GEOGG 141GEOG 3051 Principles Practice of Remote Sensing

GEOGG 141/GEOG 3051 Principles & Practice of Remote Sensing (PPRS) Active Remote Sensing: RADAR I 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 OF NEXT 2 LECTURES • • • 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: 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 • 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 • • https: //www. nrcan. gc. ca/earth-sciences/geomatics/satellite-imagery-air-photos/satelliteimagery-products/educational-resources/9371 https: //www. nrcan. gc. ca/earth-sciences/geomatics/satellite-imagery-air-photos/satelliteimagery-products/educational-resources/9309 ESA • • • RADAR tutorials: https: //earth. esa. int/web/guest/missions/esa-operational-eomissions/ers/instruments/sar/applications/radar-courses https: //core. ac. uk/download/pdf/31005519. pdf https: //sentinel. esa. int/web/sentinel/toolboxes/sentinel-1/tutorials Miscellaneous: • http: //www. radartutorial. eu/index. en. html Infoterra TERRASAR-X • • http: //www. infoterra. de/image-gallery Free data archive: http: //www. infoterra. de/terrasar-x-archive/ 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

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_Courses 20 /Radar_Course_III/radar_equation. htm and Jensen Chapter 9

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: is 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 21 _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] 22

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 23

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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 26

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 27

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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 29

Azimuth resolution (along track): RAR v La = beamwidth = /La Ψ S Arc = S Target time in beam = arc length / v = S /v. La so resolution = S /La 30

Range resolution (across track): RAR τ i. e. A-B is < PL/2 cannot resolve A & B 31

Range and azimuth resolution (RAR) Range resolution (across track) Azimuth resolution (along track) Ra = T = pulse length c = speed of light γ = depression angle (deg) 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 32

Azimuth resolution: SAR 33

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 34

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. 35

Some Spaceborne Systems 36

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 37

More recent • ASAR on Envisat (2002 -12): – https: //earth. esa. int/web/guest/missions/esa-operational-eo-missions/envisat/instruments/asar – C-band, 5 polarisation modes (HH, VV/HH, HV/HH, VH/VV), 100 to 100 s m resolution • PALSAR on ALOS: (2006 -11): – http: //www. eorc. jaxa. jp/ALOS/en/about/palsar. htm – L-band SAR 7 -100 m resolution (various modes) • PALSAR on ALOS-II: (2014 -): – http: //global. jaxa. jp/projects/sat/alos 2/ – L-band SAR 1 -3 m resolution, and lower in different modes • Terrasar-X (2007 -), Tan. DEM-X (2010 -): – – https: //directory. eoportal. org/web/eoportal/satellite-missions/t/terrasar-x#TMM 6 K 135 b. Herb X-band missions, high res (0. 24 -1 m) https: //directory. eoportal. org/web/eoportal/satellite-missions/t/tandem-x#launch Add on to Terrasar-X mission, flies few 100 m away, single-pass long baseline interfoermotetry for DEM accuracy of ~2 m 38

ESA Sentinel 1 • Launched April 2014 – first of ESA Sentinel program – long-term monitoring for climate and security • https: //directory. eoportal. org/web/eoportal/satellite-missions/copernicus-sentinel-1 • http: //www. esa. int/Our_Activities/Observing_the_Earth/Copernicus/Se ntinel-1 • Various Sentinel missions over next decade (5/6 already built) • 2 platform constellation mission (S 1 a to launch in 2016) • C-band SAR to build on ERS-2 heritage • 4 modes from 5 x 5 m (80 km swath) to 400 km swath 5 x 20 m resolution • Various interferometric modes 39

Speckle • Speckle appears as “noisy” fluctuations in brightness 40

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. 41

Speckle 42

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 43

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) 44

Speckle 45

Principle of ranging and imaging 46

Geometric effects 47

Shadow 48

Foreshortening 49

Layover 50

Layover 51

Radiometric aspects – the RADAR equation See: https: //earth. esa. int/web/guest/missions/esa-operational-eomissions/ers/instruments/sar/applications/radar-courses/content-3//asset_publisher/m. Q 9 R 7 ZVk. Kg 5 P/content/radar-course-3 -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 52

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 53

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 54

Oil slick Galicia, Spain 55

Los Angeles 56

Source: Graham 2001 Response to soil moisture 57

Crop moisture SAR image In situ irrigation Source: Graham 2001 58

Types of scattering of radar from different surfaces 59

Scattering 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