Remote Sensing “How we know what we know” A Brief Tour Dr. Erik Richard Dr. Jerald Harder LASP Remote Sensing – Space Science Teachers Summit 2008 Richard 1 Remote Sensing • The measurement of physical variables (usually light or sound) from outside of a medium to infer properties (other physical variables) of the medium. • Electro-magnetic radiation which is reflected or emitted from (or absorbed by) an object is the usual source of remote sensing data. However any media such as gravity or magnetic fields can be utilized in remote sensing. Remote Sensing – Space Science Teachers Summit 2008 Richard 2 Measurement Fundamentals • Key Instrument Components – Sensing device, or sensor – Transducer • Translates a sensed quantity (i.e. photons, acoustic waves, etc.) into a measurable quantity (e.g. voltage, current, displacement etc.) – Readout device Remote Sensing – Space Science Teachers Summit 2008 Richard 3 Everyday example: Digital camera Remote Sensing – Space Science Teachers Summit 2008 Richard 4 Functional Classes of Sensors Remote Sensing – Space Science Teachers Summit 2008 Richard 5 Element of optical sensors characteristics Sensor Spectral Characteristics Radiometric Characteristics Geometric Characteristics Detection accuracy Field of view Spectral bandwidth (λ ) Signal to noise Instan. Field of view Resolution (∆ λ ) Dynamic range Spectral band registration Out of band rejection Quantization level Alignments Polarization sensitivity Flat fielding MTF’s Scattered light Linearity of sensitivity Optical distortion Noise equivalent power Remote Sensing – Space Science Teachers Summit 2008 Richard 6 Resolving Power Na spectral lines Na D-lines D1=589.6 nm Instrument & Detector D2=589.0 nm Remote Sensing – Space Science Teachers Summit 2008 Richard 7 Schematic Wave of Radiation Electromagnetic (EM) energy at a particular wavelength l (in vacuum) has an associated frequency f and photon energy E. Thus, the EM spectrum may be expressed equally well in terms of any of these three quantities: χ c = φρεθυενχψ? ωαϖελενγτη ? λ = φ c = 299,792,458 µ / σεχ −34 ηχ η = 6.626069 ? 10 ϑ?σεχ E = η φ Ε = ? ? λ Visible Spectrum 0.4 0.5 0.6 0.7 Wavelength (µm) Remote Sensing – Space Science Teachers Summit 2008 Richard 8 The electromagnetic spectrum • Remote sensing uses the radiant energy that is reflected and emitted from Earth at various “wavelengths” of the electromagnetic spectrum • Our eyes are only sensitive to the “visible light” portion of the EM spectrum • Why do we use nonvisible wavelengths? Remote Sensing – Space Science Teachers Summit 2008 Richard 9 Passive or Active? • Passive sensor – energy leading to radiation received comes from an external source • e.g., direct Sun, reflected Sun, thermal emission etc. • Active sensor – Energy generated from within the sensor system, beamed outward, and the fraction returned is measured. • e.g. laser LIDAR, microwaves, RADAR, SONAR, etc. Remote Sensing – Space Science Teachers Summit 2008 Richard 10 Operational Classes of Sensors Remote Sensing – Space Science Teachers Summit 2008 Richard 11 Scanning or Non-scanning? • Scanning mode – Motion across the scene over a time interval (think of your video recorder) • Non-scanning – Holding the sensor fixed on the scene or target of interest as it is sensed in a brief moment (think of your digital camera) Remote Sensing – Space Science Teachers Summit 2008 Richard 12 Scanning Types Remote Sensing – Space Science Teachers Summit 2008 Richard 13 Multi or Hyper-spectral? • Multidimensional data “cube” – Spatial information – Spectral information • Full spectrum – Hyperspectral • Partial spectrum – Multispectral Remote Sensing – Space Science Teachers Summit 2008 Richard 14 EM derived information Remote Sensing – Space Science Teachers Summit 2008 Richard 15 Spectral Reflectance • Spectral reflectance is assumed to be different with respect to the type of land cover. This is the principle that in many cases allows the identification of land covers with remote sensing by observing the spectral reflectance (or spectral radiance) from a distance far removed from the surface. Remote Sensing – Space Science Teachers Summit 2008 Richard 16 Spectral Reflectance • Shown below are three curves of spectral reflectance for typical land covers; vegetation, soil and water. As seen in the figure, vegetation has a very high reflectance in the near infrared region, though there are three low minima due to absorption. Soil has rather higher values for almost all spectral regions. Water has almost no reflectance in the infrared region. Remote Sensing – Space Science Teachers Summit 2008 Richard 17 Earth’s Albedo •Albedo is defined as the reflectance using the incident light source from the Sun Remote Sensing – Space Science Teachers Summit 2008 