EEL-5432 Satellite Remote Sensing
Lecture-1: Intro to Satellite
Remote Sensing with
Microwaves
Part-I Physics
Prof. W. Linwood Jones
Central Florida Remote Sensing Lab
University of Central Florida
Orlando, FL
What is satellite microwave remote
sensing?
• A special application of microwave
communications technologies for the
purpose of collecting geophysical
information about targets (objects and
media) without making physical contact
How does it work?
• There is an interaction between
propagating EM waves and matter.
• The environment imposes a modulation on
the EM wave which becomes its identifying
characteristics
• Amplitude, phase shift, time delay and Doppler freq
• Sensors use microwave communications
technologies
• Antennas, transmitters and receivers
• Active (radar) and passive (microwave
radiometry) measurements
Outline
• Blackbody Radiation
• Microwave Radiometry
• Radiative Transfer Theory
– Microwave Emissivity
•
•
•
•
Ocean
Atmosphere
Sea Ice
Land
Thermodynamic Principal
• All matter at temperatures > absolute zero
both absorb and emit non-coherent EM
energy (noise) simultaneously
• Absorption of EM energy causes its physical or
thermodynamic temperature to rise which
increases in the emitted EM radiation
• At thermal equilibrium, the physical temperature
is constant and the rate of energy absorbed is
exactly matched by energy emitted
Blackbody Radiation
• A blackbody is a perfect emitting/absorbing
object - All incident radiation for all wavelengths
(frequencies) and incidence angles
– passes into the object (zero reflectance) and is
absorbed (zero transmittance) internally
– the energy is converted to heat that raises the
physical temperature
• Law of conservation of energy
– The sum of reflectance and absorption is unity
(100%)
– Since the absorption of a black body is unity, the
reflectance must be zero
– A true blackbody at room temperature appears black
to the eye - hence the origin of its name
ElectroMagnetic Spectrum
Blackbody Radiation Spectrum
• The blackbody emitted energy spectral flux density
varies with wavelength as determined by Planck’s
Law:
1
2πhc2
S(λ) =
ch
5
λ e λkT − 1
Where h = Planck’s constant = 6.6253 x 10-34, joule-sec
k = Boltzmann’s constant = 1.38 x 10-23 , joule/Kelvin
– For an incremental wavelength, the flux (Watts/surface
area) units are:
• W* cm-2* μm-1
• The total radiated flux is:
∫ S(λ ) dλ
Planck’s Law - Linear Plot
Units: Watts/unit area/unit wavelength (W/m2/m)
Blackbody Emission Spectrum
(log-log plot)
Planck’s Radiation Law
Region of interest
Microwave
Infrared
Visible
1 mm
300 GHz
10 cm
3 GHz
Rayleigh-Jeans Law approx. in the
microwave spectral region
0.001
0.01
0.1
Outline
• Blackbody Radiation
• Microwave Radiometry
– Measurement of Blackbody Emissions in the
Microwave Spectral Range
• Radiative Transfer Theory
– Microwave Emissivity
•
•
•
•
Ocean
Atmosphere
Sea Ice
Land
Blackbody Power Collected by a
Microwave Antenna
P
blackbody
= kT Bandwidth
b
where Tb = the blackbody brightness temp, which is equal to
the physical temp
• For non-blackbodies, microwave brightness
temperature, Tb, of a media is:
Tb = ε * T phys
where ε = the surface emissivity of the media
Radiometer Received Power
Non-blackbody Emitter
Outline
• Blackbody Radiation
