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Imaging Radar Introduction

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Title: Imaging Radar Introduction


1
Imaging Radar- Introduction
2
Types of Radars
  • Weather radar (angle scanning)
  • Side-looking Airborne radar (SLAR)
  • Synthetic Aperture Radar (SAR)
  • Imaging Radar Intereferometer
  • Ground-based radars (i.e., for range
    instrumentation)

our interest
3
Imaging Radar Definition
  • A radar which provides a visual representation of
    the geometric distribution of EM scattering
    properties of the terrain or an object under
    observation (Adam Kozma)
  • Principle Application Area (for this class)
  • remote sensing of the Earth, Moon and Planets

Examples (see JPL Imaging Radar Home Page)
  • ERS-1, J-ERS-1 (C, L)
  • SIR-C/X-SAR (L, C, X)
  • ERS-2 (C)
  • RADARSAT (C)

4
Information Available from Radar Echo
  • Range, R
  • Range rate, dR/dt
  • Position (from the pointing direction of the
    antenna)
  • Some influencing factors
  • frequency
  • polarity
  • incidence angle
  • look direction
  • terrain roughness
  • dielectric coeficient

controllable
not controllable
5
Radar Wavelength
  • Radar signals are EM waves and, as such, have a
    wavelength given by,
  • The wavelength is one of the most important
    factors influencing the radar imagery
    characteristics

6
Typical Radar Wavelengths
7
Speckle
  • SAR imagery of an area of uniform reflectivity
    shows a spatial brightness distribution of a
    granular pattern of bright and dark spots call
    speckle. This pattern has nothing to do with the
    texture of the terrain but is caused by the
    coherent nature of SAR sensor. Speckle results
    from the coherent addition of a large number of
    individual scatterers in an individual resolution
    cell (see figure).

Q
Q
I
I
LOOK g
LOOK gdg
8
Peculiarities of Imaging with EM Waves
  • We are most familiar with optical imagery (our
    eyes, photographs,) with which are associated
    wavelengths of 0.4 to 0.8 nm. Thats about the
    size of a human chromosome.
  • EM waves are thousands or millions of times
    longer. This fact is largely responsible for the
    non literal quality of SAR imagery.
  • Features that appear textured in optical imagery
    will frequently appear smooth in radar imagery.
  • Another factor is resolution. Radar imagery
    commonly has resolution cells on the order of
    meters.

9
Condition for Smoothness
smooth
rough
g is the look angle
10
Smooth???
11
Smooth???
12
Smooth???
13
Smooth???
14
Directional Properties of Radar Apertures
be measures the vertical (or elevation) angular
dimension of beam, while ba measures
the horizontal (or, sometimes called, azimuth)
dimension.
15
Radar Power Equation
Pr received power Pt transmitted
power Gt gain R1 range to target R2 return
range s target RCS Ae effective area
of receiving aperture
16
Target Radar Cross Section (RCS) sT
  • RCS is expressed in m2 or decibels relative to
    one square meter (dBsm)
  • RCS is a function of
  • target size and material
  • wavelength
  • polarization
  • man 1 m2 or 0 dBsm
  • car 3 m2 or 5 dBsm
  • truck 10 m2 or 10 dBsm

17
Terrain Radar Cross Section, Sigma Nought (s0)
18
s0 for some Common Objects
19
Radar Pulse Ranging
Accurate range measurement requires short
pulse alternatively, use a long coded pulse
(higher BW)
20
Coherent Radar can measure targets velocity
Vr
TX
R
RCV
LO
transmit
receive
After mixing filtering
Doppler Frequency
directly measures radial velocity
21
Carl Wileys Observation
  • Two fixed targets in the beam of a moving
    coherent radar can be distinguished by measuring
    the difference in their Doppler frequencies

22
Synthetic Aperture Radar (SAR)
  • Is a coherent radar
  • exploits the Doppler frequency generated by the
    motion of the antenna relative to the targets
  • is a signal processing instrument - uses heavy
    signal processing to accomplish tasks
  • by processing, can achieve resolutions far better
    than noncoherent or real-beam radars

23
Radar Incidence Geometry
24
Imaging Radar Capabilities
  • Active, all weather, day-night
  • multi frequency and multi polarization
  • large dynamic range (brightness)
  • large area coverage in short times
  • complements and/or supplements other remote
    sensing instruments

25
Imaging Radar Applications
  • Geology
  • Agiculture
  • Land use survey
  • Forestry
  • Environmental monitoring
  • Hydrology
  • Emergency response planning

26
SAR References
  • IEEE Transactions on Geoscience and Remote
    Sensing
  • Curlander, J. C., and R. N. McDonough, Synthetic
    Aperture Radar Systems and Signal Processing,
    New York, J. Wiley Sons, 1991.
  • Carrara W. G., R. S. Goodman, and R. M. Majewski,
    Spotlight Synthetic Aperture Radar Signal
    Processing Algorithms, Norwood, MA, Artech House,
    1995.
  • Jakowatz C. V., D. E. Wahl, P. H. Eichel, D. C.
    Ghiglia and P. A. Thompson, Spotlight-Mode
    Synthetic Aperture Radar A Signal Processing
    Approach, Kluwer Academic Publishers, 1996.

