401
Global Soil Moisture Estimation Using Microwave Remote Sensing
spatial resolutions (10 km) with a relatively higher temporal resolution (12–24 h).
In addition, active microwave sensors are more sensitive to surface characteristics
such as surface roughness, topographic features, and vegetation canopy than passive systems (Baghdadi et al. 2008; Ulaby et al. 1996). Examples of spaceborne
active microwave sensors for soil moisture measurements include Radarsat 2 SAR,
ENVISAT Advanced Synthetic Aperture Radar (ASAR), and Advanced Land
Observing Satellite (ALOS) Phased Array Type L-band Synthetic Aperture Radar
(PARSAR).
17.2.2 PaSSive MicRowave ReMote SenSing of Soil MoiStuRe
Passive microwave sensors measure the self-emitted and/or reflected emission from
the earth’s surface. A radiometer measures the intensity of radiations from the bare
soil surface, which is proportional to the product of the surface temperature and
the surface emissivity, or microwave brightness temperature (Engman and Chauhan
1995). The amount of energy generated at any point within the soil volume depends
on the soil dielectric properties and the soil temperature at that point. Passive microwave sensors utilize the 1–10 GHz range (L- to X-band) in the electromagnetic spectrum to estimate the soil moisture content. Moreover, L-band radiometers at 1.4 GHz
and 21-cm wavelength have shown potential for surface soil moisture measurements
(Entekhabi et al. 1994; Njoku and Entekhabi 1996; Njoku and Kong 1977; Simmonds
and Burke 1998). In comparison with active microwave sensors, passive observations
are less sensitive to surface roughness, vegetation, and topography.
Examples of spaceborne passive microwave sensors for soil moisture measurements
include the Scanning Multichannel Microwave Radiometer (SMMR) on Nimbus-7,
the Special Sensor Microwave/Imager (SSM/I) on the Defense Meteorological
Satellite Program, the Tropical Rainfall Measuring Mission Microwave (TRMM)
imager, the Advanced Microwave Scanning Radiometer-EOS (AMSR-E) on Aqua,
the Soil Moisture and Ocean Salinity (SMOS) Mission by the European Space
Agency (ESA), and the NASA Soil Moisture Active and Passive (SMAP) Mission.
17.3 CURRENT AND FUTURE SATELLITE
SOIL MOISTURE MISSIONS
17.3.1 aqua aMSR-e gloBal daily Soil MoiStuRe MeaSuReMentS
Global soil moisture is currently obtained from the AMSR-E instrument onboard the
National Aeronautics and Space Administration Earth Observing System (NASA
EOS) Aqua satellite launched on May 4, 2002. AMSR-E collects passive microwave data over the globe, allowing the production of a comprehensive dataset of soil
moisture in regions of low-density vegetation cover. The AMSR-E sensor provides
potentially enhanced soil moisture estimates in comparison with previous spaceborne radiometers such as SMMR and SMM/I. This improvement can be attributed to the higher spectral and spatial resolution that ranges from approximately
60 km at 6.9 GHz to 5 km at 89 GHz (Njoku et al. 2003). The AMSR-E global
gridded land surface products include daily measurements of surface soil moisture.
Global Soil Moisture Estimation Using Microwave Remote Sensing
spatial resolutions (10 km) with a relatively higher temporal resolution (12–24 h).
In addition, active microwave sensors are more sensitive to surface characteristics
such as surface roughness, topographic features, and vegetation canopy than passive systems (Baghdadi et al. 2008; Ulaby et al. 1996). Examples of spaceborne
active microwave sensors for soil moisture measurements include Radarsat 2 SAR,
ENVISAT Advanced Synthetic Aperture Radar (ASAR), and Advanced Land
Observing Satellite (ALOS) Phased Array Type L-band Synthetic Aperture Radar
(PARSAR).
17.2.2 PaSSive MicRowave ReMote SenSing of Soil MoiStuRe
Passive microwave sensors measure the self-emitted and/or reflected emission from
the earth’s surface. A radiometer measures the intensity of radiations from the bare
soil surface, which is proportional to the product of the surface temperature and
the surface emissivity, or microwave brightness temperature (Engman and Chauhan
1995). The amount of energy generated at any point within the soil volume depends
on the soil dielectric properties and the soil temperature at that point. Passive microwave sensors utilize the 1–10 GHz range (L- to X-band) in the electromagnetic spectrum to estimate the soil moisture content. Moreover, L-band radiometers at 1.4 GHz
and 21-cm wavelength have shown potential for surface soil moisture measurements
(Entekhabi et al. 1994; Njoku and Entekhabi 1996; Njoku and Kong 1977; Simmonds
and Burke 1998). In comparison with active microwave sensors, passive observations
are less sensitive to surface roughness, vegetation, and topography.
Examples of spaceborne passive microwave sensors for soil moisture measurements
include the Scanning Multichannel Microwave Radiometer (SMMR) on Nimbus-7,
the Special Sensor Microwave/Imager (SSM/I) on the Defense Meteorological
Satellite Program, the Tropical Rainfall Measuring Mission Microwave (TRMM)
imager, the Advanced Microwave Scanning Radiometer-EOS (AMSR-E) on Aqua,
the Soil Moisture and Ocean Salinity (SMOS) Mission by the European Space
Agency (ESA), and the NASA Soil Moisture Active and Passive (SMAP) Mission.
17.3 CURRENT AND FUTURE SATELLITE
SOIL MOISTURE MISSIONS
17.3.1 aqua aMSR-e gloBal daily Soil MoiStuRe MeaSuReMentS
Global soil moisture is currently obtained from the AMSR-E instrument onboard the
National Aeronautics and Space Administration Earth Observing System (NASA
EOS) Aqua satellite launched on May 4, 2002. AMSR-E collects passive microwave data over the globe, allowing the production of a comprehensive dataset of soil
moisture in regions of low-density vegetation cover. The AMSR-E sensor provides
potentially enhanced soil moisture estimates in comparison with previous spaceborne radiometers such as SMMR and SMM/I. This improvement can be attributed to the higher spectral and spatial resolution that ranges from approximately
60 km at 6.9 GHz to 5 km at 89 GHz (Njoku et al. 2003). The AMSR-E global
gridded land surface products include daily measurements of surface soil moisture.