Quantitative microwave backscatter measurements from the ocean surface using digital marine radar images

The capabilities for quantitative measurements of the ocean backscatter at low grazing angles of a conventional low-budget X-band marine radar associa ted with a commercial digital capture board are investigated. Details are g iven of the external and internal calibration methods designed to determine the system's transfer function. This work reveals that the accurate system transfer function can be determined only by calibrating the composite syst em in one single step. Results of an absolute calibration attempt using buo y-based corner reflectors in very calm weather conditions are presented. Th e dynamic range is deduced and found to span over a 50-dB range well suited to describe the ocean backscatter coefficient values expected at low grazi ng angles. The analysis of the overall error budget and system stability pr ovides a theoretical estimate of the system's relative radiometric resoluti on. The marine radar system is found to produce backscatter coefficient ima ges with a relative error typically of the order of 1 dB. When averaging ov er several antenna rotations, the error reduces further down to 0.6 dB, whe re it compares favorably with the performance of traditional research radar systems. It is concluded that despite the inexpensive nature of these inst ruments, marine radar systems represent a tool that is effective in routine ly providing reliable quantitative measurements of the sea surface roughnes s, which may serve to complement the synoptic views offered commonly by sat ellite or airborne radars.