Passive-Microwave The “other” microwave remote-sensing technology.
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Transcript of Passive-Microwave The “other” microwave remote-sensing technology.
Passive-Microwave
The “other” microwave remote-sensing technology
It is Possible to Detect a Signal, However Weak, at Very Long Wavelengths
Note that there is radiation from thermal bodies even at longer wavelengths (extending into the microwave region of the spectrum).
100 micrometers
Passive Microwave
Radar is active microwave; this is passive The sensor detects natural microwave
energy reflected and / or emitted from the Earth’s surface
All objects in the natural environment emit (and sometimes reflect) small amounts of passive microwave energy
Passive Microwave vs. Radar
Radar Passive Microwave
Components of the Passive Microwave Signal
Advantages of Passive Microwave
Independent of weather and clouds Day or night operation Includes imaging systems as well as non-
imaging radiometers
Characteristics of Passive-Microwave Sensors
Generally operate between 0.15 and 30 cm in wavelength (some overlap with radar wavelengths)
Wide bandwidths are typical in order to provide sufficient signal to compile an image
Spatial resolution is poor due to the need for large pixels (related to weak signal)
Imaging systems have a moving antenna
Wavelength Range for Passive Microwave
0.15-30 cm
Notice the small amount of radiant exitance at these long wavelengths
Passive-Microwave Sensing
In some ways, more like thermal-infrared than radar remote sensing (could be thought of as thermal scanning within microwave region)
The magnitude of passive-microwave emission is proportional to the product of the emissivity of the target and its surface temperature
Signal Recorded as Brightness Temperature
A measure of the total emissive characteristics of an object (it is different from kinetic temperature)
Defined as the temperature a black body in thermal equilibrium with its surroundings would have to be in order to duplicate the observed intensity (radiance) of a grey body target at a specific wavelength
Scientific unit of measure = K
Brightness Temperatures over Geographic Space Can be Imaged and Displayed
Antarctic brightness temperatures, 23 February 2004 (25 km pixels)
http://nsidc.org/daac/projects/passivemicro/amsre.html
The antenna moves sideways to produce multiple scan lines. The result is a swath of image data
The variations in intensity, when converted to photographic gray levels, yield images
Note the poor spatial resolution
Early Scanning Radiometers
Advanced Scanning Microwave Radiometer for EOS (AMSR-E)
On-board NASA’s Aqua Platform (along with MODIS)
Dedicated to observing climate and hydrology
A multi-frequency (wavelength) (6 channels), dual-polarized microwave radiometer
Provides global, continuous observation
Spatial resolution is variable (6 to 57 km)
Today’s Passive Microwave Systems
AMSR-E: Retrievable Geophysical Parameters
Water vapor Cloud liquid water Precipitation Sea-surface temperature Sea-ice concentration Snow-water equivalents Soil moisture
http://nsidc.org/data/amsre/gallery/ae_si25_ist_north.html
Brightness temperatures for arctic sea ice.
Differences are indicative of variable ice characteristics (e.g., moisture content, age, thickness)
Passive Microwave: Applications
Data Retrieval in Timely Manner
Sea Ice Concentration: Arctic
Passive Microwave: Applications
0 percent (purple) to 100 percent (white) on 07 August 2004. Antarctica is shown in grey, and the unfrozen ocean is shown in dark blue. Sea ice concentration was calculated from data measured by the Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR-E) sensor aboard NASA's Aqua satellite.
Sea ice concentration: Antarctica
AMSR-E Sea-Ice Monitoring
http://polynya.gsfc.nasa.gov/seaice_amsr_south.html
The repetitive (multi-temporal) coverage of the AMSR-E allows for the animation of scenes.
The inset shows changes in sea-ice extent over time for a localized area along the coast of Antarctica
AMSR-E Automated Iceberg Tracking
http://polynya.gsfc.nasa.gov/seaice_amsr_south.html
Date of imaging
Multi-temporal coverage allows for the tracking of large icebergs
SSM/I (Special Sensor Microwave / Imager) on DMSP (Defense Meteorological Satellite Program)
Arctic ice. Left: winter; Right: summer
http://rst.gsfc.nasa.gov/Sect8/Sect8_8.html
Greenland: Accumulated Melt, 1979-2007, DMSP/SSMI & Nimbus-7 SMMR
Source: NASA Images
The image above was made from observations collected by the Advanced Microwave Scanning Radiometer (AMSR-E) on NASA’s Aqua satellite. The map—which looks down on the North Pole—depicts sea ice extent on September 9, 2011, the date of minimum extent for the year.
