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Lecture 23 notes

  1. 1. Lecture 23 - Microwave Remote Sensing 3 December 2007 1
  2. 2. Recommended Readings• Chapter 13, pages 291-313 in Jensen 2
  3. 3. Microwave energy is largely unaffected bythe atmosphere, e.g., it has 100%transmission Figure 1-18 from Elachi, C., Introduction to the Physics and Techniques of Remote Sensing, 413 pp., John Wiley & Sons, New 3 York, 1987.
  4. 4. Unique Characteristics of Microwave Remote Sensors1. They are not affected by cloud cover and operate at night – all-weather, day-night sensing devices2. Passive microwave systems detect radiant temperature of the earth’s surface which in, varies as a function of kinetic temperature and emissivity – large emissivity variations in water and vegetation cause large variations radiant temperature3. Active microwave systems are very sensitive to variations in surface moisture content and surface roughness unique information available from these systems 4
  5. 5. Types of Microwave Remote Sensors – Microwave radiometers • Measure the emittance of EM energy within the microwave region of the EM spectrum, just like thermal IR sensors – Non-imaging RADARs 1. Altimeters – measure the elevation of the earth’s surface 2. Scatterometers – detect variations in microwave backscatter from a large area - measure variations in surface roughness, used to estimate ocean wind speed – Imaging RADARs • Synthetic Aperture Radars – map variations in microwave backscatter at fine spatial scales (10 to 50 m), used to create an image – measure variations in surface roughness and surface moisture 5
  6. 6. RADAR – Radio Detection and Ranging• Concept behind radars discovered in 1923• RADARs systems invented in the 1930s – A high powered, radio transmitter/receiver system was developed that would transmit a signal that was reflected from a distant object, and then detected by the receiver – Thus, RADAR’s initial function was to detect and determine the range to a target 6
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  8. 8. Key Components of a Radar System• Microwave Transmitter – electronic device used to generate the microwave EM energy transmitted by the radar• Microwave Receiver – electronic device used to detect the microwave pulse that is reflected by the area being imaged by the radar• Antenna – electronic component used through which microwave pulses are transmitted and received 8
  9. 9. Measurements made with a simple radar• Range to the target• Intensity of the returned pulse• Azimuth resolution• Range resolution 9
  10. 10. MicrowaveTransmitter / Receiver Antenna Target Microwave EM energy Microwave EM energy pulse transmitted by the pulse reflected from a radar target that will be detected by the radar 10
  11. 11. Microwave TargetTransmitter / Receiver 1. Transmitted pulse travels to Antenna the target 2. The target reflects the pulse, and the reflected pulse travels back to the microwave antenna / receiver, where it is DETECTED 3. The radar measures the time (t) between when the pulse was transmitted and when the reflected signal reaches the receiver – The time it takes the pulse to travel from the radar to the target and back is used to estimate the RANGE 11
  12. 12. Radar range - RThe distance, R, from the antenna to the target is calculated as R = ct / 2wherec is the speed of light (3 x 10-8 m sec -1)t is the time between the transmission of the pulse and its reception by the radar antenna 12
  13. 13. Satellite Altimeters• Altimeters are radars that measure the height of the surface of the earth• Transmit a radar pulse which is reflected from the earth’s surface• Measure the time it takes for the pulse to travel to the earth and back (t)• Height of the satellite (H) H = ct/2 where c is the speed of light• The altitude of the satellite is carefully measured using GPS and ground-based laser systems 13
  14. 14. Altimeters Altimeters measure round- trip travel time of microwave radar pulse to determine distance to sea surface From this (and additional info) we can determine η – the dynamic sea surface topography 14
  15. 15. Altimeter Missions• NASA GEOS-3, 1975-1978• NASA Seasat, 1978• NAVY Geosat, 1985-1989 (first 2 years classified)• ESA ERS-1/2, 1991-1996 and 1995-• NASA/CNES TOPEX/Poseidon, late 1992-• NASA/CNES Jason-1, late 2000- 15
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  18. 18. Measurements made with a simple radar• Range to the target• Intensity of the returned pulse• Azimuth resolution• Range resolution 18
  19. 19. The Radar Equation 19
  20. 20. σ - is the radar surface backscatter coefficientIt represents the fraction of incoming EM radiation that is scattered from the surface in the direction of the transmitted energy (hence the term “backscatter)It is equivalent to the reflection coefficient in thevisible/RIR region of the EM spectrum 20