• Microwave Radiometry
• Radiative Transfer Theory
– Microwave Emissivity
•
•
•
•
Ocean
Atmosphere
Sea Ice
Land
Microwave Brightness is the sum of
three components
Radiometric Emissions are Non-coherent
and their Powers Add
Emissivity of Physical Media
• Ocean Tb
Ocean Polarized Emission
• Ocean microwave emission is strongly
polarized
- Depends upon the
orientation of the
electric field in the
plane of incidence
Ocean Emissivity cont
• Ocean microwave emission depends upon
the dielectric properties of sea water (Debye
equation)
• Dielec constant is f(salt content, water temp &
microwave freq)
• Emissivity is calculated using EM theory
• Fresnel reflection coefficients
• f(dielec const, incidence angle & polarization)
• Emissivity: ~ 40% (H-pol) ; ~ 60% (V-pol)
• is a function of geophysical parameters:
• sea surface temperature, salinity & small scale ocean wave
roughness (surface wind speed)
Ocean Radiometric Emissions
• Surface Brightness Temperature
Tb = ε SST
• Γ = power reflection coefficient =
where
ρ
|ρ|
= Fresnel voltage coefficient
ε = Emissivity = 1- Γ
2
Ocean Surface Emissivity
To Antenna
ε is a measure of the
efficiency of transmission
of internal blackbody
radiation across the air/sea
interface
Fresnel Voltage Reflection Coefficient
Air to Water = f(θ)
cos θ 1 + e r 2 − sin θ 1
2
er2 is the complex dielectric
constant of media-2
ρV − pol = −[
er 2 cosθ1 − er 2 − sin2 θ1
er 2 cosθ1 + er 2 − sin θ1
2
]
]
Reflection coefficients
ρ H − pol = −[
cos θ 1 − e r 2 − sin 2 θ 1
Incidence angle
Sea Water Dielectric Constant
Dielectric Constant of Saline Water
• Salinity of a solution is defined as the total
mass of solid salt in grams dissolved in one
kilogram of solution (parts/thousand, ppt)
• Dielectric constant of sea water is
– f(salinity, SST, freq)
ε = ε − jε
sw
'
"
sw
sw
Dielectric Constant of Saline Water - cont-1
• Dielectric constant of sea water is
– Real part
ε −ε
ε =ε +
1+(2π f τ )
'
sw
swo
sw∞
sw∞
2
sw
– Imaginary part
2π f τ (ε − ε )
σ
ε =
+
1+( 2π f τ )
2πε f
"
sw
swo
sw∞
sw
i
2
sw
Ionic Conductivity
o
Dielectric Constant of Saline Water - cont-2
Sea surface temperature and salinity dependence of
dielectric const, εswo(T, Ssw)
ε (T ,S ) = ε (T ,0) ⋅a(T ,S )
swo
sw
swo
sw
where
ε (T ,0) = 87.134 −1.949x10 T −1.276x10 T
−1
−2
2
swo
+ 2.491x10 T
−4
3
α(T ,S ) = 1.0 +1.613x10 T ⋅ S − 3.656x10 S
−5
sw
−3
sw
+ 3.210x10 S − 4.232x10 S
−5
2
sw
−7
3
sw
sw
Dielectric Constant of Saline Water - cont-3
• The relaxation time of sea water is
τ (T ,S )= τ (T ,0) ⋅ b(T ,S )
w
sw
w
sw
where
τ (T ,0)= τ (T ), the relaxation time of pure water
w
w
and
b(T ,S ) = 1.0 + 2.282x10 T ∗ S − 7.638x10 S
− 7.60x10 S + 1.105x10 S
−5
sw
−4
sw
−6
2
sw
sw
−8
3
sw
Dielectric Constant of Saline Water - cont-4
• Ionic conductivity is
σ (T ,S ) = σ (T = 25C,S ) ⋅ e
i
sw
i
−Φ
sw
where
σ (T = 25C,S ) = S [0.18252 −1.4619x10 S + 2.093x10 S
−3
i
sw
sw
−5
sw
sw
−1.282x10 S ]
−7
3
sw
and
Φ = Δ ⋅[2.033x10 +1.266x10 ⋅ Δ + 2.464 x10 ⋅ Δ
− S (1.849x10 − 2.551x10 ⋅ Δ + 2.551x10 ⋅ Δ )]
−2
−5
−4
−7
sw
and
Δ = (25 − T ), Sea Surface temp in Celsius
2
−6
2
−8
2
Dielectric Constant of Sea Water
SST = 20 C, Salinity = 33 ppt
Real
Imaginary
Dielectric Constant of Pure Water @ 20 C
Real
Imaginary
Dielectric Constant of Sea Water
