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From: Dan Dubrick
To: All
Date: 2003-07-15 00:49:00
Subject: 7\10 ESA - Passing global sight test leaves MERIS ready for work

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European Space Agency

Press Release

Passing its global sight test leaves MERIS ready for work
 
10 July 2003
 
The MERIS sensor on ESA's Envisat environmental satellite is ready
for operational use by science and industry. A lengthy test campaign
spanning the globe has validated the complex software systems that
convert top-of-atmosphere radiation readings into useful measurements
of ocean colour, atmospheric water vapour and land cover. 
 
The Medium Resolution Imaging Spectrometer (MERIS) was launched along
with nine other instruments aboard Envisat in March last year. It
measures the solar radiation reflected off the Earth in 15 separate
bands of visible and near-infrared light.

Initial MERIS data are error-corrected and georeferenced to a quality
called 'Level 1'. Then software packages called 'processors' making
use of algorithms - mathematical instructions based on physics
models - tailor this basic data into what are known as 'Level 2'
products. 

These are specific measurements of geophysical parameters such as
oceanic phytoplankton chlorophyll concentration, cloud-top pressure
or terrestrial vegetation cover, to a maximum resolution of 300
metres from 800 km away in space.

Immediately Envisat was launched ESA began a calibration campaign to
make sure all instruments, including MERIS, were working optimally.
But the processor algorithms applied to MERIS data also needed
validation, to see how much they needed 'tuning up' for maximum
accuracy.

What this came down to was checking if what the processors reported
was really there. Some of this job could be done by cross-checking
data with similar spacecraft instruments like CNES's POLDER or NASA's
SeaWiFs, but independent verification of ground conditions was also
required.

This entailed international efforts to mount a worldwide 'sight test'
that ranged from the Greenland ice cap to African deserts, aircraft
flights above Europe, buoys in coastal zones and research vessels out
in the open ocean.

To ensure overall accuracy all the radiometer instruments used to
gather local measurements across the world were first
inter-calibrated by the UK Plymouth Marine Laboratory, one of the
leading partners in validation efforts.

Checking results on land was the simplest part of the validation
campaign, as many cross-comparisons could be done with a European
Commission land cover database. Stable desert sites in Africa that
are usually cloud-free and have not changed their surface character
in many years were also used.
 
MERIS was built primarily to chart ocean colour - a task involving a
quite different set of algorithms - so gathering in-situ data on this
and other local variables was crucial, both inside coastal zones and
out in the open ocean.

Close to land, specially equipped buoys were deployed close to
Envisat orbit times in the Baltic, North Sea, the Skagerrak and the
Mediterranean. Readings were also taken from whatever ships that were
available.
 

Journey to a cold ocean current teeming with life
 
For open ocean data, in October 2002 ESA helped fund the South
African Fisheries Research Ship Africana for a two-week cruise along
the Benguela Current off the west coast of South Africa, an upwelling
of cold nutrient-rich water. Exposed to the hot sun, the waters are
able to support an abundant population of individually-microscopic
phytoplankton, whose chlorophyll pigments MERIS was designed to see
from space.

"The phytoplankton blooms were indirectly visible to us," explained
Andre Morel of the French Observatoire Océanolgique de Villefranche,
who took part in the validation cruise. "With a high phytoplankton
chlorophyll content, oceanic waters are very dark, greenish-brown in
colour, whereas they are deep blue when chlorophyll is very low."

"Accurately measuring phytoplankton is important because it
determines the primary production of the sea, and all the food chain
up to fisheries, and it also regulates the fixation of carbon dioxide
by photosynthesis." The latter is a key variable in global warming.

The optical properties of the ocean and phytoplankton populations
were measured with surface and submersible instruments. And as with
all readings carried out - on sea and land - atmospheric data were
also recorded, including surface pressure, local aerosol properties
and how light scatters in the air. 

Raw MERIS results represent light recorded shining through the top of
the atmosphere, but typically what is being sought after is located
down at the base of the atmosphere. Indeed the signal received by the
sensor when aiming at the ocean is more than 90 per cent derived from
the atmosphere, rather than the ocean itself. So the algorithm-based
process known as atmospheric correction is crucial to ensure reliable
data.

Another algorithm is used to derive atmospheric water vapour content
from MERIS data over both land and ocean. To validate this data,
tracking GPS signals and other radio-based methods were used - the
way radio waves propagate through air changes in the presence of
water vapour in such a way it can be used for estimating its amount. 

Cloud reflectivity and cloud-top pressure are two more atmospheric
products derived from MERIS data. And to validate this, the Freie
Universität Berlin (FUB) flew survey planes through clouds in
Northern Germany.

MERIS calibration and validation activities will continue throughout
the active life of the instrument, but the data already gathered have
enabled the validation of the MERIS Level 2 products - announced by
ESA last week. To find out more about how to acquire and make use of
MERIS and other Envisat data, go to http://envisat.esa.int

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