chapter 8 microwave sensing introduction to remote sensing instructor: dr. cheng-chien...

Chapter 8Chapter 8

Microwave sensingMicrowave sensing

Introduction to Remote SensingInstructor: Dr. Cheng-Chien Liu

Department of Earth Sciences

National Cheng-Kung University

Last updated: 4 June 2003

8.18.1 Introduction Introduction

MicrowaveMicrowave• 1mm~1m not micro at all

Features:Features:• Penetration any weather condition

• Irrelevant to visible light

Active and passive, airborne and Active and passive, airborne and spacebornespaceborne

8.2 Radar development8.2 Radar development

Radio detection and ranging (RADAR)Radio detection and ranging (RADAR)• pulse of microwave energy objects echoes detect and ranging

• Nonimaging radar:e.g. Doppler radar Doppler frequency shift velocity

• Plan position indicator (PPI)circular display screenrotating antennaweather forecasting, air traffic control, navigationPoor spatial resolution not appropriate for R.S.

8.2 Radar development(Cont.)8.2 Radar development(Cont.)

Side-looking airborne radar (SLAR)Side-looking airborne radar (SLAR)• Side-looking radar (SLR)• Antenna fixed below the aircraft point to the side• Fig 8.1: SLAR image• Continuous strips depicting large ground areas• 1950s military reconnaissance

All-weather operating capabilityActive, day-or-night imaging systemDeclassification time lag non-military targets

• An active state of advancement

8.2 Radar development(Cont.)8.2 Radar development(Cont.)

Applications of SLARApplications of SLAR• A complete survey of the Darien province of Panama

Fig 8.21967Persistent cloud cover

• Mapping VenezuelaFig 8.31971Improve the accuracy of the country boundaryWater resourcessource of several rivers

• Project Radam (Radar of the Amazon)1971~1976Geologic analysis, timber inventory, transportation route location,

mineral exploration

8.2 Radar development (cont.)8.2 Radar development (cont.)

Applications of SLAR (cont.)Applications of SLAR (cont.)• Applications in ocean

Determine wind, wave and ice conditions, internal wavesStudy ocean bottom contours

Spaceborne radar remote sensingSpaceborne radar remote sensing• 1978 Seasat• Shuttle Imaging Radar• 1980s: Soviet Cosmos experiments• 1991: Almaz-1, ERS-1, JERS-1• 1991: Radarsat

8.3 SLAR system operation8.3 SLAR system operation

Fig 8.4: Operating principle of SLARFig 8.4: Operating principle of SLAR• Solid lines: radar pulse sent

• Dashed lines: return signals

• Signal from tree: later and smaller than signal from house

• The slant range SR=ct/2SR: direct distance between transmitter and object

8.3 SLAR system operation (cont.)8.3 SLAR system operation (cont.)

Fig 8.5: How to create a SLAR imageFig 8.5: How to create a SLAR image• fly speed Va

• synchronizer switch transmitter receiver

• transmitted pulse

• receive and process

• record

8.4 Spatial resolution of SLAR 8.4 Spatial resolution of SLAR systemssystems

Ground resolution cell size Ground resolution cell size • Pulse length range direction• Antenna bandwidth azimuth direction

Range resolutionRange resolution• Fig 8.6: Dependence of range resolution on

pulse lengthPulse length PLSlant-range distance ABsr=ABcosIf ABsr >1/2PL separate signal differentiableIf ABsr <1/2PL overlapped signal one large object

8.4 Spatial resolution of SLAR 8.4 Spatial resolution of SLAR systems (cont.)systems (cont.)

Fig 8.7: slant-range resolution Fig 8.7: slant-range resolution ground range resolutionground range resolution• Depression angle d

• Look angle l

• ABsr=Abcosd AB=Rr=ABsr/cosd =c/2cosd

• Example 8.1

8.4 Spatial resolution of SLAR 8.4 Spatial resolution of SLAR systems (cont.)systems (cont.)

Azimuth resolutionAzimuth resolution• Fig 8.8: Dependence of Ra, , GR

Ra=GRExample 8.2Antenna beamwidth: =/AL

• ALRaPhysical length of antenna

Brute force, real aperture, noncoherent radar e.g. =5cm, =10mrad AL=5m

if =2mrad AL=25m

Simple for design and data processing Short , short range, low altitude

8.4 Spatial resolution of SLAR 8.4 Spatial resolution of SLAR systems (cont.)systems (cont.)

Azimuth resolution (cont.)Azimuth resolution (cont.)• ALRa (cont.)

