Remote Sensing PDF

Fig. 2 Electromagnetic remote sensing of earth 1  REMOTE SENSING (LIGHT SOURCES, LIGHT INTERACTION WITH ATMOSPHERE & OBJECTS, SPECTRAL SIGNATURE & SENSORS) ELECTRO MAGNETIC RADIATION ELECTROMAGNETIC RADIATION: ❖ Two diff. model...

Fig. 2 Electromagnetic remote sensing of earth 1  REMOTE SENSING (LIGHT SOURCES, LIGHT INTERACTION WITH ATMOSPHERE & OBJECTS, SPECTRAL SIGNATURE & SENSORS) ELECTRO MAGNETIC RADIATION ELECTROMAGNETIC RADIATION: ❖ Two diff. models – Wave model & Particle Model 1. WAVE MODEL: ❖ James Clark Maxwell in 1860 conceptualized that EMR or electromagnetic radiation as wave that travels in the space at the speed of light (299792.46 km/s) ❖ EM wave consists – 2 fluctuating fields – Electric & Magnetic ❖ 2 fields are perpendicular to each other and also perpendicular to the direction of propagation ❖ Both have same amplitudes (strength) – reach their maxima & minima at the same time. ❖ EMR can transmit through space & generated wherever electric charge is accelerated ❖ 2 imp. Characteristics – Wavelength & Frequency WAVELENGTH – length of one complete wave cycle, measured as the distance between 2 successive crests/troughs ❖ Represented by λ, & measured by meters/Km/cm/micrometer etc.  Electromagnetic wave, E-Electrical Component, M-Magnetic Component E Crest E C Velocity of Light M M Trough ν = Frequency (No. of cycles passing per second of a fixed point Electromagnetic wave (comprising both magnetic and electric fields at 90° to each other) FREQUENCY – No. of cycles a wave passing a fixed point per unit time ❖ Represented by ν, & measured by Hertz/KHz/MHz/GHz etc. ❖ A wave completing one cycle per second is having a frequency of 1 Hz. Relationship between wavelength & frequency: λ = c/ν ❖ Frequency is inversely proportional to wavelength ❖ Longer the wavelength, lower the frequency λ Longer wavelength Shorter frequency Longer frequency λ Shorter wavelength PARTICLE MODEL – ❖ Light is a stream of particles called ‘PHOTONS’ – similar to subatomic particles like neutrons. ❖ Quantum theory applicable to this type of motion describes that light is transferred in discrete pockets called ‘quanta’ or ‘photons’. ❖ Photons – physical form of quantum, subatomic massless particles - basic particle for EM force. ❖ Comprise radiation emitted by the matter when it is excited thermally. ❖ Energy in photons - represented in terms of electron volts ❖ Rate of energy from one place to another – Flux & measured by ‘Watts’ ❖ Amount of energy by a photon is determined by Plank’s equation: Q=hν ❖ Q – energy measured in watts ❖ h – Plank’s constant (6.6260×10-34 J)  Light: Dual Wave & Particle nature ❖ We have, λ = c/ν = hc / hν = hc / Q i. e., Q = hc / λ ❖ Quantum energy is inversely proportional to its wavelength ❖ Photons with shorter wavelengths (at higher frequency) – more energetic) ELECTROMANGETIC SPECTRUM The electromagnetic spectrum is the range of frequencies of electromagnetic radiation and their respective wavelengths and photon energies.  Electromagnetic Spectrum – Useful Wavelength Bands ❖ Visible range - Blue - 0.4 - 0.5 m Green - 0.5 – 0.6 m Red - 0.6 – 0.7 m ❖ Infrared (IR) Near IR (NIR) 0.7 – 1.3 m Mid IR 1.3 – 3.0 m Thermal IR - beyond 3 m to 14 m ❖ Micro wave 1 mm – 1 m ❖ Earth's atmosphere absorbs energy in  rays, x rays and ultraviolet bands – hence, not used in Remote Sensing 12 Electromagnetic Radiation Ultraviolet Radiation - 0.4 micrometers ❖ Not much RS activities are done with UV since these shorter wavelengths are easily scattered by the atmosphere Electromagnetic Radiation Visible Radiation ❖ BLUE (.4-.5 micrometers) ❖ GREEN (.5-.6 micrometers) ❖ RED (.6-.73 micrometers) Electromagnetic Radiation Infrared Radiation - 0.72 - 15 micrometers 1. Near Infrared - reflected, can be recorded on film 2. Mid Infrared - reflected, can be detected using electro-optical sensors. 