1. Sensors Remote sensing sensors are designed to record radiations in one or more parts of the EM spectrum. Sensors are electronic instruments that receive EM radiation and generate an electric signal that correspond to the energy variations of different earth surface features. Imaging sensor is a device that receives EM radiation & converts is into signal that can be recorded & displayed as numerical data or an image. Strength of the signal depends upon- Energy flux: The amount of energy reflected or radiated from terrain is the energy flux Lower on darker day and higher on brighter days.
2. Altitude: For a given resolution cell, amount of energy reaching to detector is inversely proportional to square of the distance. Spectral bandwidth: Broader wavelength will give more energy to detector Instantaneous Filed of View: A small IFOV needed for high resolution restricts the signal strength. Dwell time: The time required by detector IFOV to sweep across a ground resolution cell. A long dwell time will have more energy and hence stronger siqnal scanning There are two major categories:- Whisk Broom or Opto-mechanical scanners Pushbroom or CCD scanners
3. Linear imaging & self scanning sensor (LISS I) – This payload was on board on IRS IA & IB satellite. It had four bands operating in visible & NIR region. The characteristics are given below. 7 bit Quantization level 148km Swath 0.45-0.52, 0.52- 0.59, 0.62-0.68, 0.77- 0.86 Band width(µm.) 72.5m Resolution
4. b) Linear imaging & self scanning sensor (LISS II ):- This payload was on board on IRS IA & IB satellite. It had four bands operating in visible & NIR region. The characteristics are given below. 7 bit Quantization level 74km Swath 0.45-0.52, 0.52- 0.59, 0.62-0.68, 0.77- 0.86 Band width(µm.) 36.25m Resolution
5. c) Linear imaging & self scanning sensor (LISS III):- This payload was on board on IRS IC & ID satellite. It had three bands operating in visible & NIR region & one band SWIR. The characteristics are given below. Greater than 128 Signal to noise radio 7 bit Quantization level 141 Swath 0.45-0.52, 0.52- 0.59, 0.62-0.68, 0.77- 0.86 1.55-1.77 Band width(µm.) 36.25m Resolution
6. d) Pancromatic sensor (PAN):-This payload is on board IRS IC & ID satellite. It has one band. The characteristics are given below. +/- 26 deree Steering Greater than 64 Signal to noise radio 6bit Quantization level 63 to 70 km Swath 0.50-0.75 Band width(µm.) 5.8m Resolution
7. e) Wide field sensor (WiFS):- This payload is on board IRS IC & ID satellite. It has two band operating in visible & NIR region. The characteristics are given below. Greater than 128 Signal to noise radio 7bits Quantization level 812 Swath 0.62-0.68, 0.77-0.86, 1.55-1.75(IRS P3only) Band width(µm.) 188m Resolution
8. f) Modular opto elctronic scanner (MOS):- This payload is on board IRS P3 satellite. The characteristics are given below. 24,24,24days repitivity 195km, 200km, 192km Swath 755-768,408-1010,1500-1700 Spectral range 4,13,1 Bands 1569X1395m,523X523m,523X644 Resolution (MOS A,B,C)
9. g) Ocean color monitor (OCM):- This payload is on board IRS P4 satellite. It has eight spectral band operating in visible & NIR region. The characteristics are given below. Steering angle Quantization level 1420 km Swath 402-422, 433-453, 480-500, 500-520, 545-565, 660-689, 745-785 and 845-885 nm bands Band width(µm.) 360m across Resolution
10. Sensor Technology So far, we have considered mainly the nature and characteristics of EM radiation in terms of sources and behavior when interacting with materials and objects. It was stated that the bulk of the radiation sensed is either reflected or emitted from the target, generally through air until it is monitored by a sensor. The subject of what sensors consist of and how they perform (operate) is important and wide ranging.
11. Most remote sensing instruments (sensors) are designed to measure photons. The fundamental principle underlying sensor operation centers on what happens in a critical component - the detector. This is the concept of the photoelectric effect (for which Albert Einstein, who first explained it in detail, won his Nobel Prize [not for Relativity which was a much greater achievement]; his discovery was, however, a key step in the development of quantum physics). This, simply stated, says that there will be an emission of negative particles (electrons) when a negatively charged plate of some appropriate light-sensitive material is subjected to a beam of photons.
