Remote Sensing Physical Principles Lecture Notes PDF

I. Key Concepts of Remote Sensing II. Electromagnetic Radiation III. Energy-Matter Interactions in the Atmosphere the measurement or acquisition of some property of an object or phenomenon, by a recording device that is not in physical contact with the object or phenomenon under study Remote sens...

I. Key Concepts of Remote Sensing II. Electromagnetic Radiation III. Energy-Matter Interactions in the Atmosphere the measurement or acquisition of some property of an object or phenomenon, by a recording device that is not in physical contact with the object or phenomenon under study Remote sensing is both an art and a science of acquiring, processing, and interpreting images and related data that are obtained from ground-based, air- or space-borne instruments that record the interaction between matter (target) and electromagnetic radiation. COMPONENTS OF REMOTE SENSING 1. Energy Source 2. Radiation and the Atmosphere 3. Target 4. Recording of Energy by the Sensor 5. Processing 6. Analysis COMPONENTS OF REMOTE SENSING ENERGY SOURCE/ILLUMINATION - Provides electromagnetic energy to the target COMPONENTS OF REMOTE SENSING ENERGY SOURCE/ILLUMINATION Sensors can be classified as passive or active, based on the energy source they are using. COMPONENTS OF REMOTE SENSING ENERGY SOURCE/ILLUMINATION Sensors can be classified as passive or active, based on the energy source they are using. Passive – sensors, which sense natural radiations, either emitted or reflected from the Earth Active – sensor produce their own EMR  Active & Passive Systems Passive sensing – make use of Active sensing – sense artificially naturally available signals, such produced waves after they as thermal radiation of (a) interacted with the objects to be terrestrial objects, (b) sunlight, (c) sensed. Examples: RADAR, SONAR, starlight, (d) the cosmic LiDAR, background radiation. COMPONENTS OF REMOTE SENSING ELECTROMAGNETIC RADIATION (EMR) According to wave theory, EMR travels in the form of waves. Form of energy emitted produced by oscillating electric and magnetic disturbance, or by the movement of electrically charged particles traveling through a vacuum or matter.  ELECTROMAGNETIC Radiation Energy consisting of an electrical (E) and magnetic (M) fields perpendicular with each other that travels through atmosphere at the speed of light (c) and at a certain wavelength (λ). 14 ELECTROMAGNETIC RADIATION (EMR) WAVELENGTH )‫)ג‬ μm (micrometer) , 10-6 m – Length of one wave cycle OR The distance between successive crests  ELECTROMAGNETIC Radiation Frequency (f) Number of cycles of a wave passing a fixed point per unit of time. Unit: Hertz(Hz), one cycle per second. Changes as EMR propagates through media of different densities. 16  ELECTROMAGNETIC Radiation Amplitude (A) – - equivalent to the height of each peak. - often measured as energy levels (formally known as spectral irradiance), expressed as Characterised by: watts per square meter per Frequency (f) micrometer. Wavelength (λ) f = c / λ , where c=3x108 m/s 17  Electromagnetic SPECTRUM Continuum of energy that ranges from meters to nanometers in wavelength, travels at the speed of light, and propagates through a vacuum. - refers to the entire range of EMR, which encompasses all types of EMR based on their wavelengths or frequencies. 18 19  The VISIBLE Spectrum The spectral sensitivity of the human eye extends from 0.4 μm to approximately 0.7 μm. Lots of radiation around are “invisible” to our eyes but can be detected by other RS instruments and can be used to our advantage. 