Envisat making first direct measurements of ocean surface velocities(Forwarded)

ESA News
http://www.esa.int

27 March 2006

Envisat making first direct measurements of ocean surface velocities

For more than a decade space-based radar instruments have been routinely
observing ocean surface phenomena including wind, waves, oil slicks, even
the eyes of hurricanes. Now -- employing the same principle as police
speed guns -- satellite radar has also begun to enable direct measurements
of the speed of the moving ocean surface itself.

The oceans that cover 71% of the Earth's surface are constantly in motion.
Ocean surface currents can lead to strong interaction with wind and waves,
and through the transfer of heat influence the weather. Their dynamic
range in the mesoscale is broad, with features such as current eddies and
fronts, occurrences of filaments and jets, and wind-driven coastal
upwelling or downwelling.

High-resolution surface current observations on a large scale are very
difficult, although satellite altimeters permit monitoring of sea level
anomalies that are related to surface currents. Sea surface temperature
and ocean colour measurements also throw light on ocean circulation
pattern and seasonal variations.

However Synthetic Aperture Radar (SAR) instruments such as the ASAR on
ESA's Envisat have now been demonstrated to potentially offer a more
direct and therefore very valuable source of high-resolution information
for oceanographers.

SAR sensors record microwave radar backscatter in order to identify
patterns of surface roughness. Indeed, over the ocean these patterns are
linked to varying surface winds and currents -- in particular current
shear and varying convergence zones that can focus wave energy, resulting
in increased wave steepening and enhanced radar-detectable roughness
changes. Unfortunately, interpreting radar images to accurately identify
and quantify such signatures has been easier in theory than in practice.

"Due to a general lack of sufficient high-quality in-situ observations,
the understanding of how these dynamic features contribute to the
complicated surface roughness modulation pattern often manifested in SAR
images is incomplete," explained Johnny Johannessen of the Nansen
Environmental and Remote Sensing Center (NERSC) in Bergen, Norway, who
presented results at ESA's SeaSAR 2006 workshop.

But a new light is being shed on these analyses by using neglected extra
information embedded in the radar signal: the Doppler shift in
electromagnetic waves reflected from the water surface. This shift occurs
due to changing relative velocities, experienced in everyday life in the
way the pitch of a siren on a passing ambulance goes up as it approaches,
then goes down as the vehicle recedes away. This 'Doppler Effect' also
serves as the basis of police speed-measuring radar guns.

In this instance, the shift is introduced by the relative motion between
the satellite platform, the rotation of the Earth and the velocity of the
particular facets of the sea surface from which the SAR signal scatters
back to orbit. The initial two values are well known -- particularly for
Envisat, with its very stable satellite orbit and attitude -- and can be
simply subtracted to extract the useful sea surface velocity information.

"This surface velocity, in turn, is composed of contributions from the
wind-wave induced motion, and the background surface current," Johannessen
added. "Following the Doppler shift equation an estimate of the surface
current can be obtained, providing the contributions from wave- and
wind-induced motion are first quantified and removed. It is here that the
Radar Imaging Model (RIM) comes in."

The RIM is a numerical model that combines outputs from specialised
high-resolution ocean models with fundamental equations for SAR imaging of
ocean current features and simulate appropriate radar cross section
signatures. RIM outputs provide both wave- and wind-induced ocean surface
motion for subtraction.

"Our ability to interpret and quantify surface current features imaged by
SAR has not been adequately developed," Johannessen added. "By applying
this combined Doppler and Radar Imaging Model technique -- DopRIM for
short -- things are now looking much more promising."

This new analysis is still limited to surface velocity measurements in the
line-of-sight of the radar instrument. But this new source of information
already provides a useful complement to the standard roughness pattern
analysis.

Initial tests have been carried out on tidal currents off Brest and
Cherbourg in France, with simulations compared to SAR images of the same
sites and tidal surface currents derived from the combined results.
Impressive new views of the ocean from radar measurements are now
available to open up new scientific investigations and applications.

The results so far were presented at SeaSAR 2006, which took place at
ESRIN, ESA's European Centre for Earth Observation in Frascati, Italy, at
the end of January. More than 100 researchers from 20 countries
participated in the four-day workshop.

"More evaluations are certainly needed to consolidate and validate this
DopRIM method," Johannessen said. "We believe however that the preliminary
but promising results will open new opportunities for more systematic
studies of mesoscale ocean variability based on SAR in the coming years.
Provided the method is found to be reliable it will make a significant
contribution to operational oceanography."

