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)
