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Why wait for the seventh wave?


The Royal Netherlands Navy, NLR (the Dutch Aerospace Laboratory), RADAC, Amarcon/ABB and MARIN joined in the ‘Salute’ project to work out a demonstrator system to anticipate quiescent periods for helicopter landing operations. And now the new ‘DoIT’ JIP aims to turn the system into a product. Jos Koning, j.koning@marin.nl


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urfers choose the next wave to ride by waiting for the seventh one after the previous best wave. In working


environments the objective is often to await a set of low waves to perform a motion critical operation. It would be perfect for such applications to have an automatic and accurate prediction of the approaching wave train based on measurements.


Recent commercial systems can make one- minute, deterministic wave train predictions using sea clutter information from standard pulse type navigation radars. However, continuous tuning is needed. The Salute project aimed to demonstrate the merits of Frequency Modulated Continuous Wave (FMCW) radar technology that enables direct physical measurements of wave elevations


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in addition to the well-known features of pulse type radars.


Deterministic wave train The MARIN focus was on the inversion of the radar information into a deterministic wave train, as well as the validation of the performance of the FMCW system and a stereo photographic approach for nearby waves.


FMCW technology enables the derivation of surface velocity as a function of distance to the radar antenna in small grid cells. The velocity profile of the wave surface captured in the radar beam is the combination of ship sailing speed, surface current and orbital wave velocity. Orbital wave velocity is directly related to wave height; ship speed through water is known; so in principle just ‘one small


step …’ is needed. Of course, real world systems are more complicated. Electronics have noise floors, interference from regular ship radars is present, wave crests obscure the radar view into the valleys behind them, signal strengths drop quickly with range and there is also the effect of white capping. The proper performance of the method for deriving the surface speed, versus range was considered essential. Therefore, the demon- strator was configured as a non-revolving or ‘staring’ FMCW unit. In this way the quality and performance of the captured data could be thoroughly assessed along the steps in the algorithms, although this was at the cost of detailed directional information.


The processing algorithm included a ‘valid data selection’ step followed by a ‘wave


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