Agriculture Reference
In-Depth Information
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(Ulaby, 1998; Bouman et al., 1999). Because most attention is still devoted
to the problem of deriving high-quality soil moisture and vegetation prod-
ucts, there have been few investigations on how to combine such radar
products with other data and models to obtain value-added agricultural
drought products. This chapter provides a brief overview of radar sensor
systems and the principles involved in the interaction of microwave energy
with agricultural targets.
The two main radar systems with potential for agricultural monitoring
are synthetic aperture radars (SARs) and scatterometers. While SARs offer
high ground resolution suitable for providing information on a farm level,
scatterometers allow frequent sampling (daily to weekly) at a regional scale.
Scatterometers have similar spatial and temporal sampling characteristics
as spaceborne radiometers, which are discussed in chapter 7. Progress with
the use of these two radar systems is discussed, with emphasis on how the
information could be used to monitor agricultural droughts.
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nsor Systems
Investigations into the potential for radar in geosciences began in the 1960s
and gained momentum with the launch of the SEASAT satellite in 1978.
In the 1980s and early 1990s, given the impending launch of a number
of satellite systems, a variety of aircraft and NASA space shuttle missions
were conducted in support of radar applications in land, sea, and ice mon-
itoring. Radar satellite systems were launched by Europe (ERS-1 and ERS-
2), Japan (JERS-1), and Canada (RadarSat) in the early 1990s. Recently,
the European Space Agency launched ENVISAT, which has an Advanced
Synthetic Aperture Radar (ASAR) on board. More satellites carrying radar
instruments are scheduled for launch in the next few years (ALOS in 2004;
Radarsat-2, METOP and TerraSAR in 2005).
The two broad categories of radar instruments, SAR and scatterometer,
provide data on different spatial and temporal scales. The SAR systems
provide data with much higher spatial but poorer temporal resolution than
scatterometer systems. For example, the ERS-1 and ERS-2 SAR provide
25-m spatial resolution data over an area of 100
[106
100 km with a 35-day
repeat cycle. However, due to data costs and operational constraints, SAR
acquisitions of an area are much less frequent in practice. In contrast, the
scatterometer onboard ERS-1 and ERS-2 provides 50-km spatial resolution
data with a repeat cycle of every 3-4 days. In the case of RadarSat, the
SAR can be configured in a number of modes with differing spatial and
temporal resolutions. The ScanSAR mode provides information over a 500-
km swath at 100-m spatial and 3- to 4-day temporal resolution compared
to the fine or standard modes that provide information over a 50- to 100-
km swath at high spatial (9-25 m) but low temporal (24-day) resolution.
In deciding the value of each of these different radar systems, one should
critically assess the information requirements of a particular application.
It is not only important to know what parameter is of interest, but also at
what spatial and temporal scale the information is required.
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