1 Lecture 16 – Active Microwave Remote Sensing 2 December 2008

2 Recommended Readings Chapter 7 in Campbell

3 Figure 1-18 from Elachi, C., Introduction to the Physics and Techniques of Remote Sensing, 413 pp., John Wiley & Sons, New York, Active microwave systems operate at wavelengths (3 to 70 cm) that are not influenced by the atmosphere, e.g., these wavelengths have 100% transmission

4 Lecture 16 Topics 1.Definition of RADAR 2.Measurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse, 3.Microwave or Radar backscatter 4.Factors controlling microwave backscatter a.Surface roughness b.Bragg scattering c.Variations in dielectric constant 5.Spaceborne Radar Systems and Applications a.Altimeters b.Scatterometers c.Synthetic Aperture Radar (SAR)

5 The basic concept of RADAR was discovered by scientists working at the Naval Research Laboratory who were investigating using microwave EM energy as a source for radio transmissions

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

7 Microwave Transmitter / Receiver Antenna Microwave EM energy pulse transmitted by the radar Microwave EM energy pulse reflected from a target that will be detected by the radar Target

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

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10 Common Radar Bands Band FrequencyWavelength (most common) X 8 to 12 GHz2.5 to 4.0 cm (3.0 cm) C4 to 8 GHz4 to 8 cm (6.0) L1 to 2 GHz15 to 30 cm (24.0) P0.3 to 1 GHz30 to 100 cm (65 cm)

11 Radar systems control the polarization of both the transmitted and received microwave EM energy Figure 9.6 from Jensen

12 Radar System Designation Radar systems typically have a 3 letter designation to describe the frequency-polarization of operation: 1.First letter denotes the radar frequency and wavelength (e.g., X,C, L,P – see slide 10) 2.The second letter denotes the polarization of transmitted EM waves (H for horizontal, V for vertical) 3.The third letter denotes the polarization of the received EM waves (H for horizontal, V for vertical) For example, an C-VH radar is one that transmits EM radiation at a C-band wavelength (between 4 and 8 cm), it transmits horizontally polarized EM energy and it receives vertically polarized EM energy

13 Lecture 16 Topics 1.Definition of RADAR 2.Measurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse 3.Microwave or Radar backscatter 4.Factors controlling microwave backscatter a.Surface roughness b.Bragg scattering c.Variations in dielectric constant 5.Spaceborne Radar Systems and Applications a.Altimeters b.Scatterometers c.Synthetic Aperture Radar (SAR)

14 Measurements made with a simple radar Range to the target Range resolution Azimuth resolution Intensity of the returned pulse

15 Microwave Transmitter / Receiver Antenna Microwave EM energy pulse transmitted by the radar Microwave EM energy pulse reflected from a target that will be detected by the radar Target

16 Microwave Transmitter / Receiver 1. Transmitted pulse travels to the target 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 Antenna

17 Radar range - R The distance, R, from the antenna to the target is calculated as R = ct / 2 where c is the speed of light (3 x m sec -1 ) t is the time between the transmission of the pulse and its reception by the radar antenna

18 Measurements made with a simple radar Range to the target Range resolution Azimuth resolution Intensity of the returned pulse

19 Pulse Duration (  p ) pp Radars send out pulses of EM energy, e.g., a burst of energy that lasts for a very short time period, the pulse duration

20 Pulse Duration (  p ) and Pulse Length Radar systems transmit microwave pulses with of specific durations -  p The pulse length of the system defines the range resolution (  r ) of the radar

21 Measurements made with a simple radar Range to the target Range resolution Azimuth resolution Intensity of the returned pulse

22 Microwave Transmitter / Receiver Antenna Antenna Beamwidth -   The microwave energy transmitted by a radar is focused into a beam, with an angular dimension, 

23 Antenna Beamwidth -  If the length of the antenna is L, and the microwave wavelength is, then  = / L

24 Microwave Transmitter / Receiver Antenna Azimuth Resolution -  a  The direction parallel to the antenna length is called the azimuth dimension  a is the azimuth resolution of the radar, e.g., the distance 2 targets have to be separated in order to be distinguished by the radar

