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Lecture 23 - Microwave Remote Sensing

3 December2007

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Recommended Readings

• Chapter 13, pages 291-313 in Jensen

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Figure 1-18 from Elachi, C., Introduction to the Physics and Techniques of Remote Sensing, 413 pp., John Wiley & Sons, New York, 1987.

Microwave energy is largely unaffected by the atmosphere, e.g., it has 100% transmission

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Unique Characteristics of Microwave Remote Sensors

1. They are not affected by cloud cover and operate at night – all-weather, day-night sensing devices

2. Passive microwave systems detect radiant temperature of the earth’s surface which in, varies as a function of kinetic temperature and emissivity – large emissivity variations in water and vegetation cause large variations radiant temperature

3. Active microwave systems are very sensitive to variations in surface moisture content and surface roughness unique information available from these systems

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Types of Microwave Remote Sensors

– Microwave radiometers• Measure the emittance of EM energy within the

microwave region of the EM spectrum, just like thermal IR sensors

– Non-imaging RADARs1. Altimeters – measure the elevation of the earth’s

surface2. Scatterometers – detect variations in microwave

backscatter from a large area - measure variations in surface roughness, used to estimate ocean wind speed

– Imaging RADARs• Synthetic Aperture Radars – map variations in

microwave backscatter at fine spatial scales (10 to 50 m), used to create an image – measure variations in surface roughness and surface moisture

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RADAR – Radio Detection and Ranging

• Concept behind radars discovered in 1923• RADARs systems invented in the 1930s

– A high powered, radio transmitter/receiver system was developed that would transmit a signal that was reflected from a distant object, and then detected by the receiver

– Thus, RADAR’s initial function was to detect and determine the range to a target

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Key Components of a Radar System

• Microwave Transmitter – electronic device used to generate the microwave EM energy transmitted by the radar

• Microwave Receiver – electronic device used to detect the microwave pulse that is reflected by the area being imaged by the radar

• Antenna – electronic component used through which microwave pulses are transmitted and received

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Measurements made with a simple radar

• Range to the target• Intensity of the returned pulse• Azimuth resolution• Range resolution

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Microwave Transmitter / Receiver

Antenna

Microwave EM energy pulse transmitted by the radar

Microwave EM energy pulse reflected from a target that will be detected by the radar

Target

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Microwave Transmitter / Receiver

1. Transmitted pulse travels to the target

Target

2. The target reflects the pulse, and the reflected pulse travels back to the microwave antenna / receiver, where it is DETECTED

3. The radar measures the time (t) between when the pulse was transmitted and when the reflected signal reaches the receiver –The time it takes the pulse to travel from the radar to the target and back is used to estimate the RANGE

Antenna

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Radar range - R

The distance, R, from the antenna to the target is calculated as

R = ct / 2

where c is the speed of light (3 x 10-8 m sec -1)t is the time between the transmission of the

pulse and its reception by the radar antenna

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Satellite Altimeters• Altimeters are radars that measure the height

of the surface of the earth• Transmit a radar pulse which is reflected from

the earth’s surface• Measure the time it takes for the pulse to

travel to the earth and back (t)• Height of the satellite (H) H = ct/2 where c is

the speed of light• The altitude of the satellite is carefully

measured using GPS and ground-based laser systems

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AltimetersAltimeters measure round-

trip travel time of microwave radar pulse to determine distance to sea surface

From this (and additional info) we can determine η– the dynamic sea surface topography

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Altimeter Missions• NASA GEOS-3, 1975-1978• NASA Seasat, 1978• NAVY Geosat, 1985-1989 (first 2 years

classified)• ESA ERS-1/2, 1991-1996 and 1995-• NASA/CNES TOPEX/Poseidon, late 1992-• NASA/CNES Jason-1, late 2000-

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Measurements made with a simple radar

• Range to the target• Intensity of the returned pulse• Azimuth resolution• Range resolution

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The Radar Equation

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σ - is the radar surface backscatter coefficient

It represents the fraction of incoming EM radiation that is scattered from the surface in the direction of the transmitted energy (hence the term “backscatter)

It is equivalent to the reflection coefficient in thevisible/RIR region of the EM spectrum

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Factors controlling variations in σ

• Surface roughness• Surface dielectric constant

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Surface Reflectance or Scattering

• Specular reflection or scattering

• Diffuse reflection or scattering

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Specular Reflection or Scattering

• Occurs from very smooth surfaces, where the height of features on the surface << wavelength of the incoming EM radiation

