Remote Sensing How we know what we know A Brief Tour

Transcription

1 Remote Sensing How we know what we know A Brief Tour Dr. Erik Richard Dr. Jerald Harder LASP Richard 1

2 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. Richard 2

3 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 Richard 3

4 Everyday example: Digital camera Richard 4

5 Functional Classes of Sensors Richard 5

6 Element of optical sensors characteristics Sensor Spectral Characteristics Spectral bandwidth (λ ) Resolution ( λ ) Out of band rejection Polarization sensitivity Scattered light Radiometric Characteristics Detection accuracy Signal to noise Dynamic range Quantization level Flat fielding Linearity of sensitivity Noise equivalent power Geometric Characteristics Field of view Instan. Field of view Spectral band registration Alignments MTF s Optical distortion Richard 6

7 Resolving Power Na spectral lines Na D-lines Instrument & Detector D1=589.6 nm D2=589.0 nm Richard 7

8 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: c = φρεθυενχψ? ωαϖελενγτη? E = η? φ? Ε= λ= ηχ λ χ φ c = 299, 792, 458 µ / σεχ η = ? ϑ?σεχ Visible Spectrum Wavelength (µm) Richard 8

9 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? Richard 9

10 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. Richard 10

11 Operational Classes of Sensors Richard 11

12 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) Richard 12

13 Scanning Types Richard 13

14 Multi or Hyper-spectral? Multidimensional data cube Spatial information Spectral information Full spectrum Hyperspectral Partial spectrum Multispectral Richard 14

15 EM derived information Richard 15

16 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. Richard 16

17 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. Richard 17

18 Earth s Albedo Albedo is defined as the reflectance using the incident light source from the Sun Richard 18

19 MODIS MODIS: MODerate-resolution Imaging Spectroradiometer NASA Terra & Aqua satellites Launched 1999, km polar orbits, descending (10:30 am) & ascending (1:30 pm) Sensor Characteristics 36 spectral bands ranging from 0.41 to 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 Richard 19

20 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 Richard 20

21 Blackbody Radiation Curves Richard 21

22 The Sun s spectrum UV Vis IR Radiometric definitions Irradiance : Radiant power incident per unit area upon a surface (W/m Spectral Irradiance : Irradiance per unit wavelength interval (W/m2/nm) Richard 22

23 The Sun s spectrum with Planck distributions at different temperatures UV Vis IR M. Planck Richard 23

24 Black body radiation Planck distributions Hot objects emit A LOT more radiation than cool objects QuickTimeﾪ and a YUV420 codec decompressor are needed to see this picture. I (W/m2) = σ x T4 The hotter the object, the shorter the peak wavelength T x λ max = constant Richard 24

25 Spectral Characteristics of Energy Sources and Sensing Systems Richard 25

26 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 formulaραδιαντ?ενεργψￊοφￊαν ￊοβϕεχτ Emissivity = Ραδιαντￊενεργψￊοφￊαￊβλαχκￊβοδψ ωιτηￊτηε ￊσαµ ε ￊτεµ περατυρε ￊασￊτηε ￊοβϕεχτ Richard 26

27 Atmospheric Absorption in the Wavelength Range from 1 to 15 µm Richard 27

28 Atmospheric Observation Modes Remote Sensing Space Science Teachers Summit Richard ￊￊ

29 Transmittance of the Atmosphere Transmission of solar radiation through the atmosphere is affected by Absorption Scattering The reduction of radiation intensity is called extinction (expressed as extinction coefficient, σext) Richard 29

30 Optical thickness The optical thickness of the atmosphere (τ t) is the integrated value σext with altitude τ t (l ) =?s ext dz 0 Total attenuation in a vertical path from the top of the atmosphere down to the surface Ι τ τ ( λ ) T = =ε Ιο Richard 30

31 < 2% RE Altitude (km) Atmospheric absorption of solar radiation ~99% penetrates to the troposphere stratosphere troposphere Altitude contour for attenuation by a factor of 1/e I(km) = 37% x Io Richard 31

32 Global Ozone Monitoring The Total Ozone Mapping Spectrometer (TOMS) samples backscatter UV at six wavelengths and provides a contiguous mapping of total column ozone. Richard 32

33 Composition of atmospheric transmission Richard 33

34 Atmospheric Scattering Factors influencing atmospheric transmittance Atmospheric molecules (size << λ) CO2, O3, N2, etc. Aerosols (size >λ) Water drops (fog & haze), smog, dust, etc. Richard 34

