1. Microwave remote sensing uses radar and radiometers to measure Earth's surface. 2. Radar is unaffected by clouds and can image day/night, detecting variations in surface roughness and moisture. Radiometers measure microwave brightness temperature related to kinetic temperature and emissivity. 3. Key applications include radar altimeters to measure ocean topography, scatterometers to estimate wind speed over oceans, and synthetic aperture radar for fine-scale surface mapping.

1. Lecture 23 - Microwave
Remote Sensing
3 December
2007
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2. Recommended Readings
• Chapter 13, pages 291-313 in Jensen
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3. Microwave energy is largely unaffected by
the atmosphere, e.g., it has 100%
transmission
Figure 1-18 from Elachi, C., Introduction to the Physics and
Techniques of Remote Sensing, 413 pp., John Wiley & Sons, New
3
York, 1987.

4. 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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5. 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 RADARs
1. Altimeters – measure the elevation of the earth’s
surface
2. 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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6. 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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7. 7

8. 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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9. Measurements made with a
simple radar
• Range to the target
• Intensity of the returned pulse
• Azimuth resolution
• Range resolution
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10. Microwave
Transmitter / Receiver
Antenna Target
Microwave EM energy Microwave EM energy
pulse transmitted by the pulse reflected from a
radar target that will be
detected by the radar
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11. Microwave Target
Transmitter / Receiver
1. Transmitted pulse travels to
Antenna the 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
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12. 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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13. 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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14. Altimeters
Altimeters 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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15. 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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16. 16

17. 17

18. Measurements made with a
simple radar
• Range to the target
• Intensity of the returned pulse
• Azimuth resolution
• Range resolution
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19. The Radar Equation
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20. 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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21. Factors controlling variations in σ
• Surface roughness
• Surface dielectric constant
21

22. Surface Reflectance or Scattering
• Specular reflection or scattering
• Diffuse reflection or scattering
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23. 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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24. 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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25. Radar backscattering is
dependent on the relative
height or roughness of the
surface
Figures from
http://pds.jpl.nasa.gov/
mgddf/chap5/f5-4f.gif
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26. Microwave scattering as
a function of surface
roughness is wavelength
dependent
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27. 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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28. Microwave scattering is
dependent on incidence angle
As incidence angle increases,
backscattering decreases
Figure from
http://pds.jpl.nasa.gov/
mgddf/chap5/f5-4f.gif
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29. Factors controlling σ
• Surface roughness
• Surface dielectric constant
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30. 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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31. 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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32. 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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33. 33

34. 34

35. Moderate Burn All Years
0
-2 y = 0.3299x - 18.268
-4 R2 = 0.82
ERS-2 Backscatter (dB)
-6
All Years
-8
2003-4 Validation Sites
-10
Linear (All Years)
-12
-14
-16
-18
0 10 20 30 40 50
6 cm % Volumetric Moisture
Radar backscatter (image intensity) in burned
forests is proportional to soil moisture
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36. 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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37. λ = 3 cm λ = 24 cm
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38. Smooth area – no wind
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39. L-band airborne SAR Image
Why do you have backscatter at L-band from an ocean surface?
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40. Backscatter dependence on wind speed:
L-HH Measurements
upwind
Radar backscatter from a water surface varies as a function of:
1. Wind speed
2. Look direction (upwind, downwind, cross wind)
3. Incidence angle (look direction of the sensor relative to the surface
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41. 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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42. ERS Scatterometer Accuracy
Scatterometer accuracies
determined through comparisons
made with surface data collected
by buoys
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43. 43

44. 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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45. 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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46. Key: O = Open Water, FY = First Year Ice, MY = Multi-year Ice
H = horizontal polarization, V = Vertical Polarization
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47. Key: O = Open Water, FY = First Year Ice, MY = Multi-year Ice
H = horizontal polarization, V = Vertical Polarization
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48. Scanning Multi-channel
Microwave Radiometer (SMMR)
1978 - 1987
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
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49. Special Sensor
Microwave/Imagery (SSM/I)
1987 to present
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
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50. Key Points for Lecture 23
1. Definition of a RADAR
2. Uses of a radar altimeter
3. The radar backscatter coefficient - σ
4. Source of variations in σ
– Surface roughness
– Incidence angle
– Surface dielectric constant
5. Effects of soil moisture on σ
6. Effects of wind speed on σ from water surfaces
7. ERS Scatterometer
8. Microwave Radiometers – how do they work?
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