CryoSat-2 on the road to recovery (Forwarded)

ESA News
http://www.esa.int

12 March 2007

CryoSat-2 on the road to recovery

Building a satellite in just three years is without doubt an ambitious
undertaking. Nevertheless, the decision to rebuild CryoSat and recover the
mission includes just that goal. A year on and the mission is now well on
the way to recovery, with a design that incorporates no less than 85
separate improvements.

The CryoSat-2 satellite replaces CryoSat, which was lost as a result of
launch failure in October 2005. With climate change bringing about the
threat of receding ice cover, the need to understand the extent to which
this may be happening is even more relevant today than it was when the first
CryoSat was selected for development in 1999. The International Polar Year
2007-2008 (IPY) will focus resources on making measurements of our polar
environments. The loss of the original CryoSat prevented it contributing to
this effort but the exploitation of CryoSat-2 will benefit from the IPY.

Although the impact of climate change is expected to be amplified at the
poles, it is extremely difficult to determine what effect this is having on
the polar ice cover. Recent reports of receding sea ice in the Arctic and
the break-up of the edges of the massive Antarctic ice sheet are much in the
public eye. On the other hand, there have also been reports of the ice at
the North Pole being thicker than usual.

Scheduled for launch in 2009, CryoSat-2 will measure fluctuations in the
thickness of ice on both land and sea to provide conclusive evidence as to
whether there is indeed a trend towards diminishing ice cover. Moreover,
since ice plays such an important role in the Earth system, predicting
future climate and sea level depends on the data that CryoSat-2 will
realise.

In the year since the go-ahead to build CryoSat-2, much has been achieved
and the project has just passed an important milestone, that being the
Critical Design Review during which all the changes to the design were
scrutinised. Notable among the changes, is the decision to have the
satellite carry a back-up for the main payload, the SAR/Interferometric
Radar Altimeter (SIRAL). This means fitting in a complete second set of
electronics.

Radar altimeters have been flown in space before, but the SIRAL is a radar
with a difference. Its sophisticated design encompasses enhanced resolution
and observing capabilities to meet the challenges of acquiring accurate
measurements of the thickness of floating sea ice, whilst also being able to
survey the surface of vast ice sheets accurately enough to detect small
changes.

As a result of the dual SIRAL payload and associated interfaces, and other
improvements to reliability, there has been a knock-on effect to the design
of the satellite. For example, the back-up SIRAL has to be kept warm while
it is switched off -- the additional heater power is provided by increasing
the size of the satellite's battery. Some minor shortcomings found in the
original design have been fixed and other changes were needed to replace
obsolete equipment and parts. Overall 85 separate improvements were
specified of which 30-40% have been small software changes that make the
satellite much easier to operate.

Richard Francis, ESA CryoSat-2 Project Manager said, "We hit the ground
running with CryoSat-2. After the launch failure we managed to get ourselves
organised pretty quickly so that when we got the go-ahead for CryoSat-2 we
knew exactly what to do. Our industrial team has made astonishing progress
in the last year with some flight hardware already delivered. So far
everything's on track and we expect that to continue, so that by the end of
2007 our beloved CryoSat will be almost completely reassembled."

In addition to building the new satellite, a number of field experiments to
support the mission are getting underway in the Arctic. First is the Arctic
Arc Expedition, part of the International Polar Year. The expedition's two
Belgian explorers, Alain Hubert and Dixie Dansercoer, have just left for a
punishing 110-day trek across the Arctic. Each pulling a 130-kg sledge
holding supplies and equipment, the two intrepid explorers will 'walk' from
an island off the coast of Siberia to southern Greenland -- a route never
before considered. In support of the CryoSat-2 Project they will be taking
valuable measurements of snow depth throughout their journey. Regular
updates are posted on the expedition web-site http://www.arcticarc.org .

These measurements are extremely useful for helping to validate the
technique which will be used to convert CryoSat-2's radar measurements into
accurate ice thickness data. Since direct measurements of snow lying on
Arctic sea ice are hard to come by, ESA is very pleased to benefit from this
opportunity.

The weight of overlying snow forces sea ice to float lower in the water than
it otherwise would. CryoSat-2's determination of sea-ice thickness depends
on measuring its freeboard, or how high above the sea-surface it floats.
Everybody knows that 7/8 of an iceberg is under the surface so if knowing
how much is above the surface it's easy to calculate the rest. So overlying
snow, which the radar waves go right through, reduces the measured freeboard
and so the derived thickness measurement is compromised. The kind of data
that Hubert and Dansercoer will gather during their trek will contribute to
the validation of the schemes that will be used to estimate the amount of
snow during CryoSat-2's lifetime.

As part of the CryoSat-2 validation campaign, scientists will also be
heading out to the ice on Svalbard in mid-April to carry out the 2007
component of the CryoSat-2 Validation Experiment (CryoVEx-2007). This
campaign will use ASIRAS, an airborne version of the SIRAL instrument, to
make measurements over teams of glaciologists who will be making
comprehensive in-situ measurements of snow and ice properties. This campaign
continues a series started for CryoSat and which will also be complemented
by further experiments as the remaining complexities in the exploitation of
CryoSat data are explored.

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