3. Thermal Infrared - emitted, can only be detected using electro-optical sensors Electromagnetic Radiation Microwave Radiation - radar sensors, wavelengths range from 1mm to 1m ❖ Sun – main source of EMR for remote sensing. ❖ However, all matter at temperature above absolute zero (0 K or - 273C) emits EMR. ❖ Amount of energy emitted from an object is a function of temperature of the object; M=  t4 (Boltzmann Law) where M= total energy from the surface of a matter watts m-2  = Stefan – Boltzmann Constant 5.669710-8 W m-2 W M-2 K-4 T= absolute temperature (K) of the emitting material This law is expressed for an energy source that behaves as a blackbody. Blackbody is a hypothetical, ideal radiator that totally absorbs and re-emits all energy incident on it. 17 Spectral distribution of energy radiated from black bodies at 18 various temperatures Dominant Wavelength LIGHT INTERACTION WITH THE ATMOSPHERE Atmosphere has profound effect mainly on: ❖the intensity ❖ the spectral composition of radiation available to sensors Net effect of Atmosphere varies with ❖ differences in path length ❖ magnitude of energy signal being sensed ❖ the atmospheric conditions present ❖ wave lengths involved Atmospheric modification of incoming and outgoing EM radiation includes scattering, refraction and absorption 21 Absorption ❖ Caused primarily by three atmospheric gases: ozone, carbon dioxide and water vapour ❖ Process by which radiant energy is absorbed and converted into other forms of energy. ❖ May take place at atmosphere & by the targets ❖ An absorption band is a range of wavelengths in which radiant energy is absorbed by the particles ❖ Atmosphere can close down RS activity in these bands due to cumulative effect of these wavelengths. no attenuation attenuation ❖ Thus, atmospheric gases absorb EMR in different specific regions of the spectrum ❖ They influence where in the spectrum we can ‘look’ for RS purpose. ❖ Those areas in the spectrum which are not severely affected by absorption & useful for RS activities – ATMOSPHERIC WINDOWS. ❖ These regions are less affected by absorption & nearly transparent. Atmospheric transmittance and atmospheric windows (Note that wave length scale is logarithmic.) Scattering - Unpredictable, diffusion of radiation by particles in the atmosphere - Depends on relative size of particles and the radiation wavelength Adverse effects in remote sensing - Reduces image contrast - Changes spectral signature of ground objects as seen by sensor 25 Rayleigh scattering - Atmosphere molecules and particle size 8  ❖ diffuse reflection – (Lambertian) ❖ reflection is scattered and equal in all direction (uniform) ❖ - contains information on the colour of reflecting surface In remote sensing “diffuse reflectance” properties of terrain are measured ❖ energy is reflected equally in all directions ❖ many natural surfaces act as a diffuse reflector to some extent. 36 Specular vs diffuse reflectance 37  CONCEPT OF SPECTRAL SIGNATURE Reflectance characteristics of earth surface features are quantified by measuring the portion of the incident energy that is reflected Energy of wave length () reflected Spectral reflectance = -------------------------------------------- x 100 Energy of wave length () incident 38 SPECTRAL REFLECTANCE CURVES ❖ These curves show the relationship between EM spectrum with the associated % reflectance for any given material. ❖ Plotted as a chart that represents wavelength on horizontal axis & percent reflectance on the vertical axis. ❖ Spectral reflectance - responsible for color or tone in visible band ❖ Signatures are not deterministic , but statistical in nature Spectral Signatures  SPECTRAL SIGNATURES ❖ Signal received by sensor depends on land cover 50 50 50 Spectral Signature % Reflectance unique to healthy vegetation 0 0 Water Bare Earth 0.4.6.8 1.0 1.2 1.4 Green - Highest reflectance hence we see green trees SPECTRAL REFLECTANCE OF VEGETATION: Visible band 0.4 m - 0.7 m ❖ At wave length bands 0.45 m and 0.67 m chlorophyll in plant absorbs energy ❖ Human eye perceive healthy vegetation as green Infrared band – 0.7 m to 1.3 m ❖ 40 to 50 % energy reflects – Rest is transmitted , absorption is minimal (5%) ❖ Reflectance is mainly due to the internal structure of the plant leaves. Hence discrimination among the species is possible in this band. ❖ Beyond Infrared band –1.3 m, Energy absorbed or reflected, but no transmission ❖ Dips at 1.4 m, 1.9 m and 2.7 m – because of water presence in the leaf absorbs energy ❖ Reflectance is inversely proportional to the total water present Typical vegetation reflectance spetrum Soil ❖ Less peak and valley variations in reflectance ❖ Factors effecting soil reflectance - moisture content - soil texture (sand, silt and clay) - surface roughness - presence of iron oxide - organic matter content 46 Water: ❖ Little reflectance in only blue and green wave band. ❖ Presence of turbidity increase the reflectance ❖ Absorbs most of the radiation in the near IR ❖ Middle infrared region - helps in delimiting even small water bodies. ❖ Dissolved gases and many inorganic salts do not manifest any change in the spectral response of water. 