Synthesizing and effective length of antenna Synthetic Aperture Radar (SAR) Complex Single physically short antenna+motion along the flight line

successive elements of a single, long synthetic antenna Near range fewer elements Ra=constant fn(Range)

Another view of explaining how SAR operate (Fig 8.10) Ahead of the aircraft upshifted f Behind the aircraft downshifted f

8.5 Geometric characteristics of 8.5 Geometric characteristics of SLAR imagery SLAR imagery

Different from photo and scanner imageryDifferent from photo and scanner imagery Slant-range scale distortionSlant-range scale distortion

• Fig 8.11: Slant-range vs ground-range image formatA=B=C but A1<B1<C1

GR=(SR2-H΄2)1/2

• Range scale = fn(H΄)

• Azimuth scale = fn(Vair, VCRT)

• Inertial navigator and control system strict control of flight parameters reconcile and equalize these independent scales

8.5 Geometric characteristics of 8.5 Geometric characteristics of SLAR imagery (cont.)SLAR imagery (cont.)

Relief displacementRelief displacement• Fig 8.12: RD on SLAR versus photos

Layover effect: a vertical feature lay over the closer features and appears to lean toward the nadir

• Fig 8.13: Effects of terrain relief on SLAR imagesTerrain slope steeper than lines perpendicular to the direction of the

radar pulse layover effect.D: no layover but foreshortening effectC: image of the front slope foreshortened 0B: layover effect, right side is facing away from the radar antenna no

return signal darkA: layover effect, right side is also illuminated weak return

8.5 Geometric characteristics of 8.5 Geometric characteristics of SLAR imagery (cont.)SLAR imagery (cont.)

ParallaxParallax• Fig 8.14: Flight orientation to produce parallax

a: opposite sideb: same side but different altitude altitude parallax

• Fig 8.15: Stereo SLAR image, flying same flight line at different altitude

8.6 Transmission characteristics of 8.6 Transmission characteristics of Radar signalsRadar signals

Table 8.1: Radar band designationTable 8.1: Radar band designation• Letter codes arbitrarily selected for military

security

WavelengthWavelength• atmospheric attenuation/dispersion• Precipitation echoes D6/4

D: drop diameter

• Applications: PPI range of heavy raine.g. = 1cm echo, =3cm no echo

8.6 Transmission characteristics of 8.6 Transmission characteristics of Radar signals (cont.)Radar signals (cont.)

PolarizationPolarization• The signal can be filtered in such a way that its

electrical wave vibrations are restricted to a single plane perpendicular to the direction of wave propagation

• Send: H,V Receive: H,V

Like-polarized: HH, VVCross-polarized: HV, VHCircular polarization

• Mode of polarization details see §8.8

8.7 Earth surface feature characters 8.7 Earth surface feature characters influencing radar returnsinfluencing radar returns

Geometric characteristics:Geometric characteristics:• Fig 8.16: Effect of sensor/terrain geometry

Local incident angle (Fig 8.17): i

Flat terrain: i =l

Consider earth curvature: i >l

Radar shadow complete dark and sharp

• Factors that dominate radar image0 < i < 300: topographic slope

300 < i < 700: surface roughness

700 < i : radar shadows

8.7 Earth surface feature characters 8.7 Earth surface feature characters influencing radar returns (cont.)influencing radar returns (cont.)

