# Topic 7 Passive microwave

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```							CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                                                                            1
Topic 7: Passive Microwave Systems

Topic 7: PASSIVE MICROWAVE SYSTEMS
•    Passive microwave radiation – like thermal radiation – results from thermal emission
from the earth but at much longer wavelengths: 1mm – 30 cm
•    What is measured is radiative (apparent) temperature
•    The microwave observations are strongly affected by of the target.

Planck's Formula describes the magnitude and spectral distribution of radiation emitted by a
blackbody source.

2hc 2                                         where:     M    =   exitance at wavelength 
M 
     hc                                                   h    =   Planck's constant = 6.625 x 10-34 J-sec
5 exp        1
     kT                                                   c   =   speed of light in a vacuum = 2.997 x 108m/sec
k   =   Boltzmann's constant = 1.38 x 10-23 J/°K
T   =   absolute temperature in degrees Kelvin

8.E+07                                                                                  1.4E+12
7.E+07                                                                                  1.2E+12
275 *K
Earth Emittance (w/m2)

Solar Emittance (w/m2)
6.E+07
300 *K           1.0E+12
5.E+07
325 *K           8.0E+11
4.E+07
350 *K           6.0E+11
3.E+07
5800 *K
4.0E+11
2.E+07

1.E+07                                                                                  2.0E+11

0.E+00                                                                                  0.0E+00
0          5            10             15               20               25
Wavelength (microns)

1.00
Emittance (w/m2)

0.10                                                                            275 *K
300 *K
0.01                                                                            325 *K

0.00

0.00
1000       2000   3000     4000      5000     6000      7000        8000      9000      10000
Wavelength (mm)
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                    2
Topic 7: Passive Microwave Systems

• Passive: Emission from the earth
• Wavelengths:          1 mm – 30 cm
• Frequency:          300 GHz - 0.3 GHz               30
• Atmosphere:                                              y GHz  3 cm  10 GHz
x cm
• Essentially opaque for λ < 1 mm primarily
due to H20 and O2                                          10 cm  3 GHz
• Atmospheric attenuation very low for λ > 3 cm
• Low energy  Large FOB
• Brightness Temperature (apparent temperature) is the temperature of a blackbody with the
same temperature.
Atmospheric Transmission in the microwave
1c         3m      2m                    1m
m          m       m                     m     Dotted line: oxygen contribution,
Dash-dotted line: water contribution,
Solid line: contribution from all resonance
lines in atmosphere.

Passive microwave imaging: Shafter
Airport, Bakersfield CA

Image collected using a plastic lens to focus 3 mm     Passive millimeter wave imaging system
radiation onto an array of miniature receivers.        mounted on the nose of an aircraft. Note
the white, plastic lens. Austin Richards
"Alien Vision" SPIE Press, 2001, 160 pgs.
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                    3
Topic 7: Passive Microwave Systems

Characteristics of Passive Microwave
•   All weather                                       •    Multi-channel
•   Day/night                                         •    Long record
•   Daily or better coverage                          •    Low energy / Large footprint (FOV)

Planck's Formula
2hc2
M 
     hc  
5 exp        1
     kT  
2        3
 hc         hc  1  hc      1  hc 
Taylor Series Expansion:      exp        1                       
 kT        kT  2!  kT  3!  kT 
For hc/kT << 1: exp[hc/kT] ~ 1 + hc/kT

2ck
 Rayleigh-Jeans Approximation:          M           T
4

Thus, emission from real objects on the earth's surface in the microwave region is then:

Mλ = ελ Mbbl = (2ck/λ4) ελTbb
• Linear dependence on temperature
• Much more sensitive to emissivity differences than in the thermal range.

Brightness Temperature: TB = ελTbb

• There is a large variation in emissivity of different materials at microwave wavelengths.
Example: for water, ε  0.41
for ice, ε  1.0 ==> ice appears relatively warm, water relatively cold.
• Low energy ==> poor spatial resolution

Emissivity in the microwave:
Emissivity is a function of several variables:
• Dielectric properties (e.g., electrical conductivity).
– Inherent conductivity
– Water content
– Salinity
– Frequency (wavelength)
• Angle of incidence/view
• Non-Lambertian emission
– Surface roughness
– Polarization

NOTE: A blackbody source will be Lambertian and unpolarized.
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                   4
Topic 7: Passive Microwave Systems

Dielectric properties
Earth Conductivity & Dielectric Constant
The dielectric constant and conductivity of the earth vary greatly at different locations.
• The dielectric constant is a measure of the ability of a material to store a charge from an
applied electromagnetic field and then transmit that energy. In general, the greater the
dielectric constant of a material, the slower microwave energy will move through it.
• The dielectric constant is a measurement of how well radar energy will be transmitted to
depth. It therefore not only measures velocity of propagating radar energy, but also its
strength.
• Most earth materials have a dielectric constant in the range of 1 to 4 (air=1, veg=3,
ice=3.2)
• Dielectric constant of liquid water is 80  moisture content affects brightness
temperature

Sensitivity to water content of bare soil

•   Emissivity decreases as soil moisture
increases.