20  Visible Region in EMS Near IR 0.7-1.3 μm Thermal IR >3-14 μm Mid IR 1.3-3 μm (directly related to the sensation of heat) 21 EM SPECTRUM REGIONS Different regions provide discrete information about an object. Remote sensors detect specific spectrum wavelength and frequency ranges *Visible, Infrared, Microwave 22 23 Ultraviolet (UV) Short wavelengths (~0.3 - 0.4 μm) range between visible light and X-rays Used in atmospheric science applications Visible Light Only portion to be perceived as colors Shortest (violet) and longest (red) Useful in Land Cover Classification Infrared 100 times as wide as the visible portion For Military applications Divided into two categories based on their radiation properties Reflected IR (~0.7 - 3.0 μm) Emitted or Thermal IR (~>3.0 μm) Microwave Longest λ used in RS ranging from ~0.1 mm to 1 m Influenced by moisture content Studies of meteorology, hydrology, oceans, geology, agriculture, forestry, and ice, and for topographic mapping (RADAR) Radiometric Terms and Definition 1. Radiant Energy (Q): Energy carried by EMR. It causes the detector of the sensor to do work OR respond to EMR. (Joules) 2. Radiant Flux (Φ): Total power of radiation emitted by a source in a certain amount of time. (Joules/second OR Watt) 26 Radiometric Terms and Definition 3. Radiant flux density (E or M): Radiant flux that flows through unit area of a surface 4. Radiance (L): Radiant flux density per unit of projected source area, in a particular direction defined by a solid angle 27 Wavelength, Frequency and Energy (Q) Energy-frequency relationship Energy of EM directly proportional to frequency Energy determines interaction with matter 28  Electromagnetic Radiation BLACKBODY Radiation Blackbody – Hypothetical ideal radiator that totally absorbs and reemits all energy upon it. Blackbody Radiation Emitted by all matter having a temperature above absolute zero (0 K) The intensity and spectral composition of the radiation are a function of the material type involved and the surface temperature of the object under consideration (Planck’s Law) 29  Planck’s Law Emitted energy and spectrum (wavelength) by an object is a function of its temperature. 30 Planck’s Radiation Law 31 Electromagnetic Radiation BLACKBODY Radiation 32  What determines how much energy an object radiates? Stefan-Boltzmann Law Relationship between the total emitted radiance and temperature. W = σT4 W: Total emitted radiance (usually in Watts/m2) T: Absolute temperature (K) σ: Stefan-Boltzmann constant (5.6697 x 10-8 W m-2K-4) E.g.: T=288K → W=390 W m-2 T=5800K → W=64 million W m-2 33  Wien’s Displacement Law λ = A/T Where: λ = Wavelength with max radiance (μm) A = 2897.8 μm oK T = Absolute temperature (K) Wien’s law relates temperature to wavelength at which maximum energy is emitted Wavelength at which maximum energy is emitted is the ‘color’ of emitting object (brightness temperature) 34 Blackbody Curves 35 Emissivity Most objects are NOT blackbodies They emit less than the maximum amount of energy Emissivity (sometimes calls emittance) varies with wavelength emitted radiation at   = B (T ) The ratio between the emittance of a given object and that of the blackbody at the same temperature. 36 36 Emissivity of Various Materials 37  Electromagnetic Radiation: Black Body Radiation Water is a good approximation of a black body (greybody) *grey body has an emissivity less than 1 but is constant at all wavelengths. 38  Electromagnetic Radiation: Black Body Radiation Quartz is not a good approximation of a black body (selective radiator) Selective radiator: the emissivity of the object varies with wavelength. 39 Solar Radiation (shortwave) Passes through the atmosphere - reflected in varying degrees by Earth's surface and atmosphere Detectable only during daylight Sun's visible surface has temperature approx. 6000K 99% of sun's radiation fall between 0.2 - 5.6μm (UV to part of IR) 80% are within 0.4 - 1.5μm, max radiation occurs 0.48 μm About 50% of solar radiation passes through the atmosphere and is absorbed in varying degrees by surface 40 Terrestrial Radiation (longwave) Energy emitted from the earth and atmosphere, detectable both day and night Earth's ambient temperature is approx. 300K Essentially all energy is radiated at (invisible) thermal infrared wavelengths between 4-25μm Maximum emission occurs at 9.7μm 41 Solar and Terrestrial Radiation Solar radiation passes through Earth’s atmosphere and is reflected to varying degrees About half makes it to the surface, where it is absorbed and re-emitted continuously as Terrestrial Radiation. 