25 Microwave Transmitter / Receiver Antenna Azimuth Resolution -  a Real Aperture Radar  R aa  a =  R

26 Synthetic aperture radar (SAR) is a specific type of imaging radar system – A SAR operates by continuous transmitting and receiving pulses reflected from a target the entire time the target is within the beamwidth of the system

27 The pulses transmitted and received by a SAR are linearly swept in frequency, e.g., the frequency of the pulse is lower at the beginning of the pulse than at the end A single target results in thousands of pulses that are detected and recorded by the SAR These pulses are specially processed using fourier transforms to recreate a single point on the image representing the imaged target

28 By collecting data over a very long time, a SAR creates an synthetically long antenna or aperture (L s ) – hence the term synthetic aperature radar As a result, the azimuth resolution of a SAR is independent of range to the target:  a = L /2 where L is the actual length of the antenna LsLs Slides 26 to 27 are for background only – know material on this slide Azimuth resolution for a Synthetic Aperture Radar

29 Measurements made with a simple radar Range to the target Range resolution Azimuth resolution Intensity of the returned pulse

30 The Radar Equation

31 Lecture 16 Topics 1.Definition of RADAR 2.Measurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse, 3.Microwave or Radar backscatter 4.Factors controlling microwave backscatter a.Surface roughness b.Bragg scattering c.Variations in dielectric constant 5.Spaceborne Radar Systems and Applications a.Altimeters b.Scatterometers c.Synthetic Aperture Radar (SAR)

32 Microwave (Radar) Backscatter When microwave EM energy transmitted by a RADAR system reaches the earth surface, some is absorbed by the surface and the remainder is reflected in multiple directions In microwave remote sensing, surface reflection is referred to as scattering of microwave EM energy The microwave EM energy that is scattered in the Radar’s direction of transmission is the only EM energy that is detected by the radar – this EM energy is referred to as microwave or radar backscatter

33 Radar cross section - σ Radar cross section is the area of a theoretical, perfect reflector of EM energy (e.g., a metal sphere) that would reflect the same amount of energy back to the radar as the actual target resulting in the microwave EM energy To determine the radar cross section for a detected microwave signature, engineers build targets with known cross section and use these to calibrate radar image intensity values The units for radar cross section is m 2

34 Radar scattering coefficient - σ° The radar scattering coefficient is used to describe the radar intensity per unit area of the image pixel σ° = σ / A where A is the area of the pixel

35 The Decibel unit (dB) Radar scattering coefficient is typically described using decibels, where σ° (dB) = 10 log (σ°)

36 Lecture 16 Topics 1.Definition of RADAR 2.Measurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse, 3.Microwave or Radar backscatter 4.Factors controlling microwave backscatter a.Surface roughness b.Bragg scattering c.Variations in dielectric constant 5.Spaceborne Radar Systems and Applications a.Altimeters b.Scatterometers c.Synthetic Aperture Radar (SAR)

37 Surface Reflectance or Scattering Specular reflection or scattering Diffuse reflection or scattering

38 Specular Reflection or Scattering Occurs from very smooth surfaces, where the height of features on the surface << wavelength of the incoming EM radiation

39 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

40 Figures from mgddf/chap5/f5-4f.gif Radar backscattering is dependent on the relative height or roughness of the surface

41 Microwave scattering as a function of surface roughness is wavelength dependent

42 Figure from mgddf/chap5/f5-4f.gif Microwave scattering is dependent on incidence angle As incidence angle increases, radar backscatter decreases for all surface roughnesses

43 Variation in MW backscatter from a rough surface (grass field) as a function of wavelength – As the wavelength gets longer, the backscattering coefficient drops

44 Lecture 16 Topics 1.Definition of RADAR 2.Measurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse, 3.Microwave or Radar backscatter 4.Factors controlling microwave backscatter a.Surface roughness b.Bragg scattering c.Variations in dielectric constant 5.Spaceborne Radar Systems and Applications a.Altimeters b.Scatterometers c.Synthetic Aperture Radar (SAR)

45 Microwave Scattering from a Water Surface – Bragg Scattering Water 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

46 = 3 cm = 24 cm Small surface or capillary waves present on a water surface – these waves are generated by wind

47 Smooth area – no wind

48 L-band airborne SAR Image of ship and its wake from previous slide Why do you have backscatter at L-band from an ocean surface?

