CEGEG 046 GEOG 3051 Principles Practice of Remote

CEGEG 046 / GEOG 3051 Principles & Practice of Remote Sensing (PPRS) 8: RADAR 1 Dr. Mathias (Mat) Disney UCL Geography Office: 113, Pearson Building Tel: 7670 05921 Email: mdisney@ucl. geog. ac. uk www. geog. ucl. ac. uk/~mdisney

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

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

9/8/91 ERS-1 (11. 25 am), Landsat (10. 43 am) 4

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, 1994. 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 5

Ice 6

Oil slick Galicia, Spain 7

Nicobar Islands December 2004 tsunami flooding in red 8

Paris 9

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 • If. SAR Interferometric SAR – Generates X, Y, Z from two SAR images using principles of interferometry (phase difference) 10

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, Sci. Tech Publishing. • C. Oliver and S. Quegan, Understanding Synthetic Aperture Radar Images, Artech House, 1998. • Woodhouse I H (2000) Tutorial review. Stop, look and listen: auditory perception analogies for radar remote sensing, International Journal of Remote Sensing 21 (15), 2901 -2913. • Jensen, J. R. (2000) Remote sensing of the Environment, Chapter 9. 11

Web sites Canada • http: //www. ccrs. nrcan. gc. ca/resource/tutor/fundam/chapter 3/01_e. php ESA • http: //earth. esa. int/applications/data_util/SARDOCS/space borne/Radar_Courses/ 12

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 (~10 m - 1 km) • altimeters (nadir-looking) to derive surface height variations • scatterometers to derive reflectivity as a function of incident angle, illumination direction, polarisation, etc 13

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. 14

Principle of RADAR 15

Principle of ranging and imaging 16

Radar Pulse 17

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ERS 1 and 2 geometry 19

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. 20

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

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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 24

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 25

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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 27

Azimuth resolution: synthetic aperture v La ψ R Target time spent in beam = arc length / v = Ry / v = R / v. La 28

Resolution τ 29

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

Resolution of SAR 31

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. 32

Some Spaceborne Systems 33

ERS 1 and 2 Specifications Geometric specifications Spatial resolution: along track <=30 m across-track <=26. 3 m Swath width: 102. 5 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. 1 deg. mid swath 23 deg. far swath 25. 9 deg Incidence angle tolerance: <=0. 5 deg. Radiometric specifications: Frequency: 5. 3 GHz (C-band) Wave length: 5. 6 cm 34

Speckle • Speckle appears as “noisy” fluctuations in brightness 35

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. 36

Speckle 37

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 38

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) 39

Speckle 40

Principle of ranging and imaging 41

Geometric effects 42

Shadow 43

Foreshortening 44

Layover 45

Layover 46

Los Angeles 47

Radiometric aspects – the RADAR equation Pr = (Power per unit area at target ) × Eff. scatt. area of target × Spread loss of reflected signal × Eff. Antennae area • 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 • http: //earth. esa. int/applications/data_util/SARDOCS/sp aceborne/Radar_Courses/Radar_Course_III/radar_equ ation. htm 48

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 Smooth Rough – Water content of the surface 49

Roughness Smooth, intermediate or rough? • Jensen (2002; p 314) – surface height variation h – – Smooth: h < /25 sin β Rough: h > /4. 4 sin β Intermediate β is depression angle, so depends on AND imaging geometry http: //rst. gsfc. nasa. gov/Sect 8_2. html 50

Oil slick Galicia, Spain 51

Los Angeles 52

Source: Graham 2001 Response to soil moisture 53

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

Types of scattering of radar from different surfaces 55

Scattering 56

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 Pt is transmitted by an antenna with gain Gt , the power per unit solid angle in the direction of the scatterer is Pt Gt, where the value of Gt in that direction is used. READ: http: //earth. esa. int/applications/data_util/SARDOCS/spaceborne/Radar_C ourses/Radar_Course_III/radar_equation. htm and Jensen Chapter 9 57

The Radar Equation 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 Because R = range P = power G = gain of antenna A = area of the antenna Where: The Radar scattering cross section 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. fa is absorption Ars is effective area of incident beam received by scatterer Gts is gain of the scatterer in the direction of the receiver READ: http: //earth. esa. int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar_Course_III/radar _equation. htm 58 And Jensen Chapter 9

Measured quantities • Radar cross section [d. Bm 2] • Bistatic scattering coefficient [d. B] • Backscattering coefficient [d. B] 59

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

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. 61