In this article we will discuss about:- 1. Meaning of Remote Sensing 2. Remote Sensing Classification 3. Spectral Characteristics 4. Components 5. Requirements 6. Remote Sensing Satellites 7. Communication and Data Collection 8. Detection of Electromagnetic Radiation 9. Applications.
Contents:
- Meaning of Remote Sensing
- Remote Sensing Classification
- Spectral Characteristics of Remote Sensing
- Components of Remote Sensing System
- Requirements for an Ideal Remote Sensing System
- Remote Sensing Satellites
- Communication and Data Collection from Remote Sensing System
- Detection of Electromagnetic Radiation by Remote Sensing Device
- Applications of Remote Sensing Technique
1. Meaning of Remote Sensing:
Remote sensing is the study of any object without making actual contact with the object under consideration. More precisely, it is the acquisition and measurement of data/ information on few properties of a phenomenon, object or material by a recording device; not in physical and intimate contact with the feature(s) under surveillance. In other words it is the science and art of acquiring spectral, spatial and temporal informations about material, objects, area, or phenomenon without coming into their physical contact.
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In remote sensing, the transfer of information is carried out by means of electromagnetic radiation, expressed as EMR. The electromagnetic radiation is the form of energy, revealing its presence by developing observable effects at striking the object. It spans the spectrum of wavelengths ranging from 10 mm to cosmic rays up to 1010 mm.
Remote sensing as a tool can be used to study the things at all scales, ranging from the smallest particles within the atomosphere to the universe as a whole. Remote sensing tools are associated to the researchers or scientists to conduct scientific investigations on various aspects.
2. Remote Sensing Classification
:
The remote sensing techniques are classified as under:
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1. Based on the Type of Energy Resources:
i. Passive remote sensing; and
ii. Active remote sensing.
Passive remote sensing makes the use of sensors to detect the reflected or emitted electro-magnetic radiations from the natural sources. On the other hand, the active remote sensing uses the sensors to detect the reflected responses from the objects which are irradiated from radar, or any other kind of artificially generated energy source.
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2. Based on the Wavelength Regions:
(i) Visible and reflective infrared remote sensing.
(ii) Thermal infrared remote sensing; and
(iii) Microwave remote sensing.
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The atoms and molecules in the gas emit the electromagnetic radiations. Atoms consist of a positively charged nucleus surrounded by the orbiting electrons with discrete energy. On transition of electrons from one energy state to the other state, there is emission of radiation at discrete wavelengths. During this action, there develops a kind of spectrum is called line spectrum.
The molecules develop the energy in the state of rotation and vibration forms. Transition between these two states of energy causes emission of radiation in a band spectrum. The emission from solids and liquids takes place on their heating; and resulting spectrum is in continuous form. This type of emission is called thermal emission. Thermal emission is very important source of electromagnetic radiation for remote sensing.
The electromagnetic radiation is reflected or emitted from the object. The EMR is a common source of remote sensing apart from the other sources such as gravity or magnetic fields. In remote sensing a wide range of electromagnetic spectrum ranging from a very short wavelength (Gamma ray) to a very long wavelength (Radio wave) are used. The important wavelength regions for remote sensing applications are mentioned in Table 28.1. The microwave region, which wavelength is from 1 mm to 1m is another range of electromagnetic spectrum used for remote sensing applications.
3. Spectral Characteristics
of Remote Sensing:
The sun is the only natural source for light generation on the earth system. The transmission of sunlight through the atmosphere gets affected (reduction) due to absorption and scattering of atmospheric molecules and aerosols, as result the intensity of sunlight also gets reduce. The reduction in sunlight intensity is called extinction.
Every object/matter is composed of atoms and molecules; and they have their own compositions. Accordingly, the emission/absorption of electromagnetic radiation by each of them is with a particular wavelength. In other word the reflection, absorption, penetration and emission of electromagnetic radiation power is different for different matters; in which few are very strong; few are in medium, while few are very poor.