Ice-covered areas range in color from white (highest concentration) to light blue (lowest concentration). Open water is dark blue, and land masses are gray. The yellow outline shows the median minimum ice extent for 1979–2000; that is, areas that were at least 15 percent ice-covered in at least half the years between 1979 and 2000.
Passive Microwave: Applications
Global snow depth (in cm) from AMSR-E
http://nsidc.org/data/amsre/gallery/ae_si25_ist_north.html
AMSR-E Rain-Rate Product
http://wwwghcc.msfc.nasa.gov/sport/library/gallery.html
Hurricane Katrina
The technique involves assessing the extent to which raindrops interfere with the terrestrial passive-microwave signal
Passive Microwave: Applications
This is an excellent mechanism for measuring soil moisture over large geographic areas. A problem is the coarse spatial resolution.
Soil Moisture
Using AMSR-E Soil Moisture Data to Study the Extent of the March 2004 Flood
Passive Microwave: Applications
Passive Microwave: Applications
http://sharaku.eorc.jaxa.jp/AMSR/index.html
Note timeliness
Passive Microwave for Monitoring SST
Con: weaker signal than thermal infrared, so spatial resolution is quite poor
Pro: longer wavelength; no problems with clouds
White pixels are clouds
Thermal Infrared
Passive Microwave
AMSR-E Sea-Surface Temperatures: Before and After Hurricane Gustav
Top: 28Aug08
Bottom: 1Sep08
Note the cooler waters after the hurricane
http://wwwghcc.msfc.nasa.gov/sport/library/gallery.html
TRMM: Total Rainfall Measuring Mission
Launched 1997: NASA and NASDA (Japan) TMI: Tropical Microwave Imager Provides quantitative rainfall data over a 487 mile wide
swath 5 km spatial resolution 5 frequencies (wavelengths) in the passive
microwave; dual polarization More raindrops = warmer signal; rainfall rate linked to
scene temperature
TRMM Rainfall Rate
http://trmm.gsfc.nasa.gov/images_dir/rina_28oct11_0753_utc.jpg
28 October 2011
Tropical Depression:Rina
TRMM Precipitation Radar (TPR): Cloud Heights
Tropical Depression: Rina
28 October 2011
UNL Passive-Microwave Radiometer
SNR / CALMIT “Radar Van” with a passive-microwave radiometer installed at the end of the boom
Wetland canopies at ARDC (Mead, NE) used for testing passive-microwave radiometer. Note that the instrument on the boom (shown above) is an ASD spectroradiometer; not a passive-microwave radiometer.
Passive-microwave antenna Non-imaging system
Controlled Wetland Plots
Done at the CALMIT research facility, ARDC, near Mead, NE
Water levels / depths in vegetation plots were controlled
Study done throughout the growing season, with several different water levels at each sampling
Purpose Of The EE / CALMIT Passive-Microwave Study of Wetlands
Determine if standing water could be detected beneath a full canopy of aquatic vegetation
Sensor of choice – passive microwave Rationale – water levels are related to hydrostatic
pressure, which is related to the fluxes of methane gas to the atmosphere
Higher water level = higher pressure = low flux of methane (and vice-versa)
Methane
A very efficient greenhouse gas, so it should be monitored
20 times more effective in trapping heat in the atmosphere than carbon dioxide
Wetlands are important sources of methane
Nebraska has large expanses of wetlands in the Sandhills and Rainbasin areas of the state
Water Versus No Water
Wavelength = 18.7 cm (L-band)
UNL Masters Thesis by Rick Howard, EE, 1996
Water
No Water
Angle of incidence
Growing Season Profile of TB
Note increasing brightness temperature with increasing LAI
Polarization Comparisons
Note increasing divergence of signal with increasing incidence angle
HPOL
VPOL
HPOL
VPOL
TB Response To Varying Water Depth
Note decreasing brightness temperature with increasing depth of surface water beneath the vegetation canopy
Conclusions: Proximal Sensing Study
The L-band passive-microwave radiometer has a sensitivity to several key physical parameters associated with wetland environments, such as density of canopy, height of canopy, the spatial configuration of the vegetation, and the depth of water beneath the canopy
Brightness temperature increased with increasing LAI Most importantly, the effect of varying surface-water
depth is evident in the radiometric signature (i.e., we can “see” through a full canopy of wetland vegetation)
The problem: coarse spatial resolution of satellite sensors