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SeaSAR 2006: Satellite radar reveals ever-changing face of the ocean 
Radar satellites such as ESA's Envisat and ERS-2 maintain constant watch on the Earth's surface, their signals able to cut through clouds, rain or darkness. This surveillance extends beyond the land to the 71% of the planet covered by ocean – acquiring unique imagery of the ever-shifting face of the sea that is proving a boon to oceanographers. Last week around 100 researchers from 20 countries met at ESRIN, ESA's European Centre for Earth Observation in Frascati, Italy, to discuss the use of Synthetic Aperture Radar (SAR) satellite data on the ocean. The four-day SeaSAR 2006 workshop began on 24 January.
SAR sensors - their signal returns highly sensitive to altering surface texture - provide two-dimensional representations of a wide expanse of sea surface from which a wide range of useful information can be derived. Prevailing currents and wave patterns can be tracked, and local sea state estimated.
Customised processing algorithms extract such information from SAR data, and the workshop represented an opportunity to review their development, present results from scientific research and operational applications as well as provide recommendations for future SAR algorithm and product development.
Extreme weather was the focus of much interest, including the presentation of research surveying the occurrences of extreme waves measuring more than 25 metres in height, thought to be a leading cause of ship sinkings in bad weather. Dr Susanne Lehner of the German Aerospace Center (DLR) discussed her reprocessing of a two-year archive of ERS-2 SAR wave mode data to retrieve extreme wave heights as well as conventional sea state parameters.
"Ocean wave predictions from weather centres do not make individual wave predictions," Dr Lehner said. "Instead they deal with averaged information, such as the significant wave height, an estimate of about one third of the highest wave value within a one degree box over a time of six hours."
"A ship is constructed differently depending on whether it will operate in the China Seas or the North Sea. At the moment the assumption is the North Atlantic to be the most dangerous region, while our reprocessed ERS data indicated good candidates may also be the North Pacific or the Southern Ocean where the waves can be very strong."
Lehner added that the US National Science Foundation (NSF) was considering support for a project to situate survey buoys in the Southern Ocean – a peak region for extreme waves – to record occurrences in-situ. With current supporting data only sparsely available, the extent of SAR wave mode suitabillity for extreme wave study was the subject of much debate, and one workshop recommendation was that a summary paper would be written on the subject.
SAR-based studies of hurricanes, typhoons and polar lows were also highlighted. A typical SAR satellite image covers an area of around 500 by 500 km, enough to capture complete 'mesoscale' phenomena such as tropical storms. While optical satellite images show the swirling cloud-tops of a hurricane, a SAR image pierces through the clouds to show how the wind fields shape the sea surface, and estimate their likely destructive extent.
The system has been implemented at the Center for Southeastern Tropical Advanced Remote Sensing (CSTARS) of the University of Miami – which includes an Envisat ASAR ground station - and was routinely used to extract wind fields during the last hurricane season. "In the case of Hurricane Katrina which struck the Gulf Coast last August, sustained winds of over 200 km/h were measured just prior to landfall using WiSAR," Horstmann said.
William Pichel of the US National Oceanographic and Atmospheric Administration (NOAA) presented results from a project called AKDEMO, which for the last seven years has applied SAR imagery of coastal Alaska to the analysis of storms and wind speed and direction – including 'gap winds' induced by local topography. AKDEMO results are also employed for ice edge monitoring and ship detection for fisheries enforcement, with users including the Alaska Region National Weather Service, National Ice Center and US Coast Guard.
Taking as a focus the Gulf of Tehuantepec across the isthmus of Central America, Francisco Ocampo-Torres of Mexico's Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE) recounted the application of SAR data together with in-situ buoys and surface radar to study the occurrence of strong and persistent wind jet events that occur particularly during winter.
While for the southern Gulf of Mexico, Enrico Pedroso of Brazil's Federal University of Rio de Janeiro related how SAR has been used on behalf of Mexico's PEMEX oil company in tracking changes to a naturally-occurring Cantarell Oil Seep, which requires careful monitoring in order to conserve regional ecosystems and fisheries. Floating oil dampens out small waves: with SAR signals responsive to surface texture, oil slicks typically appear noticeably darker than the surrounding water.
Non-oil surface slicks formed by algae and other natural means represent a significant false detection source, but Fabio Del Frate of Italy's Tor Vergata University presented work on neural network algorithms which progressively 'learn' to distinguish oil slicks on SAR images.
The angular and metallic surfaces of ships exhibit a high rate of signal return in contrast to surrounding water, so similar efforts are ongoing to utilise SAR for semi-autonomous ship detection systems. Dedicated algorithms can yield information on ship size, shape and even speed – based on Doppler effects extracted from displaced ship wakes in the SAR signal. Hans C. Graber of CSTARS briefed attendees on operational trials of a maritime surveillance system called OceanView, being developed with the Vexcel company, with ship probability scores assigned to candidate radar-bright objects for delivery to users in near-real time, between 30 minutes to an hour after image acquisition.
The 'phase change' of water turned to ice was the subject of the final session, with the proven ability of SAR to see ice and identify properties meaning it is increasingly employed in the high latitudes on an operational basis, including by the US National Ice Center, the Canadian Ice Service and the Norwegian Meteorological Institute - as well the High Arctic ice monitoring products provided by the Polar View consortium, part of the Global Monitoring for Environment and Security (GMES) initiative of ESA and the European Commission.
In this context, Nick Walker of Vexcel UK highlighted the usefulness of Envisat's ASAR Global Monitoring mode for Arctic and Antarctic ice monitoring. While it has relatively low 1-km spatial resolution, its wide 400 km swath can cover a large part of the Polar Regions on a daily basis, including areas kept obscure from optical satellites by clouds or seasonal darkness. He pointed out that of the 42 10 km-plus Antarctic icebergs currently recorded by the National Ice Center, 38 are being tracked using Envisat data.
Graber is also working jointly with the US Jet Propulsion Laboratory (JPL) and the Alaska SAR Facility (ASF) on a project to digitise data from NASA's 1978 Seasat spacecraft, whose brief 105-day working life represented the first SAR mission in orbit. The effort should make this historic data accessible to a fresh generation of researchers and its availability could help with the planning of more advanced SAR sensors in the future.
SeaSAR 2006 itself corresponded with the launch of a new SAR satellite: Japan's Advanced Land Observing Satellite (ALOS) whose longer-wavelength SAR design will deliver fresh opportunities for ocean-going radar research, and should help ensure many results for discussion at the next SeaSAR scheduled for January 2008.
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