Active Microwave Remote
Sensing
Lecture 11
Oct 5, 2005
Reading materials: Chapter 9
Basics of passive and active RS
†
Passive: uses natural energy, either reflected
sunlight (solar energy) or emitted thermal or
microwave radiation.
†
Active: sensor creates its own energy
„
„
„
Transmitted toward Earth or other targets
Interacts with atmosphere and/or surface
Reflects back toward sensor (backscatter)
Widely used active RS systems
†
RADAR: RAdio Detection And Ranging (read p285 for an explanation)
„
†
LIDAR: LIght Detection And Ranging
„
†
Long-wavelength microwaves (1 – 100 cm)
Short-wavelength laser light (UV, visible, near IR)
SONAR: SOund Navigation And Ranging
„
„
„
„
Sound waves through a water column.
Sound waves are extremely slow (300 m/s in air, 1,530 m/s in seawater)
Bathymetric sonar (measure water depths and changes in bottom
topography )
Imaging sonar or sidescan imaging sonar (imaging the bottom
topography and bottom roughness)
Microwaves
Band
BandDesignations
Designations
(common
Wavelength
(commonwavelengths
wavelengths
Wavelength((λλ)) Frequency
Frequency((υυ))
shown
inincm
ininGHz
shownininparentheses)
parentheses)
cm
GHz
_______________________________________________
_______________________________________________
Ka
Ka(0.86
(0.86cm)
cm)
0.75
0.75--1.18
1.18
40.0
40.0toto26.5
26.5
KK
KKu
u
XX(3.0
(3.0and
and3.2
3.2cm)
cm)
CC(7.5,
(7.5,6.0
6.0cm)
cm)
SS(8.0,
(8.0,9.6,
9.6,12.6
12.6cm)
cm)
LL(23.5,
(23.5,24.0,
24.0,25.0
25.0cm)
cm)
PP(68.0
(68.0cm)
cm)
1.18
1.18--1.67
1.67
1.67
1.67--2.4
2.4
2.4
2.4 --3.8
3.8
3.8
3.8 --7.5
7.5
7.5
7.5 --15.0
15.0
15.0
15.0--30.0
30.0
30.0
30.0--100
100
26.5
26.5toto18.0
18.0
18.0
18.0toto12.5
12.5
12.5
12.5--8.0
8.0
8.0
8.0 --4.0
4.0
4.0
4.0 --2.0
2.0
2.0
2.0 --1.0
1.0
1.0
1.0 --0.3
0.3
Two active radar imaging systems
In World War II, ground based radar was used to detect incoming planes
and ships.
Imaging RADAR was not developed until the 1950s (after World War II).
Since then, side-looking airborne radar (SLAR) has been used to get
detailed images of enemy sites along the edge of the flight field.
†
Real aperture radar
„
„
†
Aperture means antenna
A fixed length (for example: 1 - 11m)
Synthetic aperture radar (SAR)
„
„
1m (11m) antenna can be synthesized electronically into a 600m (15
km) synthetic length.
Most (air-, space-borne) radar systems now use SAR.
Advantages of Active Radar RS
Primary
†
†
†
Radar can penetrate clouds (so it’s all weather)
Acquisitions can be obtained 24/7
Provides info on surface roughness, dielectric
properties, moisture content
Secondary
†
†
†
Can penetrate vegetation, ice, snow, and dry sand
Very accurate change detection - interferometry
Can produce altimetry products: DEM’s – Digital
Elevation Models
Principle of SLAR
Radar Nomenclature and Geometry
o
Lo
k/R
i
ed
g
an
n
tio
c
e
r
Azimuth flight direction
Flightline groundtrack
Near range
Far range
Radar
Radar Nomenclature
Nomenclature
••nadir
nadir
••azimuth
azimuth(or
(orflight)
flight)direction
direction
••look
look(or
(orrange)
range)direction
direction
••range
range(near,
(near,middle,
middle,and
andfar)
far)
••depression
depressionangle
angle((γγ))
••incidence
incidenceangle
angle((θθ))
γ
••altitude
-ground-level, HH
altitudeabove
above-ground-level,
••polarization
polarization θ
Polarization
†
†
†
†
†
Unpolarized energy
vibrates in all possible
directions perpendicular to
the direction of travel.