Geometric characteristics (cont.)Geometric characteristics (cont.)• Fig 8.18: Radar reflection from various surfaces

Rayleigh criterion: SVHrms > /8cosi Rough surfacediffuse reflectorsignificant return SVHrms /8cosi Smooth surfacespecular reflectorlow return

Modified Rayleigh criterion: SVHrms> /(4.4cosi) rough SVHrms< /(25cosi) smooth Others: intermediate Table 8.2: various bands, various i

Corner reflector: double reflection bright, sparkles (see Fig 8.22 and 8.23)

2)height variancesurface(rmsSVH

8.7 Earth surface feature characters 8.7 Earth surface feature characters influencing radar returns (cont.)influencing radar returns (cont.)

Electrical characteristicsElectrical characteristics• Complex dielectric constant (DC) reflectivity

& conductivityDry natural material: DC 3~8Water: DC 80Moisture DCPlant: moisture good reflectorsMetal DC (e.g. metal bridges in Fig 8.22a)

8.7 Earth surface feature characters 8.7 Earth surface feature characters influencing radar returns (cont.)influencing radar returns (cont.)

Soil ResponseSoil Response• Soil moisture DC limit radar penetration• Extremely dry soil condition penetration of L-band

(Fig 8.30)

Vegetation responseVegetation response• Size

Vegetation canopy: leaves, stems, stalks, limbs,….Underlying soil

• WavelengthShort (2~6cm) sensing crop canopies and tree leavesLonger (10~30cm) sensing tree trunks and limbs

8.7 Earth surface feature characters 8.7 Earth surface feature characters influencing radar returns (cont.)influencing radar returns (cont.)

Vegetation response (cont.)Vegetation response (cont.)• Other factors:

MoistureLike-polarized (HH or VV) penetrates more than cross-

polarizedAlign in the azimuth directioni

8.7 Earth surface feature characters 8.7 Earth surface feature characters influencing radar returns (cont.)influencing radar returns (cont.)

Water and ice response:Water and ice response:• Smooth water surface specular reflector no

returns

• Rough water surface varying strengths of returns

• Wave moving toward or away from the radar system easier to detect

• Sea ice dielectric propertiesIce age, surface roughness, internal geometry, temperature,

snow cover…

8.8 Interpretation of SLAR imagery8.8 Interpretation of SLAR imagery

Applications of SLAR imageApplications of SLAR image• Mapping major rock units and surficial materials• Mapping geologic structure (folds, faults and joints)• Mapping vegetation types• Determining sea ice types• Mapping surface drainage features

v.s. roughnessv.s. roughness• Table 8.3• Fig 8.19

8.8 Interpretation of SLAR imagery 8.8 Interpretation of SLAR imagery (cont.)(cont.)

Intensity of return signal Intensity of return signal • High slopes facing aircraft, rough objects,

high moisture, metal, urban building (corner reflection.)

• Low smooth water, pavements, playas

• No radar shadow

Speckle:Speckle:• Grainy or salt-and-pepper pattern

• Random constructive and destructive interface random bright and dark areas

8.8 Interpretation of SLAR imagery 8.8 Interpretation of SLAR imagery (cont.)(cont.)

Multiple-look processingMultiple-look processing• Reduce speckle

• Average several independent images for the same area

• Amount of speckles (number of looks)-1/2

• The size of the resolution cell & number of looks

8.8 Interpretation of SLAR imagery 8.8 Interpretation of SLAR imagery (cont.)(cont.)

Fig 8.20: SLAR imageFig 8.20: SLAR image• Large synclinal mountain upper left and center

• Flight on the top lighter tone on the slopes facing up

• Return signals vegetation surfaces

• Banding around mountain alternation of bedrock types

• River and lake dark tone

• HV shows less contrast than HHIt’s not always possible to predict HH or HV is better

8.8 Interpretation of SLAR imagery 8.8 Interpretation of SLAR imagery (cont.)(cont.)

Fig 8.21Fig 8.21• Basaltic lava flow

C: The “Sunshine Basalt” flow Sunshine Crater Darker tone on the HV image.

