Active and Passive Microwave Remote Sensing Lecture 7 Oct 6, 2004 Reading materials: Chapter 9.

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.