2009 -10 CEGEG 046 GEOG 3051 Principles

2009 -10 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: [email protected] geog. ac. uk www. geog. ucl. ac. uk/~mdisney

OVERVIEW FOR LECTURES 8 -10 • • • Principles of RADAR, SLAR and SAR Characteristics of RADAR SAR interferometry Applications of SAR Summaries 2

LECTURE 8 PRINCIPLES AND CHARACTERISTICS OF RADAR, SLAR AND SAR • • Examples Definitions Principles of RADAR and SAR Resolution Frequency Geometry Radiometry: the RADAR equation(s) 3

References • Jensen, J. R. (2000) Remote sensing of the Environment, Chapter 9. • 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. 4

Web resources, tutorials Canada • http: //www. ccrs. nrcan. gc. ca/resource/tutor/fundam/chapter 3/01_e. php • http: //www. ccrs. nrcan. gc. ca/resource/tutor/fundam/pdf/fundamentals_e. pdf ESA • http: //earth. esa. int/applications/data_util/SARDOCS/spaceborne/Radar _Courses/ Miscellaneous: • http: //www. radartutorial. eu/index. en. html Infoterra TERRASAR-X • http: //www. infoterra. de/image-gallery/images. html 5

9/8/91 ERS-1 (11. 25 am), Landsat (10. 43 am) 6

© Infoterra Gmbh 2009: 12/1/09 1 m resolution 7

Ice 8

Oil slick Galicia, Spain 9

Nicobar Islands December 2004 tsunami flooding in red 10

Paris 11

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) 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 is an active remote sensing system & can operate day/night 14

Principle of RADAR 15

Principle of ranging and imaging 16

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ERS 1 and 2 geometry 18

Radar wavelength • Most remote sensing radar wavelengths 0. 5 -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 > 4 cm are not significantly affected by cloud cover 19

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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, but…… • In general, C-band is a good compromise • New airborne systems combine X and P band to give optimum measurement of vegetation 22

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 needed to produce radar images. • Motion of platform used to synthesise larger antenna • 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 23

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Principle of Synthetic Aperture Radar SAR Doppler frequency shift f. D due to sensor movement As target gets closer http: //www. radartutorial. eu/11. coherent/co 06. en. html 25

Azimuth resolution (along track): RAR v La = beamwidth = /La S Arc = S Target time in beam = arc length / v = S /v. La so resolution = S /La 26

Range resolution (across track): RAR τ i. e. A-B is < PL/2 cannot resolve A & B 27

Range and azimuth resolution (RAR) Range resolution (across track) Azimuth resolution (along track) Ra = Sl L = Hl L sinγ L = antenna length S = slant range = height H/sin λ = wavelength Pulse length typically 0. 4 -1 s i. e. 8 -200 m Short pulse == higher Rr BUT lower signal cos : inverse relationship with angle 28

Azimuth resolution: SAR 29

Azimuth resolution (along track): SAR See: http: //facility. unavco. org/insar-class/sar_summary. pdf La S Ra Previously, azimuth resolution Ra = S/L = H/Lsin where H = height So, for synthetic aperture of 2 Ra & nominal slant range S (H/sin ) we see Ra, SAR = S/2 Ra = L/2 So Ra, SAR independent of H, and improves (goes down) as L goes down 30

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. 31

Some Spaceborne Systems 32

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 33

Speckle • Speckle appears as “noisy” fluctuations in brightness 34

Speckle • Fading / speckle are inherent “noise-like” processes in a coherent imaging system. • Speckle = constructive / destructive interference • Averaging independent samples can effectively reduce the effects of speckle (~1/sqrt(N)) for N samples • Multiple-look filtering – separate maximum synthetic aperture into smaller sub-apertures to generate independent views of target areas based on the angular position of the targets. Looks are different Doppler frequency bands. • Averaging (incoherently) adjacent pixels. • Either approach – enhances radiometric resolution at the expense of spatial resolution. 35

Speckle 36

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 37

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) 38

Speckle 39

Principle of ranging and imaging 40

Geometric effects 41

Shadow 42

Foreshortening 43

Layover 44

Layover 45

Radiometric aspects – the RADAR equation • The brightness of features is combination of several variables / characteristics – 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/spaceborne/R adar_Courses/Radar_Course_III/radar_equation. htm 46

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 47

Roughness Smooth, intermediate or rough? • Peake and Oliver (1971) – surface height variation h – Smooth: h < /25 sin – Rough: h > /4. 4 sin – Intermediate – is depression angle, so depends on AND imaging geometry http: //rst. gsfc. nasa. gov/Sect 8_2. html 48

Oil slick Galicia, Spain 49

Los Angeles 50

Source: Graham 2001 Response to soil moisture 51

Crop moisture SAR image In situ irrigation Source: Graham 2001 52

Types of scattering of radar from different surfaces 53

Scattering 54

The Radar Equation Relates characteristics of the radar, the target, and the received signal 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 55

The Radar Equation Radar equation can be stated in 2 alternate forms: one in terms of the antenna gain G 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 56 _Course_III/radar_equation. htm and Jensen Chapter 9

Measured quantities • Radar cross section [d. Bm 2] • Bistatic scattering coefficient [d. B] • Backscattering coefficient [d. B] 57

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 58

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. 59