The pulse of
electromagnetic energy is
filtered and sent out by the
antenna may be vertically
or horizontally polarized.
The pulse of energy
received by the antenna
may be vertically or
horizontally polarized
VV, HH – like-polarized
imagery
VH, HV- cross-polarized
imagery
†
a.
K a - band, HH polarization
look direction
b.
K a - band, HV polarization
N
Lava flow in
north-center
Arizona
Slant-range vs. Ground-range
geometry
Radar
Radarimagery
imageryhas
hasaadifferent
differentgeometry
geometrythan
thanthat
thatproduced
producedby
bymost
most
conventional
conventionalremote
remotesensor
sensorsystems,
systems,such
suchas
ascameras,
cameras,multispectral
multispectralscanners
scanners
or
orarea-array
area-arraydetectors.
detectors.Therefore,
Therefore,one
onemust
mustbe
bevery
verycareful
carefulwhen
whenattempting
attempting
totomake
makeradargrammetric
radargrammetricmeasurements.
measurements.
••Uncorrected
Uncorrectedradar
radarimagery
imageryisisdisplayed
displayedininwhat
whatisiscalled
calledslant-range
slant-range
geometry,
geometry,i.e.,
i.e.,ititisisbased
basedon
onthe
theactual
actualdistance
distancefrom
fromthe
theradar
radartotoeach
eachof
ofthe
the
respective
respectivefeatures
featuresininthe
thescene.
scene.
•• ItItisispossible
possibletotoconvert
convertthe
theslant-range
slant-rangedisplay
displayinto
intothe
thetrue
trueground-range
ground-range
display
displayon
onthe
thex-axis
x-axisso
sothat
thatfeatures
featuresininthe
thescene
sceneare
areinintheir
theirproper
proper
planimetric
planimetric(x,y)
(x,y)position
positionrelative
relativetotoone
oneanother
anotherininthe
thefinal
finalradar
radarimage.
image.
†
Most radar systems
and data providers
now provide the data
in ground-range
geometry
Range (or across-track) Resolution
Rr =
†
†
t ⋅c
2 cos γ
t.c called pulse
length. It seems the
short pulse length
will lead fine range
resolution.
However, the shorter
the pulse length, the
less the total amount
of energy that
illuminates the
target.
Pulse duration (t)
= 0.1 x 10 -6 sec
t.c/2
t.c/2
Azimuth (or along-track) Resolution
S ⋅λ
Ra =
D
†
†
†
The shorter
wavelength and longer
antenna will improve
azimuth resolution.
The shorter the
wavelength, the poorer
the atmospheric and
vegetation penetration
capability
There is practical
limitation to the
antenna length, while
SAR will solve this
problem.
AAmajor
majoradvance
advanceininradar
radarremote
remotesensing
sensinghas
hasbeen
beenthe
theimprovement
improvementininazimuth
azimuthresolution
resolutionthrough
throughthe
the
development
of
synthetic
aperture
radar
(SAR)
systems.
Great
improvement
in
azimuth
resolution
development of synthetic aperture radar (SAR) systems. Great improvement in azimuth resolution
could
couldbe
berealized
realizedififaalonger
longerantenna
antennawere
wereused.
used.Engineers
Engineershave
havedeveloped
developedprocedures
procedurestotosynthesize
synthesizeaa
very
verylong
longantenna
antennaelectronically.
electronically.Like
Likeaabrute
bruteforce
forceororreal
realaperture
apertureradar,
radar,aasynthetic
syntheticaperture
apertureradar
radar
also
alsouses
usesaarelatively
relativelysmall
smallantenna
antenna(e.g.,
(e.g.,11m)
m)that
thatsends
sendsout
outaarelatively
relativelybroad
broadbeam
beamperpendicular
perpendiculartoto
the
theaircraft.
aircraft.The
Themajor
majordifference
differenceisisthat
thataagreater
greaternumber
numberofofadditional
additionalbeams
beamsare
aresent
senttoward
towardthe
the
object.
Doppler
principles
are
then
used
to
monitor
the
returns
from
all
these
additional
microwave
object. Doppler principles are then used to monitor the returns from all these additional microwave
pulses
pulsestotosynthesize
synthesizethe
theazimuth
azimuthresolution
resolutiontotobecome
becomeone
onevery
verynarrow
narrowbeam.
beam.
