“Just before launch we sit in the control room in the back row, completely
quiet,” says Palle S¯rensen, “We have simulated everything, but when
something went wrong we just rewound a little. Now that is no longer
possible. We know it is for real and everything has to work.”

Palle S¯rensen is Ground Segment Manager for the Envisat Mission and sees
to it that receiving equipment on the ground – computers, antennas,
software, communication networks – and above all the control teams are
operational. Up to 50 engineers and scientists sit at consoles in the
control room with dozens of computer monitors and a huge screen with
pictures, data, and graphics. Their job is to guide the environmental
satellite from ESOC, the European Space Operations Centre in Darmstadt.
The European Space Agency will launch Envisat early next year and operate
it for the next 10 years.

The Europe’s newest, largest and most expensive satellite will orbit the
earth 14 times a day and probe every corner of the earth for environmental
pollution and climate change. It will measure and analyse greenhouse gases
in the atmosphere, locate environmental polluters, identify ocean currents
and algae growth, and keep a watch on the ozone hole.

Its ten instruments are among the most sensitive created by modern sensor
technology. They are capable of identifying and immediately reporting the
smallest changes in natural and man-made processes on land, in water or
air. From such information scientists can derive reliable models for
climate change and even improve the forecasting of earthquakes, volcanic
eruptions and flood tides.

Such models demand a high degree of accuracy. When a satellite flies over
the same position every few days, measurement errors can be calculated
out. From an altitude of 800 kilometres Envisat will be able to see if the
earth’s surface has sunk a few millimetres – as it has in Venice, which
according to the latest satellite data is threatening to sink rather
lopsidedly into the sea – or risen as it does before a volcanic eruption.

These kinds of observation demand the highest degree of precision in
satellite orbits. “The location of Envisat is the most exact that we
know,” says S¯rensen. His colleagues will work round the clock to train
the satellite on the earth in precise paths “If we do not regulate the
orbit correctly, we shall have huge problems. But that’s why we have
simulated every possible case.”

This is particularly true of one instrument: the second-generation
synthetic aperture radar, or ASAR, which looks like a wide board on the
belly of the satellite. It will transmit a weak radar signal to earth and
receive its reflection from the surface. It will be able to calculate the
height of glaciers, detect deforestation in the rain forest and pick out
the smallest oil slick on the high seas. At a speed of almost 20 000
kilometres per hour this is no small feat. If the satellite were to lose
its exact angle of vision, the reflection would ricochet into space.

“There are different gravitational fields, a residual atmosphere, and
there is the solar wind. All of which makes a big difference as to how
often corrections are needed,” says S¯rensen, referring to automatic
procedures on board; Envisat has about 50 different independent on-board
computers. These recognise position automatically and operate gyroscopic
instruments and magnets, with which the satellite maintains its exact
orientation. “Position is survival”, adds S¯rensen.

What happens if the eight tonne vehicle goes off course? “That’s not the
way to think about it,” says S¯rensen understandingly, “If it goes off
course we see immediately on the instruments or know that the solar panel
is not delivering enough current.”

S¯rensen has worked on this mission for ten years. He has designed the
infrastructure, procured the hardware, and tested software endlessly.
After many test runs and countless simulations the fruits of this work are
tangible. “It is not the first time we have done something like this,” he
announces proudly.

In an emergency the engineers will have just 400 minutes. The satellite
can circle the earth four times without power from the sun. “After that
it’s over, the satellite would be lost,” broods S¯rensen, but adds
immediately: “we have thought through all procedures, there are rules and
exact scenarios for everything and we have done multiple simulations for
every contingency.”

Deployment of the solar panel shortly after launch has been practiced
hundreds of times. All possible emergencies and errors are stored in the
computer systems and can be called up at all

The Darmstadt engineers intervene regularly and according to plan when a
correction in the orbit becomes necessary. Every ten days, according to
the ground manager’s estimate, the orbit will have to be adjusted.”We have
a lot of experience with this”, says S¯rensen, mentioning the successful
manoeuvre to raise the orbit of the telecommunication satellite Artemis.
The Envisat team was especially happy about that because, without Artemis,
data traffic from Envisat would be problematic.