SST = 20 C, Salinity = 33 ppt
1 GHz
25 GHz
21 GHz
13 GHz
2 GHz
7 GHz
3GHz
45 GHz
35 GHz
55 GHz
75 GHz
100 GHz
17 GHz
9 GHz
5 GHz
Salinity Effect on Emissivity
1GHz
Nadir
H-pol
Salinity Effect on Emissivity
4GHz
Nadir
Ocean Roughness - Wind Vector
Fresh Water Voltage Reflection Coeff &
Emissivity
Fresnel Reflection Coeff
Emissivity
H-pol
V-pol
V-pol
H-pol
Rough Ocean Emissivity
• The total ocean emissivity is
eocean = esmooth + Δeocean
• The rough surface emissivity is an additive
term which increases with surface roughness
Δeocean = f(wind speed)
Ocean Emission is Area Weighted
Sum of Foam and Foam-free Water
Δεocean(wind speed) = FF *εfoam + (1- FF)
*εrough
Ocean Emission
Ocean Emission
Foam Emission
Effect of Wind Speed (Surface
Roughness)
W3>W2>W1
Foam Emissivity
• Foam is approx a blackbody
• Emissivity is independent of polarization
• Emissivity increases with frequency
• Foam fraction (area coverage) increases
with wind speed
FF
• 0% @ 6 m/s & 14% @ 30 m/s
WS , m/sec
Ocean Surface at High Wind Speeds
Foam
patches
Breaking
Waves
Foam
Streaks
Modified Power Reflection Coeff
• The sea surface emissivity is modified by the
action of the surface wind speed
– Surface roughness effect
• Frequency dependent
• Polarization dependent (V & H-pol)
• Incidence angle dependent
– Foam cover effect
• Frequency dependent
• Incidence angle dependent
• Surface roughness increases with wind speed
• Foam cover area fraction increases with wind
speed
Polarimetric Radiometry
• Ocean Surface Emission and Scattering
Vary With Wind Vector
- Wind Direction Dependence Arises From
Anisotropic Distribution and Orientation of Wind
Driven Waves
• Polarimetric Radiometry Measures Stokes
Vector
- Polarization Properties of Emitted/scattered
Radiation
- Contains Directional Information
TV
Tlc
T+45
TH
Trc
T-45
Upwelling Microwave
Emission
- Wind Direction Signal Is Two Orders of
Magnitude Smaller Than Background Signal
*
*
⎡ I ⎤ ⎡ E h Eh + E v E v
⎢Q ⎥ ⎢ E E * − E E *
h
h
v
v
Is = ⎢ ⎥ = ⎢
⎢U ⎥ ⎢ 2 Re E v Eh*
⎢ ⎥ ⎢
*
⎣V ⎦ ⎣ 2 Im E v Eh
⎤ ⎡ Tv + Th ⎤
⎥ ⎢
⎥
T
T
−
v
h
⎥=⎢
⎥
⎥ ⎢T45 − T−45 ⎥
⎥ ⎢
⎥
T
T
−
⎦
⎣
lc
rc
⎦
Available from “Dual Polarization” Systems
(SSM/I, SSMIS)
New Capability Available from “Polarimetric” Systems
(WindSat)
Polarimetric Radiometry
AV-H signal
TV
Tlc
T+45
TH
Trc
S. Soisuvarn, Z. Jelenak, and W. L. Jones, "An Ocean Surface Wind Vector Model Function for a
Space borne Microwave Radiometer," IEEE Trans. Geosci. Remote Sens., vol. 45, Oct. 2007.
T-45
Atmosphere Emissivity & Tb
Atmospheric Microwave Tb is the
sum of two components
Atmospheric Emissivity (Absorption)
• Atmospheric microwave emission is
isotropic and non-polarized
• Resonant absorption by water vapor
(22 GHz, 183 GHz, & 325 GHz),
• Resonant absorption by oxygen (60
GHZ & 120 GHz)
• Non-resonant absorption by
hydrometers (cloud water and rain)
Oxygen Absorption Coefficient
Resonant Freq
~ 60 GHz
Resonant Freq
~ 120 GHz
Water Vapor Absorption Coefficient
Resonant Freq
= 22.2 GHz
Cloud Liquid Water Absorption Coeff
High
CLW
Med
CLW
Low
CLW
Atmospheric BrightnessTemperature
(incremental volume)
• Tb emitted by a given volume of atmosphere
= emissivityatmos * physical tempatmos
= absorptionatmos * physical tempatmos
• Power transmission coefficient through a
