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High-frequency surface wave radar (HFSWR) has been widely adopted as a useful remote-sensing tool for sea-state monitoring. There are limitations to the performance because of various unwanted clutters and interferences that contaminate radar signals. Among all types of unwanted signals, radio frequency interference (RFI) is dominant since the frequency bands 3-30-MHz, the operation range of HFSWR, are shared by many radio services. If RFI occupies the areas in the Doppler spectrum that are essential for the estimation of oceanographic parameters, for example, wind speed, current speed, wave height etc., it will significantly reduce the quality of the data. The characteristics of RFI in time, space and Doppler domains are analysed for sea-state monitoring, and a robust and general signal processing method consisting of image recognition, segmentation processing and subspace projection for cancellation of RFI is proposed. The effectiveness of this approach is validated using data obtained with the Pisces HF radar, which is a high-performance radar developed for long-range wave measurement, operating in the lower half of the HF band (5-10-MHz).