Sunday, December 10, 2006

Radar and wave measurement (Part 1)

Last Thursday I was invited to a demonstration of electronic equipment able to measure wave height as well as detect oil slicks on the water with radar assistance. The venue took place at Aveiro Pilot Station, where the equipment had been previously installed. It consisted of a standard marine navigation radar with antenna, some extra hardware connected and a windows based software. Several pilots of different ports were present as well as some Aveiro port officials. The company representative started with a brief description of the equipment capabilities and modus operandi. Some scientific aspects were briefly discussed.

It does not come as a surprise to me that radar can be able to effectively measure wave heights and direction. We all had, while sailing, to adjust our anti sea clutter controls in order to avoid the PPI being blurred with the swell echoes. Before this adjustment is performed, is relatively easy to know the swell direction and to establish some direct relationship between the strength of the swell echoes and the swell height.

So I think it was a matter of time before someone would use common marine x-band radars as a sensor to survey ocean wave fields. The wave field images provided by the radars are sampled and analysed by a wave monitoring system that can be mounted on a ship or at coastal locations (in this case the antenna was mounted on the top of the pilot station). The measurement is based on the backscatter of microwaves from the ocean surface, which is visible as sea clutter on the radar screen. From this observable sea clutter, a numerical analysis is carried out. The directional wave spectrum, the surface currents and sea state parameters such as wave periods, wave lengths, and wave directions can be derived. To provide absolute wave heights, the response of the nautical radar must be calibrated with data from an oceanographic buoy, which, unfortunately, the company representative was unable to do. Weather conditions that day were terrible with the southwesterly wind gusting 30 to 40 knots and a 4 to 5 meter swell at the port entrance. The swell was great for testing purposes although the rain had some affect on the strength of the echoes.

As for oil slick detection it is more or less the same principle, but with the use of a different algorithm. The radar signal is sensitive to the roughness of the sea surface, which is modulated by wind speed and direction. The suppression of the capillary waves by oil from either anthropogenic sources, such as an oil spill, or from natural biological slicks, reduces the surface roughness resulting in less radar backscatter and darker image tones.
(to be continued)

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