2009-10 CEGEG046 / GEOG3051Principles & Practice of Remote Sensing (PPRS)8: RADAR 1

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

2

OVERVIEW AGENDA

• Principles of RADAR, SLAR and SAR

• Characteristics of RADAR

• SAR interferometry

• Applications of SAR

• Student summaries

3

LECTURE 1PRINCIPLES AND CHARACTERISTICS OFRADAR, SLAR AND SAR

• Examples

• Definitions

• Principles of RADAR and SAR

• Resolution

• Frequency

• Geometry

• Radiometry

49/8/91 ERS-1 (11.25 am), Landsat (10.43 am)

5

The image at the topwas acquired throughthick cloud cover by theSpaceborne ImagingRadar-C/X-bandSynthetic ApertureRadar (SIR-C/X-SAR)aboard the spaceshuttle Endeavour onApril 16, 1994.

The image on thebottom is an opticalphotograph taken by theEndeavour crew underclear conditionsduring the second flightof SIR-C/X-SAR onOctober 10, 1994

6

Ice

7

Oil slickGalicia, Spain

8

Nicobar Islands

December2004

tsunamiflooding in

red

9

Paris

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)

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: Activeand Passive (3 vols), 1981, 1982, 1986

• S. Kingsley and S. Quegan, Understanding Radar Systems, SciTechPublishing.

• C. Oliver and S. Quegan, Understanding Synthetic Aperture RadarImages, Artech House, 1998.

• Woodhouse I H (2000) Tutorial review. Stop, look and listen: auditoryperception analogies for radar remote sensing, International Journal ofRemote Sensing 21 (15), 2901-2913.

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

12

Web sites

Canada

• http://www.ccrs.nrcan.gc.ca/resource/tutor/fundam/chapter3/01_e.php

• ftp://ftp2.ccrs.nrcan.gc.ca/ftp/ad/MAS/fundamentals_e.pdf

ESA

• http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/

13

What is RADAR?

• Radio Detection and Ranging

• Radar is a ranging instrument

• (range) distances inferred from time elapsed betweentransmission of a signal and reception of the returnedsignal

• imaging radars (side-looking) used to acquire images(~10m - 1km)

• altimeters (nadir-looking) to derive surface heightvariations

• scatterometers to derive reflectivity as a function ofincident angle, illumination direction, polarisation, etc

14

What is RADAR?

• A Radar system has three primary functions:

- It transmits microwave (radio) signals towards ascene

- It receives the portion of the transmitted energybackscattered from the scene

- It observes the strength (detection) and the timedelay (ranging) of the return signals.

• Radar provides its own energy source and, therefore,can operate both day or night. This type of system isknown as an active remote sensing system.

15

Principle of RADAR

16

Principle ofranging andimaging

17

Radar Pulse

18

19

ERS 1 and 2geometry

20

Radar wavelength

• Most remote sensing radars operate at wavelengthsbetween 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 orinto a surface layer is increased with longerwavelengths. Radars operating at wavelengths > 2 cmare not significantly affected by cloud cover. Raindoes become a factor at wavelengths < 4 cm.

21

22

Comparison of C band and L band SAR

C-band

L-band

23

24

Choice of wave length

• Radar wavelength should be matched to the size ofthe 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 giveoptimum measurement of vegetation

25

Synthetic Aperture Radar (SAR)

• Imaging side-looking accumulates data along path –ground surface “illuminated” parallel and to one sideof the flight direction. Data, processing is needed toproduce radar images.

• The across-track dimension is the “range”. Near rangeedge is closest to nadir; far range edge is farthestfrom the radar.

• The along-track dimension is referred to as “azimuth”.

• Resolution is defined for both the range and azimuthdirections.

• Digital signal processing is used to focus the imageand obtain a higher resolution than achieved byconventional radar

26

27

Principle ofSyntheticAperture RadarSAR

Dopplerfrequency due tosensormovement

28

Azimuth resolution: synthetic aperture

Target

time spent in beam = arc length / v =

R v = R / vLa

v

R

29

Resolution

τ

30

Range and azimuth resolution (RAR)

cos2

TcRr

T = duration of the radar pulsec = speed of lightγ = depression angle

Range resolution (across track)

L

SRa

L = antenna lengthS = slant range = height/sinλ = wavelength

Azimuth resolution (along track)

cos : inverse relationship with angle

31

Resolution of SAR

32

Important point

• Resolution cell (i.e. the cell defined by the resolutionsin the range and azimuth directions) does NOT meanthe same thing as pixel. Pixel sizes need not be thesame thing. This is important since (i) theindependent elements in the scene are resolutionscells, (ii) neighbouring pixels may exhibit somecorrelation.