layer of the atmosphere is
Tau = 1 - emissivityatmos
= 1 - absorptionatmos
Upwelling Atmos Tb
• Integral equation to calc up-welling (down-welling)
atmospheric emission (brightness temperature) is:
Atten through atmos layers
∞
T (θ ) = secθ ∫ K(z')T (z') e
up
− τ ( z',∞ )secθ
dz'
0
Amtos absorption = f(alt)
Amtos temp = f(alt)
Downwelling Atmos. Tb
Reflection at Air/Sea Interface
To Antenna
Downwelling
Atmos. Tb
Reflected Atmos. Tb
Absorbed Atmos Tb
Total Atmospheric Tb
T
b−atmos
•τ
•Γ
=T
upwelling
+ (T
downwelling
∗Γ)∗ τ
= Atmospheric transmissivity
= Sea surface power reflection coefficient
Total Ocean Tb
Sum of direct ocean emission and
atmospheric components
Microwave Brightness is the sum of
three components
Radiometric Emissions are Non-coherent
and their Powers Add
Block Diagram Representation of
Radiative Transfer
Microwave Ocean Apparent Temp
Tap = Tup + La ∗ (Tb + Tscat )
Tscat = (1− ε) ∗ Tsky
= (1− ε) ∗ (Tdn + La ∗ Tex )
Tup ≈ (1− La ) ∗ Tatmos ≈ Tdn
La = Atmospheric loss (transmission coefficient)
Tex = cosmic brightness = 2.7 K
(1 - La) = Atmos absorption
(1 - ε) = Fresnel power reflection
Example Ocean Brightness Temp
Rain over Ocean
• Rain over the oceans “warms” the
brightness temperature
• The effect is a function of the rain rate
(mm/hr)
– The surface emission is reduced and the
atmospheric emission is increased
– For heavy rain, the atmosphere is totally
opaque and the surface cannot be seen
• For this case the atmospheric emissivity is unity
and the Tb saturates at ~ 275 K
Ocean Rain Tb is Freq Sensitive
Ocean Rain Tb @ 13.4 GHz
WindSat/Coriolis Hurricane Rainfall
Imagery
Katrina at Landfall
Operational Data Products Generated at FNMOC in Near Real Time
Emissivity of Physical Media
• Sea Ice
• Land
Sea Ice Emissivity
• Sea ice microwave emission is weakly
polarized and depends upon ice type and
concentration
• First year (FY) ice is saline and has a emissivity
near unity ( > 95% )
• Multi-year, MY, sea ice has undergone a
thaw/freeze cycle and has much less salt content
• Emissivity of MY ice is less than FY ice
• Microwave brightness contrast is > 100 K
between open water and FY and MY sea ice
Retrieved Sea Ice Conc
10/5/96
12/6/96
1/18/97
5/9/97
Land Emissivity
• Microwave emission from dry bare soil has
nearly unity emissivity (radiometrically hot)
• Tb decreases with increasing free water
content (soil moisture)
• Vegetation cover also masks the surface
emissivity for frequencies greater than 2 GHz
• It is not possible to discriminate between
vegetation types
• Microwave emission at millimeter
wavelengths (freq > 30 GHz) from snowcovered land depends upon snow depth
WindSat 37 GHz H-pol Tb
EEL-5432 Satellite Remote Sensing
Intro to Satellite Remote
Sensing with Microwaves
Part-II Application
Prof. W. Linwood Jones
Central Florida Remote Sensing Lab
University of Central Florida
Orlando, FL
Satellite Passive Radiometer
Microwave Imager
WindSat Conical Scanning
Microwave Imager
WindSat Radiometer on
Coriolis Satellite
Microwave Radiometer
V ∝ (T + T ) ⋅G kB
out
ant
rec
rec
WindSat Flight Build
WindSat in TVAC Chamber
WindSat Feed Horn Array
Coriolis Satellite at Launch Site
WindSat Payload Configuration
Height
Width
Mass
Power
Spin Rate
10.5 ft
8.25 ft
661 lbs.