12. The electrons can then be made to flow as a current from the plate, are collected, and then counted as a signal. A key point: The magnitude of the electric current produced (number of photoelectrons per unit time) is directly proportional to the light intensity. Thus, changes in the electric current can be used to measure changes in the photons (numbers; intensity) that strike the plate (detector) during a given time interval.
13. The kinetic energy of the released photoelectrons varies with frequency (or wavelength) of the impinging radiation. But, different materials undergo photoelectric effect release of electrons over different wavelength intervals; each has a threshold wavelength at which the phenomenon begins and a longer wavelength at which it ceases .
14. Now, with this principle established as the basis for the operation of most remote sensors, let us summarize several main ideas as to sensor types (classification) in diagram: The first is a functional treatment of several classes of sensors, plotted as a triangle diagram, in which the corner members are determined by the principal parameter measured: Spectral; Spatial; Intensity.
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16. optical-mechanical-electronic radiometers and scanners, leaving the subjects of camera-film systems . The common components of a sensor system are shown in this table (not all need be present in a given sensor, but most are essential):
17. The two broadest classes of sensors are Passive (energy leading to radiation received comes from an external source, e.g., the Sun; the MSS is an example) and Active (energy generated from within the sensor system is beamed outward, and the fraction returned is measured; radar is an example).
18. Sensors can be non-imaging (measures the radiation received from all points in the sensed target, integrates this, and reports the result as an electrical signal strength or some other quantitative attribute, such as radiance) or imaging (the electrons released are used to excite or ionize a substance like silver (Ag) in film or to drive an image producing device like a TV or computer monitor or a cathode ray tube or oscilloscope or a battery of electronic detectors . (since the radiation is related to specific points in the target, the end result is an image [picture] or a raster display [for example: the parallel horizontal lines on a TV screen]).
19. Radiometer is a general term for any instrument that quantitatively measures the EM radiation in some interval of the EM spectrum. When the radiation is light from the narrow spectral band including the visible, the term photometer can be substituted. If the sensor includes a component, such as a prism or diffraction grating, that can break radiation extending over a part of the spectrum into discrete wavelengths and disperse (or separate) them at different angles to an array of detectors, it is called a spectrometer .
20. One type of spectrometer (used in the laboratory for chemical analysis) passes multiwavelength radiation through a slit onto a dispersing medium. The term spectroradiometer is reserved for sensors that collect the dispersed radiation in bands rather than discrete wavelengths. Most air/space sensors are spectroradiometers.
21. Sensors that instantaneously measure radiation coming from the entire scene at once are called framing systems The eye, a photo camera, and a TV vidiocon belong to this group. The size of the scene that is framed is determined by the apertures and optics in the system that define the field of view, or FOV . If the scene is sensed point by point (equivalent to small areas within the scene) along successive lines over a finite time, this mode of measurement makes up a scanning system . Most non-camera sensors operating from moving platforms image the scene by scanning.
22. The Airborne Sensor Facility provides remote sensing systems development for the Airborne Science Program; calibration and validation of the Earth Observing System's earth science investigations; and system operation, data processing, calibration and maintenance of Code Y facility remote sensing instruments Airborne Sensor
23. Introduction CIMSS (cooperative institute of metrological satellite studies) has developed and flown a variety of airborne earth-observing sensors since 1985. The data acquired by these sensors is used to develop, test, and validate processing algorithms which can eventually be used for spaceborne sensors. For example, a cloud detection algorithm can be developed using data acquired by an airborne imager and tested over a variety of earth scenes while a similar spaceborne sensor is still under development. When the spaceborne sensor is launched, the processing algorithm can be applied to the data with confidence because it has already been proven with airborne data
24. Aircraft Platforms Airborne sensors fly on a variety of different platforms, each with its own unique advantages. High altitude (> 20 km) aircraft provide the widest perspective view of the earth scene below, and can closely simulate the behavior of spaceborne sensors by virtue of being above most of the atmosphere. These platforms typically are flown by a single pilot because of the challenges of life support at high altitude. Medium altitude (> 10 km) aircraft provide a shirt-sleeve environment where scientists can ride along with their sensors to monitor their performance in real-time. Unpiloted aircraft provide the opportunity for longer flights at medium to high altitudes.
