A surface wave radar system including a receive antenna array (20, 22) for generating receive signals, and a data processing system (24) for processing received data representing the receive signals to mitigate ionospheric clutter. The received data is range and Doppler processed, and a spatial adaptive filter (52) is trained using training data selected from the processed data. The training data includes ionospheric clutter data and excludes cells which contain target data and substantial sea clutter. The processed data is filtered using the filter (52), which may be based on loaded sample matrix inversion. The antenna array (20,22) may be two-dimensional having an L or T shape.