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2009-10 CEGEG046 / GEOG3051 Principles & Practice of Remote Sensing (PPRS) 8: RADAR 1. Dr. Mathias (Mat) Disney UCL Geography Office: 113, Pearson Building Tel: 7670 0592 1 Email: mdisney@ucl.geog.ac.uk www.geog.ucl.ac.uk/~mdisney. OVERVIEW AGENDA. Principles of RADAR, SLAR and SAR

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2009 10 cegeg046 geog3051 principles practice of remote sensing pprs 8 radar 1

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

overview agenda
OVERVIEW AGENDA
  • Principles of RADAR, SLAR and SAR
  • Characteristics of RADAR
  • SAR interferometry
  • Applications of SAR
  • Student summaries
lecture 1 principles and characteristics of radar slar and sar
LECTURE 1PRINCIPLES AND CHARACTERISTICS OF RADAR, SLAR AND SAR
  • Examples
  • Definitions
  • Principles of RADAR and SAR
  • Resolution
  • Frequency
  • Geometry
  • Radiometry
slide5

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

nicobar islands
Nicobar Islands

December 2004 tsunami flooding in red

definitions
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)
references
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.
web sites
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/
what is radar
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
what is radar1
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.
slide19

ERS 1 and 2

geometry

radar wavelength
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.
choice of wave length
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
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

Synthetic Aperture Radar (SAR)
principle of synthetic aperture radar sar
Principle of Synthetic Aperture Radar SAR

Doppler frequency due to sensor movement

azimuth resolution synthetic aperture
Azimuth resolution: synthetic aperture

v

R

Target

time spent in beam = arc length / v =

Ry / v = Rl / vLa

range and azimuth resolution rar
Range and azimuth resolution (RAR)

Range resolution (across track)

Azimuth resolution (along track)

S

l

=

R

a

L

L

=

antenna length

S

=

slant range = height/sin

λ

=

wavelength

cos : inverse relationship with angle

important point
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.
ers 1 and 2 specifications
ERS 1 and 2 Specifications

Geometric 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 

speckle
Speckle
  • Speckle appears as “noisy” fluctuations in brightness
speckle1
Speckle
  • Fading and speckle are the inherent “noise-like” processes which degrade image quality in a coherent imaging system.
  • Local constructive and destructive interference appears in the image as bright and dark speckles, respectively.
  • Using independent data sets to estimate the same ground patch, by average independent samples, can effectively reduce the effects of speckle. This can be done 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.
speckle3
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
multi looking
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)
radiometric aspects the radar equation
Radiometric aspects – the RADAR equation
  • The brightness of features in an image is usually 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
returned energy
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

Rough

Smooth

roughness smooth intermediate or rough
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 imaging geometry

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

50

response to soil moisture
Response to soil moisture

Source: Graham 2001

crop moisture
Crop moisture

SAR image

In situ irrigation

Source: Graham 2001

the radar equation
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

the radar equation1

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

The Radar Equation

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

And Jensen Chapter 9

measured quantities
Measured quantities
  • Radar cross section [dBm2]
  • Bistatic scattering coefficient [dB]
  • Backscattering coefficient [dB]
the radar equation point targets
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
calibration of sar
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.