35 Scattering Rayleigh scattering Scattering by atmospheric molecules with size << λ Scattering coefficient σs 1 σs? 4 l 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) Richard 35

36 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) Richard 36

37 Radiometry con t Irradiance Radiant flux incident upon a surface per unit area (Wm-2) Radiant emittance Radiant flux radiated from a surface per unit area (Wm-2) Radiance Radiant intensity per unit projected area in a radial direction (Wm-2sr-1) Richard 37

38 Understanding the Earth s Energy Budget Solar radiation is the Earth s only incoming energy source. The balance between the Earth s incoming and outgoing energy controls daily weather as well as longterm weather patterns (i.e. climate). Since we are dealing only with electromagnetic radiation as a heat transfer mechanism, we can start by applying the basic laws of radiation physics to begin to understand the Earth-Sun system and the Earth s energy budget Richard 38

39 Radiation Balance Richard 39

40 Radiation Balance Richard 40

41 Radiation Balance Richard 41

42 Earth s Energy Balance Richard 42

43 So, just how bright is the Sun? If T = 5780 Sun s surface Then the Sun s emission from the photosphere is I Sun = σ?ξￊτ 4 ISun ~ 63,000,000 W/m2 (6.3 kw / cm2) What does this mean for Earth? Remote Sensing Space Science Teachers Summit Richard

44 Surface areaￊ=?4π Ρ12ΑΥ 2 Surface areaￊ=?4πρσυν 63 MW/m2 here rsun = 696, 000?κµ How much here? R1AU = 149, 600, 000?κµ I@ Earth? 1360?Ω / µ 2 Historically know as Earth s Solar Constant Richard 44

45 It is ridiculous to try to measure variations in a constant - Dove & Maury (ca. 1890) famous oceanographers Richard 45

46 SORCE Solar Radiation and Climate Experiment A Mission of Solar Irradiance for Climate Research Launched January 25, 2003 Daily measurements of Total Solar Irradiance (TSI) Solar Spectral Irradiance (SSI) 0.1 nm-27nm & nm Remote Sensing Space Science Teachers Summit Richard

47 Total Irradiance Monitor (TIM) Four Radiometers TIM Instrument Detector Head Board Heat Sink Vacuum Door Shutter Precision Aperture Light Baffles Radiometer (Cone) Vacuum Shell Richard 47

48 1360 W/m2 QuickTimeﾪ and a YUV420 codec decompressor are needed to see this picture. Richard 48

49 30 year TSI record from space The constant variable Richard 49

50 Solar Cycle 0.1% = 1.4 W/m2 T of ~1.5 C on Sun Richard 50

51 Clouds and the Earth s Radiant Energy System (CERES) NASA, TRMM, Terra & Aqua launches 1997, 1999, km orbit (35 inclination), 705 km polar orbits, descending (10:30 a.m.) & ascending (1:30 p.m.) Sensor Characteristics 3 spectral bands» Shortwave ( µm)» Window (8-12 µm)» Total (0.3->200 µm) Spatial resolution:» 20 km ±78 cross-track scan and 360 azimuth biaxial scan 0.5% calibration accuracy onboard blackbodies & solar diffuser CERES Swath Movie Richard 51

52 CERES Results Longwave (thermal) radiation Longwave (thermal) & simultaneous Shortwave (reflecte Richard 52

53 If the Sun had no magnetic field it would be as boring as most astronomers seem to believe it is - R. Leighton Astrophysicist, CalTech Richard 53

54 The Sun s magnetism is ultimately responsible for all manifestations of solar activity Sunspots CME s Flares Erupting prominences Coronal loops Richard 54

55 The Sun s spectrum UV Vis IR Richard 55

56 Magnetic Fields and Sunspots P. Zeeman G. E. Hale λ G.E. Hale, June 1908 Richard 56

57 The formation of sunspots Animation Hale provided the first proof that sunspots are the seats of strong magnetic fields QuickTimeﾪ and a YUV420 codec decompressor are needed to see this picture. TRACE image Richard 57

58 The Sun s Magnetic Cycle Hale s polarity Law (1919) Well-organized large scale magnetic field Changes polarity approximately every 11 years (22 year magnetic cycle) N S S N t=0 t = 3 yrs t = 9 yrs t = 11 yrs Richard 58

59 Seeing the Sun s magnetic fields QuickTimeﾪ and a YUV420 codec decompressor are needed to see this picture. SOHO MDI Magnetograms Richard 59