CryoSat-2 set to shed new light on climate change

28 April 2010 | By Ellie Zolfagharifard

On 8 April, on the barren steppes of Kazakhstan, a UK-led team of space engineers held its collective breath as it watched an adapted SS-18 ballistic missile tear into the sky over the Baikonur Cosmodrome.

The instrumentation on the earlier generation of satellites was simply not designed to deal with the ice-sheet task

Prof Duncan WIngham, UCL

Attached to the former Soviet launcher was CryoSat-2, the European Space Agency’s (ESA’s) first satellite dedicated to monitoring polar ice. For the next three years, it will orbit Earth at an altitude higher than 700km while recording changes in the thickness of the planet’s ice sheets.

Scientists hoping to understand climate change believe the mission could provide valuable information on changes in the polar regions that have, up until now, been patchy and unreliable. ’The instrumentation on the earlier generation of satellites was simply not designed to deal with the ice-sheet task,’ explained Prof Duncan Wingham, of University College London, who first proposed the satellite in 1999.

’Over the past few decades, we’ve used satellite radars designed to measure the sea surface. The fact that we’ve got signals out of places that aren’t sea surface is in part luck and in part due to the fact that we’ve had to work hard, even to get out what I would regard as poor-quality data.’

ESA’s current Envisat satellite suggests that the extent of sea ice in the Arctic is diminishing by around 2.7 per cent per decade, while the remaining ice on the surface is also thinning. With recent scandals feeding scepticism over global warming, validating this data could be one of the planet’s best hopes of uniting efforts against climate change.

But despite the urgent need to extend and improve this data, launching the CryoSat probe into space hasn’t been easy. Five years ago, engineers on the mission faced bitter disappointment as they watched years of work sink into the sea when the original CryoSat crashed just minutes after launch.

Reliving the moment, Dr Richard Francis, CryoSat’s project manager, described how the team coped with the disappointment – by abandoning the control centre and heading to the bar. ’There were a lot of us in that bar, both from ESA and industry, and the common sentiment was that we had to carry on with the mission and build the satellite again.’

In February 2006, ESA had come up with a plan to rebuild CryoSat from existing budgets by cutting other projects that it deemed less critical. The result was CryoSat-2: a satellite with the same capabilities as its predecessor, but with better electronics and redundancy in all its major components.

’The first improvement was that we put a duplicate radar in,’ said Francis. ’As soon as you put in another radar, you introduce some further complications. So we had to change the data pathway and improve some of the electronics to accommodate the redundant radar and make the entire satellite easier to operate. What we were left with was a reliable radar system that could for the first time precisely measure changes at the margins of the ice sheets.’

Known as the SAR Interferometer Radar Altimeter (SIRAL), CryoSat-2’s instrument measures the distance between the satellite and the ice to calculate its thickness to an accuracy of 1cm. Conventional radar altimeters do this by sending short radar pulses to the nearest point on the ground and measuring the time it takes for the signal to travel back to the satellite. However, accuracy is limited as the radar altimeters cannot determine the exact location of the returning echo.

To pin down this location in the foreand aft-direction, SIRAL uses a synthetic aperture technique to measure the angle of signal on arrival. To do this at Earth’s poles, CryoSat-2 must fly at an unusually high inclination, reaching latitudes of 88˚ north and south. This is greater than any satellite currently in space and will result in an additional area coverage of 4.6 million km2 – more than the area covered by all 27 EU member countries.

Fixed panels of solar arrays power CryoSat-2. ’This is another unusual feature,’ added Francis. ’Other satellites have solar arrays that are rotating on an arm so they constantly face the sun. With CryoSat, we wanted to reduce the number of moving parts. This is partly because they are susceptible to failure, but also because they are expensive. In the 1980s, we used to say every single moving mechanism, be it a hinge or whatever, will cost you at least £1m. I’m sure that’s even more today.’

The only moving parts on CryoSat-2 are the valves controlling its propellant movement. But to remove the hinges and install flat solar arrays, the engineering team had to use one of the world’s most efficient solar cells. ’These are triple-junction gallium arsenide cells,’ said Francis. ’They are around 28 per cent efficient, which is quite remarkable. In space, without the atmosphere, you are getting around 1,600W per m2 out of the sun. With 28 per cent of that, you’re in pretty good shape.’

If all goes well, CryoSat-2 will send a flood of scientific data about the planet’s ice sheets to an antenna located in the far north of Sweden. The additional information about the polar regions will allow scientists to undertake more sophisticated analysis interactions of the ocean with the atmosphere.

Alan O’Neill, director of the National Centre for Earth Observation, said: ’The key point to make about polar regions is not only are they early indicators of climate change, but also they are not distant from what affects people’s lives and the weather that affects the rest of the planet. They are connected by Earth’s “bodily fluids” (the ocean and atmosphere) and changes in the polar regions can significantly affect circulation patterns in both parts of that system.’

To ensure CryoSat-2’s data is accurate, ESA is also carrying out large-scale expeditions to the Arctic and Antarctic to understand natural sources of error. These could range from changes in snow wetness, the weight of snow on top of sea ice, and variations in snow and ice density over a particular area. The land-based measurements will involve equipment such as ground-radar and neutron probes, as well as more traditional techniques such as digging in snow pits. A number of polar explorers inspired by the CryoSat mission have also volunteered to take measurements of snow-depth in their journeys across the Arctic.

’I think the days when we send huge satellites into space are numbered,’ said Francis. ’In the future there will probably be a tendency towards smaller satellites and ones with increased capability. In fact, my next job is to build the future Jason satellite based on CryoSat. And from there there’s nothing stopping us building other satellites with an even higher inclination to better understand the polar regions.