25. Passive Sensors Passive airborne sensors measure electromagnetic radiation which is either reflected from (solar energy) or emitted by (thermal energy) the earth scene below the aircraft. Sensor types include imagers which collect tens of bands of spectral information at high spatial resolution, and sounders which collect thousands of bands of spectral information at low to moderate spatial resolution. Aircraft Platforms used by CIMSS include: ER-2 :A high-altitude platform with a flight crew of 1 pilot( photos ), DC-8 :A medium-altitude platform with a flight crew of 4, and a science crew of up to 18 ( photos ), Proteus :A high altitude piloted (1 pilot) or unpiloted platform ( photos ).
26. Passive Sounders used by CIMSS include: HIS : A nadir viewing interferometer with > 2000 spectral channels (ER-2), Scanning-HIS : A scanning interferometer with > 2000 spectral channels (ER-2 or DC-8), NAST-I : A scanning interferometer with > 2000 spectral channels (ER-2). Passive Imagers used by CIMSS include: MAS : A scanning spectrometer with 50 spectral bands (ER-2), MASTER : A scanning spectrometer very similar to MAS (ER-2 or DC-8), MAMS : A scanning radiometer with 11 spectral bands (ER-2).
27. Active Sensors Active airborne sensors send pulses of electromagnetic energy (e.g., a laser beam) towards the earth scene below the aircraft, and then measure the energy which is scattered back towards the sensor (the same principle as radar). Active Sensors used by CIMSS (cooperative institute of metrological satellite studies) include: CLS : A nadir pointing laser radar, also known as lidar (ER-2).
28. IfSAR :Interferometric Synthetic Aperture Radar (IfSAR or InSAR) is an aircraft-mounted sensor designed to measure surface elevation, which is used to produce topographic imagery. Radar pulses are aimed at targets on the Earth, and . Airborne Sensors LIDAR (Light Detection And Ranging) is an active sensor, similar to radar, that transmits laser pulses to a target and records the time it takes for the pulse to return to the sensor receiver. This technology.
29. Airborne Multispectral Systems Airborne multispectral (MS) camera systems are currently being deployed by many private sector vendors. These sensors are complex systems incorporating multiple cameras, different storage... more Thermal Infared Radiometer Airborne thermal infrared radiometers (TIR) are used to map and measure thermal characteristics of landscapes and seascapes. These instruments are often components of complex remote sensing systems that... more Hyperspectral Systems Hyperspectral sensors are passive sensors that acquire simultaneous images in many relatively narrow, contiguous and/or non-contiguous spectral bands through the ultraviolet, visible and infrared... more
30. Characteristics of sensors Scanning mechanism A scanning system employs detectors with a narrow field of view, which sweeps across the terrain to produce an image. When photons of EM energy radiated or reflected from earth surface feature encounter the detector, an electrical signal is produced that varies in proportion to the number of photons. There are four common scanning modes: whiskbroom scanning, Along-track or pushbroom scanning, Circular scanning and Cross-track
31. In whiskbroom a mirror of fixed size rotates along an axis and the energy received from the ground is focused on to a spectrometer or grating for dispersion into different spectral bands. After dispersion the reflected radiation is focused on an array of photovoltaic cells and energy sensed in different spectral bands is than calibrated. Whiskbroom scanning
32. These sensors do not use opto-mechanical device and have no moving parts. The energy coming from ground directly falls on the array of charge-coupled devices {CCD), which calibrate the received energy and change it to digital counts. This method allows sensing of energy for larger time and hence results in better signal. Along-track or Pushbroom scanning
33. The scan motor and mirror are mounted with a vertical axis of rotation that sweeps a circular path on the terrain. Only forward motion of the sweep is recorded to produce images. An advantage of this system is the distance between scanner and terrain is constant and all the ground resolution cells have the same dimension. Circular scanners are used for reconnaissance purposes in helicopters and low flying aircrafts. The axis of rotation is tilted to point forward and acquire images of the terrain well in advance of the aircraft position. Circular scanning
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35. Airborne Thermal Scanning Airborne scanners, which are similar to satellite data in that they record digital data that can be easily manipulated by computers, have the added benefit of a much smaller spatial resolution. In addition they can be flown at different altitudes and times of day required by specific research needs. Modern airborne scanners have thermal capabilities that may be able to locate individual buried site features.
