SeaSAR 2006: Satellite radar reveals ever-changing face of the ocean(Forwarded)

ESA News
http://www.esa.int

1 February 2006

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.

From the patterns that winds scour on the sea surface, their speed and
direction can be calculated, while even human-made features such as ships
and oil spills can be detected with superior sensitivity to optical
satellite sensors. Sea ice is also visible, with differing SAR signal
properties returned from varied ages and types of ice.

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.

"The engineering community however needs additional types of information:
if you want to build a ship or protect a harbour you don't just want to
know the average statistics but also know what happens in the highest sea
states. They need an accurate picture of that in order to create accurate
numerical models.

"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 size, shape and orientation of signature 'wind streaks' found in a SAR
image can be used to derive wind intensity and direction for even extreme
weather events. Jochen Horstmann of the Institute for Coastal Research of
the GKSS Research Center in Geesthacht, Germany presented results from a
system called Wind Fields from SAR (WiSAR).

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.

Down in the Gulf of Mexico, NOAA had also used SAR imagery to coordinate
responses to the 'gigantic impact' of last year's hurricanes, with the
NOAA Office of Response and Restoration utilising satellite radar views of
possible oil spills in the Mississippi Delta after Hurricane Rita to plan
follow-up survey flights.

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.

A number of regional oil slick monitoring systems were presented based on
this principle, covering areas including the Mediterranean and North Seas.
Markku Similä of the Finnish Institute of Marine Research recounted
development of an operational algorithm for SAR imagery intended to serve
the Baltic Sea area, with studies being made of how high and low wind
speeds, varying radar incidence angles and differing algorithms changed
detection rate.

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 workshop also heard how SAR imagery can be used to indirectly peer
beneath the waves, with it capable of deriving coastal bathymetry for
areas with water of less than 30 metres in depth, and identifying surface
features indicative of underwater 'internal waves' -- oscillations caused
by different marine layers coming into contact. Another recommendation of
the workshop is that areas of persistent internal waves by added to
Envisat's 'background mission', meaning that new imagery of them would be
acquired routinely.

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.

"I think it is commendable that ESA is using this event to reach out and
understand from the perspective of the users what things are working and
what aren't," commented Graber after the workshop's end.

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.

Related news

* Japan's ALOS in orbit: ESA will deliver its data to European researchers
http://www.esa.int/esaEO/SEM0SXMZCIE_index_0.html
* Providing GMES services at the ends of the Earth -- interview with Dr
Charles Randell
http://www.esa.int/esaEO/SEMGHVVLWFE_index_0.html
* Ship-sinking monster waves revealed by ESA satellites
http://www.esa.int/esaEO/SEMOKQL26WD_economy_0.html
* FRINGE scientists use radar vision to see the Earth move
http://www.esa.int/esaEO/SEM913VZJND_index_0.html

Related Missions

* Envisat overview
http://www.esa.int/esaEO/SEMWYN2VQUD_index_0_m.html
* ERS overview
http://www.esa.int/esaEO/SEMGWH2VQUD_index_0_m.html

In Depth

* SeaSAR 2006 Workshop
http://earth.esa.int/workshops/seasar2006/
* EO Principal Investigator Portal
http://eopi.esa.int/
* GMES
http://www.esa.int/esaLP/LPgmes.html

Related links

* DLR
http://www.dlr.de/
* CSTARS
http://www.rsmas.miami.edu/groups/cstars/index.html
* US National Oceanic & Atmospheric Administration (NOAA)
http://www.noaa.gov/
* CICESE
http://www.cicese.mx/
* Federal University of Rio de Janeiro
http://www.ufrj.br/
* Institute of Marine Research
http://www.imr.no/english/main
* Vexcel UK
http://www.vexcel.co.uk/
* The Polar View
http://www.northernview.org/

[NOTE: Images supporting this release are available at
http://www.esa.int/esaEO/SEMGMCNZCIE_economy_1.html ]