•   Emissivity is highly wavelength dependent.

In general:
high dielectric constant  low emissivity
water:        80     ~0.50
bare soil:      1 - 4 ~0.95

M. Brogioni, G.Macelloni, S.Paloscia,
HU

P.Pampaloni, S.Pettinato, E.Santi
IFAC-CNR – Florence, Italy
H
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                        5
Topic 7: Passive Microwave Systems

Polarization notation
The direction of polarization is specified relative to the plane containing the incident, reflected
and refracted rays.

Microwav                                                      Polarization is defined determined by
antenna              V
the microwave antenna
H

V
emission
θi
H

H = horizontal polarization      the electric field is perpendicular
(perpendicular polarization)   to the vertical plane.

V = vertical polarization           the electric field is parallel to the
(parallel polarization)           vertical plane

Emissivity Variables
•   Water content
•   Polarization
•   Salinity
•   Angle of incidence
•   Frequency
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                     6
Topic 7: Passive Microwave Systems

Sensitivity to Plant Water Content

Desert: high emissivities                   Vegetation: low emissivities
Fatima Karbou & Catherine Prigent
http://cimss.ssec.wisc.edu/itwg/itsc/itsc14/presentations/session6/6_7_Karbou.pdf

Emissivity in North America
18 GHz, Horizontal Polarization, Day                  19 GHz, Horizontal Polarization, Night

Prigent, C., Rossow, W. B., and Matthews, E. (1998) Global maps of microwave land
surface emissivities: Potential for land surface characterization, Radio Sci., 33, pp. 745-
751.

Moncet, Galantowicz, Liang, and Lipton (2006) A land surface emissivity database for
conically scanning microwave sensors. American Meteorological Society.
http://ams.confex.com/ams/pdfpapers/100359.pdf
HU                                                         UH
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                                         7
Topic 7: Passive Microwave Systems

Atmospheric effects: Brightness temperature from a reflecting surface
emitting sources @ Ti (e.g.,
µ wav
sky,
-senso
e
sun, moon, clouds, deep space)                              r

 

Surface @
Ts
For an opaque surface: TB = Ts + Ti() = Ts + (1–)Ti()

•      if   1 there will be very little influence on the observed brightness by reflectance
from other sources of moderate brightness temperature
•      if  is low, reflectance from other sources can have a strong effect.
•      There is a wide range of emissivities in the μwave
fresh water < 0.5                 ice  1
ocean water  0.4               earth > 0.85
Absorption:
• for  < 1 mm, H2O and O2 are strong absorbers
• for  > 3 cm the air is quite clear
• Clouds (water vapor, liquid water) both absorb and re-emit radiation.

• Atmospheric transmittance = a
a + a + a = 1
TB   a  Ts                   1   a  Ta
• atmospheric absorption:                                                   attenuated          energy absorbed
a = (1– a) =  if a = 0                                           brightness           and reradiated
temperature           by the medium
[ in µ-wave, a  0 ]
of the surface       in the optical path
• for thermal equilibrium, aTa = aTa

external sources @ Ti                         µ-wave sensor

1,Ta1                  2,Ta2
θ   θ

Surface @ Ts
TB = {     [1 Ti +                (1 – 1 ) Ta1 ] 2            +      2Ts }       +     (1 – 1 ) Ta2
External energy                 External energy                     Energy                  Energy
which is not                      which is                        emitted             absorbed and
absorbed, i.e., it               absorbed and                        by the             re-radiated by
reaches the                    re-emitted by                      surface            the atmosphere
earth's surface                the atmosphere                      at temp                 at temp
at temp T1                     at temp Ta1                          Ts                     Ta2
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                     8
Topic 7: Passive Microwave Systems

The detector for microwave radiation is an antenna. The apparent temperature observed at the
antenna -- the antenna temperature -- is related to the brightness temperature, TB, by:
U                      U

Tant 
 T G d d

B

where  is the IFOV of the system and G is the antenna gain.