The transmission of electromagnetic radiation through atmosphere to/from different matters lying on the earth’s surface, takes place in the form of reflection, scattering, diffraction, refraction, absorption, transmittance and dispersion. For example – a leaf looks green because of the reason that the chlorophyll absorbs blue and red spectra of sunlight; and reflects only green colour. This unique characteristic of matter is called spectral characteristics.
Spectral Reflectance & Colour Readability:
The quantity of energy reflected, absorbed and transmitted from the earth’s objects depend on the wavelength, which mainly depends on the type of materials and their conditions. The range of wavelength varies for matter to matter. The difference in reflection, absorption and transmittance distinguishes the object’s features.
In brief:
i. The reflected, absorbed and transmitted energy varies with the wavelengths. Because of this effect the two features become distinguishable in one spectral range; and different on another wavelength bands.
ii. The spectral variation within visible spectrum portion results into development of visual effect, called colour.
A curve representing the relationship between spectral reflectance of an object and the wavelength is called spectral reflectance curve. Its configuration provides inside characteristics of the object; and also supports for making choice on selection of wavelength regions to acquire remote sensing data for a particular use. The discrimination of colour based on the wavelengths of spectral reflectance is presented in Table 28.2.
4. Components of Remote Sensing System:
Although, the remote sensing includes a wide array of technologies and types, but they all are based on certain common concepts with the same basic components.
The basic components of remote sensing system are given below:
1. Target
2. Energy source
3. Transmission path, and
4. Sensor.
The target is the object or material being studied. All the components in the system work together, to measure and record the information about the target without making physical contact. The energy source illuminates or provides electromagnetic energy to the target.
The energy interaction with the target depends on the target properties and the radiation. It also acts as a medium for transmitting the information from target to the sensor. The sensor is a remote device to collect and record the electromagnetic radiation.
Sensors are also used to measure the given-off energy or emitted energy by the target; reflected-off energy of the target; or transmitted energy from the target. After recording of energy, the resulting set of data is transmitted to the receiving station.
At receiving station, the data is processed to a usable format, i.e., in the form of image. The image is then interpreted to extract the informations about target. The interpretation of image can be done visually or electronically with the help of computers and image processing softwares.
Apart from above components, the others are detailed as below:
1. Energy Source:
These are the –
i. Passive System – It includes the sun and irradiance from the earth’s materials, mainly.
ii. Active System – It includes the irradiance from man-made energy sources, i.e., radar etc.
2. Platform:
It is the device used to carry the sensors. The vehicles such as truck, aircraft, space shuttle, satellite etc., are the main devices used for cartage of sensors.
3. Sensors:
These are the device to detect the EMR (electro-magnetic radiation), such as camera, scanner etc.
4. Detectors:
These are for handling the signal data. The data may be the photographic, digital or in any other forms.
5. Processor:
This is for processing of the signal data. The data may be photographic, digital etc.
6. Institution:
For proper execution of remote sensing at various stages, there is requirement of organization. The organization may be the national or international, universities, research centres etc.
These components are described as under:
1. Remote Sensing Platforms:
The vehicles/carriers used for cartage of sensors are called platforms. The satellites and aircraft are typical platforms used in remote sensing. The radio-controlled aeroplanes, balloon kits and ladder trucks or cherry pickers are mainly used for this purpose. The balloon kits are used for low altitude remote sensing sensors and cherry pickers are for ground investigations. The selection of suitable platform is done on the basis of altitude determining the ground resolution. The ground resolution is dependent on the instantaneous field of view (IFOV) of sensor on the board of platform.
Different types of platforms are described as under:
i. Ground-Based Platforms:
A platform which fix the sensor at the earth’s surface is called ground-based platform. These platforms are fixed at the earth; and the mounted sensors are used to measure the environmental parameters such as air temperature, wind characteristics, water salinity, earthquake intensity etc. The ground-based sensors can also be placed on the tall structures like towers, buildings etc. to elevate the platform. These sensors are less expensive to operate and maintain, as compared to the aircraft or satellite sensors.
