1. Summer Session 14 July 2011

2. Interaction of EM Radiation with the Atmosphere

3. Key components of VIS/NIR remote sensing Constituents of the atmosphere that will interact with EM radiation: Gases Water – –Water vapor –Water droplets –Ice particles Particulate matter – smoke, dust, other particles VIS/NIR Satellite EM energy ATMOSPHEREATMOSPHERE

4. 90 km

5. Atmospheric Gases Nitrogen – N 2 – 78% Oxygen – O 2 – 21% Argon – Ar – 1% H 2 0 – 0 to 7% Atmospheric trace gases (less than 0.1% each) Carbon dioxide - CO 2, Ozone – O 3, Methane – CH 4, Carbon Monoxide – CO, Nitrous Oxide – N 2 O, Chlorofluorocarbons (CFCs), and many others Primarily Absorb and Scatter EM Radiation. *water can reflect as well.

6. Water in the atmosphere Water is present in a variety of forms in the atmosphere Gas/vapor, droplets, ice crystals Its form determines the manner in which it reacts with EM radiation

7. Particulate Matter Inorganic and organic particles are suspended in the atmosphere from a variety of sources Dust storms, pollution, fires, volcanic eruptions These particles interact with EM energy From a recent Science article (authored by two UMD Geographers, Drs. Kaicun Wang and Shunlin Liang): “Visibility in the clear sky is reduced by the presence of aerosols, whose types and concentrations have a large impact on the amount of solar radiation that reaches Earth's surface... Visibility has increased over Europe, consistent with reported European ‘brightening,’ but has decreased substantially over south and east Asia, South America, Australia, and Africa, resulting in net global dimming over land (Wang, Dickinson, and Liang 2009, p. 1468).” Aerosols can interrupt the passage of light energy through the atmosphere to Earth.

8. Dust cloud south of Iceland Observed by MODIS

9. Smoke plume over Eastern US observed by MODIS in July 2002 from Forest Fires (red dots) in Quebec

10. Landsat Image of Mt. Pinatubo Eruption

11. Why is atmosphere important in RS of land and ocean surfaces? The constituents of the atmosphere are highly variable both spatially and temporally. These constituents interact with EM energy. Performing quantitative analyses of satellite remote sensing imagery requires an understanding of atmospheric effects. Sophisticated computer models have been developed to quantify the effects of the atmosphere and to normalize remote sensing data for its effects.

12. What do gases and particles in the atmosphere do to EM radiation? FIVE THINGS: 1. Refract 2. Reflect 3. Absorb 4. Scatter 5. Transmit Important!

13. Basic EM energy/matter interactions Incident EM Radiation Refraction Reflection Scattering Transmitting Absorption Earth surface

14. Index of refraction - n n = c / c n where c is the speed of light in a vacuum, and c n is the speed of light within a substance such as water or air n of water is 1.33 n of air is 1.000296 **n, on Earth, will always be greater than 1, because light never travels as fast as it does in a vacuum.

15. c, n v c a, n a vv aa sun ATMOSPHEREATMOSPHERE v = in a vacuum a = in the atmosphere  = angle

16. Multiple changes in direction as the light passes through portions of the atmosphere which vary in optical density.

17. Reflection – the process by which incoming EM radiation is reflected of the surface of an object Incoming Radiation Outgoing Radiation

18. Absorption The process by which EM radiant energy is absorbed by a molecule or particle and converted to another form of energy

20. Scattering The process whereby EM radiation is absorbed and immediately re-emitted by a particle or molecule – energy can be emitted in multiple-directions Incoming EM energy Scattered energy Note: No EM energy is lost during scattering

21. Types of Scattering Rayleigh scattering Mie scattering Non-selective scattering The type of scattering is controlled by the size of the wavelength relative to the size of the particle Scattering is more important for short- wave radiation than long-wave radiation

22. Rayleigh Scattering Occurs when the wavelength is MUCH LARGER than the particle size

23. Rayleigh scattering ~ 1 / 4 Blue light is scattered 5 times as much as red light UV radiation is not scattered by the upper atmosphere because it is absorbed by the OZONE Layer

24. 90 km Most Rayleigh scattering occurs in the top 10 km of the stratosphere, e.g., at the ozone layer

25. Summary of Rayleigh Scattering Occurs at the molecular level The degree of Rayleigh scattering is inversely proportional to the fourth power of the EM wavelength Most Rayleigh scattering occurs in the upper 10 km of the stratosphere