27
NASA JPL Imaging Radar Home Page
http//southport.jpl.nasa.gov/
  • How to get data
  • Applications
  • Commercial Applications of Space Based SAR
  • Imaging Radar Interferometry
  • Instruments
  • Characteristics of many current and recent
    operational commercial systems
  • Imaging Radar Reports
  • What is Imaging Radar?
  • SAR References
  • Excellent SAR Bibliography
  • SAR Imagery Gallery

28
Suggested exercise
  • Log on to the IEEE Explore page and do a search
    in IEEE GRS on keyword ERS 1. You will see a
    good size list of abstracts discussing a wide
    range of applications for SAR imagery. Try other
    keywords suggested to you in the lecture.
  • Go to the JPL Imaging Radar Homepage. Look for
    the report on Commercial Applications of Space
    Borne SAR. This report discusses many other uses
    for imaging radar.

29
Imaging Radar- SAR Principles
30
SAR Measurement Model
  • Observables q g
  • los constant angle-angle line which passes
    through target
  • Observables Rt qD
  • SC intersection of the isoRange sphere and the
    isoDoppler cone at target

EO Sensor
Radar Sensor
31
SAR Collection Geometry
L
Level flight path
Slant plane
Ground plane
Image patch center
See Appendix C in Jakowatz C. V., D. E. Wahl, P.
H. Eichel, D. C. Ghiglia and P. A. Thompson,
Spotlight-Mode Synthetic Aperture Radar A
Signal Processing Approach, Kluwer Academic
Publishers, 1996.
32
Slant Range Resolution
33
Linear FM Signal (CHIRP)
The CHIRP signal (the transmitted signal) is
given by gt st(n,t)-gtAtcos(2PifctPiK
(t-nT)2)
n 0 corresponds to the first pulse and
nnsamp-1 corresponds to the last.
It is customary to express this real signal as a
complex analytic signal (where tft-nT) gt
sx(n,t)-gtA0Rect(tf/taup)exp(j(2Pif
cPiK(tf2)))
34
Linear FM Signal (CHIRP) cont.
Define a rectangle function to represent the time
dependence of the pulse (use "Rect" for the inert
form) gt rectx-gtconvert(piecewise(abs(x)gt1/2,0,ab
s(x)lt1/2,1), Heaviside)
35
Linear FM Signal (CHIRP) cont.
36
The Received Signal
The received signal will be a replica of the
transmitted except the amplitude will be
modified and the time will be delayed by the
total time from the antenna to the target and
back. It is customary to assume that the antenna
is stationary while the pulse is transmitted and
received and moves in discrete increments between
pulses. Under this assumption and substituting
2Rt/c for the two way delay time, the received
signal has the form
37
Reference Signal
The reference function used to deramp the returns
is equivalent to a signal that returns from the
image center at R0
38
Deramped Signal
The demodulated (deramped) signal, formed by
multiplying the returned by the reference signal
is
39
Signal Phase Terms
Doppler frequency term
Range frequency term
Residual video phase term
40
Sampled Signal
where,
41
Deramp Processing
Transmit frequency
Near range
Far range
time
fc
t0
Frequency after deramp
A/D interval
Near range
Far range
42
Simple Range Compression Examples
43
Summary
  • Dispersed (stretch) waveforms may be used in
    place of CW burst to obtain high range resolution
    in imaging radars
  • A linear FM chirp is an example of a dispersed
    waveform commonly employed in radars, due to its
    desirable pulse compression properties
  • The bandwidth of the FM chirp is given by
  • The resulting slant-range resolution is given by
  • The ground-range resolution is
  • For equal bandwidths, an FM chirp pulse is
    dispersed in time by a factor equal to its
    time-bandwidth product,

44
Summary (cont)
  • Using the FM chirp allows greater transmitted
    energy per pulse (for a fixed level of microwave
    tube power) compared to that of the CW burst
  • The steps of deramp processing are
  • demodulation with in-phase and quadrature
    versions of the FM chirp, delayed appropriately
  • low-pass filtering
  • range compression (Fourier transformation)

45
Next Lecture
  • How to generate cross-range resolution in an
    imaging radar
  • some public domain imagery examples
  • radar image interpretation exercises
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