CEGEG046 / GEOG3051 Principles & Practice of Remote Sensing (PPRS) 8: RADAR 1 Dr. Mathias (Mat) Disney UCL Geography Office: 113, Pearson Building Tel:

2 OVERVIEW AGENDA Principles of RADAR, SLAR and SAR Characteristics of RADAR SAR interferometry Applications of SAR Summaries

3 PRINCIPLES AND CHARACTERISTICS OF RADAR, SLAR AND SAR Examples Definitions Principles of RADAR and SAR Resolution Frequency Geometry Radiometry

4 9/8/91 ERS-1 (11.25 am), Landsat (10.43 am)

5 The image at the top was acquired through thick cloud cover by the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) aboard the space shuttle Endeavour on April 16, The image on the bottom is an optical photograph taken by the Endeavour crew under clear conditions during the second flight of SIR-C/X-SAR on October 10, 1994

6 Ice

7 Oil slick Galicia, Spain

8 Nicobar Islands December 2004 tsunami flooding in red

9 Paris

10 Definitions Radar - an acronym for Radio Detection And Ranging SLAR – Sideways Looking Airborne Radar –Measures range to scattering targets on the ground, can be used to form a low resolution image. SAR Synthetic Aperture Radar –Same principle as SLAR, but uses image processing to create high resolution images IfSAR Interferometric SAR –Generates X, Y, Z from two SAR images using principles of interferometry (phase difference)

11 References Henderson and Lewis, Principles and Applications of Imaging Radar, John Wiley and Sons Allan T D (ed) Satellite microwave remote sensing, Ellis Horwood, 1983 F. Ulaby, R. Moore and A. Fung, Microwave Remote Sensing: Active and Passive (3 vols), 1981, 1982, 1986 S. Kingsley and S. Quegan, Understanding Radar Systems, SciTech Publishing. C. Oliver and S. Quegan, Understanding Synthetic Aperture Radar Images, Artech House, Woodhouse I H (2000) Tutorial review. Stop, look and listen: auditory perception analogies for radar remote sensing, International Journal of Remote Sensing 21 (15), Jensen, J. R. (2000) Remote sensing of the Environment, Chapter 9.

12 Web sites Canada 3/01_e.phphttp:// 3/01_e.php ESA borne/Radar_Courses/ borne/Radar_Courses/

13 What is RADAR? Radio Detection and Ranging Radar is a ranging instrument (range) distances inferred from time elapsed between transmission of a signal and reception of the returned signal imaging radars (side-looking) used to acquire images (~10m - 1km) altimeters (nadir-looking) to derive surface height variations scatterometers to derive reflectivity as a function of incident angle, illumination direction, polarisation, etc

14 What is RADAR? A Radar system has three primary functions: - It transmits microwave (radio) signals towards a scene - It receives the portion of the transmitted energy backscattered from the scene - It observes the strength (detection) and the time delay (ranging) of the return signals. Radar provides its own energy source and, therefore, can operate both day or night. This type of system is known as an active remote sensing system.

15 Principle of RADAR

16 Principle of ranging and imaging

17 Radar Pulse

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19 ERS 1 and 2 geometry

20 Radar wavelength Most remote sensing radars operate at wavelengths between 0.5 cm and 75 cm: X-band: from 2.4 to 3.75 cm (12.5 to 8 GHz). C-band: from 3.75 to 7.5 cm (8 to 4 GHz). S-band: from 7.5 to 15 cm (4 to 2 GHz). L-band: from 15 to 30 cm (2 to 1 GHz). P-band: from 30 to 100 cm (1 to 0.3 GHz). The capability to penetrate through precipitation or into a surface layer is increased with longer wavelengths. Radars operating at wavelengths > 2 cm are not significantly affected by cloud cover. Rain does become a factor at wavelengths < 4 cm.

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22 Comparison of C band and L band SAR C-band L-band

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24 Choice of wave length Radar wavelength should be matched to the size of the surface features that we wish to discriminate – e.g. Ice discrimination, small features, use X-band – e.g. Geology mapping, large features, use L-band – e.g. Foliage penetration, better at low frequencies, use P-band In general, C-band is a good compromise New airborne systems combine X and P band to give optimum measurement of vegetation

25 Synthetic Aperture Radar (SAR) Imaging side-looking accumulates data along path – ground surface “illuminated” parallel and to one side of the flight direction. Data, processing is needed to produce radar images. The across-track dimension is the “range”. Near range edge is closest to nadir; far range edge is farthest from the radar. The along-track dimension is referred to as “azimuth”. Resolution is defined for both the range and azimuth directions. Digital signal processing is used to focus the image and obtain a higher resolution than achieved by conventional radar

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27 Principle of Synthetic Aperture Radar SAR Doppler frequency due to sensor movement Use Doppler frequency shift (relative to reference pulse) due to sensor movement to recombine multiple pulses into a single coherent image from an apparently much larger (synthesised) aperture

28 Azimuth resolution: synthetic aperture Target time spent in beam = arc length / v = R  v = R / vL a v R ψ LaLa

29 Resolution τ

30 Range and azimuth resolution (RAR) Range resolution (across track) L S R a  L =antenna length S = slant range = height/sin  λ =wavelength Azimuth resolution (along track) cos : inverse relationship with angle T = duration of RADAR pulse c = speed of light γ= depression angle

31 Resolution of SAR

32 Important point Resolution cell (i.e. the cell defined by the resolutions in the range and azimuth directions) does NOT mean the same thing as pixel. Pixel sizes need not be the same thing. This is important since (i) the independent elements in the scene are resolutions cells, (ii) neighbouring pixels may exhibit some correlation.

