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 monitoring. 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 x 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 500km 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 100km 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.
The radar satellite systems of the 1990s were primarily designed for sea and ice monitoring rather than for applications on land. Thus, the frequency (or wavelength), polarization, incidence angle, and sampling characteristics (both in space and time) of these satellite radar systems were not always ideal for monitoring land-surface processes. The latest generation of spaceborne radars have more advanced technical capabilities such as multiple polarization imaging and higher spatial resolution. It is expected that these new features will considerably enhance the usefulness of these sensors for agricultural applications. To facilitate a better understanding of the capabilities and limitations of past and new radar systems, the effects of system and target parameters on radar backscatter are discussed in the next section.
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