active and passive microwave remote sensing lecture 7 oct 6, 2004 reading materials: chapter 9
TRANSCRIPT
Active and Passive Microwave Remote Sensing
Lecture 7
Oct 6, 2004
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
Active microwave (RADAR: RAdio Detection And Ranging, read p285 for an explanation) Long-wavelength microwaves (1 – 100 cm)
LIDAR (LIght Detection And Ranging) Short-wavelength laser light (UV, visible, near IR)
SONAR (SOund Navigation And Ranging) Sound waves through a water column. Sound waves extremely slow (300 m/s in air, 1,530 m/s in sea-water) Bathymetric sonar (measure water depths and, hence changes in
bottom topography ) Imaging sonar or sidescan imaging sonar (imaging the bottom
topography and bottom roughness) It is not our focus in this remote sensing class.
Microwaves
Band Designations(common wavelengths Wavelength () Frequency ()shown in parentheses) in cm in GHz_______________________________________________Ka (0.86 cm) 0.75 - 1.18 40.0 to 26.5K 1.18 - 1.67 26.5 to 18.0Ku 1.67 - 2.4 18.0 to 12.5X (3.0 and 3.2 cm) 2.4 - 3.8 12.5 - 8.0C (7.5, 6.0 cm) 3.8 - 7.5 8.0 - 4.0S (8.0, 9.6, 12.6 cm) 7.5 - 15.0 4.0 - 2.0L (23.5, 24.0, 25.0 cm) 15.0 - 30.0 2.0 - 1.0P (68.0 cm) 30.0 - 100 1.0 - 0.3
Band Designations(common wavelengths Wavelength () Frequency ()shown in parentheses) in cm in GHz_______________________________________________Ka (0.86 cm) 0.75 - 1.18 40.0 to 26.5K 1.18 - 1.67 26.5 to 18.0Ku 1.67 - 2.4 18.0 to 12.5X (3.0 and 3.2 cm) 2.4 - 3.8 12.5 - 8.0C (7.5, 6.0 cm) 3.8 - 7.5 8.0 - 4.0S (8.0, 9.6, 12.6 cm) 7.5 - 15.0 4.0 - 2.0L (23.5, 24.0, 25.0 cm) 15.0 - 30.0 2.0 - 1.0P (68.0 cm) 30.0 - 100 1.0 - 0.3
1. Active microwave remote sensing
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 the world war II). Since then, the side-looking airborne radar (SLAR) has been used to get detail image of enemy sites along the edge of the fight 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.
Principle of SLAR
transmitted pulse
backscattered pulse
antenna
TransmitterDuplexer
• sends and receives
Pulse Generator
CRT Display or Digital Recorder
Receiver
b.
a. antenna
transmitted pulse
backscattered pulse
antenna
TransmitterDuplexer
• sends and receives
Pulse Generator
CRT Display or Digital Recorder
Receiver
b.
a. antenna
A CRT (cathode ray tube)shows a quick-look display
Radar NomenclatureRadar Nomenclature
• • nadirnadir•• azimuth (or flight) directionazimuth (or flight) direction•• look (or range) directionlook (or range) direction•• range (near, middle, and far)range (near, middle, and far)•• depression angle (depression angle ())•• incidence angle (incidence angle ())•• altitude above-ground-level, altitude above-ground-level, HH•• polarizationpolarization
Radar NomenclatureRadar Nomenclature
• • nadirnadir•• azimuth (or flight) directionazimuth (or flight) direction•• look (or range) directionlook (or range) direction•• range (near, middle, and far)range (near, middle, and far)•• depression angle (depression angle ())•• incidence angle (incidence angle ())•• altitude above-ground-level, altitude above-ground-level, HH•• polarizationpolarization
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.
b.
look direction
N
Ka - band, HH polarization
Ka - band, HV polarization
a.
b.
look direction
N
Ka - band, HH polarization
Ka - band, HV polarization
Lava flow in north-center Arizona
Slant-range vs. Ground-range geometry
Radar imagery has a different geometry than that produced by most conventional remote sensor systems, such as cameras, multispectral scanners or area-array detectors. Therefore, one must be very careful when attempting to make radargrammetric measurements.
• Uncorrected radar imagery is displayed in what is called slant-range geometry, i.e., it is based on the actual distance from the radar to each of the respective features in the scene.
• It is possible to convert the slant-range display into the true ground-range display on the x-axis so that features in the scene are in their proper planimetric (x,y) position relative to one another in the final radar image.
Radar imagery has a different geometry than that produced by most conventional remote sensor systems, such as cameras, multispectral scanners or area-array detectors. Therefore, one must be very careful when attempting to make radargrammetric measurements.
• Uncorrected radar imagery is displayed in what is called slant-range geometry, i.e., it is based on the actual distance from the radar to each of the respective features in the scene.
• It is possible to convert the slant-range display into the true ground-range display on the x-axis so that features in the scene are in their proper planimetric (x,y) position relative to one another in the final radar image.
Most radar systems and data providers now provide the data in ground-range geometry
Range (or across-track) Resolution
cos2
ctRr
Pulse duration (t)= 0.1 x 10 -6 sec
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.
t.c/2 t.c/2
Azimuth (or along-track) Resolution
D
SRa
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.
Synthetic Aperture Radar -
SAR
A major advance in radar remote sensing has been the improvement in azimuth resolution through the development of synthetic aperture radar (SAR) systems. Great improvement in azimuth resolution could be realized if a longer antenna were used. Engineers have developed procedures to synthesize a very long antenna electronically. Like a brute force or real aperture radar, a synthetic aperture radar also uses a relatively small antenna (e.g., 1 m) that sends out a relatively broad beam perpendicular to the aircraft. The major difference is that a greater number of additional beams are sent toward the object. Doppler principles are then used to monitor the returns from all these additional microwave pulses to synthesize the azimuth resolution to become one very narrow beam.
