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

Dr. Mathias (Mat) DisneyUCL GeographyOffice: 113, Pearson BuildingTel: 7670 05921Email: mdisney@ucl.geog.ac.ukwww.geog.ucl.ac.uk/~mdisney

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OVERVIEW AGENDA

• Principles of RADAR, SLAR and SAR• Characteristics of RADAR• SAR interferometry• Applications of SAR• Summaries

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PRINCIPLES AND CHARACTERISTICS OF RADAR, 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)

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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, 1994.

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, 1994

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Ice

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Oil slickGalicia, Spain

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Nicobar IslandsDecember

2004 tsunami

flooding in red

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Paris

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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)

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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, 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.

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

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Web sites

Canada• http://www.ccrs.nrcan.gc.ca/resource/tutor/fundam/chapter

3/01_e.phpESA• http://earth.esa.int/applications/data_util/SARDOCS/space

borne/Radar_Courses/

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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

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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.

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Principle of RADAR

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Principle of ranging and imaging

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Radar Pulse

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ERS 1 and 2geometry

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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.

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Comparison of C band and L band SAR

C-band

L-band

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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• In general, C-band is a good compromise• New airborne systems combine X and P band to give

optimum measurement of vegetation

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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

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Principle of Synthetic Aperture Radar

SARDoppler 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

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Azimuth resolution: synthetic aperture

Target

time spent in beam = arc length / v =

Rv = R / vLa

v

R

ψ

La

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Resolution

τ

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Range and azimuth resolution (RAR)

Range resolution (across track)

LS

Ra

L = antenna length S = slant range = height/sinλ = wavelength

Azimuth resolution (along track)

cos : inverse relationship with angle

T = duration of RADAR pulsec = speed of lightγ= depression angle

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Resolution of SAR

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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.

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Some Spaceborne Systems

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ERS 1 and 2 SpecificationsGeometric 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 20.1deg. 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 

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Speckle

• Speckle appears as “noisy” fluctuations in brightness

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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.

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Speckle

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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

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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)

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Speckle

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Principle of ranging and imaging

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Geometric effects

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Shadow

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Foreshortening

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Layover

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Layover

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LosAngeles

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Radiometric aspects – the RADAR equation

• 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

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

Pr = (Power per unit area at target )

Eff. scatt. area of target

Spread loss of reflected signal

Eff. Antennae area

× × ×

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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

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RoughnessSmooth, intermediate or rough?

• Jensen (2002; p314) – surface height variation h– Smooth: h < /25sin β– Rough: h > /4.4sin β– Intermediate– β is depression angle, so depends on AND imaging

geometry

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

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Oil slickGalicia, Spain

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LosAngeles

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Response to soil moisture

Sou

rce:

Gra

ham

200

1

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Crop moisture

SAR image

In situ irrigation

Source: Graham 2001

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Types of scattering of radar from different surfaces

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Scattering

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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 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/Radar_Course_III/radar_equation.htm and Jensen Chapter 9

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The Radar Equation

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

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

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

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Measured quantities

• Radar cross section [dBm2]

• Bistatic scattering coefficient [dB]

• Backscattering coefficient [dB]

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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

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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.