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Diffuse Reflectors or Scatterers

• Most surfaces are not smooth, and reflect incoming EM radiation in a variety of directions

• These are called diffuse reflectors or scatterers

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Figures from http://pds.jpl.nasa.gov/ mgddf/chap5/f5-4f.gif

Radar backscattering is dependent on the relative height or roughness of the surface

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Microwave scattering as a function of surface roughness is wavelength dependent

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Variation in MW backscatter from a rough surface (grass field) as a function of

wavelength – As the wavelength gets longer, the backscattering coefficient drops

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Figure from http://pds.jpl.nasa.gov/ mgddf/chap5/f5-4f.gif

Microwave scattering is dependent on incidence angle

As incidence angle increases, backscattering decreases

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Factors controlling σ

• Surface roughness• Surface dielectric constant

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Dielectric Constant

• The dielectric constant is a measure of the electrical conductivity of a material

• Degree of scattering by an object or surface is proportional to the dielectric constant of the material –– σ ~ dielectric constant

• To some degree, dielectric constants are dependent on microwave wavelength and polarization

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Dielectric Constants of Common Materials

• Soil – 3 to 6• Vegetation – 1 to 3• Water – 80

– For most terrestrial materials, the moisture content determines the strength of scattering of microwave energy

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Dielectric constant as a function of soil moisture

λ = 21.4 cm

Figure E.47 from Ulaby, Moore, and Fung, Microwave Remote Sensing, Volume III.

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Moderate Burn All Years

y = 0.3299x - 18.268R2 = 0.82

-18

-16

-14

-12

-10

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-6

-4

-2

0

0 10 20 30 40 50

6 cm % Volumetric Moisture

ERS

-2 B

acks

catte

r (d

B)

All Years2003-4 Validation SitesLinear (All Years)

Radar backscatter (image intensity) in burned forests is proportional to soil moisture

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Microwave Scattering from a Water Surface

Water has a dielectric constant of 80• All scattering from water bodies in the

Microwave region of the EM Spectrum is from surface scattering as no EM energy penetrates the water surface

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λ = 3 cm λ = 24 cm

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Smooth area – no wind

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L-band airborne SAR Image

Why do you have backscatter at L-band from an ocean surface?

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Backscatter dependence on wind speed: L-HH Measurements

upwind

Radar backscatter from a water surface varies as a function of:1. Wind speed2. Look direction (upwind, downwind, cross wind)3. Incidence angle (look direction of the sensor relative to the surface

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ERS Scatterometer

Resolution = 50 km Swath Width = 500 km

Obtains backscatter measurements looking upwind, cross-wind, and downwind

Empirical Algorithms used to estimate wind speed and direction

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ERS Scatterometer Accuracy

Scatterometer accuracies determined through comparisons made with surface data collected by buoys

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Microwave Radiometers

• Land and water surfaces not only emit EM energy that can be detected in thermal IR wavelengths, but also in microwave wavelengths (1 cm to > 1 m)

• Microwave radiometers have the ability to measure the brightness temperature (TB) of the earth’s surface

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Microwave Radiometers

• Recall TB = ε Tkin

• The microwave emissivity (ε) of the different materials found on the earth’s surface varies greatly

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Key: O = Open Water, FY = First Year Ice, MY = Multi-year IceH = horizontal polarization, V = Vertical Polarization

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Key: O = Open Water, FY = First Year Ice, MY = Multi-year IceH = horizontal polarization, V = Vertical Polarization

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Scanning Multi-channel Microwave Radiometer (SMMR)

Wavelength Polarizations Pixel Size

0.81 cm H,V 30 x 30 km

1.40 cm H,V

1.70 cm H,V

1.80 cm H,V

4.60 cm H,V 159 x 159 km

1978 - 1987

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Special Sensor Microwave/Imagery (SSM/I)

Wavelength Polarizations Pixel Size

0.35 cm H,V 16 x 14 km

0.81 cm H,V 38 x 30 km

1.35 cm V 60 x 40 km

1.55 cm H,V 70 x 45 km

1987 to present

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Key Points for Lecture 231. Definition of a RADAR2. Uses of a radar altimeter3. The radar backscatter coefficient - σ4. Source of variations in σ

– Surface roughness– Incidence angle– Surface dielectric constant

5. Effects of soil moisture on σ6. Effects of wind speed on σ from water surfaces7. ERS Scatterometer8. Microwave Radiometers – how do they work?