Lecture 11: Radar and Lidar Remote Sensing

Report
An Introduction to Radar and
Lidar Remote Sensing
Credit to: Weile Wang
Gustav Klimt (1862-1918), Der Park
With materials from Drs. Jeff Dozier (UCSB),
Howard Zebker (Stanford), Jacob van Zyl
(JPL), Alan Strahler (Boston U.), Ralph
Dubayah (U. Maryland), Michael Lefsky (U.
Colorado), Guoqing Sun (U. Maryland), and
many others.
Outline
 Radar Basics, PPI, SLAR, SAR, InSAR;
 Radar Equation, Imaging Geometry, Geometric Distortion,
Speckle, Polarization, Interferometry;
 Lidar, Waveform, Footprint, Forest Structure Measurement;
 SRTM, LVIS, SMAP, GRACE, DESDynI, and Echidna.
Active and passive remote sensing
• Passive: uses natural energy, either reflected
sunlight or emitted thermal or microwave
radiation
• Active: sensor creates its own energy
– Transmitted toward Earth
– Interacts with atmosphere and/or surface
– Reflects back toward sensor (backscatter)
Common active remote sensing systems
• Radar (RAdio Detection And Ranging)
– long-wavelength microwaves (1-100cm)
– recording the amount of energy back-scattered from the terrain
• Lidar (LIght Detection And Ranging)
– short-wavelength laser light (e.g., 0.90 µm)
– recording the light back-scattered from the terrain or atmosphere
• Sonar (SOund Navigation And Ranging)
– sound waves through a water column
– recording the amount of energy back-scattered from the water
column or the bottom
Microwave Bands
Frequency  
c

where c  speed of light
=3.00  108 m s -1
Useful trick
30
  GHz  
  cm 
What is Radar?
TRANSMITTER
RADAR PULSE
CIRCULATOR
"TARGET"
RECEIVER
•
•
RADAR = Radio Detection And Ranging
Since radar pulses propagate at the speed of light, the difference to the “target”
is proportional to the time it takes between the transmit event and reception of
the radar echo
Ranging: Distance Measurement
?
?
c = speed of light
= 3.00 × 108 m/s
Mapping Multiple Objects: PPI Radar Display
PPI=Plan Position Indicator
The Radar Equation (1)



• Gt is the “Antenna Gain”;
• σ is the “cross section” of the target.

The radar equation (2)
• The radar equation represents the physical dependences of
the transmit power, that is the wave propagation up to the
receiving of the echo-signals. The power PE returning to
the receiving antenna is given by the radar equation,
depending on the transmitted power Pt, the slant range R,
and the reflecting characteristics of the aim (described as
the radar cross-section σ). At known sensibility of the
radar receiver the radar equation determines the achieved
by a given radar set theoretically maximum range.
Furthermore one can assess the performance of the radar
set with the radar equation.
• Suggest reading:
http://www.radartutorial.eu/01.basics/rb13.en.html
The radar equation (3)
• antenna gain: Since a spherical segment
emits equal radiation in all direction (at
constant transmit power), if the power
radiated is redistributed to provide more
radiation in one direction, then this results
an increase of the power density in direction
of the radiation. This effect is called
antenna gain.
Imaging Radar: Side-Looking Airborne Radar
Imaging Geometry
azimuth refers to the along-track dimension parallel to the flight direction.
Swath width refers to the strip of the Earth’s surface from which data are collected by
a side-looking airborne radar (SLAR)
Forming an image
Radar Reflections from Flat Ground
• The Earth plane
surrounding a radar
antenna has a significant
impact on the vertical
polar diagram.
The combination of the
direct and re-reflected
ground echo changes the
transmitting and receiving
patterns of the antenna.
Nomenclature
•
•
•
•
•
•
•
nadir
azimuth flight direction
look direction
range (near and far)
depression angle (γ)
incidence angle (θ)
altitude above-groundlevel, H
• polarization
Radar geometry
Range resolution
Rr 
Calculate Rr
c
2cos 