D: The “Pisgah Basalt” flow Pisgah CraterA: lighter tone on the HV image greater density of

vegetationB: boundaryPlaya (dry lakebed) dark tone

Gravel road

8.8 Interpretation of SLAR imagery 8.8 Interpretation of SLAR imagery (cont.)(cont.)

Fig 8.22a: small urban areaFig 8.22a: small urban area• Large building corner reflection high return• Metallic bridge high return• River water dark• Rectangle field• Bed sedimentary rock

Fig 8.22b: horizontally bedded sedimentary Fig 8.22b: horizontally bedded sedimentary rocks with loess coverrocks with loess cover• Potential soil erosion strip farming contour lines

8.8 Interpretation of SLAR imagery 8.8 Interpretation of SLAR imagery (cont.)(cont.)

Fig 8.23:Fig 8.23:• IRIS:

high resolution mode 3m 6m resolutionWide swath mode: 18m 10m resolution

• Malldark• Potomac Riverblack• Airport

Fig 8.24: Multiwavelength SLAR imageFig 8.24: Multiwavelength SLAR image• Wooded area: diffuse reflectors in both X and L band• Cultivated fields: diffuse reflectors in X band but acts

as near-specular reflectors in the L band

8.9 Experimental Radar remote 8.9 Experimental Radar remote sensing from spacesensing from space

Seasat-1Seasat-1• 1978-1978, (99 days)

• 800km, near-polar orbit

• SAR, L-band (23.5cm), HH polarization

• Swath: 100 km

• Resolution: 25m25m (range azimuth)

• Original goals:Global sea surface wave fieldPolar sea ice conditions.

• Also revealed applications toOcean: internal waves, current boundaries, eddies, fronts, bathymetric features,

storms, rainfalls, windrows.Land: geology water resources, land cover mapping, agricultural assessment.

8.9 Experimental Radar remote 8.9 Experimental Radar remote sensing from space (cont.)sensing from space (cont.)

Seasat-1 imagesSeasat-1 images• Fig 8.25: Seasat SAR image of the English Channel

near the straits of Dover.Tidal variations 7m, 1.5m/sSand bars hazardous

• Fig 8.26: Pack iceBanks Island (lower right-hand portion)Brighter ice areas rough surface older iceDarker areas open water or recently frozen ice3 days 15km

Fletcher’s Ice Island: 7km 12 km, 157km in 2 month

• Fig 8.27: Appalachian mountains. L-band, 1:575,000Sidelighting auticlines and synclines

8.9 Experimental Radar remote 8.9 Experimental Radar remote sensing from space (cont.)sensing from space (cont.)

Shuttle Imaging RadarShuttle Imaging Radar• SIR-A: 1981

260km

SAR. L-band (23.5cm), HH polarization9.4m antenna, 470~530 look angleResolution: 40m 40m (range azimuth)Swath: 50km

Fig 8.28: Saudi Arabia & Iraq. (MSS vs SAR) Dry river channels smooth, dry layer of wind- deposited silt lots radar return dark Outcropping carbonate rocks rough angular surface strong radar return.

Fig 8.29: Eastern China White spots villages Levee

Fig 8.30: Sahara Desert Penetration of L-band wave in extremely dry material underlying bedrock structure

8.9 Experimental Radar remote 8.9 Experimental Radar remote sensing from space (cont.)sensing from space (cont.)

Shuttle Imaging Radar (cont.)Shuttle Imaging Radar (cont.)• SIR-B: 1984

Tiltable antenna (150 ~ 600) Assessing the effect of various incident angles Stereo images

Resolution Azimuth: 25m

Range: 14m at 600~ 46m at 150

Fig 8.31: Montreal Developed area bright area River dark area Bridge Long, striplike patterns of agricultural fields (lower right)