A major advance in radar remote sensing has been the improvement in azimuth resolution through the development of synthetic aperture radar (SAR) systems. Great improvement in azimuth resolution could be realized if a longer antenna were used. Engineers have developed procedures to synthesize a very long antenna electronically. Like a brute force or real aperture radar, a synthetic aperture radar also uses a relatively small antenna (e.g., 1 m) that sends out a relatively broad beam perpendicular to the aircraft. The major difference is that a greater number of additional beams are sent toward the object. Doppler principles are then used to monitor the returns from all these additional microwave pulses to synthesize the azimuth resolution to become one very narrow beam.
Azimuth resolution isconstant = D/2, it isindependent of the slantrange distance, , andthe platform altitude.
9 8 7 6 5 4 3 2 1
time n
time n+4time n+3
time n+2
pulses of microwave energy
interference signal
radar hologram
a.b. c.
d. e.
8 7
6.5 7
9 9 8 9 8 7
78 9 78 9 6.5 6.5 7
time n+1
object is a constant distance from the flightline
9 8 7 6 5 4 3 2 1
time n
time n+4time n+3
time n+2
pulses of microwave energy
interference signal
radar hologram
a.b. c.
d. e.
8 7
6.5 7
9 9 8 9 8 7
78 9 78 9 6.5 6.5 7
time n+1
object is a constant distance from the flightline
Fundamental radar equation
t
Amount of backscatter per unit area
sin8h
Intermediate
L-band 23.5 cm
C-band 5.8 cm
X-band 3 cm
a. b. c.
L-band 23.5 cm
C-band 5.8 cm
X-band 3 cm
a. b. c.
Response of A Pine Forest Stand to X-, C- and L-band Microwave EnergyResponse of A Pine Forest Stand to X-, C- and L-band Microwave EnergyResponse of A Pine Forest Stand to X-, C- and L-band Microwave EnergyResponse of A Pine Forest Stand to X-, C- and L-band Microwave Energy
Penetration ability to forest
Penetration abilityto subsurface
hw90hw90
RailwayRailway
Radar Radar ImageImage
ETM+ ETM+ ImageImage Xie et al., 2004
Roughness andPenetration ability tosubsurface
SIR-C/X-SAR SIR-C/X-SAR Images of a Portion Images of a Portion
of Rondonia, of Rondonia, Brazil, Obtained on Brazil, Obtained on
April 10, 1994April 10, 1994
SIR-C/X-SAR SIR-C/X-SAR Images of a Portion Images of a Portion
of Rondonia, of Rondonia, Brazil, Obtained on Brazil, Obtained on
April 10, 1994April 10, 1994
Penetration abilityto heavy rainfall
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
Shuttle Imaging Radar (SIR-C) Image of MauiShuttle Imaging Radar (SIR-C) Image of MauiShuttle Imaging Radar (SIR-C) Image of MauiShuttle Imaging Radar (SIR-C) Image of Maui
Shadows and look direction
Radar Noise – SpeckleSpeckleSpeckle is a grainy salt-and-pepper is a grainy salt-and-pepper pattern in radar imagery present pattern in radar imagery present due to the coherent nature of the due to the coherent nature of the radar wave, which causes random radar wave, which causes random constructive and destructive constructive and destructive interference, and hence random interference, and hence random bright and dark areas in a radar bright and dark areas in a radar image. The speckle can be reduced image. The speckle can be reduced by processing separate portions of by processing separate portions of an aperture and recombining these an aperture and recombining these portions so that interference does portions so that interference does not occur. This process, called not occur. This process, called multiple looksmultiple looks or non-coherent or non-coherent integration, produces a more integration, produces a more pleasing appearance, and in some pleasing appearance, and in some cases may aid in interpretation of cases may aid in interpretation of the image but at a cost of degraded the image but at a cost of degraded resolution. resolution. N (D/2)N (D/2)
a.
b.
c.
1 - Look radar image
4 - Look radar image
16 - Look radar image
a.
b.
c.
1 - Look radar image
4 - Look radar image
16 - Look radar image
N, number of looksD, antenna length
Another way to remove speckle noise
G-MAPG-MAP
Blurred objectsBlurred objectsand boundaryand boundary
Gamma Maximum A Posteriori Filter
Xie et al., 2004
Statistical algorithmsGeometric algorithms
Striping Noise and Removal
CPCACPCA
Combined Principle Combined Principle Component AnalysisComponent Analysis
Xie et al., 2004
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)
Advantages of active radar
All weather, day or night Some areas of Earth are persistently cloud covered
Penetrates clouds, vegetation, dry soil, dry snow Sensitive to water content (soil moisture),
roughness Can measure waves
Sensitive to polarization Interferometry
2. Passive microwave remote sensing
Principals
While dominate wavelength of Earth is 9.7 um, a continuum of energy is emitted from Earth to the atmosphere. In fact, the Earth passively emits a steady stream of microwave energy, though it is relatively weak in intensity.
A suit of radiometers developed can record it. They measure the brightness temperature of the terrain or the atmosphere. This is much like the thermal infrared radiometer for temperature.
A matrix of brightness temperature values can then be used to construct a passive microwave image.
Jeff Dozier
Some important passive microwave radiometers
Special Sensor Mirowave/Imager (SSM/I) It was onboard the Defense Meterorological
Satellite Program (DMSP) since 1987 It measure the microwave brightness
temperatures of atmosphere, ocean, and terrain at 19.35, 22.23, 37, and 85.5 GHz.
TRMM microwave imager (TMI) It is based on SSM/I, and added one more
frequency of 10.7 GHz.