26. Mie Scattering Occurs when the wavelength  particle size

27. Mie Scattering Occurs with particles that are actually 0.1 to 10 times the size of the wavelength Primary Mie scatterers are dust particles, soot from smoke Mie scatterers are found lower in the Troposphere

28. For further discussion of this slide, see http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c5

29. Non-Selective Scattering Occurs when the wavelength is MUCH SMALLER than the particle size

30. Non-Selective Scattering Its name derives from the fact that all wavelengths (visible/near IR) are equally affected Particles are very large, typically water droplets and ice crystals of fog banks and clouds Particles are 10 times the size of the wavelength, > 20 um in size When all wavelengths are scattered or reflected equally, you get pure white light (clouds)!

31. sun Reflected Refracted Scattered Absorbed Transmitted

32. Atmospheric Extinction Extinction is a term used to account for the loss or attenuation* of radiant energy as light passes through the atmosphere, and includes both scattering and absorption The amount of atmospheric transmittance depends on the amount of extinction *sunglasses are attenuators  they lessen the intensity of visible and UV light.

33. Atmospheric Extinction I o - the unattenuated light intensity L - the path length through the atmosphere I - attenuated light intensity

34. Extinction Coefficient -   = b m + b p + k where b m is the Rayleigh or molecular scattering coefficient b p is the Mie scattering coefficient (due to the airborne particles) k is the absorption coefficient

35. Atmospheric windows

37. Figure 1-18 from Elachi, C., Introduction to the Physics and Techniques of Remote Sensing, 413 pp., John Wiley & Sons, New York, 1987. Transmission = 100% – absorption (in the context of the atmosphere)

38. Interaction of EM Radiation with the surface

39. Key components of VIS/NIR remote sensing 1. Sun is EM Energy Source 2. Energy emitted from sun described by Stephan/Boltzman Law, Planck’s formula, and Wien’s Displacement Law 3. EM Energy interacts with the atmosphere 4. EM energy reflected from Earth’s Surface VIS/NIR Satellite EM energy 5. EM Energy interacts with the atmosphere

40. Radiation Budget Equation – Earth’s Surface Three things can happen to incident EM energy [  i ] when it interacts with a feature 1. Reflected 2. Absorbed 3. Transmitted ii The degree to which EM energy is reflected, transmitted, and absorbed is dependent on the wavelength of the EM energy

41. Radiant Flux -  The fundamental unit to measure electromagnetic radiation is radiant flux -   is defined as the amount of energy that passes into, through, or off of a surface per unit time Into = absorbed Through = transmitted Off of = reflected Radiant flux (  ) is measured in Watts (W)

42. Radiation Budget Equation  i = R + A + T R is the amount of energy reflected from the surface A is the amount of energy absorbed by the surface T is the amount of energy transmitted through the surface  i is the incident radiation (radiant flux) for a given wavelength

43. Radiant Flux Density  Radiant flux density is simply the amount of flux per unit area Radiant flux density =  /area

44. Irradiance versus Exitance Irradiance (E) is the amount of incident radiant flux per unit area of a plane surface in Watts per square meter (W m –2 ) Exitance (M) is the amount of radiant flux per unit area leaving a plane surface in Watts per square meter (W m –2 ) They both incorporate radiant flux per unit area, but their directionality is what distinguishes them.

45. Hemispherical reflection, absorption, transmission Hemispherical reflection, absorption, and transmission refer to what happens to all energy that comes in If it is not absorbed, it can be reflected or transmitted in any direction into a hemisphere  energy is conserved, not lost!

46. Hemispherical reflectance (r ), absorptance (  ), and transmittance (  ) r = R /  i  = A /  i  = T /  i A ratio of radiant flux reflected, transmitted, or absorbed from the surface to the radiant flux incident to it. r +  +  =1 recall that:  i = R + A + T

47. Reflectance There are several types of surfaces, whose texture and composition influence the way in which light is reflected. Specular reflectors/surfaces Diffuse reflectors/surfaces Lambertian reflectors/surfaces

48. Specular Reflectance Occurs from very smooth surfaces, where the height of features on the surface << wavelength of the incoming EM radiation In specular reflection, all energy is reflected in one direction angle of incidence = angle of exitance

49. Diffuse Reflectance Most surfaces are not smooth, and reflect incoming EM radiation in a variety of directions These are called diffuse reflectors

