Since 1972, when the first ERTS (Earth Resources Technology Satellite) better known under the name Landsat-1 was launched, remote sensing from space has evolved towards more operational oriented applications. Nevertheless, one of the limiting factors in the use of remote sensing for the majority of applications has been the problem of the availability of data over a certain area for a particular time period. This problem was partly resolved by the launch of Spot in 1986 and its off-nadir viewing capabilities. Nevertheless, in tropical areas there has always been a pronounced lack of data due to the almost continual cloud cover in these regions.
With the launch of ERS-1 in 1991, and the recent launch of ERS-2 in 1995, to be complemented by the scheduled launching of a further satellite (RADARSAT I) of the same type by Canada in late 1995, a remote sensing tool with new characteristics has been made available to the users of various Earth observation data related applications.
This availability of data over a longer time span will ensure that projects using radar data in an operational way will not suffer from a lack of spaceborne information well into the next century.
With this in mind, the European Space Agency (ESA) decided to launch a campaign aimed at making potential radar data users aware of the availability and capabilities of ERS-SAR images for various applications. This publication on the use and role of ERS data for Central African countries is part of the campaign and is also part of the joint ESA and UN effort to support sustainable development in developing countries with remote sensing techniques. The installation of a mobile ERS receiving station by DLR (German Aerospace Establishment) in collaboration with ESA in Libreville (Gabon) in June 1994 finally created the means for acquiring ERS-SAR imagery of Central Africa. These images are shown in the following chapters and emphasize the great potential of SAR data as a very useful tool for sustainable development in Central African countries.
Since the launch of the first remote sensing satellite about 25 years ago, satellite images have played a considerable role depending on the application. The speed of satellite data integration has not only been different from application to application, but also from region to region. While meteorological forecasting applications started to integrate satellite data very early in their operational activities, other fields of potential spaceborne image use remained underdeveloped for a longer time. This is not surprising, as hardware and software capabilities, necessary for the processing of a high resolution data set occupying for instance 70 megabytes, were not affordable for a normal user even ten years ago. Thus, current developments in computing, through the speedy increase of computing power coupled with falling prices are also allowing a certain democratization of satellite imagery use. Ten years ago, a potential user interested in the processing of images paid 10,000 US $ for hardware and about the same amount for software. Now, image processing hardware can be obtained for about 5,000 $. The necessary software (e.g. GRASS) can be obtained free in the public domain directly via Internet.
Consequently, this publication is not intended for a specific user group (either scientific or operational), but rather for the general information of users interested in the processing of high resolution imaging radar. However, as the examples given in this report are primarily related to land applications, the target group may be described as follows:
The importance of transnational activities as, a priori, very promising for satellite data applications should, in particular be mentioned here. The use of radar imagery for the mapping, monitoring and assessment of natural resources is not only of interest for the sustainable development of a particular region (e.g. the African Development Bank) but can also help to reduce territorial conflicts between neighbouring countries and increase mutual trust.
In order to provide those users who are not familiar with remote sensing in general nor specifically with the interpretation of radar data, with a better understanding of the potential of SAR imagery, the following chapter explains some basic principles. For more detailed descriptions, a bibliography related to this issue is given in the Annex.
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