What is the role of the Lead Investigator?

After having proposed the mission to begin with, my role now is to maintain the scientific integrity of the mission. When you make a proposal for such a mission you have a stated estimated performance – my job has been to ensure that whatever happens during the complexities of the engineering as you go from a paper design to a real satellite that we go on meeting all the performances.

I am also involved a lot in the ground segment design, and the software elements that convert radar echoes into height information. Because CryoSat is a new sensor and not the same as the old one I keep a close on what's going on there and we at UCL have developed quite a bit of that software.  
 

	CryoSat measuring freeboard sea-ice

In addition, I provide a link between the project and the ice science community who will be the main users of CryoSat data, and many of who will be helping in validating results.

How did the CryoSat mission come about?

ESA announced the Earth Explorers competition! Meanwhile here at UCL we had just realised it was possible to measure sea ice thickness and by inference ice mass from space. It was clear to us even back in 1998 that was happening with sea ice mass was becoming an increasingly critical issue. So ESA's announcement and the discovery we made here were almost simultaneous in time.

We had been demonstrating the approach was feasible using ESA's ERS class of radar altimeters, but at the same time the flaws in trying to use that technology to solve this problem were immediately clear.

What were the problems with these other satellites?

First of all the orbits were inappropriate and just didn't cover a large part of the Arctic. Secondly the resolution of those sensors, which were designed to measure the marine geoid – or mean sea surface – was just too poor to learn much about the ice.
 
 

CryoSat Workshop

We don't measure the thickness directly – we can't see through to the bottom of the ice. Instead, in order to measure the sea ice you have to distinguish what's an echo from the ice and what's an echo from the water, and differencing these two quantities, following the principle of Archimedes, is how we take the measurement, arriving at what is called the 'freeboard'.

In order to do that you have to distinguish the ice and water, and the poorer the spatial resolution the harder that is to do, which translates to the larger the piece of ice you need to unambiguously determine it. Most of the Arctic ice floes are between 500 metres and five kilometres in size. The resolution of the ERS radar altimeter is around about ten kilometres – in short one can see only the very largest ice floes, which in a rough and ready way mean one is measuring only 10% of the ice.

On CryoSat, which is Europe's first dedicated ice mission, the resolution will be a kilometre, so we should measure around 80% of the total ice mass.

Existing radar altimeters also similar problems measuring the edge of an ice sheets on land, because the ice sheet gets steeper as you go to the coast and the steeper it is, the harder it is for a poor resolution sensor to determine a clean echo: the echoes start to get increasingly complicated and until they get to the point where you can't really interpret them.

Similarly, the Antarctic Peninsula is very mountainous so there is only about 30% coverage at the moment. It is only by improving the resolution with CryoSat that we can push out into the marginal regions of the ice sheet.

Why are these land ice sheet margins of such interest?
 
 

	Topography of Antarctic ice sheets and estimated sea-ice thickness in the Arctic

These are important because most of the mass turnover of an ice sheet happens near the edges – snow falls within about a hundred kilometres of the coast and this then is then recycled back out to sea by the ice flow. So it's likely that a great deal of dynamic action will happen around the edges. Current work suggests that the ocean at least is driving some of this thinning. If it is affecting behaviour of ice at the edges then we may miss quite a bit of that with the current sensors.

It is easy to see why ice scientists are taking an interest in CryoSat – what about ordinary European citizens and taxpayers?

From my own experience, people definitely recognise the importance of understanding what we are doing to the polar environment. There is enormous interest in what we are doing to the environment, and agreement that we need to find out more – even though the cost of CryoSat to the individual European taxpayer is absolutely tiny.
 
 

Ocean circulation in the Atlantic Ocean

Are there reasons other than simply wanting to understand the Arctic and Antarctic? The answer to that question is definitely yes. There is no doubt that the water density in the Arctic and sub-Arctic has been changing in a very dramatic way over the last 20 years, turning fresher and less dense – this is straightforward fact.

When sea ice is frozen the salt in it gets ejected, on the same principle that salt is thrown on the roads to stop them freezing – the salt doesn't want to freeze at the same temperature as the water. So sea ice is frozen freshwater: one suggestion is that this ice has dramatically declined in the last 30 years, and this has effectively put a load of freshwater in the ocean.

The amount of water involved is truly enormous, and it would appear at least that sea ice melting is intimately involved. Now as these changes start to leak out of the Arctic and move south into the Greenland Sea, they begin to interfere with the convective, downwelling processes that are happening there.
 
 

	Prof. Duncan Wingham, and the Rt. Hon. Charles Kennedy

These processes are in part responsible for the northward flow of the North Atlantic Drift that keeps us warmer in winter, so there are possibilities that if we are not careful with what we do with the ice, we may significantly alter our weather in winter. If you move along a line of latitude from Europe to North America in winter you find yourself in a landscape of frozen rivers – it is the North Atlantic Drift that keeps that from happening here.

Talk of the North Atlantic Drift and Gulf Stream shutting down altogether is just straightforward nonsense - because you would have to turn off the global wind field for that to happen - but some of the northward transport of warm water is due to the sinking of water in the Greenland Sea and return flow at depth and we could moderate that process, and there are certainly reasons to start making good predictions about whether it will start altering our climate.
 
 

NASA's ICESat

Have satellites contributed to our knowledge of the Polar Regions so far?

Back in 1995 we just didn't know what was happening in Antarctica at all, so in ten years we do not have some idea of how the Antarctic ice sheet is evolving. What we see is a great deal of variability, thickening and thinning, and although we think we know why this is happening we don't have any real capability today to predict how they will evolve over time.

Up in the Arctic again, the satellites have been the basic tool with which we've started to realise the Arctic is changing quite dramatically, although it would be wrong to overstate this since, for example, the freshening of the oceans is certainly not something we can study by satellite.

I've said before that if you're working with satellites in this area you're working in a golden age, just because of the richness of the data that the satellites bring.

In particular CryoSat should complement the US ICESat mission now flying, which uses a laser instead of microwave radar to measure ice sheets. If we can get the two missions to overlap for a while we should learn a lot about the relative merits of the two approaches.
 
 

	Artist's impression of CryoSat

Do you have any further mission proposals in mind?

No, I must say we're just too busy concentrating on this one, and it's an activity driven by opportunity really. It's very, very difficult to think about doing another one until we at least have CryoSat up there and we can show it's doing the job it was designed to do. CryoSat is an experimental sensor, and they'll be quite a lot to learn about the signals coming back from it.

On the other hand I'm pretty confident that if CryoSat is a success they'll be another one: history tells us that if one agency flies a successful sensor that other agencies will follow it. So I'm more concentrating on trying to make CryoSat a success than worrying about what happens afterward.