50. Lambertian Surface A perfectly diffuse reflector is called a Lambertian surface A Lambertian surface reflects equally in all directions

51. Take a Break Here

52. Two things to consider when relating surface reflectance curves to satellite observations of radiant flux 1. The viewing angle of the sensor may not be fixed throughout time 1. Between different imaging dates for the same location 2. For the same location, on different dates 2. A given surface type’s reflection may not be constant over different viewing angles

53. Further information on this slide can be viewed at http://snrs.unl.edu/agmet/908/brdf_definition.htm If a surface were Lambertian, viewing angle would not matter because reflection would be equal in any direction

54. Further information on this slide can be viewed at http://snrs.unl.edu/agmet/908/brdf_definition.htm Most surfaces are not Lambertian; therefore, reflection is dependent upon viewing angle

55. Reflectance from a Grass Field – Effects of differing viewing angles

56. Bidirectional Reflectance Distribution Function (BRDF) A mathematical description of how reflectance varies for all combinations of illumination (determined by where the sun is) and viewing angles (determined by where the sensor is) at a given wavelength. “Defines the reflectance of a surface in multiple viewing directions based on a specified irradiance azimuth and zenith angles” Note: I have placed online an optional reading by Walthall (1985), which is considered to be the best model for estimating the effects of bidirectional reflectance. Go look at the graphics he creates of BRDF.

58. Anistrophy Factor and Reflectance Factor Anistrophy Factor – the ratio of the radiance at a specific viewing geometry divided by the radiance at a nadir viewing geometry. Nadir = 0 o (but you can picture it as perpendicular to the Earth’s surface) Reflectance Factor - the ratio of the radiance of the actual surface to the radiance of an ideal Lambertian surface illuminated and viewed in the same manner as the surface of interest.

59. BRDF examples backscattering forward scattering (sun behind observer)(sun opposite observer) http://geography.bu.edu/brdf/brdfexpl.html Photographs by Don Deering

60. The Hotspot – backscatter of light from the surface in the direction of the illumination sensor Occurs when the azimuth and zenith angles of the sensor are the same as those of the sun.

61. Goniometer – a radiometer used to measure BRDF

63. EM Radiation at the sensor

64. Key components of VIS/NIR remote sensing 1. Sun is EM Energy Source 2. Energy emitted from sun described by Stephan/Boltzman Law, Planck’s formula, and Wien’s Displacement Law 3. EM Energy interacts with the atmosphere 4. EM energy reflected from Earth’s Surface VIS/NIR Satellite EM energy 5. EM Energy interacts with the atmosphere

65. Radiant Flux -  The fundamental unit to measure electromagnetic radiation is radiant flux -   is defined as the amount of energy that passes into, through, or off of a surface per unit time Into = absorbed Through = transmitted Off of = reflected Radiant flux (  ) is measured in Watts (W) Refreshing your memory…

66. Radiant Flux Density  Radiant flux density is simply the amount of flux per unit area Radiant flux density =  /area Refreshing your memory…

67. Irradiance versus Exitance Irradiance (E) is the amount of incident radiant flux per unit area of a plane surface in Watts per square meter (W m –2 ) Exitance (M) is the amount of radiant flux per unit area leaving a plane surface in Watts per square meter (W m –2 ) They both incorporate radiant flux per unit area, but their directionality is what distinguishes them. Refreshing your memory…

68. Detection of flux by a remote sensing system θ – Sensor viewing angle Area as seen by the sensor = A cos θ Satellite Radiometer A – area on ground being sensed

69. Solid angle of the sensor Flux from a surface is actually being emitted or reflected in all directions equally, i.e., it is being distributed into a hemisphere The radiometer intercepts a fraction of the exitance from a surface, this fraction is defined by the solid angle, Ω, of the sensing system, which is defined by the area of the sensing surface and the distance to the target area  ice cream cone!

70. Figure 2-10 from Elachi, C., Introduction to the Physics and Techniques of Remote Sensing, 413 pp., John Wiley & Sons, New York, 1987. Same as Figure 2-21 in Jensen

71. Radiance - L Radiance – the radiant intensity per unit of projected source area in a specified direction... Watts m -2 sr -1 (steradian). What satellites detect. Concerned with a particular wavelength at a time. Example: You are in an airplane looking down through a telescope at the Earth. The information (radiance/energy) seen through the telescope (projected source area – the ground) travels through the telescope (solid angle) and back to your eye (the remote sensing system).