Synthetic
Aperture
Radar SAR
Azimuth resolution is
constant = D/2, it is
independent of the slant
range distance, λ , and
the platform altitude.
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
Animation of the Doppler Effect
pulses of
microwave energy
9
a.
8
7
6
5
4
object is a
3
constant distance
from the flightline
2
time n
1
c.
b.
8
7
time n+2
time n+1
interference signal
radar hologram
9
9
8
9
7
6.5
time n+4
time n+3
9
8
7
d.
6.5
9
8
7
e.
6.5
7
8
7
Fundamental radar equation
t
Amount of backscatter per unit area
Intermediate
h=
λ
8 sin γ
Penetration ability to forest
Response
-, CC- and
-band Microwave
Responseof
ofAAPine
PineForest
ForestStand
StandtotoXX-,
andLL-band
MicrowaveEnergy
Energy
L-band
23.5 cm
a.
C-band
5.8 cm
b.
X-band
3 cm
c.
Penetration ability
into subsurface
Penetration ability
to heavy rainfall
SIR
-C/X-SAR
SIR-C/X-SAR
Images
Images of
of aa Portion
Portion
of
of Rondonia
Rondonia,,
Brazil,
Brazil, Obtained
Obtained on
on
April
April 10,
10, 1994
1994
Penetration of Ice
A Study of Ice Thickness on the Jamapa
Glacier, Pico de Orizaba, Mexico
†
†
†
†
A ground-based radar system (GPR) typically
used in shallow ground surveys
Can penetrate ice an order of magnitude
greater due to dielectric properties
400 mHz antenna – approx. 75 cm
wavelength
100 mHz systems are flown over Antarctica
to penetrate 100’s of meters
Penetration of Ice
A Study of Ice Thickness on the Jamapa
Glacier, Pico de Orizaba, Mexico
Penetration of Ice
A Study of Ice Thickness on the Jamapa
Glacier, Pico de Orizaba, Mexico
Output of GPR
shotpoint 8
Surface
Approx.
14
meters
of ice
Bedrock
Radar Shadow
†
Shadows in radar images can enhance the geomorphology and texture of the terrain.
Shadows can also obscure the most important features in a radar image, such as the
information behind tall buildings or land use in deep valleys. If certain conditions are
met, any feature protruding above the local datum can cause the incident pulse of
microwave energy to reflect all of its energy on the foreslope of the object and
produce a black shadow for the backslope
†
Unlike airphotos, where light may be scattered into the shadow area and then
recorded on film, there is no information within the radar shadow area. It is black.
†
Two terrain features (e.g., mountains) with identical heights and fore- and backslopes
may be recorded with entirely different shadows, depending upon where they are in
the across-track. A feature that casts an extensive shadow in the far-range might have
its backslope completely illuminated in the near-range.
†
Radar shadows occur only in the cross-track dimension. Therefore, the orientation of
shadows in a radar image provides information about the look direction and the
location of the near- and far-range
Shadows and look direction
Shuttle
-C) Image
ShuttleImaging
ImagingRadar
Radar(SIR
(SIR-C)
Imageof
ofMaui
Maui
Major Active Radar Systems
†
†
†
†
†
†
†
†
†
†
Seasat, June 1978, 105 days mission, L-HH band, 25 m resolution
SIR-A, Nov. 1981, 2.5 days mission, L-HH band, 40 m resolution
SIR-B, Oct. 1984, 8 days mission, L-HH band, about 25 m resolution
SIR-C, April and Sept. 1994, 10 days each. X-, C-, L- bands multipolarization
(HH, VV, HV, VH), 10-30 m resolution
JERS-1, 1992-1998, L-band, 15-30 m resolution
(Japan)
RADARSAT, Jan. 1995-now, C-HH band, 10, 50, and 100 m
(Canada)
ERS-1, 2, July 1991-now, C-VV band, 20-30 m
(European)
AIRSAR/TOPSAR, 1998-now, C,L,P bands with full polarization, 10m
NEXRAD, 1988-now, S-band, 1-4 km,
TRMM precipitation radar, 1997, Ku-band, 4km, vertical 250m (USA and
Japan)