Envisat will gather so much data that even the two 160 gigabyte hard disks
would overflow after a few hours. It will therefore transmit its load of
data every time it orbits the earth, when it races over the ESA ground
station at Kiruna in northern Sweden. There will be just ten minutes to
empty the on-board storage because after that Envisat has already
disappeared over the horizon. If Kiruna is out of commission a second
station at Svalbard in Norway is available.

As an alternative to the giant receiving dishes on the ground Artemis,
the telecommunications satellite parked high above the earth, will serve
as a transmission station. Because Artemis is at an altitude 45 times
higher than Envisat, every orbit gives it almost three quarters of an
hour’s “eye contact” with environmental satellites and it can very easily
receive all data Envisat might want to unload. Afterwards Artemis sends
the data to the ground station in its field of view.

>From there an incessant stream of data will flow directly to ESA’s data
processing centre, ESRIN, in Frascati, near Rome. In its lifetime ENVISAT
will gather a petabyte of data. That is 1 followed by 15 zeros or a hard
disk with the storage capacity of a million PCs. Software experts have
developed hundreds of programs to make the valuable information available
as effectively and quickly as possible.

Preferential treatment of individual users is ruled out from the
beginning. The ESRIN data is sent directly to six processing and
archiving centres in England, Germany, Italy, France, Sweden and Spain.
That guarantees equal information status for all participants and
long-term security of archiving.

These centres will supply thousands of scientists worldwide with raw data
for climate research, catastrophe prevention, weather forecasting and
environmental observation. Thanks to the fast channels of transmission
data, like the analysis of the ozone measuring instrument GOMOS, will
often be available in three hours.

In order to meet the needs of all users, the experts in Frascati have
developed four levels of quality for data security. Where extreme
accuracy is required, it might be a month before ESRIN releases the data.
That is how long Envisat needs to retrace the same orbit over the same
place and confirm the accuracy of its measurements.

The participating scientific institutes receive data at cost,
approximately what it costs to copy, with the stipulation that it must be
used for scientific purposes and published in its entirety. In this
category are all researchers who have proposed an idea to a scientific
committee and whose projects have been accepted. An impressive community
of several thousand earth observers in Europe and worldwide has taken
shape in the last few years. The ESRIN centre will serve as the data
distribution coordinator for them.

Envisat also has interesting economic potenaial. Analysts believe that
environmental observation could one day become a branch of industry on the
same scale as satellite navigation. This could well be since, as with GPS,
the technology is likely to stimulate the imaginations of users.

In order to sound out the market for data, two consortia are receiving
select data from the Envisat Mission: EMMA under the leadership on the
Italian company Eurimage and SARCOM under the direction of the French
company SPOT. This could be interesting for ocean-going vessels. Envisat
data could help them avoid especially stormy seas and high winds and steer
clean of pack ice. Details of water with high plankton content will help
freighters avoid filling their ballast in the wrong place and being sent
back to sea by harbour authorities.

The data from three instruments – ASAR, MERIS and AATSR – will be made
available to everyone via the internet and on CD-Rom. Regional forecasts
of very high UV radiation or warnings about carpets of poisonous algae
at certain beaches can be factored in by everyone planning a holiday. And
weekend sailors will soon be able to consult special maps to see where
wind and surf are to be found.

Ten turbulent years await the data managers in Frascati and the space
helmsmen in Darmstadt. “We have to guide and manage everything,” says
S¯rensen, but usually the job is “everyday work.” he says contentedly.
And then he lets slip: “Meanwhile we are already working on the next
generation of environmental satellites – and they will be a lot smaller.”
The relief in his voice is unmistakable.

Note to Editors : This Information Note is the second of a series devoted
to the Envisat programme and its applications. Pictures relating to
Envisat are available in low and high resolution on the Web at
http://www.esa.int (under Image Gallery), or in the electronic version of
the relevant Information Note.