42 COMPONENTS OF REMOTE SENSING INTERACTION WITH THE ATMOSPHERE As the energy travels from its source to the target, it will come in contact with and interact with the atmosphere it passes through.  COMPONENTS OF REMOTE SENSING INTERACTION WITH THE TARGET All EMR must travel through the atmosphere and along the way several things can happen to the radiation that alter it in some way (by redirection or change in energy level)  COMPONENTS OF REMOTE SENSING INTERACTION WITH THE TARGET Atmosphere affects: speed of radiation wavelength, intensity spectral distribution. Energy-Matter Interactions in the Atmosphere Energy enters the atmosphere and is selectively scattered or absorbed. EMR may also be diverted from its original direction due to refraction. Interactions include: Refraction Scattering Absorption Reflection Sun emits a range of energy centered on the visible bands 48  Refraction Bending of light when it passes from one medium to another Occurs because the media with different densities and the speed of EMR is different in each 49  Scattering Scattering process disperses radiation in all directions Important scattering agents include gaseous molecules, suspended particulates, clouds Differs from reflection in that the direction is unpredictable 51  Rayleigh Scattering Molecular scattering Occurs when particles have diameters small relative to wavelength (d < 0.1λ) Wavelength-dependent – favoring short wavelengths (blue and violet light) 52 Rayleigh Scattering the shorter wavelengths of light (blue and violet) are scattered much more than the longer wavelengths (red, orange) (4-5x) Causes blue sky & red/orange sunsets 53 Rayleigh Scattering Why is the sky red during sunrise/sunset? 54 Rayleigh Scattering At sunrise and sunset, sunlight travels through a greater thickness of the atmosphere, meaning the light must pass through more air and particles. By the time sunlight reaches the observer, most of the shorter wavelengths (blue, violet) have been scattered out of the line of sight. Why is the sky red during sunrise/sunset? 55 Mie Scattering Non-molecular scattering Not wavelength-dependent, affects all wavelength more equally Occurs when particles have diameter is on the order of the wavelength (0.1λ < d < 10λ) (or ~equal to the wavelength of incoming radiation) Amount of scattering is typically greater than Rayleigh and wavelengths affected are longer 59 Nonselective Scattering For particles significantly larger than wavelength (d > 10λ) Not wavelength dependent Manifest as whitish or grayish haze – all wavelengths scattered equally Occurs in lowermost portion of atmosphere 60 Absorption Process by which radiant energy is absorbed and converted into other forms of energy Takes place in the atmosphere or at the surface Occurs when atmospheric particles DO NOT ALLOW EMR to be fully transmitted 61 Absorption Absorption band – range of wavelengths (or frequencies) in EMS within which radiant energy is absorbed by substances Cumulative effect of absorption by various constituents can cause the atmosphere to close down completely in certain regions Other regions, atmosphere is transparent 62 63  Transmission Transmission of radiation occurs when radiation passes through a substance without significant attenuation. Ability of a medium to transmit energy is measure as the transmittance (t): t = Transmitted Radiation Incident Radiation Varies greatly with wavelength. 64 Energy-Matter Interactions in the Atmosphere 65  Atmospheric Windows An atmospheric window is a part of the spectrum which is transparent to EM radiation Windows are crucial for life and remote sensing as they allow us to see through the atmosphere Visible: 0.3 - 0.8μm IR: 8 - 12μm The idea that the atmosphere blocks out some wavelengths while allowing others to pass through is a key concept in remote sensing. We must design our satellite sensors to operate within these windows! 66 Atmospheric Windows 67 Atmospheric “Windows” in the Visible and IR Regions 68 Microwave region atmospheric window 69