47 Comparison of reflectance spectra of a few land covers ❖ Spatial effects - same type of features have different characteristics at different geographic location at given point of time - identified by - shape, size & texture of objects ❖ Temporal effects – changes in reflectivity or emissivity of a feature over time - helps in the study of changes in the growing cycle of a crop 49 Polarization – refers to the changes in polarization of radiation reflected or emitted by an object - helps to distinguish the objects – useful in microwave remote sensing 50  Wayanad Floods-2024 Source: NRSC SENSORS Sensors – Mounted on the platform – capture the reflected energy from the objects Classification 1. Based on the type of illumination: Two types - Active or passive sensors ❖ Passive: sensors measure the amount of energy reflected from the earth’s surface ❖ Active: sensor emits radiation in the direction of the target, it then detects and measures the radiation that is reflected or backscattered from the target. ❖ Most systems rely on the sun to generate all the EM energy needed to image terrestrial surfaces - passive sensors. ❖ Other sensors generate their own energy, called active sensors, transmits that energy in a certain direction and records the portion reflected back by features within the signal path.  Active Remote Sensing ❖ Transmit their own signal and measure the energy that is reflected or scattered back from the target ❖ Advantages: ability to “see” regardless of time of day or season; use wavelengths not part of solar spectrum; better control of the way target is illuminated Ex. of active RS - Satellite Radar Altimetry ❖ Satellite radar altimeters work on the principle of active remote sensing. ❖ Transmit very 'sharp' electromagnetic pulses of a predetermined wavelength and frequency (a typical radar altimeter operates at 13.5 GHz) ❖ The satellite's altitude (i.e. absolute height above the surface) is obtained by measuring the time required for the pulse to travel from the altimeter antenna to the earth's surface and back to the satellite's receiver.  Passive Remote Sensing ❖ Measure natural radiation emitted by target or/and radiation energy from other sources reflected from the target ❖ examples: passive microwave radiometers, LandSat, SPOT Examples of Passive Sensors: ❖ Advanced Very High Resolution Radiometer (AVHRR) Sea Surface Temperature ❖ Sea-viewing Wide Field-of-View Sensor (SeaWiFS) Ocean Color Classification 2. Based upon the process of scanning: whiskbroom scanner: ❖visible / NIR / MIR / TIR ❖point sensor using rotating mirror, build up image as mirror scans ❖Landsat MSS, TM Pushbroom scanner: ❖mainly visible / NIR ❖array of sensing elements (line) simultaneously, build up line by line ❖SPOT (Satellite Pour l’Observation de la Terre) Classification 3. Based upon the EMR range: 1. Optical Sensors: ❖ Sensors that operate in the optical portion of spectrum, which extends from approximately 0.3 to 14 mm. ❖ Can do more with these data because \. ❖ look at differences in colors ❖ look at differences over time ❖ Applications: meteorological, ocean monitoring (i.e. chlorophyll absorption). Optical Sensors ❖ show how much energy from the sun was being reflected or emitted off the Earth's surface when the image was taken. ❖ Clear water reflects little radiation, so it looks black. ❖ Pavement and bare ground reflect a lot of radiation, so they look bright. ❖ Urban areas usually look light blue-grey. ❖ Vegetation absorbs visible light but reflects infrared, so it looks red 2. Microwave Sensors ❖ sensors that operate in the microwave portion of the spectrum ❖ advantages: capable of penetrating atmosphere under virtually all conditions, different view of the environment. ❖ disadvantage: Radar instruments have a hard time identifying water bodies because the wavelength is much longer than the general character of the surface roughness ❖ applications: sea ice and snow, geologic features, ocean bottom contours, other planets. Satellite orbits  Characteristics of Satellite Orbits 1. Orbital period 2. Altitude 3. Apogee and perigee 4. Inclination 5. Nadir, zenith and ground track 6. Swath 7. Side lap and overlap  Orbital Period Time taken by a satellite to complete one revolution around the earth Spatial and temporal coverage of the imagery depends on the orbital period It varies from around 100 minutes to 24 hours  Altitude The height of the satellite in a orbit from the point directly beneath it on the surface of the earth Low altitude ( altitude < 2000 km) Moderate altitude High altitude (altitude ~36000 km)  Apogee and Perigee Apogee: The point at which the satellite is at the farthest distance from the earth in an orbit is defined as the apogee. Perigee: The point at which the satellite is at the closest distance from the earth in an orbit is defined as the perigee.  Inclination Inclination of an orbit is its deviation from the equator measured in the clockwise direction. This is usually 99 degrees for remote sensing satellites.  Nadir, Zenith and Ground Track Nadir – It is the point on the earth’s surface where the imaginary radial line between the center of the earth and the satellite meet.  Nadir, Zenith and Ground Track Zenith: It is the point directly opposite to the nadir point above the satellite Ground Track: The projection of a satellites orbit on the earth’s surface.  Swath The width of the area on the earth’s surface which has been ‘sensed’ by a satellite during a single pass.  Overlap Areas of the earth’s surface which are common in two consecutive images along the flight path.  Sidelap Overlapping areas in the images of the earth’s surface on two adjacent flight paths. Sensors in Indian remote sensing satellites ❖ Linear Self-scanning Sensors (LISS) – for multi spectral scanning. This scanner has four generations – LISS-I, LISS-II, LISS-III, & LISS-IV. ❖ Panchromatic (PAN) sensors to collect data in single Band ❖ Wide Field Sensors (WiFS) – to collect data in a wide swath with two bands ❖ Advanced Wide Field Sensors (AWiFS) - to collect data in a wide swath with four bands ❖ Ocean Color Monitor (OCM) – Operating in 8 narrow spectral bands for oceanographic applications ❖ Multispectral Optoelectronic Scanner (MOS) – in 3 & 19 bands for oceanographic applications ❖ Multi-frequency Scanning Microwave Radiometer (MSMR) – for passive RS in 4 different frequencies ❖ Synthetic Aperture Radar (SAR) for active microwave RS  PAN LISS III WiFS IRS 1C Sensors overview  Sun Synchronous Orbits Earth observation satellites usually follow the sun synchronous orbits. A sun synchronous orbit is a near polar orbit whose altitude is such that the satellite will always pass over a location at a given latitude at the same local solar time. In this way, the same solar illumination condition (except for seasonal variation) can be achieved for the images of a given location taken by the satellite. POLAR ORBITING SATELLITES ❖ Polar-orbiting satellites are those in which the position of the satellite’s orbital plane is kept constant relative to the sun. ❖ Landsat satellite series, IRS Series  Geostationary Orbit (appr.36.000 km) ❖ Geostationary orbiting satellites are those that remain stationary relative to a point on the surface of the earth ❖ Communications and meteorological satellites Satellite Remote Sensors: TYPES OF ORBITS Sun-synchronous polar orbits ❖Circle the planet in a roughly north-south ellipse while the earth revolves beneath them - a particular place is imaged ❖Global coverage, fixed crossing, repeat sampling ❖Typical altitude 500-1,500 km Ex: Terra/Aqua, Landsat Non-Sun-synchronous orbits ❖Tropics, mid-latitudes, or high latitude coverage, varying sampling ❖Typical altitude 200-2,000 km Ex: TRMM, ICESat Geostationary orbits ❖Regional coverage, continuous sampling ❖Over low-middle latitudes, altitude 35,000 km Ex: GOES, INSAT Thank you Optical image of Montreal area during ice storm of 1998. Ice snow and clouds appear as various colors of white, vegetation is green.