36. Thermal data are useful for locating archaeological resources because they can actually measure the heat difference in the ground. Heat differences in the soil can be an indicator of buried stone structures, as these can act as passive solar collectors during the day, soaking up the heat, and then releasing this heat in the afternoon and evening. They also can mark small variations in vegetation and soils.
37. ARIES Scanner System Description The Aries scanner system is a French airborne digital radiometer system constructed . It has two channels, normally set with one in the area of the visible and near infrared, and the other in the thermal portion of the spectrum . Internal calibration of the thermal scanner can provide apparent temperature recording capability.
38. The spatial resolution of the data is dependent on aircraft elevation, but 1-2 meter data are typical for missions such as ours. For this project, the ARIES scanner system was mounted in a single engine Pilatus aircraft, and three corridors within the research area were flown on March 12, 1987.
39. A RIES scanner showing destroyed villa at star This image shows the Gallo-Roman villa site which is now a lake. The gravel mines continue to consume the land adjacent to the river. There is a large, semicircular soil mark to the left of the star. The regular shape and large size make it likely that this is not a natural feature, but no field testing has been done yet.
40. Remote sensing observation platforms Airborne and space borne platforms has been in use in remote sensing of earth resources Airborne platforms- In early days Aircraft were mostly used as RS platforms for obtaining photographs. Aircraft carrying the RS equipment should have maximum stability, free from vibration and fly with uniform speed. In India three types of aircraft are currently used for RS operation’s Dakota,AVRO and Beach-craft super king Air 200. The RS equipment's available in India are multi spectral scanner, ocean color radiometer, aerial cameras for photography in B/W, color & near infrared etc.
41. Air craft operations are very expensive and moreover for periodical monitoring of constantly changing phenomenon like crop growth, vegetation cover etc. aircraft based platform cost and time effective solutions.
42. Space based platforms Platforms in space are very less affected by atmospheric drag, hence orbit can be well defined Entire earth or any designated portion can be covered at specified intervals synoptically, which is immensely useful for management of natural resources Satellite is a platform that carries the sensor and other payload required in RS operation. It is put into the earth’s orbit with the help of launch vehicles The space borne platforms are broadly divided into two classes ( i ) Low altitude near polar orbiting satellite (ii) High altitude Geo-stationary satellite
43. In general, in remote sensing a given area of the earth's surface is observed by the sensor and number of measurements are made. Each measurement corresponds to an element area on the surface over a number of spectral bands. Measurements are also made at fixed and regular interval of time. It would be instructive to understand as to how we get information from the remotely sensed data using spectral signatures which are linked to various kinds of resolutions. Four different types of resolution are considered in the remote sensing Spatial Resolution Spectral Resolution Radiometric Resolution Temporal Resolution Resolution
44. Spatial resolution It is measure of area or the smallest dimensions on the earth surface oyer which an independent measurement can be made. A qualitative measure of spatial resolution is the amount of detail that can be observed on an image. In many of the remote sensors, a small element area is observed at a time by means of suitable electronic sensors, which incorporates lenses in order to focus the in coming radiation on to the detectors and such a field of view of the sensor is called instantaneous field of view (IFOV). There are various factors like IFOV, dwell time (the time for which sensor looks on the element area), sampling frequency, which contribute significantly to the overall spatial resolution of the sensor
45. This refers to the fineness of details seen on an image. It describes the minimum size of the objects on the ground which can be separately identified and measured. This is ultimately decided by the size of pixel or picture element in an image. It is a function of the spectral contrast between objects in the scene and their background, of the size/shape of the objects, and the signal‑to‑noise ratio of the sensor system. Spatial resolution of Landsat MSS is 79 x 57 m, and that of LISS II 36m .