pulse length  speed of light
2cos  depression angle 
Side-looking airborne radar (SLAR)
H 
Ra 
L  cos 
H is the height of the antenna,
(height of the airplane)
L is the geometric length of the antenna,
λ is the wavelength of the transmitted pulses, and
θ is the incidence angle(1) L · cos θ
Azimuth resolution
S 
L
slant range  wavelength
antenna length
Ra 
Question:
Why is wavelength
important in
determining Ra?
• The equation shows, that with increasing
altitude decreases the azimuthal resolution
of SLAR. A very long antenna (i.e., large L)
would be required to achieve a good
resolution from a satellite. Synthetic
Aperture Radar (SAR) is used to acquire
higher resolution.
• For an SLAR with the following characteristics:
λ = 1 cm,
L = 3 m,
H = 6000 m,
θ = 60°, and
tp = 100 ns,
has got a resolution of
Ra = ??? and
Rr = ??? m
• Note: The same SLAR on a platform in a height of 600 km
would achieve an azimuth-resolution of Ra = ???.
Synthetic aperture radar (SAR)
• A Synthetic Aperture Radar (SAR), or SAR, is a
coherent mostly airborne or spaceborne
sidelooking radar system which utilizes the flight
path of the platform to simulate an extremely large
antenna or aperture electronically, and that
generates high-resolution remote sensing imagery.
• Read
http://www.radartutorial.eu/20.airborne/ab07.en.ht
ml
•
•
•
•
The SAR works similar of a phased array, but contrary of
a large number of the parallel antenna elements of a
phased array, SAR uses one antenna in time-multiplex.
The different geometric positions of the antenna elements
are result of the moving platform now.
The SAR-processor stores all the radar returned signals, as
amplitudes and phases, for the time period T from position
A to D. Now it is possible to reconstruct the signal which
would have been obtained by an antenna of length v · T,
where v is the platform speed. As the line of sight
direction changes along the radar platform trajectory, a
synthetic aperture is produced by signal processing that
has the effect of lengthening the antenna. Making T large
makes the „synthetic aperture” large and hence a higher
resolution can be achieved.
As a target (like a ship) first enters the radar beam, the
backscattered echoes from each transmitted pulse begin to
be recorded. As the platform continues to move forward,
all echoes from the target for each pulse are recorded
during the entire time that the target is within the beam.
The point at which the target leaves the view of the radar
beam some time later, determines the length of the
simulated or synthesized antenna. The synthesized
expanding beamwidth, combined with the increased time a
target is within the beam as ground range increases,
balance each other, such that the resolution remains
constant across the entire swath.
The achievable azimuth resolution of a SAR is
approximately equal to one-half the length of the actual
(real) antenna and does not depend on platform altitude
(distance).
Radar Image Elements
Roughness
Smooth h 
Rough h 

25sin 

4.4 sin 
Sources of radar
backscattering from a
vegetation canopy
Question:
Does the strength of the
backscattering vary with
frequencies?
Strength of scattering from a pine stand depends on frequency
Polarization
• 1st letter is
transmitted
polarization,
2nd is received
– Can have
VV, HH
(like)
– HV, VH
(cross)
Polarization with
radar
a.
Ka - ban d, HH p ol arizati on
lo o k di rect io n
b.
Ka - ban d, HV po lari zati on
N
Polarization with radar
•
RADARSAT, C-band radar (5.4 GHz) with HH, VV, HV, and VH
polarizations
Geometric Distortions
Geometric Distortions
(or: Slant-range distortion)
• Foreshortening
• Layover
• Shadow
See handout
Slant-range distortion
The slant-range distortion occurs because the radar is measuring the distance to
features in slant-range rather than the true horizontal distance along the
ground. This results in a varying image scale, moving from near to far range.
• Foreshortening occurs when the radar beam reaches the base of a tall feature
tilted towards the radar (e.g. a mountain) before it reaches the top. Because the
radar measures distance in slant-range, the slope (from point a to point b) will
appear compressed and the length of the slope will be represented incorrectly
(a' to b') at the image plane.
•
Layover occurs when the radar beam reaches the top of a tall feature (b)
before it reaches the base (a). The return signal from the top of the feature will
be received before the signal from the bottom. As a result, the top of the
feature is displaced towards the radar from its true position on the ground, and
„lays over” the base of the feature (b' to a').
•
The shadowing effect increases with greater incident angle θ, just as our
shadows lengthen as the sun sets.
Foreshortening
Layover
• Extreme case of
foreshortening,
when incidence
angle is less than
slope angle toward
radar (i.e. θ<α)
– cannot be
corrected
– got to be careful
in the mountains
Shadow
• When slope away from radar is steeper than the
depression angle, i.e. –α > γ
Speckle: Random Interference
•
Grainy salt-and-pepper pattern in
radar imagery
– Caused by coherent nature of the
radar wave, which causes
random constructive and
destructive interference, and
hence random bright and dark
areas in a radar image
•
Reduced by multiple looks
– processing separate portions of
an aperture and recombining
these portions so that
interference does not occur
InSAR: Adding the Z-dimension
Landsat overlaid on topography from SRTM – Malaspina Glacier, Alaska
InSAR Geometry
 interferometric phase
 incidence angle
 antenna angle
B baseline length
 wavelength
 range
From interferometry   
B 2    

