What is Remote Sensing (RS)?

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1 GMAT x600 Earth Observation / Remote Sensing Topic 2: Electromagnetic Radiation A/Prof Linlin Ge l.ge@unsw.edu.au What is Remote Sensing (RS)? Remote Sensing is a technology for sampling electromagnetic radiation to acquire and interpret non-immediate geospatial data from which to extract information about features, objects, and classes on the Earth's land surface, oceans, and atmosphere (and, where applicable, on the exteriors of other bodies in the solar system, or, in the broadest framework, celestial bodies such as stars and galaxies). Outline Basic radiation quantities Wave and quantum properties Polarization Coherent and monochromatic radiation Doppler effects Blackbody radiation Solar radiant energy Definition of Basic Radiation Quantities Electromagnetic Radiation E.M. Radiation Quantity Radiant Energy Radiant Flux Incident Flux Reflected Flux Symbol Q Φ Φ i Φ i Unit Joule, J Watt, W (J/s) Watt, W Watt, W Quantity Irradiance Exitance Radiant Intensity Radiance Symbol E M I L Unit Watt / metre 2, W/m 2 Watt/metre 2 W/m 2 Watt/Steradian W/SR Watt/Steradian/ metre 2 W/(SR * m 2 ) Most important medium for remote sensing Only form of energy transfer that can take place through free space, with velocity C = 3 * 0 8 m/s Source to sensor Directly Indirectly : Reflection or Re-radiation > 0 0 K (-273( 0 C) Gamma rays (Yellow book P23) X-rays Ultra-violet Visible (blue, green, red) Infrared (near-,, and thermal-infrared) Microwave (X-,, C-, C, S-, S, L-, L, and P-band) P Radio

2 Nature of Radiation Energy Ability to do work. Exist in variety of forms: chemical, electrical, heat, mechanical, etc. In the course of work being done energy transferred by: Conduction atomic or molecular collisions Transfer by contact, RS? Convection corpuscular mode of transfer. Molecules physically moved. Radiation only form of energy transmitted without intervening medium. Primary concern in Remote Sensing Generation of Radiation Either: Electron: Atomic transition - Short wave: infrared, visible, ultraviolet. Molecule: Vibrational or Rotational - Electron orbital changes -> > shortest wavelengths - Vibrational -> > short and intermediate infrared - Rotational -> > long wavelength Infrared and short wavelength microwave Wave Model of E.M.R Electro-magnetic Radiation (E.M.R) conceived of as wave motion due to regular variations in both electric and magnetic fields surrounding a charged particle. Two force fields, electrical and magnetic, inseparable. If E.M. waves intercepted by matter, result will depend on both magnetic and electric properties of the matter. E.M.R cont. E and B are mutually orthogonal and perpendicular to direction of wave advancement. E.M.R cont. Wave Wave propagate through empty space (i.e., vacuum) C = λ 0 ƒ where C = 3 x 0 8 m/s λ 0 = wavelength ; ƒ = frequency Waves Waves propagate through a material V = λƒ where ƒ does not change λ changes with V V = C / n, n refractive index Note: Maxwell s s equations govern all propagation of E.M. waves. Describe time and space inter-relationship relationship of electric and magnetic fields. Complex Waves Principle of superposition: amplitude of combined waves sum of separate waves Complex wave made up of sinusoidal components - spectral components. 2

$(explained by wave theory) Diffraction Effects Diffraction MUST always occur when wave cut off by sensor aperture. One limiting factor to measurement of E.M.R.$

3 Complex Waves cont. Change in direction of some radiation due to propagation of wave past edge. (explained by wave theory) Diffraction Effects Diffraction MUST always occur when wave cut off by sensor aperture. One limiting factor to measurement of E.M.R. Particle or Quantum Properties of E.M.R. Wave theory fails to account for certain significant phenomena. Particle model treats radiation as comprised of discrete packages. Quanta Photon Short wave trains or bursts of energy, smallest package of energy possible. Q = h*f = h * c / λ where h is Planck s s constant = x 0-34 J. sec X-ray vs radar/radio Quantum Properties cont. Energy delivered on a probabilistic basis. Probability of full delivery proportional to flux density at that place. Flux Density time rate with which radiation passes a spatial position Large number of photons time rate of energy delivered, as wave theory predicts. Instant to instant fluctuating at receiver. Limit to precision of measurements photon /noise limited. Wave Particle Duality Photon : Quantised, statistical properties of radiation. Wave : Overall average effects. Atomic particles have wave properties. Light waves have particle properties. Polarization Direction of electric field conventionally used to define direction of wave polarization. Plane, Plane, circular and elliptical polarizations. Random Random polarization particularly in visible spectrum. Note: (both Plane and circular polarizations are special cases of elliptical polarization) 3