  21. 21. Factors controlling variations in σ• Surface roughness• Surface dielectric constant 21
  22. 22. Surface Reflectance or Scattering• Specular reflection or scattering• Diffuse reflection or scattering 22
  23. 23. Specular Reflection or Scattering• Occurs from very smooth surfaces, where the height of features on the surface << wavelength of the incoming EM radiation 23
  24. 24. Diffuse Reflectors or Scatterers• Most surfaces are not smooth, and reflect incoming EM radiation in a variety of directions• These are called diffuse reflectors or scatterers 24
  25. 25. Radar backscattering is dependent on the relative height or roughness of the surfaceFigures fromhttp://pds.jpl.nasa.gov/mgddf/chap5/f5-4f.gif 25
  26. 26. Microwave scattering asa function of surfaceroughness is wavelengthdependent 26
  27. 27. Variation in MW backscatter from a rough surface (grass field) as a function ofwavelength – As the wavelength gets longer, the backscattering coefficient drops 27
  28. 28. Microwave scattering isdependent on incidence angleAs incidence angle increases,backscattering decreases Figure from http://pds.jpl.nasa.gov/ mgddf/chap5/f5-4f.gif 28
  29. 29. Factors controlling σ• Surface roughness• Surface dielectric constant 29
  30. 30. Dielectric Constant• The dielectric constant is a measure of the electrical conductivity of a material• Degree of scattering by an object or surface is proportional to the dielectric constant of the material – – σ ~ dielectric constant• To some degree, dielectric constants are dependent on microwave wavelength and polarization 30
  31. 31. Dielectric Constants of Common Materials• Soil – 3 to 6• Vegetation – 1 to 3• Water – 80 – For most terrestrial materials, the moisture content determines the strength of scattering of microwave energy 31
  32. 32. Dielectric constant as a function of soil moisture λ = 21.4 cm Figure E.47 from Ulaby, Moore, and Fung, Microwave Remote Sensing, Volume III. 32
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  35. 35. Moderate Burn All Years 0 -2 y = 0.3299x - 18.268 -4 R2 = 0.82ERS-2 Backscatter (dB) -6 All Years -8 2003-4 Validation Sites -10 Linear (All Years) -12 -14 -16 -18 0 10 20 30 40 50 6 cm % Volumetric Moisture Radar backscatter (image intensity) in burned forests is proportional to soil moisture 35
  36. 36. Microwave Scattering from a Water SurfaceWater has a dielectric constant of 80• All scattering from water bodies in the Microwave region of the EM Spectrum is from surface scattering as no EM energy penetrates the water surface 36
  37. 37. λ = 3 cm λ = 24 cm 37
  38. 38. Smooth area – no wind 38
  39. 39. L-band airborne SAR ImageWhy do you have backscatter at L-band from an ocean surface? 39
  40. 40. Backscatter dependence on wind speed: L-HH Measurements upwindRadar backscatter from a water surface varies as a function of:1. Wind speed2. Look direction (upwind, downwind, cross wind)3. Incidence angle (look direction of the sensor relative to the surface 40
  41. 41. ERS Scatterometer Resolution = 50 km Swath Width = 500 km Obtains backscatter measurements looking upwind, cross-wind, and downwind Empirical Algorithms used to estimate wind speed and direction 41
  42. 42. ERS Scatterometer Accuracy Scatterometer accuracies determined through comparisons made with surface data collected by buoys 42
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  44. 44. Microwave Radiometers• Land and water surfaces not only emit EM energy that can be detected in thermal IR wavelengths, but also in microwave wavelengths (1 cm to > 1 m)• Microwave radiometers have the ability to measure the brightness temperature (TB) of the earth’s surface 44
  45. 45. Microwave Radiometers• Recall TB = ε Tkin• The microwave emissivity (ε) of the different materials found on the earth’s surface varies greatly 45
  46. 46. Key: O = Open Water, FY = First Year Ice, MY = Multi-year Ice H = horizontal polarization, V = Vertical Polarization 46
  47. 47. Key: O = Open Water, FY = First Year Ice, MY = Multi-year Ice H = horizontal polarization, V = Vertical Polarization 47
  48. 48. Scanning Multi-channelMicrowave Radiometer (SMMR) 1978 - 1987Wavelength Polarizations Pixel Size 0.81 cm H,V 30 x 30 km 1.40 cm H,V 1.70 cm H,V 1.80 cm H,V 4.60 cm H,V 159 x 159 km 48
  49. 49. Special SensorMicrowave/Imagery (SSM/I) 1987 to present Wavelength Polarizations Pixel Size 0.35 cm H,V 16 x 14 km 0.81 cm H,V 38 x 30 km 1.35 cm V 60 x 40 km 1.55 cm H,V 70 x 45 km 49
  50. 50. Key Points for Lecture 231. Definition of a RADAR2. Uses of a radar altimeter3. The radar backscatter coefficient - σ4. Source of variations in σ – Surface roughness – Incidence angle – Surface dielectric constant5. Effects of soil moisture on σ6. Effects of wind speed on σ from water surfaces7. ERS Scatterometer8. Microwave Radiometers – how do they work? 50