Consistent, long-term measurements are needed of the key physical variables that define earth-system processes. A full set of observations requires different orbits. For global coverage, polar orbits will view the entire earth over the course of many orbits over several days. Low-inclination orbits will permit observation of a portion of the earth over several days, with the observations on successive days being made at different times of day. Geostationary orbits permit continuous observations in time, but only for a limited view of the earth. Because a geostationary satellite progresses in its orbit at the same rate as the earth's rotational rate, it can provide a fixed view of the earth's sphere that is determined by its selected position above the equator.
As a satellite in a polar or near-polar (high-inclination) orbit passes over the earth, the earth's rotation shifts the satellite ground track westward, so that after a period, which varies for different types of satellites, the entire earth is covered and the cycle begins again. If orbits are sun synchronous, the satellite passes over each latitude at the same local time, providing consistent lighting and allowing easier comparison between data and images taken of the same area on different days.
GEOGRAPHIC INFORMATION SYSTEM (GIS)
A geographic information system is a computer-assisted system for the acquisition, storage, analysis, and display of geographic data. GIS technology integrates common database operations such as query and statistical analysis with the unique visualization and geographic benefits offered by maps (Burrough, 1990).
Maps have traditionally been used to explore the earth and to exploit its resources. GIS technology is an expansion of cartographic science that takes advantage of computer science technologies, enhancing the efficiency and analytical power of traditional methodologies (Coulson et al., 1991; Ballestra et al., 1996).
GIS has become an essential tool in the effort to understand complex processes at different scales: local, regional, and global. In GIS, the information coming from different disciplines and sources, such as traditional or digital maps, databases, and remote sensing, can be combined in models that simulate the behavior of complex systems. Remote sensing is a very important contributor of information to a GIS (Maracchi, Pérarnaud, and Kleschenko, 2000).
In agrometeorological applications, the preliminary basic information is often provided by historical archives of different disciplines such as geography, meteorology, climatology, and agronomy. Data collected directly in the field are very important, because they provide the ground truth. Meteorological stations, field data collection (ecophysical observations, agronomic practices, insect attacks, diseases, soil, etc.), and direct territorial observations are fundamental to all the possible agrometeorological applications (Maracchi, Pérarnaud, and Kleschenko, 2000). Where there is a lack of information, models can be used to complete the information to assist in the understanding of the real situation (Rijks, Terres, and Vossen, 1998). An important component is the incorporation of digital elevation models (DEMs) (Moore, Grayson, and Ladson, 1991), which are three-dimensional representations of the landscape. This allows consideration of many other parameters, including hydrology and sunshine duration and intensity.
In a GIS all this information can be linked and processed simultaneously, obtaining a syntactical expression of the changes induced in the system by the variation of a parameter. The technology allows for the contemporary updating of geographical data and their relative attributes, producing a fast adaptation to real conditions and obtaining answers in near-real time.
Public agencies, research laboratories, academic institutions, and private and public services have established their own information systems incorporating GIS. Because of the increasing pressure on land and water resources and land-use management and forecasting (crop, weather, fire, etc.), GIS has become an irreplaceable and powerful tool at the disposal of decision makers.
In developed countries, agricultural and environmental GISs are used to plan the types and times of agricultural practices and regional management activities, and for monitoring devastating events and evaluating agricultural losses. Maracchi, Pérarnaud, and Kleschenko (2000) gave as an example the evaluation of fire risk in Tuscany, Italy. The final map produced was the result of the integration of satellite data with regional data, through the implementation of GIS technology (Romanelli, Bottai, and Maselli, 1998).
In developing countries, the data used for the production of information layers are often unreliable or even lacking. Implementation of GISs must take a different approach from those used in developed countries. A first phase should use sufficiently simple systems to answer specific problems. Completion of the different information layers should be gradual, eventually creating a fully operational GIS (Maracchi, Pérarnaud, and Kles-chenko, 2000). An example of a preliminary information system is given by the SISP (Système Integré de Suivi et Prevision des rendements, an integrated information system for monitoring cropping season by meteorological and satellite data) for Niger. The SISP was developed to enable monitoring of the cropping season and to evaluate an early warning system with useful information about the evolution of crop conditions (Di Chiara and Maracchi, 1994). Longley and colleagues (2001) have recently written a useful textbook on the subject of GIS, and MicroImages tutorial on GIS is available at <http://www.microimages.com/>.
GLOBAL POSITIONING SYSTEM (GPS)
GPS is a satellite-based navigation system developed and maintained by the U.S. Department of Defense. A constellation of 24 satellites broadcasts continuous timing signals that GPS receivers are designed to monitor. When a GPS receiver detects at least three of these satellites above the horizon, the unit can derive its position on the earth's surface by triangulation and provide map coordinates for the user. Most GPS receivers can collect a stream of map coordinates collected at intervals and save them as a file for later use. Appropriate software can import such log files and use them as data sources for input into GIS mapping software. GPS receivers are used in a variety of scientific, commercial, and industrial applications and support varying degrees of accuracy. The U.S. military no longer purposefully degrades the accuracy of the GPS signal for civilian receivers, so accuracy depends primarily on the quality and configuration of the receiver. GPS-derived data can be used to establish geographic location for mapping features of interest and to track vehicles, field agents, or other moving entities.
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