The amount of radiation obtained is limited by:
• the antenna bandwidth (Δλ or Δν)
• the beamwidth (size of the area at which the antenna looks)

The bandwidth is the wavelength or frequency range over which the antenna is sensitive.
U           U

The beamwidth is the angular interval over which the antenna's power pattern exceeds one-
U           U

half of its maximum value ("half-power" beamwidth). (This is the "IFOV" of the microwave
system)
Microwave systems use antennas to focus and direct radiation. The most widely used narrow
beam antennas are reflector
antennas. The antenna shape is generally a paraboloid of revolution.
For full earth coverage from a geostationary satellite, a horn antenna is used. Horns are also used
as feeds for reflector antennas.
Phased array antennas may be used to produce multiple beams or for electronic steering. Phased
arrays are found on many non-geostationary satellites.

Microwave sensors – Dipole Antennas
A dipole antenna is a straight electrical conductor measuring
1/2 wavelength from end to end and connected to a radio-
frequency (RF) feed line. Although this antenna is one of the
simplest, it constitutes the main RF radiating and receiving
element in various sophisticated types of antennas.

A horn antenna derives its name from
the characteristic flared appearance. The
flared portion can be square,
rectangular, or conical. A horn antenna
must be a certain minimum size relative
to the wavelength of the incoming or
outgoing electromagnetic field. If the
horn is too small or the wavelength is
too large (the frequency is too low), the
antenna will not work efficiently.

Like the dipole antenna, the horn antenna is typically used as the main receiving or transmitting
element of a more sophisticated antenna.
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                   9
Topic 7: Passive Microwave Systems

The widest dimension of a waveguide is called the "a" dimension and determines the range of
operating frequencies.
The narrowest dimension determines the power-handling capability and is called the "b"
dimension.
Horn antenna, waveguide and feed

Dipole Antennas: polarization
The polarization of a dipole antenna is determined by its orientation.
A dipole antenna erected vertically (or a horn antenna with it's "a" dimension oriented vertically)
is "vertically polarized".
A dipole antenna erected horizontally (or a horn antenna with it's "a" dimension oriented
horizontally) is "horizontally polarized".

Dipole Antennas: polarization & beam pattern
A vertical dipole antenna is responsive to vertically polarized radiation (of the appropriate
wavelength) in all azimuthal directions (φ). The response in the zenith direction (θ) decreases as
the sin of θ.

A horn antenna is inherently more directional, but is not necessarily well-focused.
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                      10
Topic 7: Passive Microwave Systems

Microwave antennas – Antenna Focus
Antenna focusing can be accomplished with a parabolic dish which restricts the direction of
waves detected by the dipole (or horn) antenna at the focus.

Microwave antennas– Dish antenna (paraboloid)
• Dipole and horn antennas are commonly used as the active element in a dish antenna.
• The dipole or horn is pointed toward the center of the dish.
The use of a horn, rather than a dipole antenna, at the focal point of the dish minimizes
loss of energy (leakage) around the edges of the dish reflector. It also minimizes the
response of the antenna to unwanted signals not in the favored direction of the dish.

Microwave antennas – Truncated paraboloid
 The reflector is parabolic in the horizontal plane  focused into a narrow
beam horizontally.
 The reflector is truncated vertically  the beam spreads out vertically
 This fan-shaped beam is used for the accurate determination of bearing.
• Since the beam is spread vertically, it will detect aircraft at different
altitudes without changing the tilt of the antenna.
• The truncated paraboloid also works well for surface search radar
applications to compensate for the pitch and roll of the ship.
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                11
Topic 7: Passive Microwave Systems

Antennas: beam patterns from: http://www.tmeg.com/tutorials/antennas/antennas.htm

Microwave antennas – dipole phased arrays
HU                                            UH

•   Whenever two or more simple antenna elements (e.g. dipoles) are brought together and
driven from a source of power (a transmitter) at the same frequency, the resulting antenna
pattern becomes more complex due to interference between the signals
transmitted/detected separately from each of the individual elements.
•   At some points, this interference may be constructive causing the transmitted signal to be
increased. At other points, the interference may be destructive causing a decrease or even
a cancellation of transmitted energy in that direction.
Here two dipole antennas are placed close to each other and
The resulting antenna pattern is narrower or sharper in the
weaker than it would have been for either dipole alone.
The ratio of the strength of the signal at the pattern maximum
(i.e. at T1) to the signal for a single antenna element is called
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                     12
Topic 7: Passive Microwave Systems

the pattern gain. Pattern gain is accomplished at the expense of power transmitted in other
directions.

