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PHARUS (phased array universal SAR) is an airborne SAR concept which is being developed in the Netherlands. The PHARUS system differs from other airborne SARs by the use of a phased array antenna, which provides both for the flexibility in the design as well as for a compact, light-weight instrument that can be carried on small aircraft. The concept allows for the construction of airborne SAR systems on a common generic basis but tailored to specific user needs and can be seen as a preparation for future spaceborne SAR systems using solid state transmitters with electronically steerable phased array antenna. The whole approach is aimed at providing an economic and yet technically sophisticated solution to remote sensing or surveying needs of a specific user. The solid state phased array antenna consists of a collection of radiating patches; the design flexibility for a large part resides in the freedom to choose the number of patches, and thereby the essential radar performance parameters such as resolution and swath width. Another consequence of the use of the phased array antenna is the system's compactness and the possibility to rigidly mount it on a small aircraft. The use of small aircraft of course considerably improves the cost/benefit ratio of the use of airborne SAR. Flight altitude of the system is flexible between about 7,000 and 40,000 feet, giving much operational freedom within the meteo and airspace control limits. In the PHARUS concept the airborne segment is complemented by a ground segment, which consists of a SAR processor, possibly extended by a matching image processing package. (A quick look image is available in real-time on board the aircraft.) The SAR processor is UNIX based and runs on easily available hardware (SUN station). Although the additional image processing software is available, the SAR processing software is nevertheless designed to be able to interface with commercially available image processing software, as well as being able to ingest raw data from other SARs on the input side. The combination of the airborne and the ground segment, augmented by the transfer of technological knowledge needed to operate the system, will provide for an autonomous capability of the system user/owner. The PHARUS project has so far resulted in the construction of a C-band, VV-polarized research SAR (PHARS) with a 1- look resolution of 1.5 multiplied by 5 meter (5 multiplied by 5 meter at 7 independent looks) and a swath width of 6 km. This system has been extensively used for research and application projects in Europe, for purposes of mapping, land use inventory, change detection, coastal bathymetry, ship detection and ocean wave measurement. The next system recently completed is a fully polarimetric C-band system with adjustable resolution and swath width (the latter up to 20 km); this system is expected to be operational autumn 1995. The polarimetric capability will provide for a much enhanced discerning power (discrimination between e.g. forest/cultivated, various forest types, etc.). Discrimination by polarimetric signature is an alterative approach, with different possibilities and limitations, to e.g. the use of several frequencies. This paper gives an overview of the SAR research system and the results obtained with this system. The PHARUS design and use are discussed.
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