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High frequency radars (HFR) use HF waves (3 MHz to 30 MHz). They interact with the sea surface and are well-suited radars to perform remote sensing of the sea. Moreover, HFR coverage range is not limited by the radio horizon: it is possible to keep watch over the sea up to few hundred kilometers from the coast line (surface wave mode) or up to few thousands kilometers (space wave mode). Oceanographic parameters (i.e. wave height, surface current velocity, wind direction and wind velocity) are derived from the so-called sea spectrum. Moreover, HFR can be used for maritime surveillance of the Exclusive Economic Zone (EEZ). In that case, the sea spectrum is an unwanted signal because it can mask targets. So, sea spectrum extraction is an important topic of HFR signal processing since it is a key point for remote sensing accuracy and target detection features. Wavelet-based processing allows an efficient extraction.