Passive Microwave Remote Sensing of Soil Moisture and Regional Drought Monitoring - Monitoring and Predicting Agricultural Drought: A Global Study - page 97

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table 7.1, these are multifrequency systems that include a 6.9-GHz channel
with 60-km spatial resolution. AMSR holds great promise for estimating
soil water content in sparsely vegetated regions and is the best possibility
in the near term for mapping soil water. Based on published results and
supporting theory (Owe et al., 1992; Njoku and Li, 1999), this instrument
should be able to provide information about soil water content in regions
of low vegetation cover, with less than 1 kg/m 2 vegetation water content.
Aqua and ADEOS-II can provide observations with nominal equatorial
crossing times of 1330 and 1030, respectively.
As opposed to previous passive microwave satellite missions, Aqua and
ADEOS-II include soil moisture as a product. On Aqua it is a standard
product, and on ADEOS-II it is a research product. The algorithm planned
for use with Aqua is a variation of the multichannel approach described
in Njoku and Li (1999). Several types of soil moisture products are to be
produced. These include a daily swath product and a global composite. The
swath products include a retrieval of soil moisture for each pixel observed.
Results will be composited to a standard grid to generate a global map of
surface soil moisture with a nominal spatial resolution of 25 km. Following
a period of calibration/validation, the soil moisture products should be
available on a daily basis. Examples of the types of products can be found
on the Web ( http://sharaku.eorc.nasda.go.jp/AMSR/index e.htm).
Windsat was launched in 2003 and includes a multifrequency passive
microwave radiometer system with a C-band channel. This system includes
the AMSR and other frequencies and offers additional polarization op-
tions. The equatorial crossing time is 0630 h. It is a prototype of one com-
ponent of the next generation of operational polar orbiting satellites that
the United States will be implementing by 2010. Experience gained by using
these science missions will provide the basis for future operational prod-
ucts. Research programs are underway to develop and implement space-
based systems with a 1.4-GHz channel that would provide improved global
soil moisture information. The challenge with low-frequency passive mi-
crowave remote sensing from satellite platforms has been to achieve a use-
ful spatial resolution subject to the constraints of antenna size. Toward
that goal, the European Space Agency is developing a sensor system called
the Soil Moisture Ocean Salinity (SMOS) mission (Wigneron et al., 2000).
SMOS will use synthetic aperture radiometry techniques to overcome the
resolution-antenna-size problem. It is scheduled for launch after 2007.

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D rought-Related Investigations
As described in previous sections, passive microwave remote sensing of soil
moisture and associated applications such as drought monitoring have been
limited by the availability of reliable synoptic daily products. This is the
result of sensor system limitations, which are expected to be overcome in
the near future. The following examples illustrate the types of information