Richard 18 MODIS • MODIS: MODerate-resolution Imaging Spectroradiometer • NASA Terra & Aqua satellites – Launched 1999, 2002 – 705 km polar orbits, descending (10:30 am) & ascending (1:30 pm) • Sensor Characteristics – 36 spectral bands ranging from 0.41 to 14.385 µm – Cross-track scan mirror with 2330 km swath width – Spatial resolutions • 250 m (bands 1-2) • 500 m (bands 3-7) • 1000 m (bands 8-36) – 2% reflectance calibration accuracy movie Remote Sensing – Space Science Teachers Summit 2008 Richard 19 Black Body Radiation • An object radiates unique spectral radiant flux depending on the temperature and emissivity of the object. This radiation is called thermal radiation because it mainly depends on temperature. Thermal radiation can be expressed in terms of black body theory. • Black body radiation is defined as thermal radiation of a black body, and can be given by Planck's law as a function of temperature T and wavelength Remote Sensing – Space Science Teachers Summit 2008 Richard 20 Blackbody Radiation Curves Remote Sensing – Space Science Teachers Summit 2008 Richard 21 The Sun’s spectrum UV Vis IR Radiometric definitions Irradiance : Radiant power incident per unit area upon a surface (W/m2) Spectral Irradiance : Irradiance per unit wavelength interval (W/m2/nm) Remote Sensing – Space Science Teachers Summit 2008 Richard 22 The Sun’s spectrum with Planck distributions at different temperatures UV Vis IR M. Planck Remote Sensing – Space Science Teachers Summit 2008 Richard 23 Black body radiation • Planck distributions Hot objects emit A LOT more radiation than cool objects QuickTimeﾪ and a 2 4 YUV420 codec decompressor I (W/m ) = σ x T are needed to see this picture. The hotter the object, the shorter the peak wavelength λ T x max = constant Remote Sensing – Space Science Teachers Summit 2008 Richard 24 Spectral Characteristics of Energy Sources and Sensing Systems Remote Sensing – Space Science Teachers Summit 2008 Richard 25 Emissivity • In remote sensing, a correction for emissivity should be made because normal observed objects are not black bodies. Emissivity can be defined by the following formula- Ραδιαντ?ενεργψￊοφￊανￊοβϕεχτ Emissivity= Ραδιαντￊενεργψￊοφￊαￊβλαχκￊβοδψ ωιτηￊτηεￊσαµ εￊτεµ περατυρεￊασￊτηεￊοβϕεχτ Remote Sensing – Space Science Teachers Summit 2008 Richard 26 Atmospheric Absorption in the Wavelength Range from 1 to 15 µm Remote Sensing – Space Science Teachers Summit 2008 Richard 27 Atmospheric Observation Modes Remote Sensing – Space Science Teachers Summit Richard ￊￊ Transmittance of the Atmosphere • Transmission of solar radiation through the atmosphere is affected by – Absorption – Scattering • The reduction of radiation intensity is called σ extinction (expressed as extinction coefficient, ext) Remote Sensing – Space Science Teachers Summit 2008 Richard 29 Optical thickness τ • The optical thickness of the atmosphere ( t) σ is the integrated value ext with altitude τ (l ) = s dz t ? ext 0 Total attenuation in a vertical path from the top of the atmosphere down to the surface Ι −τ (λ) T = = ε τ Ιο Remote Sensing – Space Science Teachers Summit 2008 Richard 30 Atmospheric absorption of solar radiation ~99% penetrates ) m to the troposphere k ( e d u t i t < 2% R l E A stratosphere troposphere Altitude “contour” for attenuation by a factor of 1/e I(km) = 37% x Io Remote Sensing – Space Science Teachers Summit 2008 Richard 31 Global Ozone Monitoring • The Total Ozone Mapping Spectrometer (TOMS) samples backscatter UV at six wavelengths and provides a contiguous mapping of total column ozone. Remote Sensing – Space Science Teachers Summit 2008 Richard 32 Composition of atmospheric transmission Remote Sensing – Space Science Teachers Summit 2008 Richard 33 Atmospheric Scattering • Factors influencing atmospheric transmittance – Atmospheric molecules (size << λ) • CO2, O3, N2, etc. – Aerosols (size >λ) • Water drops (fog & haze), smog, dust, etc. Remote Sensing – Space Science Teachers Summit 2008 Richard 34 Scattering • Rayleigh scattering – Scattering by atmospheric molecules with size << λ σ – Scattering coefficient s 1 σ ? s l 4 The strong wavelength dependence of the scattering (~λ-4) means that blue light is scattered much more than red light. Scattering by aerosols with larger size than the wavelength is called Mie scattering (think of a movie projector with dust) Remote Sensing – Space Science Teachers Summit 2008 Richard 35 Radiometry • Radiant energy – Energy carried by EM radiation (J) • Radiant flux – Radiant energy transmitted per unit time (W) • Radiant intensity – Radiant flux from a point source per unit solid angle in a radial direction (W sr-1) Remote Sensing – Space Science Teachers Summit 2008 Richard 36 Radiometry con’t • Irradiance – Radiant flux incident upon a surface