49 Bragg Scattering from Water Surfaces Wind creates small waves on the ocean surface (capillary waves) which in the absence of wind will continue to propagate If wind continues, waves will grow in size and increase in wavelength and height to become ultra-gravity waves and eventually gravity waves A water surface affected by wind will have a spectrum of surface waves, e.g., multiple wavelengths and heights

50 Bragg Scattering from Water Surfaces Microwave EM energy has been shown through wave tank experiments to constructively interfere or resonate with surface capillary and ultra-gravity waves This phenomenon is known as Bragg Scattering

51 Bragg Scattering

52 Backscatter dependence on wind speed L-HH Measurements upwind Incidence angle Wave tank studies show that variations in radar backscatter from water surfaces is proportional to wind speed

53 Lecture 16 Topics 1.Definition of RADAR 2.Measurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse, 3.Microwave or Radar backscatter 4.Factors controlling microwave backscatter a.Surface roughness b.Bragg scattering c.Variations in dielectric constant 5.Spaceborne Radar Systems and Applications a.Altimeters b.Scatterometers c.Synthetic Aperture Radar (SAR)

54 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 – –  ~ dielectric constant To some degree, dielectric constants are dependent on microwave wavelength and polarization

55 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

56 At microwave wavelengths, the refraction coefficient (n) is determined by the dielectric constant of the material (  ) n = square root (  ) The reflection coefficients are determined from the refraction coefficient (see slides in Lecture 15) At an incidence angle of 0 º, the reflection coefficients are r v = r h = [(n-1) / (n+1)] 2

57 Dielectric constant as a function of soil moisture = 21.4 cm Figure E.47 from Ulaby, Moore, and Fung, Microwave Remote Sensing, Volume III.

58 Radar backscatter as a function of soil moisture Figure from Ulaby, Moore, and Fung, Microwave Remote Sensing, Volume III. = 3.3 cm

59 Lecture 16 Topics 1.Definition of RADAR 2.Measurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse, 3.Microwave or Radar backscatter 4.Factors controlling microwave backscatter a.Surface roughness b.Bragg scattering c.Variations in dielectric constant 5.Spaceborne Radar Systems and Applications a.Altimeters b.Scatterometers c.Synthetic Aperture Radar (SAR)

60 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

61 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  – the dynamic sea surface topography

62 Altimeter Missions NASA GEOS-3, NASA Seasat, 1978 NAVY Geosat, (first 2 years classified) ESA ERS-1/2, and NASA/CNES TOPEX/Poseidon, late NASA/CNES Jason-1, late 2000-

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65 Lecture 16 Topics 1.Definition of RADAR 2.Measurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse, 3.Microwave or Radar backscatter 4.Factors controlling microwave backscatter a.Surface roughness b.Bragg scattering c.Variations in dielectric constant 5.Spaceborne Radar Systems and Applications a.Altimeters b.Scatterometers c.Synthetic Aperture Radar (SAR)

66 ERS Scatterometer Resolution = 50 km Obtains measurements looking upwind, cross- wind, and downwind Empirical Algorithms used to estimate wind speed and direction

67 Backscatter dependence on wind speed: L-HH Measurements upwind

68 ERS Scatterometer Accuracy

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70 Lecture 16 Topics 1.Definition of RADAR 2.Measurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse, 3.Microwave or Radar backscatter 4.Factors controlling microwave backscatter a.Surface roughness b.Bragg scattering c.Variations in dielectric constant 5.Spaceborne Radar Systems and Applications a.Altimeters b.Scatterometers c.Synthetic Aperture Radar (SAR)