Fig 8.32: Mt. Shasta. a: 600, b:i=300

Young lava flow unvegetated angular chunks of basalt

Older lava flows darker, more vegetated

8.9 Experimental Radar remote 8.9 Experimental Radar remote sensing from space (cont.)sensing from space (cont.)

Shuttle Imaging Radar (cont.)Shuttle Imaging Radar (cont.)• Fig 8.33: perspective views of Mt. Shasta

Generated from Fig 8.32Successive views taken counterclockwise around the mountain

• Fig 8.34: Stereopair (450 and 540)Small stereo convergence angle (90) but excellent imageSnow cover dark

• Fig 8.35: northern FloridaFlat, 45m mean elevation, sandy soil overlay weathering, limestone,

sinkhole lakesWater bodies (W), clear-cut areas(C), powerline right-of-way (P), roads

( R), Pine forest (F), Cypress-tupelo swamps (S) Cypress-tupelo swamps: dark (580) light (450) lighter (280)

Specular reflection from the standing water + tree trunks complex corner reflector effect

8.9 Experimental Radar remote 8.9 Experimental Radar remote sensing from space (cont.)sensing from space (cont.)

Shuttle Imaging Radar (cont.)Shuttle Imaging Radar (cont.)• SIR-C/X-SAR

1994X-band(3cm), C-band(6cm), L-band (23cm)Antenna + shuttle pointableSwath: 15-90km

Resolution: 10~200m

Plate

• Cosmos-1870Experiment1987

8.10 ALMAZ-18.10 ALMAZ-1

ALMAZ-1ALMAZ-1• 3-31-1991~10-17-1992

• USSR

• Commercial basis

• Altitude: 300km 360km

• S-band (10cm)

• L= 300 ~ 600

• Resolution:10m ~ 30m

• 2 antennas

• Swath: 350km 2

• Radiometric scanner (RMS)

Fig 8.36: Almaz radar imageFig 8.36: Almaz radar image

8.11 ERS Satellite Program8.11 ERS Satellite Program

Agency: ESA (European Space Agency)Agency: ESA (European Space Agency) Orbit: 777Orbit: 777kmkm, Sun synchronous, Sun synchronous Design life: 3 yearsDesign life: 3 years ERS-1: 1991ERS-1: 1991 ERS-2: 1995ERS-2: 1995

8.12 Sensors onboard ERS-18.12 Sensors onboard ERS-1

C-band AMI (active microwave instrument)C-band AMI (active microwave instrument) Ku-band radar altimeterKu-band radar altimeter An along-track scanning radiometerAn along-track scanning radiometer 3 modes of AMI3 modes of AMI

• IMAGE• WAVE• WIND

8.13 ERS-1 AMI image 8.13 ERS-1 AMI image interpretationinterpretation

AMI vs SIR-A, -B and Almaz-1AMI vs SIR-A, -B and Almaz-1• Shorter : C-band• Steeper i: i =230

• VV

Fig 8.37: ERS-1 radar imageFig 8.37: ERS-1 radar image• Canada/USA border• Milk River

Fig 8.38:Fig 8.38:• Center pivot irrigation area moisture lighter• Marsh roughness, moisture corner reflection

lighter

8.13 ERS-1 AMI image 8.13 ERS-1 AMI image interpretation (cont.)interpretation (cont.)

Fig 8.39: mountainous region.Fig 8.39: mountainous region.• Effect of layover steep l

• The Pacific Ocean

Fig 8.40: area of regenerating forest Fig 8.40: area of regenerating forest clearcut.clearcut.• Clear cut pasture smooth grass surface

visible• Reprocess clearcut light-colored• River

8.13 ERS-1 AMI image 8.13 ERS-1 AMI image interpretation (cont.)interpretation (cont.)

Fig 8.42: internal waves from the Atlantic Fig 8.42: internal waves from the Atlantic Ocean to the Mediterranean Sea.Ocean to the Mediterranean Sea.• 2km