The area coverage of these platform is not as large as the airborne platforms. The ground-based sensors are often used to record the detail informations about the surface, which are compared with the informations collected from the aircraft or satellite sensors for truthing. The ground-based sensors can be used to supplement or ground truthing of measurements taken from the airborne or satellite sensors.
ii. Aerial Platforms:
These platforms are most often mounted on fixed-wing aircraft, and sometimes also on the airborne platforms such as balloons, rockets and helicopters. The aircrafts are used to collect the detail images of the earth surface; and also facilitate for collecting the data of any portion of the earth surface at any time. In this system there is provision to elevate the sensor above the earth surface for increasing the aerial coverage; and monitoring very large area of the earth surface, which is impossible with the ground-based sensors.
The airborne remote sensing was introduced in the early 1900’s when airplanes were used during the World Wars to conduct surveillance of the enemy. Nowadays, the camera-mounted aircrafts are also used to monitor the land use practices; to locate the forest fires and produce accurate maps of inaccessible and remotely located areas on the earth. The weather balloons and rockets are also used as a means for the direct measurement of upper atmosphere’s properties.
iii. Satellite Platforms:
The satellite-mounted sensors were introduced in the early 1960’s by the researchers for research purposes. Nowadays, the satellites are capable to provide the informations about the earth to monitor global change and understand the planet. The manufacturing of satellites and placing them into orbit is very difficult and expensive task. Satellites are remotely operated from the ground surface. The data from satellite sensors are transmitted to the earth surface. The weather satellite for predicting the weather condition was the first application of satellite in remote sensing.
These satellites are now suitably used for study of weather extremes such as hurricanes and mid-latitude cyclones. Prior to development of satellites the informations on storms were used to collect from the ground observations that was not so authentic. Similarly, the development of land use satellites has created a kind of very authentic technology to evaluate the effects of tropical rain forests and climatic change on agricultural production; how the deserts advance and withdraw and how the polar ice caps are retreated.
In the year 1972 the Landsat program was started with the launch of Landsat-1 satellite. At now Landsat-7 has been installed to collect daily images of various parts of the earth’s surface. The salient features of few important platforms are outlined in Table 28.3.
2. Sensors:
The sensors used in remote sensing are the active and passive sensors. The active sensors detect, reflect or emit the electromagnetic radiation from natural sources. The passive sensors detect the reflected responses from the objects that are irradiated from artificially generated energy sources (radar). Tables 28.4 and 28.5 illustrate the types of sensors falling under active and passive groups along with their important features.
It refers to the ability of remote-sensing system (including lens, antennae, display, exposure, processing and others) to render a sharp image. There are several applications of remote sensing sensors; and each sensors are manufactured for very specific purposes. The design and placement of sensor is based on the target characteristics that are to study, and the informations required from the target.
The R.S. applications are for specific demands such as the informations about area coverage, the frequency with which measurements are to be taken, and the type of energy to be detected. For all these purposes, a sensor must be capable to provide the spatial, spectral and temporal resolutions to meet the requirements.
The resolutions of remote sensing are of different types, given as under:
i. Spectral Resolution:
This resolution refers to the width or range of each spectral band measured by the sensor. In order to detect the vegetative stress, the sensor which is sensitive to a narrow spectral band, is very suitable because the differences in spectral signatures at a specific wavelength are easily detected. A panchromatic sensor which covers a wide spectral range, is not being suitable for such types of tasks.
A narrow band sensor in the red portion of spectrum is found better for detecting the vegetative stress. The spectral resolution of sensor is predicted based on the bandwidths of EMR (Electro-Magnetic Radiation) of different channels, used. A high spectral resolution is achieved by the narrow band widths. A high spectral resolution provides more accurate signature for discrete objects as compared to the broad bandwidth resolution.
ii. Radiometric Resolution:
This type of resolution is predicted on the basis of number of discrete levels into which the signals are divided.
iii. Spatial Resolution:
It refers to the details that can be detected by the sensor. For detail mapping of land use practices, a greater spatial resolution is required than for the observations of large-scale storm detection. The land use satellites such as Landsats generally have greater spatial resolution than the global, weather satellites.
