By Helen Briggs
BBC News science reporter, Plesetsk
Europe's Cryosat spacecraft is about to launch on a three-year mission to study the Earth's ice caps.
Cryosat has the task of filling in the data gaps
The satellite's main objective is to test and quantify the prediction that global warming is causing ice to thin at the poles.
Scientists hope the data will give a clearer picture of the impact of rising temperatures on ice and, ultimately, global sea levels.
Climate models suggest that as the Earth gets warmer, the planet's ice cover will shrink.
But while there are already some signs this is under way, scientists want conclusive evidence.
CRYOSAT - ICE OBSERVATORY
The spacecraft is the first of Esa's Earth Explorer missions
Its focus is polar ice melting
Data will test and quantify global warming predictions
Launch set for Saturday, 8 October; on a Rockot vehicle from Plesetsk, Russia
The satellite is currently in the final stages of preparation at its launch site, the Plesetsk Cosmodrome in northern Russia.
Blast-off is set for 8 October on a modified intercontinental ballistic missile (ICBM). Under the Strategic Arms Reduction Treaty (Start) agreed in the early 1990s, Russia is "destroying" some of its ICBMs by using them to place satellites in orbit.
On Friday, engineers finished final tests and attached the satellite to the upper part of its rocket, a newly built Breeze module.
Speaking from the cleanroom which houses the spacecraft, chief mission scientist, Professor Duncan Wingham, who proposed the mission seven years ago, said: "When I see the Cryosat satellite, I have a faint sense of disbelief that there it is after all this time, ready to go, sitting on the top stage of the rocket.
The spacecraft will ride a modified Cold War missile
"The overwhelming feeling looking at it is the realisation that upwards of 300 people have been involved for about four years in putting that whole satellite together," the University College London, UK, researcher told the BBC News website.
"So although one also thinks about the satellite simply as a producer of data, I'm also acutely conscious of the huge effort that people have made to get us to this stage."
On the edges
In the next few days, Cryosat will be taken out of the assembly building and rolled along the railway track to the launch pad a few kilometres away in the depth of the forest.
It will then be attached to the main part of the rocket, which itself will have to be tested and fuelled.
Cryosat mission manager Pascal Gilles said: "It's the end of the story for the engineers who have worked on the building of the satellite but the start for the scientists who will work on the mission. All we can do now is wait for a successful launch."
THE EARTH'S ICE COVER
Two types of polar ice: The ice that covers land and the ice that floats on the sea
The large Greenland and Antarctic ice sheets cover 10% of the Earth's land area
Surface measurements and overflights suggest Greenland's ice is undergoing melting
Data from the Antarctic points to melting at the edges and some growth in central regions
Considerable thinning of Arctic sea ice has been recorded by nuclear subs since the 1950s
Observations from space are more extensive, but overall there remains a paucity of data
After a commissioning period of six months, Cryosat will begin its task of monitoring the huge ice sheets that blanket the surface of Greenland and Antarctica, and the thin crust of sea ice that floats on the oceans around the poles.
Its primary instrument is a radar altimeter, a device for measuring the height of ice by studying the echoes of radar pulses beamed to the surface. Once the height is known, the mass of ice can be deduced from its known density.
Cryosat is expected to provide a very much larger number of measurements than its predecessors and thereby a more detailed picture of how the ice is changing in even small regions of the Arctic Ocean.
A specially designed double antenna should improve accuracy at the edge of the ice sheets, where much of the melting seems to be happening.
The European satellites ERS-1 and ERS-2 have shown that it is possible in principle to map changes in the topography of the Antarctic ice sheets and estimate sea-ice thickness in the Arctic.
However, there is no coverage for large parts of each pole and the outer edges of the ice sheets.
"There have been earlier satellites such as ERS that have proved that the technique works," said Professor Wingham.
Professor Wingham must hope now for a trouble-free launch
"But these still have not provided us with anything like the accuracy that is needed in, for example, climate models or even to do simple things like determine what the change in mass is from year to year."
The Cryosat radar is principally designed to deal with the floating sea ice.
A US space agency (Nasa) satellite, ICESat, launched in 2003, has a primary mission goal of measuring the continental ice sheets of Antarctica and Greenland using a laser altimeter.
The combined data should give the clearest picture yet of how the Earth's ice sheets are responding to climate change and the likely implications for global sea levels.
The Cryosat launch is set for Saturday, 8 October