Press Release

A voyage from space to sea with Envisat

By SpaceRef Editor
February 8, 2002
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From its vantagepoint in sun-synchronous orbit, the European Space
Agency’s new spacecraft Envisat, whose launch is scheduled end of February
2002, will tirelessly sweep the Earth’s surface and atmosphere, using a
suite of ten different scientific instruments. Over a 35-day cycle, the
satellite’s orbit will cover the entire planet, and then start all over
again. Two thirds of the time it will be over water. Because of the sheer
size of the oceanic currents, the complexity of thermal exchanges, and
ocean-atmosphere coupling, the ocean is a crucial factor in explaining the
way our planet’s climate operates and how it is changing.

Beginning with the first brief American Seasat mission in 1978, satellite
observation has created a revolution in our understanding of how the ocean
behaves. Since the 1990s, a continuous stream of scientific data has been
flowing from ESA’s ERS satellites and the joint French-US Topex/Poseidon
altimetry satellite. This first generation is now being succeeded by
Envisat’s instruments designed not only to advance science but also to
lead to a range of operational applications services.

The data obtained with a radar altimeter, RA-2, will ensure continuity
with the original RA system on the ERS satellites. It will also provide a
valuable complement to Topex/Poseidon and its follow-on, the small
satellite Jason, launched on 7 December 2001. The two systems provide
altimetric observation of the ocean from different orbits. A low satellite
repetition ensures that Envisat, like the ERS spacecraft before it,
obtains excellent spatial resolution, needed to study complex phenomena
like eddies and describe the distribution of water mass with precision.
Topex/Poseidon and Jason, on the other hand, with a repetitivity of just
ten days, will provide better temporal resolution.

CLS, a French company, is handling the operational exploitation of
altimetry measurements from both Envisat and Jason. “We will integrate
data from these two sources, to get the maximum benefit,” explains
Philippe Gaspar, Head of satellite oceanography at CLS. “The combinatorial
algorithm has been in use since 1998, allowing CLS to offer
state-of-the-art data products”.

Hydrosphere, atmosphere, ionosphere: a close relationship

The products developed by CLS will be of interest, first and foremost, for
scientists who use them as inputs for ocean modelling and forecasting. In
conjunction with atmospheric models, this forms a starting point for
predicting climate change. “It is now possible to forecast some major
events such as El Niño, but smaller-scale phenomena such as the North
Atlantic oscillation pose much more of a problem.” Gaspar said.

To obtain the best performance from a forecasting model, initial
conditions must be described as precisely as possible, and this is where
satellite data is indispensable. “Before the advent of satellites,
forecasting of this type would have been unthinkable,” as Gaspar reminds
us.

Radar altimetry measurements are correlated with high-precision
orbitographical data from an instrument called “DORIS”, and with those of
a microwave radiometer, so that errors caused by atmospheric water can be
corrected. The use of a dual-frequency altimeter further makes it possible
to correct for the distorting effects of the ionosphere. In this way two
significant sources of error are eliminated, in the order of 40 to 50 cm
for water vapour and 20 to 30 cm for the ionosphere.

The continuity of altimetry data is vital, as Gaspar explains. “The
usefulness of the scientific work would be seriously impaired if the
service was interrupted. And once commercial applications are on the
market, continuity will become absolutely indispensable.”

Managing the traffic at sea

In addition to oceanography and climate studies, the radar altimeter has
an important contribution to make to marine traffic management. Dispersion
of the reflected radar signal gives information on the height of waves,
with a precision of ±25 cm, while the advanced synthetic aperture radar
(ASAR) determines wave direction. Taken together with information on major
currents, this will make it possible to optimise routing of maritime
traffic for speed and fuel economy, a substantial boon to international
shipping. The ASAR also tracks drifting ice and monitors the pack ice and
its limits.

Two other Envisat instruments are designed primarily to study the
hydrosphere. MERIS (for Medium Resolution Imaging Spectrometer) is a
fifteen-band optical system spanning the visible and near-infrared
spectrum, and will be used for ocean colour measurements. The AATSR
radiometer covers more of the IR spectrum. It will measure ocean surface
temperatures, continuing the work of earlier ATSR systems on the ERS
satellites. These two instruments together will have major applications in
studies of plankton and fish movements.

As instrument data comes in, Envisat puts it provisionally into its
onboard mass-memory devices. Transmission may be to one of the ground
stations in Kiruna, Sweden, or Svalbard, Norway or via a high-speed link
to the Artemis data relay satellite, which will be positioned in
geostationary orbit and forward the data to ESA’s establishment ESRIN,
located in Frascati, Italy, for near-real time processing.

The data gathered will be made available to the world scientific community
at archiving centres distributed throughout Europe and linked with very
high speed leased lines. A data server will be put on line for access via
the Internet. Two consortiums have been set up with data
processing/commercialisation experts: Sarcom, led by Spot Image, and
Emma, by Eurimage, will handle the commercial exploitation of the
application products developed from Envisat data.

By concentrating such an array of instruments on a single orbital
platform, it becomes possible to integrate the information received in
near-real time. This opens exciting opportunities for new products,
building on recent progress in our understanding of the mechanisms, which
determine the behaviour of the Earth’s oceans and atmosphere. Thanks to a
continuous flow of data, these products could revolutionise the management
of our Planet and our environment.

Note to the Editors: all pictures relating to Envisat are available under
http://www.esa.int. The present information note is part of a series of
articles devoted to the Envisat programme and its applications.

For further information, please contact :

ESA Media Relations Office

Tel: +33(0)1.53.69.7155

Fax: +33(0)1.53.69.7690

SpaceRef staff editor.