The spatial resolutions in terms of geometric properties of imaging system are referred as Instantaneous Field of View (IFOV). The IFOV is the maximum angle of view for the sensor to detect the electromagnetic energy, effectively.
iv. Temporal Resolution:
This resolution refers to the time interval between measurements. For detection of severe thunderstorms the measurements are taken at the frequency of few minutes. Similarly, for detection of crop production or insect infestation the seasonal measurements are taken, while for geological mapping a single measurement is recorded. These detections are performed using the temporal resolutions. The spatial resolution of Landsat 4/5 is 16 days.
5. Requirements for an Ideal Remote Sensing System
:
The requirements for an ideal remote-sensing system are outlined as under:
1. Uniform Energy Source:
For an ideal remote sensing system the uniformity in energy source is very important to provide the energy at constant and known rate, and high level of output irrespective of time and place for the entire range of wavelengths.
2. Non-Interfering Atmosphere:
This is also an important requirement for an ideal remote sensing system. It is required, mainly to maintain a uniform energy from the source, whether the energy is on its way to the earth’s surface or coming from it.
3. Series of Energy/Matter Interaction at the Earth’s Surface:
In an ideal remote sensing system such interactions are essentially required to create reflected/emitted signals, which are not only wavelength selective, but are also known, invariant and unique to each feature of the earth surface.
4. Super Sensor:
The sensor should be simple and reliable; require no high power or space, and accurate along with economical to use.
A super sensor is that which meets following requirements:
i. Highly sensitive to all wavelengths.
ii. Yield detail spatial data on absolute brightness or radiance from the objects throughout the spectrum.
5. Real-Time Data Handling System:
In this respect the following features are required to get satisfied:
i. Instant radiance versus wavelength response over the terrain element should be generated.
ii. The data should be processed in an interpretable format and recognized to the terrain elements from which it was generated.
iii. The processing should be done instantaneously; and provide timely information.
iv. The derived data should provide insight into the physical-chemical-biological state of each feature under consideration.
6. Multi Data Users:
For an ideal remote sensing system the provision of multi data users is very important. The persons involved should have comprehensive knowledge about their field, remote-sensing data acquisition and their analysis techniques.
6. Remote Sensing Satellites
:
A device equipped at remotely installed sensor to observe the earth is called remote-sensing satellite. Sometimes, it is also called earth observation satellite. These satellites are characterized by their altitude, orbit and sensors equipped. Table 28.6 enlists various remote sensing satellites.
7. Communication and Data Collection
from Remote Sensing System:
Communication refers to the delivery of collected data from remote sensing system to the end users, after their retrieving. This needs to be done very quickly for immediate use, such as in case of forecasting of severe thunderstorm. Also, the transmission, reception, processing and distribution of collected data from satellite sensor should be carefully done to satisfy the users’ demand. The mode of transmission of data depends on the type of platforms have been used.
For example – the ground-based remote sensing platforms transmit the data using ground-based communication systems such as radio and microwave transmission or computer networks. Few systems are so which stores the data on platform; and allow the users to manually collect from the platform. The data collected by aircraft, is stored on board and retrieved once as the aircraft lands. The satellites data is very difficult to obtain, because they remain in space for their entire operational lifetime.
The satellite data are transmitted back to the earth to a receiving station, where they are processed for communicating to the end users. The collected data from satellite are send to the earth receiving station by a number of ways, e.g., directly to the receiving station if it is within its line of sight.
If the satellite is not in the line of sight of ground station then satellite stores the data on board and dump thereafter when it is back in the sight of ground station. The satellite finally relays the data to the ground receiving station for transmission through a series of communication satellites lying in the orbit around the earth.
The transmission of data from one satellite to another is continued until it has been reached to the ground receiving station. Many satellites use a combination of the methods. The NOAA satellites, which are for meteorological investigations consist of continuous transmission of low resolution data that is received from the ground station within radio range; and also use board storage to store high resolution data, which is transmitted to the specific ground station.
The satellite data received by the ground station are in the raw digital format. They are processed into correct systematic, geometric and atmospheric distortions to the imagery; and again they are translated into a standardized format. These data may be in the form of tape, disk or CD.
