Passing its global sight test leaves MERIS ready for work (Forwarded)

ESA News
http://www.esa.int

10 July 2003

Passing its global sight test leaves MERIS ready for work

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

* Europe mosaic
http://www.esa.int/esaCP/SEMM9EXO4HD_FeatureWeek_0.html

Related links

* Envisat
http://envisat.esa.int/
* Envisat MERIS instrument
http://envisat.esa.int/instruments/meris/
* Plymouth Marine Laboratory
http://www.pml.ac.uk/
* FRS Africana
http://www.environment.gov.za/mcm/offshore/africana.htm
* Freie Universität Berlin - Institute for Space Sciences
http://www.fu-berlin.de/iss/
* Observatoire Océanologique de Villefranche
http://www.obs-vlfr.fr/index6.html

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/export/esaCP/SEM4..._index_1.html]
The Benguela current is a cold ocean current off the west coast of South Africa.
It is an upwelling from deep beneath the sea; the combination of its cool
nutrient-rich waters and the hot Sun supports a high population of
phytoplankton. Taken last October, this MERIS image of the Benguela current
depicts concentrations of phytoplankton chlorophyll. Meanwhile researchers were
sailing in the area to validate the accuracy of MERIS algorithms used to derive
this image from raw data.

Credits: ESA/ACRI

[Image 2:
http://www.esa.int/export/esaCP/SEM4...html#subhead1]
MERIS is an instrument optimised for ocean colour, looking down on the world
from 800km away, aboard ESA's Envisat environmental satellite. MERIS measures
the solar radiation reflected off the Earth in 15 separate bands of visible and
near-infrared light.

Credits: ESA

[Image 3:
http://www.esa.int/export/esaCP/SEM4...html#subhead3]
South African Fisheries Research Ship Africana carried ESA-funded researchers
and equipment out to the Benguela Current off the west coast of South Africa
last October, as part of the global MERIS validation campaign.

Credits: South African Department of Environmental Affairs and Tourism - Marine
and Coastal Management

[Image 4:
http://www.esa.int/export/esaCP/SEM4...html#subhead4]
This BOUSSOLE buoy was designed and built by French company ACRI on behalf of
ESA, specifically for the MERIS validation campaign. Only four and a half metres
of the buoy can be seen -- the other 20 metres stays underwater. A Kevlar cable
anchors it to the sea floor. As a floating platform for scientific instruments
such as radiometers, it played an important role in the MERIS validation campaign.

Credits: ESA