narrowing of the pattern.
Four dipole antennas placed near each other and monitored by a
pattern than that for the 2-dipole case.
The resulting antenna pattern is narrower or sharper in the
Sidelobes also appear in the total antenna pattern:
• characteristic feature of most complex antenna arrays.
• generally an undesirable characteristic of an antenna system
• It is theoretically possible to suppress side lobes completely in an array of antenna elements
if the excitation of each element is controllable.
• The process of shaping the antenna pattern so as to eliminate sidelobes is called tapering.
• Eliminating sidelobes results in less total gain at the pattern maximum, however, and it

The angle at which the pattern maximum occurs can be
each of the antenna elements.
• With all elements in-phase, the pattern maximum will
• By adjusting the relative phase, the peak of the main lobe
can be shifted (or steered) to a new angle relative to
• In general, the maximum signal strength at the new
pointing angle (T4 in Figure 3 to the left) is close to but
When the pattern is steered to a new direction, the shape and direction of any sidelobes that may
have originally been present changes.
If the pattern is steered too far relative to the element spacing, a new lobe (called a grating lobe)
will appear with a peak in its pattern nearly equal to the main lobe.
The point where this occurs is the maximum useful steering angle.
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                 13
Topic 7: Passive Microwave Systems

Increasing the sensitivity of microwave radiometers:
- increasing the IFOV (beamwidth)
(typical solution. The spot size of a typical satellite microwave radiometer is
many kilometers.)
- increasing the bandwidth
(can probably be effective for some applications but is often not a realistic
option.)
- integrating over longer periods
(good for ground systems, not very useful for aircraft systems)

Types of microwave scanner

The conical scan insures a constant           Note that the scan line is not perpendicular to the
incidence angle at the surface  removes      nadir path  the scan rate is relatively slow.
one variable of emissivity.

Microwave sensors - filters:
The 301-1X series of ceramic bandpass filters are four-pole filters
primarily used in CDMA transmitters and receivers for base station
applications. Features include a bandwidth of ±15 MHz and center
frequencies of 881.5 and 836.5 MHz. The small size of these
surface mount filters provide a low profile and low mass for
efficient assembly integration.

Trak Communications
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                   14
Topic 7: Passive Microwave Systems

SMMR - Scanning Multi-channel Microwave Radiometer
(1978 – 1987: Seasat and Nimbus
HU   http://podaac.jpl.nasa.gov:2031/SENSOR_DOCS/smmr.html   UH

•   sea ice concentration,
•   5 freq.: 6.63, 10.69, 18.0, 21.0 and 37.0 GHz
•   snow cover,
•   sea surface temp.,
•   snow moisture,
•   low altitude winds,
•   rainfall rates, and
•   water vapor and cloud liquid water content,
•   differentiation of ice types.
•   sea ice extent,

Elliptical antenna reflector is approximately 110 cm x 80 cm.

SSM/I
aboard Defense Meteorological
Satellite Program (DMSP) satellites.

linearly polarized, 5-frequencies:
19.35 GHz       (69 x 43 km)
22.235 GHz (50 x 40 km)
37.0 GHz        (37 x 28 km)
85.5 GHz        (15 x 13 km)

Ocean surface winds,
Column water vapor,
Cloud liquid water.

http://podaac.jpl.nasa.gov:2031/sensor_docs/ssmi.html
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                      15
Topic 7: Passive Microwave Systems

March 2002-present: Envisat
• Two channels: 23.8 and 36.5 GHz.
• Measurement of the integrated atmospheric
water vapor column and cloud liquid water
content, as correction terms for the radar
altimeter signal.
Also provides the precipitable water and cloud
liquid content along the satellite track
http://envisat.esa.int/instruments/mwr/

MWR: Differential absorption

• measure the strength of the water-
vapor emission-line at 22 GHz

• differential measurements to
eliminate Earth emission

May 2002-present: AQUA
• conically scanning
• total power passive microwave radiometer
• 12 channels @ 6 frequencies:
6.925 GHz (56 km);      10.65 GHz (38 km)
18.7 GHz (21 km);       23.8 GHz (24 km)
36.5 GHz (24 km):       89.0 GHz (12 km)
• Horizontally and vertically polarized radiation are
measured separately at each frequency.
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                      16
Topic 7: Passive Microwave Systems

TMI - TRMM Microwave Imager
Tropical Rainfall Measuring Mission
1997 – present: TRMM - http://trmm.gsfc.nasa.gov/overview_dir/tmi.html

Observation Frequency    10.7, 19.4 21.3, 27 and 85.5 GHz
Polarization               Vertical/Horizontal
(21.3 GHz Channel: H only)

Horizontal Resolution                6 - 50 km

Scan Mode                Conical Scan (49 deg)

Purpose            Designed to provide quantitative
rainfall information over a wide
swath : water vapor, cloud water,
and rainfall intensity.