Polarization - Plane Polarization - Plane y x z Transverse waves oscillating (a) in the z-direction, and (b) at an angle in the y-z plane Phase difference = 0 and Ey = Ez Image copyright:

4 Polarization - Plane Polarization - Plane y x z Transverse waves oscillating (a) in the z-direction, and (b) at an angle in the y-z plane Phase difference = 0 and Ey = Ez Image copyright: David M. Harrison, Dept. of Physics, Univ. of Toronto Polarization - Circular Phase difference = 90 o Ez = Ey Image copyright: Left-handed and right-handed Polarization - Elliptical Phase difference = 90 o Ez Ey Left-handed and right-handed Image copyright: Polarization - Summary Plane polarized radiation incident on surface will reflect in different amounts depending upon the direction of polarization relative to the surface (depolarize). Horizontal and vertical polarizations are used in microwave radar (HH, VV, HV, VH). Parallel and perpendicular polarizations used by optics allow prediction of effect of any other direction of polarization. 4

EMR & polarisations Longitudinal: sound waves Transverse: EMR Plane Circular Elliptical Random/unpolarised Week 3 Week 3 Coherent Radiation Coherent if there is a regular or systematic relationship

5 EMR & polarisations Longitudinal: sound waves Transverse: EMR Plane Circular Elliptical Random/unpolarised Week 3 Week 3 Coherent Radiation Coherent if there is a regular or systematic relationship between amplitudes (i.e. highly correlated). The coherent waves can be in phase or out of phase. Receiving detector will indicate more power at some locations and less power at others. In Phase vs. Out of Phase Two waves are in phase Two waves are 80 o out of phase Incoherent Radiation Amplitudes related in random fashion. Power of combined radiation is sum of each separately. Diffusely reflected solar radiation is incoherent measured flux received at a sensor from a heterogeneous surface will be simple sum due to each point surface. Detector will indicate average power at any position. 5

Monochromatic Radiation Microwave Radar and Lasers Reflected radiation can be highly coherent No power to 4 x average power, depending on sensor position relative to the objects.

6 Monochromatic Radiation Microwave Radar and Lasers Reflected radiation can be highly coherent No power to 4 x average power, depending on sensor position relative to the objects. Characteristic speckle pattern. Pattern, function of geometric arrangement of point source reflectors, not generally resolvable. Revealed through appropriate analysis. Doppler Effect Alteration of E.M.R. frequency due to relative motion between observer and source. - red shift?? Doppler Effect You hear the high pitch of the siren of the approaching ambulance, and notice that its pitch drops suddenly as the ambulance passes you. That is called the Doppler effect. Visualise Doppler Effect: Hear Doppler Effect from F! (mp3) More details will be discussed together with SAR later on in this course. Doppler Effect cont. When relative velocity is much less than C then change in frequency, Δƒ,, is given by Δƒ = ƒ u cosө / c where u = relative velocity between source and receiver Ө = angle between direction of motion of source and a line connecting source and observer Airborne or spaceborne microwave radar has twice Doppler shift (source and detector re-radiation). radiation). Applications of Doppler Effects In astronomy: to predict the rate of expansion of the universe (u); Positioning: to determine the location of a point on the Earth by the change in frequency of an emitted signal from an overpassing satellite (theta); and SAR: to improve the ground resolution of imaging radar (more( in GMAT9606 Microwave Remote Sensing, 7-7 Sept 2009). 6

Emission of Radiation Blackbody Radiation Atmospheric windows Emission of Radiation All bodies with temperatures above absolute zero generate or emit energy in radiant form.

(Lillesand and Kiefer, 2000, Remote sensing and image interpretation.) Temperature scales Celsius ( 0 C): named after A. Celsius, a Swedish astronomer; Kelvin( 0 K): named after W.

7 Emission of Radiation Blackbody Radiation Atmospheric windows Emission of Radiation All bodies with temperatures above absolute zero generate or emit energy in radiant form. Blackbody is a perfect radiator and absorber. Real material cannot emit thermally at a rate in excess of a blackbody. Blackbody absorbs and converts all incident radiant energy into heat energy. (Lillesand and Kiefer, 2000, Remote sensing and image interpretation.) Temperature scales Celsius ( 0 C): named after A. Celsius, a Swedish astronomer; Kelvin( 0 K): named after W. Kelvin, a British physicist; Celsius ( 0 C) Kelvin( 0 K) Absolute Ice & zero water Room temperature Boiling point Sun Four Laws of Concern Stefan-Boltzman Boltzman s Law Planck s s Law Wien s Displacement Law Yellow book P27 Kirchhoff s Law - Yellow book P30 Stefan-Boltzman Boltzman s Law The total emitted radiance, M or Exitance,, in Watts per square metre (W/m 2 ) is proportional to the fourth power of its absolute temperature in degrees Kelvin. (for entire wavelength spectrum) M = Power/Area = σ T Where σ = x 0-8 W/m 2 / o K 4 4 7