72. Simple model for estimating total radiance (L s ) at a satellite radiometer Satellite radiometer E o – incident solar irradiance LsLs θoθo Assumptions -No atmosphere -Lambertian surface (the amount of radiance in a single direction is the reflected radiance divided by  ) θvθv θ o – Solar Zenith Θ v – View Zenith

73. Sources of variation in total radiance (L s ) at a satellite radiometer Satellite radiometer E o – incident solar irradiance LsLs θoθo All these factors can vary between data collections with our satellite sensor To estimate r from L s, need to quantify E o (solar radiance at top of atmosphere), θ o, and θ v θvθv

74. Sources of variation in L s in the simple model θ o – solar zenith angle will vary depending upon when the satellite collects data (time of day) as well as season of the year – means that E o (incident solar irradiance) will vary θ v - view angle of the sensor can be controlled by the operator (satellites can “point” their sensors at wide angles) – can remain constant or may vary r – what we are trying to measure or infer - may be constant, or may vary

75. Complex model for estimating total radiance (L s ) at a satellite radiometer Satellite radiometer E o – incident solar irradiance r – hemispherical reflectance LsLs In reality, estimating L s is much more complicated because we have to account for the effects of the atmosphere

76. Accounting for effects of the atmosphere When we add the atmosphere to our model, we increase the complexity of our model: L s – the energy detected by the satellite L t – the energy leaving the atmosphere (top of atmosphere) L i - the total radiance from the area of interest at the earth’s surface When no atmosphere is present, L s = L t = Li When atmosphere is present, L s  L t  Li

77. L s is the total radiance at the sensor E o is the solar irradiance at the top of the atmosphere T  is the atmospheric transmittance (v and o) r is the surface reflectance L i is the total radiance from the area of interest at the earth’s surface L t is the total radiance from the area of interest at the top of the atmosphere

78. Path Radiance Atmospheric scattering results in much EM radiation entering into the field of view of the radiometer This indirect EM radiation is referred to as path radiance Radiometer Atmospheric Scattering Path radiance

79. L p is the total path radiance from multiple scattering, in this case, from sky irradiance

80. Refining our Definition of L s The radiance reaching the radiometer comes from two sources – directly from the target (L t ) and from path radiance (L p ) L s = L t + L p But what contributes to our path radiance?

81. Diffuse Sky Irradiance Atmospheric scattering results in much EM radiation indirectly reaching the earth’s surface This indirect EM radiation reaching the earth’s surface is referred to as diffuse sky irradiance Radiometer Atmospheric Scattering Path radiance Diffuse sky irradiance

82. E d is the diffuse irradiance from scattering within the atmosphere – sky irradiance

83. Sensor Point Response Function Sensors are not precise enough where they only detect energy from the target area of interest All sensors detect some energy coming from areas adjacent to the area of interest Figure from Cahoon et al. 2000. Wildland fire detection from space: Theory and application. Pages 151-169 in J. L. Innes, M. Beniston, and M. M. Verstraete, editors. Biomass Burning and its Inter-Relationship with the Climate System. Kluwer Academic Publishers, Dordrecht.

84. Sensor Point Response Function Figure from Cahoon et al. 2000. Wildland fire detection from space: Theory and application. Pages 151-169 in J. L. Innes, M. Beniston, and M. M. Verstraete, editors. Biomass Burning and its Inter-Relationship with the Climate System. Kluwer Academic Publishers, Dordrecht. In the AVHRR radiometer, 50% of signal comes from intended area, and 50% from outside of this area

85. r n is the surface reflectance from the nearby area

86. L s - Total Irradiance at the Sensor L s = L t + L p Path radiance (L p ) is a significant contributor to the total radiance measured by the sensor A great deal of research has been devoted to develop software programs to estimate path radiance for a variety of sensors

87. When it’s all put together... *Know this Diagram!

88. Lab 2 – Intro to ENVI (our software!) Always answer all questions! Use your book, the internet, the ENVI help, your brains, and one another to answer the questions. When in doubt, just try– “I am not sure what the cause of the variations in the field are, but here is a description of them: yada yada yada, and here is my guess: yada yada yada.” ENVI is a great software, but it can be confusing at first. Don’t get discouraged! The instructions for lab submission can be found on ELMS

90. Extinction Coefficient -  I / I o = e -  L where I is the attenuated light intensity I o is the unattenuated light intensity L is the path length through the a uniform medium such as the atmosphere  is the extinction coefficient in the units of inverse distance