1998 – present: NOAA 15, 16, 17 http://pm-esip.msfc.nasa.gov/amsu/index.phtml

AMSU-A
• 15-channel total power microwave radiometer
• 23.8 GHz to 89.0 GHz
AMSU-B
• 5-channel total power microwave radiometer
• two channels centered nominally at 89 GHz and 150 GHz
• three channels centered around the 183.31 GHz water vapor line

Operates with either a 48 km or 16 km resolution
Measures total precipitable water, rain rate, cloud liquid water, snow cover, sea ice, total
precipitable water, and cloud liquid water

Individual AMSU-A channels are carefully chosen
to detects microwave radiation from a discrete layer
within the earth's atmosphere.

This allows the development of a tropospheric
water vapor profile.

http://amsu.ssec.wisc.edu/explanation.html
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                             17
Topic 7: Passive Microwave Systems

An L-Band (~20 cm) Radiometer Mapping System for small aircraft
Applications:
• ocean surface temperature and salinity
• soil water content
• mapping of ice extent
Scanning Low Frequency Microwave Radiometer (SLFMR); the dual-channel
infrared radiometer makes an almost direct measurement of the ocean surface
temperature. A Global Position System (GPS) receiver is used to geolocate measurements

WATER: Sea Surface Temperature TMI (TRMM Microwave Imager):

Global SST

3 day composite

no clouds

http://www.eorc.nasda.go.jp/TRMM/imgdt/day_tmi2/index.htm
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                    18
Topic 7: Passive Microwave Systems

WATER: Sea Ice

Ice (ε ≈ 1.0) is much more emissive than the water
(ε ≈ 0.4) making it possible to estimate the areal

coverage of the
ice based on the
composite
emissivity.
CEE 6100 / CSS 6600 Remote Sensing Fundamentals       19
Topic 7: Passive Microwave Systems

WATER: AMSR-E SST & Surface winds
• SST fields are shown as gray contours
in this figure superimposed on the
ASMR-E wind field.
• Emissivity varies with surface
roughness which, in turn, varies with
wind speed.
• Because the different frequencies
interact differently with different
roughness scales it is possible to
estimate wind speed independently of
temperature
• Over the cold Malvinas Current, the
wind speed is lower than over the
warm Brazil Current waters.
• This is an example of the close
coupling that exist between the
surface wind and SST due
momentum fluxes in the marine boundary layer.

WATER: AMSU Surface air temperature
Comparing brightness temperatures from frequencies
– some of which do not penetrate all the way to the
ground – allows estimates of the temperature above
the water surface.

http://pm-esip.msfc.nasa.gov/cyclone/
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                20
Topic 7: Passive Microwave Systems

ATMOSPHERE: water vapor

www.stcnet.com/projects/NVAP/NVAP.html
Comparing brightness temperatures from frequencies that reach to different altitudes also allows
estimates of water vapor content at selected altitudes.

ATMOSPHERE: water vapor profiles

TRMM                                                 ATMOSPHERE: Katrina rainfall profile
CEE 6100 / CSS 6600 Remote Sensing Fundamentals                                                                  21
Topic 7: Passive Microwave Systems

ATMOSPHERE: water vapor

Forsythe, Jones and Vonder Haar (2004) Water vapor profile retrievals from satellite microwave sounding. 13 th
Conference on Satellite Meteorology and Oceanography, Norfolk, Va.

Comparison of rawinsonde and NOAA-15 AMSU retrievals for 2000. 13289 matchups. A)
Temperature and (B) Mixing ratio. Different colored lines indicate rawinsonde, retrieval,
retrieval with zenith angle less than 15 degrees, and retrieval with cloud liquid water less than
0.03 mm, respectively. Temperature retrieval first guess error from the NESDIS statistical
algorithm also shown.

ATMOSPHERE: Global precipitation estimates

3 Hourly Global Rainfall                                Week of Global Rainfall Accumulation
These images represent a merger of all available SSM/I and TMI microwave precipitation
estimates plus estimates from geostationary infrared satellites.

```
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