46. Spectral resolution The EM energy reaching a remote sensor from the earth surface encompasses the complete EM spectrum. The spectral resolution characterizes the ability of a sensor to resolve EM energy received in a given spectral bandwidth. A system, which measures a large number of bands, which encompasses narrow range of EM spectrum is said to have a high spectral resolution.
47. It is a measure of the discreteness of the spectral bands and the sensitivity of the sensor to distinguish between grey levels. In Landsat MSS, three of the sensors were sensitive over 0.1 m wavelength range and the fourth over 0.3 m range (0.8 ‑ 1.1 m). More number of narrow spectral bands gives rise to greater ability to discriminate various earth surface features.
48. Radiometric resolution The radiometric resolution is a measure of how many grey levels are measured between pure black and pure white spectral band. It characterizes the ability to distinguish the finer variations of the reflected or emitted radiation from the different objects in a given spectral band. The radiometric resolution is measured in 'bits'. An 8-bit system (28 = 256) records 256 grey levels, in which black is recorded by a digital number of zero and pure white by digital number of 255.
49. This determines the distinguishability of ground features differing only slightly in their reflectance or radiance. It is obtained by dividing the total range of the signal into a large number of just discriminable levels (Slater, 1980). For example, output of Landsat MSS bands 4,5,6 is divided into 128 levels whereas Landsat Thematic Mapper output is divided into 256 levels
50. Temporal Resolution It defined as the revisit time over a given area by a remote sensor at regular interval This depends upon the frequency at which the data are collected over the same scene. Landsat 1, 2, 3 had 18‑day cycle whereas Landsat 4, 5 have 16‑day cycle. This allows monitoring of dynamic phenomena like crops.
51. Identification of objects by means of image interpretation often involves combination of all these four parameters i.e. spatial, spectral, radiometric and temporal. Various methods are employed which are either visual/manual or computer based. Human interpretation is primarily based on the capability to distinguish variability of the spatial parameter which includes shape, size, pattern, texture, shadow, association etc. and to some extent on radiometric parameter (i.e. tone) and on spectral parameter (e.g. colour). However, the latter is rather difficult because of ambiguity in spectral reflectance‑colour relationships.
52. Human interpreters can also not distinguish more than 8‑10 tonal variations. In contrast computer‑based image analysis methods can make much better use of the spectral, radiometric and temporal parameters but utilisation of spatial parameter is the weakest link. To sum up, human interpretation has an edge over the current computer‑based image analysis/interpretation methods as far as spatial parameter is concerned whereas the machine‑oriented methods are far superior in exploiting the spectral, radiometric and temporal parameters.
59. The average or bulk properties of electromagnetic radiation interacting with matter are systematized in a simple set of rules called radiation laws . These laws apply when the radiating body is what physicists call a blackbody radiator . Generally, blackbody conditions apply when the radiator has very weak interaction with the surrounding environment and can be considered to be in a state of equilibrium. Although stars do not satisfy perfectly the conditions to be blackbody radiators, they do to a sufficiently good approximation that it is useful to view stars as approximate blackbody radiators. Radiation Laws
60. RADIATION LAWS Light has a dual nature being both CONTINUOUS and DISCRETE . DISCRETE Newton and planck propagated the theory of light as discrete units which travel in straight lines. Planck discovered that light is absorbed and emitted in discrete units called QUANTA or PHOTONS . The size of each unit is directly proportional to the frequency of the energy's radiation. Planck's equation explains the PHOTOELECTRIC EFFECT . The impact of quanta upon certain metal surfaces cause the emission of electrons.
61. STEFAN - BOLTZMANN LAW Defines the relationship between total emitted radiation ( W ) in watts/cm2 and temperature ( T ) expressed in kelvin ( K ). Hot blackbodies emit more energy per unit area than cool ones. As temperature increases, the total amount of energy emitted also increases And the wavelength of peak emission becomes shorter
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65. Thermal remote sensing Black body (BB) is a perfect absorber and perfect radiator at all wavelength. True BBs do not occur but clean deep water between 8 – 12 m is very close Materials that absorb and radiate only a certain fraction compared to a black body are called Grey bodies. Grey body curve is identical to Bb with lower values. Selective radiators also radiate a fraction of BB but this function varies with wavelength. Radiance emittance curve of selective radiator can look quite different from BB curve.
