Wind direction retrieval based on X-band nautical radar in long-pulse mode

This paper proposes the spaced-averaging method over the range to verify the feasibility of wind direction retrieval using nautical radar in long-pulse mode, and compares the wind direction retrieval results under different point taking methods. Long-pulse electromagnetic waves are high in energy and have a long detection range, and in actual navigation nautical radar mainly works in the long-pulse mode. Different from previous nautical radars that retrieve the wind direction in the short-pulse mode, the returned electromagnetic waves received by the radar antenna in the long-pulse mode reflect a wider range of sea surface roughness. As a result, there is a large overlap area between two adjacent points in radar image. To reduce the errors caused by the overlapping areas, this study uses the spaced-averaging method, where different spacings are selected for averaging. Images acquired by a standard HH-polarised X-band nautical radar operating at grazing incidence show a single peak in the upwind direction. A harmonic function is least-squares fitted to range-averaged radar return as a function of antenna look direction to determine the wind direction. Before fitting, the wind field scale is extended by data shift to reduce the fitting error. The results show that the X-band nautical radar can also extract the wind direction very well in the long-pulse mode. In addition, the overlap zone has no effect on the extraction of wind direction and a stable wind direction can be obtained regardless of how the points are taken.