8. Detection of Electromagnetic Radiation
by Remote Sensing Device:
It is carried out by using a device called radiometer. The radiometer is sensitive to different ranges of electromagnetic radiations. It is used to measure the energy levels in different ranges of wavelengths. A narrow band wavelength within the part of electromagnetic spectrum is called channel. The radiometers are designed to use for a specific channel on the basis of target information provided by the channel. In multi-spectral remote sensing system the use of radiometer is very common.
In multi-spectral RS system each sensor is tuned to a particular band of wavelength (channel) to provide the spectral data about the target. The radiometers placed on the aircrafts/satellites scan the earth, and measure the radiation level, which is reflected/emitted from the object in the atmosphere or ground surface. This information is transmitted back to the earth, and is converted into an image form. The images are different for different objects, because different objects have different spectral characteristics or spectral signatures.
The satellites such as Landsat and Spot with radiometers provide multi-spectral data. The objects, i.e., different types of land surfaces such as concrete, asphalt, crops, meadow, forest, water, desert etc., exhibit their individual spectral signature.
Even within single category of land use, there is difference in signature. In cropped land there is clear-cut difference in corn, soybean, wheat and other crops because of their signature variations (spectral pattern). Furthermore, the difference in signature is extended depending on other characteristics of the crop such as the crop health sufferings from drought, water logging, disease effects or pest infestation.
Broadly, the detection procedure of EMR can be of following types:
i. Passive detection; and
ii. Active detection.
i. Passive Detection:
In passive detection the sensor measures the energy levels of naturally emitted, reflected or transmitted by the target. The used sensors for this purpose are called passive sensors. The sun’s radiative energy is the main, which gets either reflected because of its visible wavelength, or absorbed/re-emitted due to thermal infrared wavelength. The passive detector works only when there is availability of naturally occurring energy. The detection of object is only happened, when sun illuminates the target. The quantum of solar radiation availability at polar latitudes affects the detection.
At lower latitudes, there is limitation about use of passive detectors because the presence of clouds, dusts, smokes and other particles in the atmosphere at lower latitudes block the reflected energy for reaching to the sensor. However, this type of problem can be removed by using such a sensor, which is capable of detecting the radiation in different portions of electromagnetic spectrum. The weather satellite with visible and thermal infrared channels can provide imagery of the cloud patterns, both at the day and night hours.
The Thematic Mapper (primary sensor) on Landsat satellites is an example of the passive sensor. This sensor includes total seven bands, or channels. Each channels are being sensitive to different ranges of electromagnetic radiations; generally, sensitive to narrow portions of visible and near infrared portion of the spectrum, with one sensitive band to thermal infrared. Using this sensor the detection of differences in crop production, soil moisture and mineral contents in the soils can be suitably done.
In active detection the remote sensing system contains its own energy source for illumination of the target. The sensors used for active detection are called active sensors. These sensors work by bursting the radiation at the target; and measuring the energy interactions by the target.
The active sensors detect the reflection energy by measuring the angle of reflection or time taken for the energy to return. These sensors are capable to record the measurements anytime, i.e., the day, night or the season. The active system requires to generate fairly large amount of energy to illuminate the targets, accurately.
The Doppler radar is a ground-based active remote sensing technology, in which radio energy is emitted in a radial pattern as the transmitter rotates. Sensors measure the reflection or echoes of the energy due to dust particles, raindrops, and even birds. The echoes are plotted on a regional map to locate the centres of storms; wind speed of storm, and also notifying the areas of potentially severe weather.
The atmospheric sounder is another form of active collection, in which various forms of energy is used. The energy may be in the form of lasers, microwaves or radar. Function of these energy sources is to take measurements on atmospheric density at a certain altitude; and providing detail data about variety of phenomena such as wind speed, pollution level, atmospheric composition etc.
These sounders may be the ground-based. Also, they can be mounted on airborne or satellite platform to measure the informations through the atmosphere. The measured data from sounding can be used for development of 3-dimensional models related to atmospheric components, which could be utilized for predicting future weather patterns.