• Different salinities different layer• Tide current

Fig 8.43: St. Lawrance RiverFig 8.43: St. Lawrance River• Wind roughened surface lighter tone• Influence of an atmospheric front on water surface

roughness pattern (upper-left lower-right)• Small river mixed with St. Lawrence River

temperature difference arc shape

8.13 ERS-1 AMI image 8.13 ERS-1 AMI image interpretation (cont.)interpretation (cont.)

Fig 8.44: oil slickFig 8.44: oil slick• Oil films dampening wave darker

8.14 JERS-18.14 JERS-1

JERS-1JERS-1• 1992• 568km, sun-synchronous orbits• SAR, L-band (23cm), HH polarization• Swath: 75km

• Resolution: 18m

• Expected lifetime: 2 years

Fig 8.45: Mt. FujiFig 8.45: Mt. Fuji• Snow-covered dark• Lake dark• Forest lighter-toned

8.15 Radarsat:8.15 Radarsat:

RadsatRadsat• 1995• 798km, sun-synchronous orbit• SAR, C-band (5.6cm). HH polarization• Swath and resolution:

Table 8.4: Radarsat beam selection modesFig 8.46: Radarsat imaging mode

• Stereo coverage• Data storage and transmit• Applications

8.16 Spaceborne radar system 8.16 Spaceborne radar system summarysummary

Table 8.5Table 8.5

8.17 Radar remote sensing of Venus8.17 Radar remote sensing of Venus

Magellan spacecraftMagellan spacecraft• 1989• Elliptical polar orbit: 2100km above the poles to 175km above the

equator• SAR, s-band• Swath 16,000km long, 25km wide• Resolution: 75m

• Fig 8.47: Mead impact crater (d=280km)• Fig 8.48: radar stereopair of the crater Geopert-Meyer

Same side at =150, 280

The edge of a ridge beltPlanetary scientists

Fig 8.49: Sapas MonsFig 8.49: Sapas Mons

8.18 Elements of passive microwave 8.18 Elements of passive microwave sensingsensing

Passive microwave sensing vs thermal sensingPassive microwave sensing vs thermal sensing• Similar principles blackbody radiation theory (Fig 8.50)• Use antenna

Fig 8.51: Components of a passive microwave Fig 8.51: Components of a passive microwave signalsignal• Emitted from the surface = fn(T, material)• Emitted from the atmosphere• Reflected from the surface• Transmitted from the surface

Passive microwave sensingPassive microwave sensing• fn(surface electrical, chemical and textural characteristics, bulk

configuration and shape, viewing angle)

8.19 Passive microwave sensors8.19 Passive microwave sensors

Microwave radiometersMicrowave radiometers• Basic configuration (Fig 8.52)

Switch rapid, alternate sampling between the antenna signal and a calibration temperature reference signal

Amplify weak signalReadout and recording

• Trade-off between antenna beamwidth and system sensitivity

• Apparent antenna temperatureThe system is calibrated in terms of the temperature that a blackbody

located at the antenna must reach to radiate the same energy as collected from the ground

8.19 Passive microwave sensors 8.19 Passive microwave sensors (cont.)(cont.)

ScannersScanners• Scan transverse to the direction of flight

• Mechanically, electronically, multiple antenna array

• Fig 8.53: Passive microwave imageLooks like thermal image, but bright coldAgricultural fieldsStriping irrigation Density moisture

8.20 Applications of passive 8.20 Applications of passive mmicrowave sensingicrowave sensing

AdvantagesAdvantages DisadvantagesDisadvantages MeteorologyMeteorology OceanographyOceanography GeologyGeology

8.21 LIDAR8.21 LIDAR

Fig 8.54: Principle of lidar bathymetryFig 8.54: Principle of lidar bathymetry Fig 8.55: Lidar returns measured over Fig 8.55: Lidar returns measured over

a forest canopya forest canopy Laser-induced fluorescence (LIF)Laser-induced fluorescence (LIF)

• Single-channel laser source + multi-channel receivers

• Distinguish several plant groups