71 Spaceborne SAR Systems ERS-1/2 SARS Launched in 1991 and 1995 by the European Space Agencies, still in operation C-VV system (6 cm wavelength) 100 x 100 km image, 25 m pixel size Repeat frequency every 2 weeks ENVISAT ASARs Launched in 2002 as follow-on to ERS 1/2 SARs Multiple swath widths and pixel sizes 30 to 1000 m pixels, 100 to 400 km swath widths C band SAR with multiple polarizations – VV, HH, HV, VH

72 Spaceborne SAR Systems Radarsat 1/2 SARS Canadian Space Agency Launched in 1995, 2007 C band (6 cm) SAR, HH polarization 100 km swath, 25 m pixel standard mode, multiple other modes (variable pixel size, swath widths

73 Spaceborne SAR Systems JERS SAR Launched by Japanese Space Agency in 1992 Operated for 4 years L-band HH SAR 30 m pixel, 75 km swath, 44 day repeat cycle ALOS PALSAR Launched by Japanese Space Agency in 2006 L-HH or L-VV SAR m pixel size 40 to 350 km swath width Repeat frequency of 40 days

74 Readings on SAR applications Kasischke, E.S., K.B. Smith, L.L. Bourgeau-Chavez, E.A. Romanowicz, S.M. Brunzell, and C.J. Richardson, Effects of Seasonal Hydrologic Patterns in South Florida Wetlands on Radar Backscatter Measured from ERS-2 SAR Imagery, Remote Sens. Env., 88, , Kasischke, E.S., L.L. Bourgeau-Chavez, and J.F. Johnstone, Assessing spatial and temporal variations in surface soil moisture in fire-disturbed black spruce forests using spaceborne synthetic aperture radar imagery - implications for post-fire tree recruitment, Rem. Sens. Environ., 108, 42-58, doi: /j.rse , Harrell, P.A., E.S. Kasischke, L.L. Bourgeau-Chavez, E. Haney, and N.L. Christensen, Evaluation of approaches to estimating of aboveground biomass in southern pine forests using SIR-C imagery, Remote Sensing Env., 59, , 1997.

75 Sources of backscatter / attenuation  c - volumetric scattering from the canopy  s – direct scattering from the ground surface  m – multiple bounce scattering between the ground and canopy  c – attenuation coefficient for the canopy cc Because of their long wavelengths, EM energy from a SAR has multiple interactions with a land surface covered by vegetation – With short, herbaceous vegetation, EM has multiple scattering sources

76 Case study 1 Monitoring burned forests in Alaska See Kasischke et al. 2007

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82 Case study 2 Monitoring of hydrologic conditions in southern Florida wetlands See Kasischke et al. 2003

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84 ERS SAR Imagery

85 Site 4 Marl Prairie Site 5 Cypress DomeSite 6 Pine Flatwood Site 11 Wet Marsh Site 12 Hatrack Cypress Examples of Wetland Types in South Florida

86 When water covers the ground surface, there is no surface backscatter (  s ) As water depth increases, the amount of canopy backscatter (  c ) and multipath backscatter (  m ) decrease

87 Backscatter vs. Soil Moisture When the wetlands dry out and standing water is absent, backscatter is proportional to soil moisture

88 Case Study 3 SAR Monitoring of forests See Harrell et al. 1997

89 cc tt In forested canopies, additional backscatter terms are present  t - direct backscatter from tree trunks  d – multi-path or double bounce-scattering between the ground and tree trunk

90 Landsat TM (June) Wetlands as Viewed with VIS/IR and Synthetic Aperture Radar Imagery Radarsat (C-HH) Double bounce scattering leads to higher radar scattering in flooded forests

91 SAR backscatter is quite sensitive to variations in aboveground biomass in forests. In our studies, biomass ranged from 0.1 to 44.4 kg/sq m See Harrell et al. 1997