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