66. Thermal remote sensing Emissivity is the fraction of energy radiated by material compared to that of BB at given wavelength. = M T / M bbT Most materials are selective radiators. Emissivity spectrum can be used to determine the composition of object similar to spectral reflectance curve. Kirchoff’s law for opaque material = = 1 - Where and are emissivity and reflectance at given wavelength
67. Thermal remote sensing Radiant temperature: It is the sensed temperature of any object. Kinetic temperature: It is the actual temperature of the object. Radiant temperature calculated is smaller than the true Kinetic temperature (actual temperature measured using thermometer) due to emissivity less than 1 and incomplete radiation. Trad = 1/4 Tkin
70. Various spectral bands Wavelength ( m) Principal Application 0.52 - 0.60 Green Green reflectance peak, assessment 0.63 - 0.69 Red Designed to sense chlorophyll absorption region, plant species differentiation 0.76 - 0.90 Near Infra-red Vegetation types biomass 1.55 - 1.75 Mid infra-red Vegetation moisture content, soil moisture content, snow & cloud differentiation 0.45 - 0.52 Blue Soil and vegetation discrimination
71. INDIAN SPACE PROGRAMME Primary objective is to promote the development and application of space science &technology for socio-economic benefits of the nation IRS Program Timely and accurate information on natural resources Inventory, planning and efficient management of natural resources Primary objectives Design and develop 3-axis stabilized Polar Sun Synchronous satellite with state of art cameras in VNIR for earth resource applications on an operational basis Establish and routinely operate ground based systems for data reception and generation of data products For natural resources survey and efficient management Coverage cycle Varies hourly to yearly based on application Hourly for disaster monitoring About 20 days cycle for agriculture Yearly for geological applications
72. No other developing country has mounted so sustained and so broad an effort to promote civilian use of satellite imagery We have important role to play in this program as an international partner Local Time 10:30 Hrs for IRS-1C/1D: Atmosphere is clearest after morning mist and well before Atmospheric convection started 12:00 Noon for IRS-P4 : High Sun angle for shallow water and turbidity Low Sun angle – Max topographical enhancement for geomorphic studies
73. Evolution of Indian RS The romance with remote sensing began in 1975 with the setting up of LANDSAT data reception centre to learn the art of data reception, analysis and utilization Parallelly India initiated technological development program for satellite and launch vehicle, so that we could enhance and sustain space application program in a self supporting manner Experimental RS satellites Bhaskara-I and II launched in 1979 and 1981 First generation operational RS satellites IRS-1A and IRS-1B launched in 1988 and 1991 The Second generation Global missions, IRS-1C and IRS-1D launched in 1995 and 1997 Through IRS-P series, IRS-P2, P3 and P4 were launched Incidentally, IRS-P3 and P4 extended the applications into Ocean also Future plan include Resource Sat, RISAT, Cartosat and Metsat Unique application of IRS: Integrated Mission for Sustainable Development (IMSD) EOSAT (US) entered in to commercial contract with DOS for distribution of IRS data
74. RADIATION LAWS Light has a dual nature being both CONTINUOUS and DISCRETE . DISCRETE Newton and planck propagated the theory of light as discrete units which travel in straight lines. Planck discovered that light is absorbed and emitted in discrete units called QUANTA or PHOTONS . The size of each unit is directly proportional to the frequency of the energy's radiation. Planck's equation explains the PHOTOELECTRIC EFFECT . The impact of quanta upon certain metal surfaces cause the emission of electrons.
75. STEFAN - BOLTZMANN LAW Defines the relationship between total emitted radiation ( W ) in watts/cm2 and temperature ( T ) expressed in kelvin ( K ). Hot blackbodies emit more energy per unit area than cool ones. As temperature increases, the total amount of energy emitted also increases And the wavelength of peak emission becomes shorter
