1. 2009-10 CEGEG046 / GEOG3051 Principles & Practice of Remote Sensing (PPRS) 8: RADAR 1
Dr. Mathias (Mat) Disney
UCL Geography
Office: 113, Pearson Building
Tel:

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

3. 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)

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
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, 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), 4 4

5. Web resources, tutorials
Canada
ESA
Miscellaneous:
Infoterra TERRASAR-X 5 5

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

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

8. Ice

9. Oil slick Galicia, Spain

10. Nicobar Islands
December 2004 tsunami flooding in red

11. Paris

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)

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

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

15. Principle of RADAR

16. Principle of ranging and imaging

18. ERS 1 and 2
geometry

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

22. 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

23. 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

25. Principle of Synthetic Aperture Radar SAR
Doppler frequency shift fD due to sensor movement
As target gets closer

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

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

28. Range and azimuth resolution (RAR)
Range resolution (across track)
Azimuth resolution (along track) S l H l R = = a L
L sinγ L =
antenna length S =
slant range = height H/sin λ =
wavelength
Pulse length typically 0.4-1s i.e m
Short pulse == higher Rr BUT lower signal
cos : inverse relationship with angle

29. Azimuth resolution: SAR

30. Azimuth resolution (along track): SAR
See: La S Ra
Previously, azimuth resolution Ra = S/L = H/Lsin where H = height
So, for synthetic aperture of 2Ra & nominal slant range S (H/sin) we see
Ra, SAR = S/2Ra = L/2
So Ra, SAR independent of H, and improves (goes down) as L goes down 30 30

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:  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. Speckle
Speckle appears as “noisy” fluctuations in brightness

35. 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.

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. 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

47. 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

48. Roughness Smooth, intermediate or rough?
Peake and Oliver (1971) – surface height variation h
Smooth: h < /25sin
Rough: h > /4.4sin 
Intermediate
 is depression angle, so depends on  AND imaging geometry 48 48

49. Oil slick Galicia, Spain

50. Los Angeles

51. Response to soil moisture
Source: Graham 2001

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

53. Types of scattering of radar from different surfaces

54. Scattering

55. 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: and Jensen Chapter 9

56. 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: and Jensen Chapter 9

57. Measured quantities Radar cross section [dBm2]
Bistatic scattering coefficient [dB]
Backscattering coefficient [dB]

58. 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

59. 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.