Slide 1

GEOGG141/GEOG3051 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

Slide 2

2 OVERVIEW OF NEXT 2 LECTURES Principles of RADAR, SLAR and SAR Characteristics of RADAR SAR interferometry Applications of SAR Summaries

Slide 3

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

Slide 4

4 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, SciTech 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.

Slide 5

5 Web resources, tutorials Canada http://www.ccrs.nrcan.gc.ca/resource/tutor/fundam/chapter3/01_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/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 Free data archive: http://www.infoterra.de/terrasar-x-archive/

Slide 6

6 9/8/91 ERS-1 (11.25 am), Landsat (10.43 am)

Slide 7

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

Slide 8

8 Ice

Slide 9

9 Oil slick Galicia, Spain

Slide 10

10 Nicobar Islands December 2004 tsunami flooding in red

Slide 11

11 Paris

Slide 12

12 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)

Slide 13

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

Slide 14

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

Slide 15

15 Principle of RADAR

Slide 16

16 Principle of ranging and imaging

Slide 17

17

Slide 18

18 ERS 1 and 2 geometry

Slide 19

19 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

Slide 20

20 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 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: http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses /Radar_Course_III/radar_equation.htm and Jensen Chapter 9

Slide 21

21 The Radar Equation The cross-section 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 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 Where: The Radar scattering cross section R = range P = power G = gain of antenna A = area of the antenna Because READ: http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar _Course_III/radar_equation.htm and Jensen Chapter 9 http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar _Course_III/radar_equation.htm

Slide 22

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

Slide 23

23 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

Slide 24

24

Slide 25

25

Slide 26

26 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

Slide 27

27 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

Slide 28

28

Slide 29

29 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/co06.en.html

Slide 30

30 Azimuth resolution (along track): RAR Target time in beam = arc length / v = S /v = S /vL a so resolution = S /L a v S Arc = S LaLa = beamwidth = /L a

Slide 31

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

Slide 32

32 Range and azimuth resolution (RAR) Range resolution (across track) L S R a L =antenna length S = slant range = height H/sin =wavelength Azimuth resolution (along track) cos : inverse relationship with angle L sin H Pulse length typically 0.4-1 s i.e. 8-200m Short pulse == higher R r BUT lower signal T = pulse length c = speed of light = depression angle (deg)

Slide 33

33 Azimuth resolution: SAR

Slide 34

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

Slide 35

35 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.

Slide 36

36 Some Spaceborne Systems

Slide 37

37 ERS 1 and 2 Specifications Geometric specifications Spatial resolution: along track /4.4sin Intermediate is depression angle, so depends on AND imaging geometry http://rst.gsfc.nasa.gov/Sect8/Sect8_2.html

Slide 55

55 Oil slick Galicia, Spain

Slide 56

56 Los Angeles

Slide 57

57 Response to soil moisture Source: Graham 2001

Slide 58

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

Slide 59

59 Types of scattering of radar from different surfaces

Slide 60

60 Scattering

Slide 61

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.