CEGEG046 / GEOG3051 Principles & Practice of Remote Sensing (PPRS) 8: RADAR 1

Transcription

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

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

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

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

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,

6 Ice 6

7 Oil slick Galicia, Spain 7

8 Nicobar Islands December 2004 tsunami flooding in red 8

9 Paris 9

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) 10

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. 11

12 Web sites Canada chapter3/01_e.php ESA spaceborne/radar_courses/ 12

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 13

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. 14

15 Principle of RADAR 15

16 Principle of ranging and imaging 16

17 Radar Pulse 17

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19 ERS 1 and 2 geometry 19

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. 20

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

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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 24

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 25

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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 27

28 Azimuth resolution: synthetic aperture v L a ψ R Target time spent in beam = arc length / v = Rψ / v = Rλ / vl 28 a

29 Resolution τ 29

30 Range and azimuth resolution (RAR) Range resolution (across track) Azimuth resolution (along track) R r = Tc 2cos! R a = S λ L T = duration of RADAR pulse T = o f the radar c = speed of light c = speed of light γ= depression angle ã = depression angle L = antenna length S = slant range = height/sinγ λ = wavelength cos : inverse relationship with angle 30

31 Resolution of SAR 31

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. 32

33 Some Spaceborne Systems Launch Agency properties resolution swath ERS-1 ERS (-1997) 1995 ESA C-VV 25 m 100 km Radarsat 1995 CSA C-HH m km JERS NASDA L-HH 18 m 76 km SIR-C/X-SAR 1994 (2x10 days) NASA DARA / ASI L,C, X polarimetric 30 m km 33

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 34

35 Speckle Speckle appears as noisy fluctuations in brightness 35

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. 36

37 Speckle 37

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 38

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) 39

40 Speckle 40

41 Principle of ranging and imaging 41

42 Geometric effects 42

43 Shadow 43

44 Foreshortening 44

45 Layover 45

46 Layover 46

47 Los Angeles 47

48 Radiometric aspects the RADAR equation P r = (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 spaceborne/radar_courses/radar_course_iii/ radar_equation.htm 48

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 49

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

51 Oil slick Galicia, Spain 51

52 Los Angeles 52

53 Response to soil moisture Source: Graham

54 Crop moisture SAR image In situ irrigation Source: Graham

55 Types of scattering of radar from different surfaces 55

56 Scattering 56

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: Radar_Courses/Radar_Course_III/radar_equation.htm and Jensen Chapter 9 57

58 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 R = range P = power G = gain of antenna A = area of the antenna Because 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. 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 READ: radar_equation.htm And Jensen Chapter 9 58

59 Measured quantities Radar cross section [dbm 2 ] lim E! = r # $ 4" r 2 s i E 2 2 Bistatic scattering coefficient [db] "! 0 4 = lim r r $ % Acos# i 2 s 2 E i E 2 Backscattering coefficient [db]! 0 4" r = r # $ A 2 s 2 E lim i E 2 59

60 The Radar Equation: Point targets Power received 1 1 P = P G " r t t 2 2 4! R 4! R A r G t is the transmitter gain, A r is the effective area of receiving antenna and σ the effective area of the target. Assuming same transmitter and receiver, A/G=λ 2 /4π 2 2 " G! P = P $ r t 3 4 ( 4# ) R 60

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. 61