Perhaps surprisingly, a key to the success of CryoSat-2 actually lies on the ground. Malcolm Davidson provides some insight into the effort that goes into the validation experiments that are carried out in some of the harshest environments on Earth to ensure the data are as accurate as possible.
Malcolm Davidson, a Canadian national, heads ESA's Earth Observation Campaigns Section. He is responsible for coordinating a wide range of field experiments on the ground and in the air to support the development of new missions, as well as the validation of satellite data products against independent 'ground truth' measurements once the satellite missions have been launched.
Malcolm joined ESA's European Space Research and Technology Centre (ESTEC) in the Netherlands, in April 2002. He obtained a BSc in Physics from the University of Toronto, an MSc in Remote Sensing and Imaging Processing from the University of Edinburgh and a PhD in Physics from the University of Bonn. Prior to joining the Agency he worked at CESBIO in Toulouse, France.
ESA: What is your role within the CryoSat-2 team?
Within the team, I am responsible for the validation programme. This includes organising a series of field experiments in the polar regions and ensuring that the results of the campaigns are fed back to ESA to include in the mission's data-processing procedure. For a satellite mission aimed at providing global maps of ice thickness change over time, down to centimetre level, these validation activities are an essential element of the mission.
Since validation for the CryoSat mission can only be done out in the field, it means travelling to inhospitable regions in the Arctic and Antarctic to collect measurements of ice and snow properties on the ground and from aircraft. We then compare these data with what CryoSat-2 'sees' from space. It's my responsibility to prepare and coordinate these dedicated validation experiments with participating scientists. I then follow the analysis and ensure the results are fed back to ESA.
ESA: Where could errors in the data come from?
Great care has gone into the design of the CryoSat mission and we know that any uncertainties in the data will be relatively small. However, there are still a number of ways in which errors could affect the determination of trends in ice thickness over time.
For example, the presence of snow lying on top of the ice is something we have to consider – it's the thickness of the underlying ice we want to measure, not the snow. The influence this overlying snow has on the measurements taken by CryoSat has to be assessed very carefully. In dry snow conditions, for instance, CryoSat will take measurements slightly below the actual surface of the snow because the signal penetrates into the snow. If the snow is wet, however, the opposite is true because the signal will not penetrate the surface.
Through the validation campaigns we now have a clear understanding of how to interpret the data we will receive from CryoSat.
ESA: Is this validation work particularly tricky?
Well, it's not easy because the ideal locations for carrying out the experiments are, by their very nature, very cold and difficult to reach. It means, therefore, there are a lot of logistical challenges to solve in having a group of scientists camping out on the ice for up to several months at a time and making sure all the equipment is in the right place.
ESA takes the lead in coordinating these huge undertakings to ensure that the experiments are coherent and properly executed. The campaigns involve taking simultaneous measurements from the air and on the ground. On the ground, sophisticated instruments such as ground-radar and neutron probes, as well as more traditional techniques, such as digging snow pits and drilling through the ice, are used to measure snow and ice properties. Data are also taken from an aircraft carrying a radar instrument similar to SIRAL on CryoSat-2.
By comparing ground readings with airborne measurements, then the airborne measurements with CryoSat-2, we are finally able to estimate the accuracy of the data. Actually, these large-scale expeditions to the polar regions are needed both before launch, as essential input to the software that will transform CryoSat-2's measurements into ice-thickness maps, and after launch in order to make a direct comparison between ground and satellite measurements.
ESA: Who carries out these validation measurements?
We rely on expert polar scientists to take the actual measurements, who we select through an Announcement of Opportunity for CryoSat calibration and validation. Their participation and expertise is essential to the success of validation, especially as taking these measurements is physically demanding. The campaigns are carried out in the north of Greenland, the very far north of Norway and north of Canada, as well as on the Antarctic ice sheet. Needless to say, the conditions are tough.
Interestingly, we have also had some voluntary contributions from polar explorers. Inspired by the mission, several teams have made measurements, following a protocol provided by ESA, throughout their expeditions across the polar sea ice.
ESA: What do you enjoy most about working on the CryoSat mission?
The creative side of CryoSat validation work appeals to me the most. Many of the experiments are unique. They have never been carried out before, certainly not at this scale and the concepts had to be designed from the ground up, together with the participating scientists.
Working directly with the scientists is also something I appreciate. My hope is that, through this validation work, CryoSat-2 will produce the most accurate measurements of ice-thickness change ever.
This is one in a series of interviews with a few of the key people that are involved in the CryoSat mission. Please check back as the list will be added to over the coming weeks.