33 Some Spaceborne Systems

34 ERS 1 and 2 Specifications Geometric specifications Spatial resolution: along track <=30 m across-track <=26.3 m Swath width: km (telemetered) 80.4 km (full performance) Swath standoff: 250 km to the right of the satellite track Localisation accuracy: along track <=1 km; across-track <=0.9 km Incidence angle: near swath 20.1deg. mid swath 23deg. far swath 25.9deg Incidence angle tolerance: <=0.5 deg. Radiometric specifications: Frequency: 5.3 GHz (C-band) Wave length: 5.6 cm

35 Speckle Speckle appears as “noisy” fluctuations in brightness

36 Speckle Fading / speckle - “noise-like” processes due to coherent imaging system. Local constructive and destructive interference Average multiple independent samples, can effectively reduce the effects of speckle e.g. by Multiple-look filtering, separates the maximum synthetic aperture into smaller sub-apertures generating independent looks at target areas based on the angular position of the targets. Therefore, looks are different Doppler frequency bands. Averaging (incoherently) adjacent pixels. Reducing these effects enhances radiometric resolution at the expense of spatial resolution.

37 Speckle

38 Speckle Radar images are formed coherently and therefore inevitably have a “noise-like” appearance Implies that a single pixel is not representative of the backscattering “Averaging” needs to be done

39 Multi-looking Speckle can be suppressed by “averaging” several intensity images This is often done in SAR processing Split the synthetic aperture into N separate parts Suppressing the speckle means decreasing the width of the intensity distribution We also get a decrease in spatial resolution by the same factor (N) Note this is in the azimuth direction (because it relies on the motion of the sensor which is in this direction)

40 Speckle

41 Principle of ranging and imaging

42 Geometric effects

43 Shadow

44 Foreshortening

45 Layover

46 Layover

47 Los Angeles

48 Radiometric aspects – the RADAR equation Brightness is a combination of several variables. We can group these characteristics into three areas which fundamentally control radar energy/target interactions. They are: –Surface roughness of the target –Radar viewing and surface geometry relationship –Moisture content and electrical properties of the target aceborne/Radar_Courses/Radar_Course_III/radar_equ ation.htm P r = (Power per unit area at target ) Eff. scatt. area of target Spread loss of reflected signal Eff. Antennae area ×××

49 Returned energy Angle of the surface to the incident radar beam –Strong from facing areas, weak from areas facing away Physical properties of the sensed surface –Surface roughness –Dielectric constant –Water content of the surface Smooth Rough

50 Roughness Smooth, intermediate or rough? Jensen (2002; p314) – surface height variation h –Smooth: h < /25sin β –Rough: h > /4.4sin β –Intermediate –β is depression angle, so depends on AND imaging geometry

51 Oil slick Galicia, Spain

52 Los Angeles

53 Response to soil moisture Source: Graham 2001

54 Crop moisture SAR image In situ irrigation Source: Graham 2001

55 Types of scattering of radar from different surfaces

56 Scattering

57 The Radar Equation The fundamental relation between the characteristics of the radar, the target, and the received signal is called the radar equation. The geometry of scattering from an isolated radar target (scatterer) is shown. When a power P t is transmitted by an antenna with gain G t, the power per unit solid angle in the direction of the scatterer is P t G t, where the value of G t in that direction is used. READ: ourses/Radar_Course_III/radar_equation.htm and Jensen Chapter 9

58 The Radar Equation The cross-section σ is a function of the directions of the incident wave and the wave toward the receiver, as well as that of the scatterer shape and dielectric properties. f a is absorption A rs is effective area of incident beam received by scatterer G ts is gain of the scatterer in the direction of the receiver We may rewrite the radar equation as two alternative forms, one in terms of the antenna gain and the other in terms of the antenna area Where: The Radar scattering cross section R = range P = power G = gain of antenna A = area of the antenna Because READ: _equation.htm _equation.htm And Jensen Chapter 9

59 Measured quantities Radar cross section [dBm 2 ] Bistatic scattering coefficient [dB] Backscattering coefficient [dB]

60 The Radar Equation: Point targets Power received G t is the transmitter gain, A r is the effective area of receiving antenna and  the effective area of the target. Assuming same transmitter and receiver, A/G= 2 /4 

61 Calibration of SAR Emphasis is on radiometric calibration to determine the radar cross section Calibration is done in the field, using test sites with transponders.