8 Planck s s Law It gives the intensity radiated by a blackbody at a temperature T as a function of wavelength or frequency. Where M 2 2πc h 5 λ e / λ λ = hc kt C is the speed of light = 3 x 0 8 m/s T is the absolute temperature in degrees Kelvin h is Planck s constant = x 0-34 J.s k is Boltzmann s constant =.3807 x 0-23 J/K M λ is spectral exitance at wavelength λ, Watts/ m 3 Planck s s Law cont. The equation can be simplified to: M C λ = 5 C2 / λt λ e Where C = x 0-6 W m 2 C 2 = m o K Planck s s Law cont. Spectral exitance of a blackbody at a given temperature is not the same at all wavelengths. For every long and very short wavelengths, M λ is low. Red hot >> White hot Example Radiation from the Sun Example Red Hot Object Wien s Displacement Law When the temperature of a blackbody increases, the overall radiated energy increases and the peak of the radiation curve moves to shorter wavelengths (why?). The wavelength, λ M, for which spectral exitance is a maximum can be calculated by Wien s Displacement Law. 8

9 Wien s Displacement Law cont. λ Max = C / T ; where C = x 0-3 m o K Stefan- Boltzman s Law 4 M = Power/Area = σ T Integration Relationships M λ Planck s Law: M λ =f(t, λ) C = 5 C2 / λt λ e Differentiation Wien s Displacement Law λmax = C / T Emissivity No real body is a perfect emitter, its exitance is less than a blackbody. The spectral emissivity, ε,, is defined as: ε λ M λ (material) = M (blackbody) λ ε nearly independent of temperature for most common materials and for terrestrial environment temperatures. Significant changes when material undergoes a change of state. Emissivity cont. Kirchhoff s Law States that under conditions of thermal equilibrium, the spectral emissivity of a material must be equal to the spectral absorptance of that material (why?). Good approximation even when thermal equilibrium not present but temperature differences not extreme. Common practice to determine ε by measuring spectral absorptance. Kirchhoff s Law cont. Ideal Radiator Perfect Absorber Object with zero exitance perfect reflector or so- called White Body This relationship can only be applied in portion of spectrum where materials are opaque. Frequently occurs in thermal portion of spectrum. 9

(0%) (photosynthesis) 30% (40%) Green 0.

then p + α = or α = p α ε Radiation Summary In the shorter wavelength

10 Kirchhoff s Law cont. Leaf vs Soil Leaf Soil Reflectance Absorptance Transmittance 20% (50%) 50% (0%) (photosynthesis) 30% (40%) Green 0.5µm (near infrared µm) 25% (35%) 75% () 0% () Kirchhoff s Law cont. If surface opaque, T = 0, then p + α = or α = p α ε Radiation Summary In the shorter wavelength region where solar energy predominates transmission of radiation must be considered. At room temperature (~300 o K) spectral radiant exitance has a peak wavelength of ~0 μm m between 7 and 5 micrometres,, so called Thermal Infrared Region. Radiation in this band can be interpreted as temperature. 0

11 Radiant Energy From The Sun Hubble Telescope? Radiant Energy From The Sun cont. Emits radiation approximately as a blackbody at 5900 o K. Wavelength of maximum emission intensity is given by Wien s Displacement Law as ~ 0.5 micrometres. Absorption by the sun s s atmosphere and other effects means that radiation at top of earth s atmosphere not exactly equivalent to blackbody. NASA's Hubble Space Telescope Radiant Energy From The Sun cont. Radiant Energy From The Sun cont. Solar irradiance at the top of the atmosphere Values derived for spectral irradiance at the top of the atmosphere assume the mean distance of the earth from the sun. (at aphelion earth/sun distance ~.034 distance at perihelion) By simple geometry represents an increase in solar irradiance between aphelion and perihelion of 6.8% or change from mean of 3.4%. Spectral solar irradiance at the top of the atmosphere, July st (aphelion) through January st (perihelion) [W/m 3 ]. Band July st Aug st Sept st Oct st Nov st Dec st Jan st July st Aug st Sept st Oct st Nov st Dec st Jan st Band 4 Band 5 Band 6 Band 7 (Average Oct values derived from P.N. Slater Remote Sensing )

Landsat MSS bands Summary Band 4 5 Wavelength (µm)( 0.5-0.6 0.6 0.6-0.7 0.

12 Landsat MSS bands Summary Band 4 5 Wavelength (µm)( Coherent and monochromatic radiation Doppler effects Blackbody radiation Solar radiant energy

Dr. Linlin Ge The University of New South Wales

Dr. Linlin Ge The University of New South Wales GMAT 9600 Principles of Remote Sensing Week2 Electromagnetic Radiation: Definition & Physics Dr. Linlin Ge www.gmat.unsw.edu.au/linlinge Basic radiation quantities Outline Wave and quantum properties Polarization