Satellite Synthetic Aperture Radar in Archaeology and Cultural Landscape: An Overview

The availability of very high resolution (VHR) synthetic aperture radar (SAR) data, such as TerraSAR-X and Cosmo Sky Med launched in 2007, opened a new era in spaceborne SAR remote sensing, including archaeology remote sensing. They provide powerful tools, based on active sensors fromspace operating in the microwave frequency range, which are useful to extract information about the contemporary landscape and make it possible, in some conditions, to infer changes in the former environment and to detect archaeological remains. The availability of VHR active and passive satellite data has grown so rapidly that new problems have arisen, linked mainly to methodological aspects of data analysis and interpretation. Compared with optical images, SAR data processing is characterized by higher complexity. This is particularly evident for archaeological purposes, which historically was limited by the low spatial resolution of early sensors. Despite this drawback, early applications of SAR in archaeology date back to the 1980s and undoubtedly enabled numerous important discoveries and provided new insights in vast deserted areas, as in the case of the Sahara (El-Baz et al., 2007). Nevertheless, these early applications from both aerial and space platforms were mainly demonstrative experimentations made by National Aeronautics and Space Administration (NASA) researchers; but archaeological investigations based on spaceborne SAR were limited to 'operative' applications due to the scarce public availability of data and also due to the complexity of data processing and software. Today the use of satellite SAR in archaeology is still in its experimental stage, even though it undoubtedly offers great potential for manifold applications ranging from the detection of features and sites, to reconstruction of palaeolandscapes and enhancement and preservation of archaeological remains. The current worldwide availability of commercial VHR satellite SAR, along with numerous data processing tools offered by a number of commercial image processing (PCI, EVI) and open source softwares, now makes the use of these data easier and more affordable. The VHR SAR data can provide a major contribution to overcome limits of passive optical data; being active sensors they are able to 'see through' clouds and dusty conditions, to sense a target at any time of day or night, and, to some extent, 'penetrate' vegetation and soil depending on sensor bands, surface characteristics (ice, desert sand, close canopy, etc.) and conditions (moisture content). The main critical aspect today, especially for archaeology and cultural landscapes, is that there is still a lack of correspondence between the great amount of spaceborne SAR data and effective methods to extract information linked to traces of past human activity. The main challenges to be addressed in the future are: setting up of systematic investigations in different geographical areas, environments and land cover; the development of effective and user friendly tools to extract subtle cultural features and patterns; and the definition of protocols for supporting a widespread use of satellite SAR in archaeology.