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 associated with a commercial digital capture board are investigated. Details are given 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 system in one single step. Results of an absolute calibration attempt using buoy-based corner reflectors in very calm weather conditions are presented. The 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 grazing angles. The analysis of the overall error budget and system stability provides a theoretical estimate of the system’s relative radiometric resolution. The marine radar system is found to produce backscatter coefficient images with a relative error typically of the order of 1 dB. When averaging over several antenna rotations, the error reduces further down to 0.6 dB, where it compares favorably with the performance of traditional research radar systems. It is concluded that despite the inexpensive nature of these instruments, marine radar systems represent a tool that is effective in routinely providing reliable quantitative measurements of the sea surface roughness, which may serve to complement the synoptic views offered commonly by satellite or airborne radars.