Cp. Gommenginger et al., Quantitative microwave backscatter measurements from the ocean surface using digital marine radar images, J ATMOSP OC, 17(5), 2000, pp. 665-678
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.