2    B sin     
2
ht  h p   cos 
Can you derive the
equation? Extra credit (point)
2
The following materials are FYI. Not required
for exam.
Shuttle Radar Topography Mission
Links to movies
SRTM Global Coverage
SRTM Elevation + Landsat Imagery
Perspective with Landsat Overlay: Antelope Valley, California
From Radar to Lidar
•
•
LIDAR = Light Detection
And Ranging
Using laser instead of
microwave
Measuring Forest Structure
Continuous Waveform, Large Footprint
Discrete Waveform, Small Footprint
Canopy Topography
Ground-Based Lidar (Echidna)
A real Echidna—in the forest
Data Examples
Airborne Lidar Instrument: LVIS
Space-borne: ICESat and GLAS
Synthesis of Lidar, Radar, and optical sensors
Global Map of Tree Height
Other Relative Sensors: GRACE
GRACE: Gravity Recovery And Climate Experiment
GRACE: The 2012 Drought over US
Soil Moisture Active & Passive (SMAP)
SMAP Instruments
Radar
•Frequency: 1.26 GHz
•Polarizations: VV, HH, HV
•Data collection:
•High-resolution/high-rate data collected for ground SAR processing
•Low-resolution real-aperture data collected continuously
Radiometer
•Frequency: 1.41 GHz
•Polarizations: H, V, U
•Relative accuracy: 1.3 K
•Data collection: Continuous over full scan
DEformation, Ecosystem Structure, and Dynamics of Ice
DESDynI Instruments
4. Instrument Design & Performance
L-Band Synthetic Aperture Radar
Multi-beam Lidar
Laser
Radiators
Interferometric SAR
Dual-Pol 3-Beams
Quad-Pol 6-Beams
Right or Left Point
Lasers
Flight
Direction
~350km
Beam Spacing
1 km
Star Tracker
Summary
 Radar Basics, PPI, SLAR, SAR, InSAR;
 Radar Equation, Imaging Geometry, Geometric Distortion,
Speckle, Polarization, Interferometry;
 Lidar, Waveform, Footprint, Forest Structure Measurement;
 SRTM, LVIS, SMAP, GRACE, DESDynI, and Echidna.
• For an SLAR with the following characteristics:
λ = 1 cm,
L = 3 m,
H = 6000 m,
θ = 60°, and
tp = 100 ns,
has got a resolution of
Ra = 40 m and
Rr = 17.3 m
• Note:
Homework-8
1.
2.
3.
4.
Derive the radar equation.
Derive the raindrop size equation from the radar equation you derived in
question 1.
The SLAR on a platform in a height of 600 km would achieve an
azimuth-resolution of Ra = ?. (other needed variable are the same given
in class activity)
(extra credit) NASA Tropical Rainfall Measuring Mission (TRMM) has
a single frequency radar at the Ku-band 13.8 GHZ particularly sensitive
to moderate rain rates. With a single frequency, the TRMM radar is able
to retrieve drop size. Assume that raindrops range from 1/100 inch
(.0254 centimeter) to 1/4 inch (.635 centimeter) in diameter. Antenna
Gain is 1.698, instrument size is 0.5 m, plot the relation of ratio of Pt/Pr
vs. raindrop size, assume height of rain is 1 km.

similar documents