33

Some Spaceborne Systems

Launch Agency properties resolutionsw ath

ERS-1ERS-2

1991 (-1997)1995

ESA C-VV 25 m100 km

Radarsat 1995 CSA C-HH 10-100 m40-500 km

JERS 1992-1998 NASDA L-HH 18 m76 km

SIR-C/X-SAR 1994 (2x10 days) NASADARA / ASI

L,C , Xpolarim etric

30 m15-90 km

34

ERS 1 and 2 Specifications

Geometric specificationsSpatial resolution: along track <=30 macross-track <=26.3 mSwath width: 102.5 km (telemetered)80.4 km (full performance)Swath standoff: 250 km to the right of the satellitetrackLocalisation accuracy: along track <=1 km;across-track <=0.9 kmIncidence angle: near swath 20.1deg.mid swath 23deg.far swath 25.9degIncidence angle tolerance: <=0.5 deg.

Radiometric specifications:Frequency: 5.3 GHz (C-band)Wave length: 5.6 cm

35

Speckle

• Speckle appears as“noisy” fluctuations inbrightness

36

Speckle

• Fading and speckle are the inherent “noise-like” processes whichdegrade image quality in a coherent imaging system.

• Local constructive and destructive interference appears in theimage as bright and dark speckles, respectively.

• Using independent data sets to estimate the same ground patch,by average independent samples, can effectively reduce theeffects of speckle. This can be done by:

• Multiple-look filtering, separates the maximum synthetic apertureinto smaller sub-apertures generating independent looks attarget 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.

37

Speckle

38

Speckle

• Radar images are formed coherently andtherefore inevitably have a “noise-like”appearance

• Implies that a single pixel is not representative ofthe backscattering

• “Averaging” needs to be done

39

Multi-looking

• Speckle can be suppressed by “averaging” severalintensity images

• This is often done in SAR processing

• Split the synthetic aperture into N separate parts

• Suppressing the speckle means decreasing the widthof the intensity distribution

• We also get a decrease in spatial resolution by thesame factor (N)

• Note this is in the azimuth direction (because itrelies on the motion of the sensor which is in thisdirection)

40

Speckle

41

Principle ofranging andimaging

42

Geometric effects

43

Shadow

44

Foreshortening

45

Layover

46

Layover

47

LosAngeles

48

Radiometric aspects – the RADAR equation

• The brightness of features in an image is usually acombination of several variables. We can group thesecharacteristics into three areas which fundamentallycontrol 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

• http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar_Course_III/radar_equation.htm

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

50

RoughnessSmooth, intermediate or rough?

• Peake and Oliver (XX) – surface height variation h

– Smooth: h < /25sin β

– Rough: h > /4.4sin β

– Intermediate

– β is depression angle, so depends on AND imaginggeometry

http://rst.gsfc.nasa.gov/Sect8/Sect8_2.html

51

Oil slickGalicia, Spain

52

LosAngeles

53

Response to soil moisture

Sou

rce

:G

raham

2001

54

Crop moisture

SAR image

In situ irrigation

Source: Graham 2001

55

Types ofscattering ofradar fromdifferentsurfaces

56

Scattering

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 scatteringfrom an isolated radar target (scatterer) is shown.When a power Pt is transmitted by an antenna with gain Gt , the power per unitsolid angle in the direction of the scatterer is Pt Gt, where the value of Gt in thatdirection is used.

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

58

The Radar Equation

The cross-section σ is a function of the directions of the incident wave and thewave toward the receiver, as well as that of the scatterer shape and dielectricproperties.

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

We may rewrite the radar equation as two alternative forms, one interms of the antenna gain and the other in terms of the antennaarea

Where: The Radar scattering cross section

R = rangeP = powerG = gain of antennaA = area of the antenna

Because

READ:http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar_Course_III/radar_equation.htmAnd Jensen Chapter 9

59

Measured quantities

• Radar cross section [dBm2]

• Bistatic scattering coefficient [dB]

• Backscattering coefficient [dB]

lim | |

| |r r

E

E

s

i4 2

2

2

0 4 2 2

2

lim

cos

| |

| |r

r

Ai

E

E

s

i

0 4 2 2

2

lim | |

| |r

r

A

E

E

s

i

60

The Radar Equation: Point targets

• Power received

• Gt is the transmitter gain, Ar is the effective area ofreceiving antenna and the effective area of the target.Assuming same transmitter and receiver, A/G=2/4

Pr

PtG

tR R

Ar

1

42

1

42

Pr

Pt

G

R

2 2

43 4

( )

61

Calibration of SAR

• Emphasis is on radiometric calibration todetermine the radar cross section

• Calibration is done in the field, using test siteswith transponders.