sensing, optical sensors detect solar radiation reflected or scattered from the earth, resembling photographs taken by a camera high up in space. The wavelength region usually extends from the visible and near-infrared to the short-wave infrared.
- There are also infrared sensors measuring the thermal infrared radiation emitted from the earth, from which the land or the sea surface temperature can be derived.
Microwave Remote Sensing
- There are some remote sensing satellites which carry passive or active microwave sensors. The active sensors emit pulses of microwave radiation to illuminate the areas to be imaged. The images of the earth surface are formed by measuring the microwave energy scattered by the ground or sea back to the sensors.
- These satellites carry their own flashlight emitting microwaves to illuminate their targets. So, the images can be acquired day and night.
- Microwaves have an additional advantage as they can also penetrate clouds. Images can be acquired even when there are clouds covering the earth surface. A microwave imaging system which can produce high resolution image of the earth is the synthetic aperture radar (SAR). The intensity in a SAR image depends on the amount of microwave reflected by the target and received by the SAR antenna.
- Remote sensing satellites are of various uses some are:
- Measuring forest cover and tree cover, assessing the health of the forest cover
- Measuring crop acreage and farm productivity
- Wasteland mapping
- Used in watershed development programme
- Used in drought prone area programme
- Command area development programme
- Ground water management
- Coastal zone management
- Mangroves management
- Mineral exploration
- Water, sea, land and air pollution
- Continental shelf management
- Tsunami early warning
- Urban planning
- Encroachment, illegal construction
- Sustainable development is fragile ecosystems
C) Placement of remote sensing satellites:
- These satellites are place in polar orbits, which have a constant angle with respect to the sun i.e.polar sun synchronous orbit. So it is easy to compare two photographs. As if the angle is different than it won’t be easy to compare two photographs as the illumination would be different.
D) Indian remote sensing satellites:
- The initial versions are composed of the 1 (A,B,C,D). The later versions are named based on their area of application including Ocean Sat, Carto Sat, Resource Sat. Some of the satellites have alternate designations based on the launch number and vehicle (P series for PSLV).
||Date of Launch
||17 March 1988
||29 August 1991
||29 September 1997
||27 May 1999
||Technology Experiment Satellite (TES)
||22 October 2001
||IRS P6 (Resourcesat-1)
||17 October 2003
||IRS P5 (Cartosat 1)
||5 May 2005
||IRS P7 (Cartosat 2)
||10 January 2007
||28 April 2008
||20 April 2009
||23 September 2009
||12 July 2010
||12 October 2011
||26 April 2012
||25 Feb 2013
||22 June 2016
||15 Feb 2017
||23 June 2017
||12 Jan 2018
||22 May 2019
||27 Nov 2019
- It was an ocean resource management sat for finding potential fishing zones for fishermen, coastal region and continental shelf management. Payload was known as OCM (ocean colour monitoring) camera. In 2009 ocean sat 2 was launched to replace oceansat1.
Carto sat 1, 2….3
- They are used for map making purposes, as a payload they have a pan-chromatic camera which provides high resolution pictures. These pictures are used for urban planning, encroachment control etc
Technology experiment satellite (TES)
- Launched in 2001 it was India’s first spy satellite
- Launched after Mumbai attacks, these are official spy satellites to keep track of oceans around India, these are radar imaging type, they send microwave signals on earth when they hit, they are reflected back thus working as a radar. They have cloud penetration capabilities and can-do imaging without sunlight because of microwave remote sensing capabilities.
- Satellite with ARgos and ALtiKa is a cooperative altimetrytechnology mission ofISROand CNES (Space Agency of France). SARAL performs altimetric measurements designed to study ocean circulation and sea surface elevation.
Megha-Tropiques is a satellite mission to study the water cycle in the tropical atmosphere in the context of climate change.A collaborative effort between India and France. Megha-Tropiques was successfully deployed into orbit by a PSLV rocket in October