9. Applications of Remote Sensing Technique
:
Various applications are outlined as under:
1. Assessment of Soil Moisture:
Passive microwave remote sensing techniques are used to measure surface soil moisture based on the fact that at the microwave frequencies the most striking feature of the emission from the earth’s surface is very heavy contrast between the water and land. The remote sensing cannot replace the ground-based methods for providing high quality profile data at a point. Its advantage is for mapping at regional, continental and even global scales and possibly on a repetitive basis.
Recently, it has been shown that the repetitive measurement of microwave brightness temperature can yield sub-surface soil hydraulic properties. For most satellite systems the revisit time can be a critical problem in the studies involving rapidly changing conditions such as surface soil water content. Currently, all passive microwave sensors on satellite platforms are operated at high frequencies (7 GHz).
Advanced Microwave Scanning Radiometer (AMSR) satellite systems with 6.9 GHz channel predicts the soil water content in the regions of low vegetation level. However, the AMSR is not suitable to map the soil water content, but it is the best tool to approximate the same.
2. Assessment of Water Quality:
The suitability of remote sensing technique for monitoring the water quality depends on the ability to measure the change in spectral signature scattered back from the water; and relate them in terms of empirical or analytical models to quantify the water quality parameters. The optimal wavelength used to measure the water quality parameters depends on the substance being measured; constituents concentration; and the sensor’s characteristics.
The main factors affecting the water quality in water bodies are the suspended sediments (turbidity), algae (chlorophylls, carotenoids), chemicals (nutrients, pesticides, metals), dissolved organic matter (DOM), thermal releases, aquatic vascular plants, pathogens and oils. The suspended sediments, algae, DOM, oils, aquatic vascular plants, and thermal release change the energy spectra of the reflected solar and/or emitting thermal radiation from the surface water.
However, the most chemicals and pathogens do not directly affect or change the spectral or thermal properties of surface waters. In fact they are inferred indirectly from measurements of other water quality parameters affected by these chemicals.
Remote sensing technique can also be used to measure the chlorophyll concentrations, spatially and temporally, both. Similar to the case of suspended sediment measurements, the studies of chlorophyll in water, based on the RS technique can be done; and the empirical relationships between radiance/reflectance in narrow bands or band ratios and chlorophyll can be developed. Also, a variety of algorithms and wavelengths have been developed to map the chlorophyll concentrations of oceans and the fresh waters.
Studies have also shown that the broad wavelength spectral data available on current satellites (Landsat, SPOT) do not permit discrimination of chlorophyll in waters with highly suspended sediments due to dominance of spectral signal of the suspended sediments.
Research on the relationship between chlorophyll and the narrow band spectral details at the “red edge” of the visible spectrum has shown a linear relationship between the chlorophyll and the difference between the emergent energy in the primarily chlorophyll scattering range (700-705 nm) and the primarily chlorophyll absorption range (675-680 nm).
3. Assessment of Land Cover:
The Landsat data can be used for developing the land use and land cover plan. For this purpose, the multi-temporal coverage is required during crop season. A minimum of four images for entire season is desirable for creating signature files, for classification.
Also, a set of ground data is collected for validation of the classification, by selecting the predominant crops as well as the vegetative cover causing confusion in classification. The base-classification is created using four visible and Near-IR bands (Landsat 3, 4, 5 and 7) for various classes in the scene.
The signatures for each land cover class are developed using non-supervised classification for each set of area of interest. Signatures based on all AOI’s are combined into one signature file for each area; and are used within supervised classification. After preparation of supervised classification, a table is generated based on the classification performance, where ground data was acquired.
4. Assessment of Leaf Area Index:
This is carried out by developing land cover map of the area. The MODIS imagery (250 m resolution) is used for developing the LAI image for every clear acquisition available for the growing season. For this purpose few models have also been developed, they can be used. The one-dimensional canopy reflectance model and SAIL (Scattering by Arbitrarily Inclined Leaves) are the examples.
They provide simulated canopy reflectance. The SAIL model calculates reflectance in the sensor direction as the function of canopy parameters and acquisition and sun angles; and canopy parameters such as leaf angle distribution, single leaf reflectance and transmittance and soil reflectance, mainly. This model runs in the inversion mode using canopy reflectance as the input; and the output is the canopy LAI.