Passive Microwave Remote Sensing of Soil Moisture and Regional Drought Monitoring - Monitoring and Predicting Agricultural Drought: A Global Study - page 95

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as severely affected are often not detected until later in the growth cycle.
The PD information would be of particular use in these situations.

Sa tellite Passive Systems
A passive microwave instrument of particular note is the Special Satellite
Microwave/Imager (SSM/I) onboard the Defense Meteorological Satellite
Platforms. These polar orbiting satellites have been in operation since 1987
and provide high frequencies and two polarizations (table 7.1) except for
22 GHz (V only). Spatial resolution of the SSM/I is very coarse, as shown
in table 7.1.
The SSM/I utilizes conical scanning, which provides measurements at
the same viewing angle at all beam positions on a swath of 1200 km.
This makes data interpretation more straightforward and simplifies image
comparisons. There have been as many as four SSM/I satellites in operation
at any given period. Therefore, frequent and even multiple daily passes are
typical for most regions of the earth. Data from the SSM/I are publicly
available ( http://www.saa.noaa.gov/).
Interpreting data from the SSM/I to extract surface information requires
accounting for atmospheric effects on the measurement. When one consid-
ers the atmospheric correction, the significance of vegetation attenuation,
and the shallow contributing depth of soil for these high frequencies, it be-
comes apparent that the data are of limited value for estimating soil water
content. However, data from the SSM/I can be used under some circum-
stances, such as in arid and semiarid areas with low amounts of vegetation.
Figure 7.1 includes the results obtained over the southern Great Plains of
the United States, from Jackson (1997), using SSM/I data. Value-added
products from the SSM/I sensors include a wide range of atmospheric and
oceanic variables. However, for reasons noted above, there have been few
attempts to generate standard land surface products.
Another current satellite option is the Tropical Rainfall Measurement
Mission (TRMM) Microwave Imager (TMI). It is a five-channel, dual-
polarized passive microwave radiometer with a constant viewing angle.
The lowest TMI frequency is 10 GHz (table 7.1), about half that of the
SSM/I. The TMI has higher spatial resolution as compared to the SSM/I.
TRMM is not a polar orbiting satellite and only provides coverage of
the tropics, which includes latitudes between 38°N and 38°S for the TMI
instrument. However, a unique capability of the TMI is its ability to collect
data daily, and in many cases more often, within certain latitude ranges.
This could facilitate multitemporal and diurnal analyses. These data are
publicly available (http://trmm.gsfc.nasa.gov/data dir/data.html). Jackson
and Hsu (2001) have retrieved soil moisture from TMI observations. These
studies showed the potential of the improved spatial resolution, higher
temporal repeat coverage, and lower frequency as compared to the SSM/I.
Although no longer in operation, another satellite instrument of inter-
est is the scanning multifrequency microwave radiometer (SMMR). This

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