These measurements would be very useful for determination of surface
drift, important for oil dispersion, wave-current interaction and
pollution transport. The method could complement the use of additional
information sources, such as 3-D ocean models, atmospheric forcing fields
and in-situ devices such as high-frequency radar stations, as well as
differing satellite sensors, wavelength and polarisation characteristics.

This includes future experimental SAR satellites, such as TerraSAR-X and
Cosmo-Skymed, planned to join the currently operating Envisat, ERS-2,
Canada's Radarsat and Japan's just-launched Advanced Land Observing
Satellite (ALOS). ESA is also planning a series of operational satellites
known as Sentinels -- designed to support the Global Monitoring for
Environment and Security (GMES) initiative of the Agency and the European
Union -- with one being dedicated to ocean applications.

Envisat ASAR wave data sharpening Meteo France ocean forecasts

Also presented at SeaSAR 2006 was the latest in an ambitious project to
assimilate another Envisat ASAR product -- Wave Mode data, which yields
wind speed and wave spectra over the open ocean -- into the sea state
prediction models of French national weather forecaster Meteo France. The
forecaster already assimilates altimetry data from Envisat sensor, the
Radar Altimeter-2 (RA-2), and has been working on ASAR assimilation since
2003.

"The work is still part of our research and development activities but
hopefully we will step forward into operational mode soon," said Lofti
Aouf of Meteo France. "The impact of the assimilation of ASAR wave spectra
processed with an upgraded algorithm on wave parameters -- wave height,
period and direction -- stays efficient for more than three days into the
forecast period, following the end of the assimilation.

"We developed an assimilation system which conjointly uses RA-2
significant wave height and ASAR directional wave height. The results
indicated a significant impact on mean wave parameters, results being
validated with independent wave data, such as wave height from the
French-US Jason-1 satellite altimeter.

"During a one-month test run the results showed a correction of root mean
square (RMS) error of 20%, with the impact duration induced by the
combined assimilation being further enhanced. This task has important
consequences on related research topics such as air-sea interaction
processes, including heat fluxes transfers and current circulation."

Harald Johnsen of Norwegian company NORUT IT explained how an upgraded
ASAR Wave Mode algorithm had been developed following detailed comparisons
of ASAR coverage across the world's oceans with in-situ data from buoys
and platforms, as well as numerical ocean and atmospheric models, to
arrive at a database of co-located spectra. The cross-comparison revealed
a number of problems and ambiguities in terms of swell estimation which
the processing algorithm was then upgraded to remove.

Validation of the results clearly shows an increase in meaningful spectra
-- as opposed to 'non-physical spectra' previously observed -- and an
improved flagging of ambiguous data, as well as a decreased average
deviation between the data and wave forecasting system of the European
Centre for Medium-Range Forecasting (ECMWF). Reprocessing of the entire
ASAR Wave Mode archive is now being undertaken at the Centre ERS
d'Archivage et de Traitement (CERSAT) operated by the French Research
Institute for Exploitation of the Sea (IFREMER).

Envisat ASAR wave data analysis over coastal areas

ASAR measurements are also providing a unique ability to estimate the wave
climate over coastal areas. The processing of SAR images in general -- and
Envisat's ASAR in particular -- offers potential for imaging large areas
with a high resolution capability to provide fine information about the
spatial variations of the wave field.

As presented during SeaSAR, ASAR measurements revealed details of the
swell transformation that can be related to the underlying bathymetry
and/or coastal currents. Encouraging results have been obtained to produce
SAR-derived maps of significant wave heights at a resolution of about 1
km, to be combined with simple ray-tracing models.

Related news

* Satellite data used to warn oil industry of potentially dangerous eddy
http://www.esa.int/esaEO/SEMKURMVGJE_economy_0.html
* SeaSAR 2006: Satellite radar reveals ever-changing face of the ocean
http://www.esa.int/esaEO/SEMGMCNZCIE_economy_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

* US National Oceanic & Atmospheric Administration (NOAA)
http://www.noaa.gov/
* BOOST Technologies: Wave measurements using SAR modes
http://www.boost-technologies.com/esa/sar_wave.html
* BOOST Technologies: Doppler measurements of ocean currents
http://www.boost-technologies.com/esa/ws_doppler.html

[NOTE: Images supporting this release are available at
http://www.esa.int/esaEO/SEMKZSMVGJE_economy_1.html ]