311 Watts
31.6 rpm
WindSat Payload Configuration
GPS Antenna
Cold Sky
Reflector
Stationary
Warm Load
Feed Bench
Feed Horns
(11)
Rotating
Electronics
Feed Bench
Radiator
BAPTA
Stationary
Deck
Stationary
Electronics
Spacecraft
Interface
Momentum Wheel
Rotating
Geophysical Measurements
(Retrievals)
• Geophysical parameter retrieval involves
“inversion” of multi-spectral TB
measurements
• Number of required TB observations is at least
equal to the number of parameters
• Brightness measurements at different EM
wavelengths, polarizations, and incidence
angles constitute independent radiometric
measurements
Geophysical Measurements
(Retrievals)
• Tb is a function of primarily four physical
parameters
• Sea surface temperature (SST), oceanic wind
speed, atmos water vapor and cloud liquid
water
∂T
∂T
∂T
∂T
T =
∗SST +
∗WSpd +
∗WV +
∗CLW
∂SST
∂WSpd
∂WV
∂CLW
B
B
B
+T ( other phys pars)
o
B
B
Example Ocean/Atmos Parameter
Linear Algorithm
• 6 radiometric observations and solve for 4 unknown
geophysical quantities
•
TBV 5.4GHz = 0.55 SST + 0.15 Wind Speed + 0.15 H20cloud + 0.02 H20vapor + 154.6
•
TBV 10.8GHz = 0.40 SST + 0.14 Wind Speed + 0.42 H20cloud + 0.05 H20vapor + 160.1
•
TBV 19.5GHz = 0.22 SST + 0.07 Wind Speed + 1.20 H20cloud + 0.04 H20vapor + 192.1
•
TBH
•
TBH 10.8GHz = 0.17 SST + 1.43 Wind Speed + 0.80 H20cloud + 0.15 H20vapor + 75.8
•
TBH 19.5GHz = 0.20 SST + 0.62 Wind Speed + 2.25 H20cloud + 0.12 H20vapor + 94.9
5.4GHz
= 0.25 SST + 1.08 Wind Speed + 0.18 H20cloud + 0.03 H20vapor + 71.1
Microwave Retrieved Sea Surface
Temperature (Feb’05)
Microwave Retrieved Sea Surface
Wind Speed (Feb’05)
Microwave Retrieved Atmospheric
Water Vapor
Microwave Retrieved Atmospheric
Cloud Liquid
Microwave Retrieved Atmospheric Rain
WindSat Geophysical Retrievals
WindSat View of Hurricane Isabel
Approximate Look Angle
Approximate
Wind Direction
- Wind direction signature is clearly evident in WindSat data
WindSat Ocean Retrievals
• Physically-based Algorithm Using
Nonlinear Optimization (NRL)
– Uses Physical Forward Model
– Solves for All EDRs Simultaneously
– Climatology a priori for TS and V
– Regression a priori for W
φR
– Constant a priori for L
Regression φR + 90
Retrieval φR
• Retrieved EDRs
– Ocean Surface Wind (W, φR)
– Sea Surface Temperature (TS)
– Columnar Water Vapor (V)
– Cloud Liquid Water (L)
– Rain Rate (R) (Based on GPROF)
φR + 180
φR + 270
a priori
TS, W, V, L
Retrieval
TS, W, V, L
Four Retrievals
TS, W, V, L, φR
Median Filter
Stage 1
Stage 2
Final
Retrieval
WindSat Near Real Time
Retrieval
WindSat Wind Field
OE NRT 1.9.1 20060209 - Ascending
WindSat/Coriolis Provides
Operational
Data
Products
Today
Hurricane Gordon - 14 Sep 2006
Imagery
Wind Field
Operational Data Products Generated at FNMOC in Near Real Time
NRL-Monterey Tropical Cyclone Web Page
WindSat Retrievals
WindSat Sea Surface
Temperature
WindSat Sea Ice Retrievals
• WindSat Channels Sensitive to Sea Ice Properties
• Sea Ice Concentration Based on SSM/I NASA Team
Algorithm
– Modified for WindSat Channels and Geometry
– Modified Weather Filter
10.7 H
18.7 H
3 day composite 2004 03 15-17
37.0 H
WindSat Level-3 Sea Ice Products
WindSat Global Soil Moisture & Vegetation Water
•
NRL WindSat physicallybased retrieval performs
simultaneous soil moisture
and vegetation retrievals
•
No direct vegetation
calibration/validation.
•
Global soil moisture
patterns are consistent
with dry/wet patterns of
climate regimes.
•
1 – 12 Sep. 2003
1 – 12 Sep. 2003
Good agreement between
WindSat retrieved
Vegetation Water Content
and AVHRR derived Green
Vegetation Fraction
Volumetric
Soil
Moisture
(Fraction)
Vegetation
Water
Content
(kg/m2)
AVHRR Green
Vegetation Fraction
1 – 8 Sep. 2003
WindSat NRT Wind Field
Summary
•
Satellite microwave radiometers provide
valuable geophysical measurements
•
Remote sensing satellites provide global
coverage each day
•
•
Atmosphere
•
•
•
Day/night all weather
Water vapor, cloud liquid water, precipitation
Sea Ice and land snow cover
Ocean
•
Surface winds, sea surface temperature