The essential groundwork
Like any scientific measurements, to be truly useful the results have to be assessed to establish if they contain uncertainties. The aim of the CryoSat mission is to measure global change in ice thickness down to a few centimetres per year – a difficult task since there are a number of reasons why the data could, potentially, hold errors.
For instance, different seasons cause snow and ice properties to change, which could mask or influence the signal reflected back to the satellite’s SIRAL instrument. Another example of uncertainty could be due to snow lying on top of sea ice. The weight of the snow pushes the ice floes lower into the water, so that CryoSat, which translates the height of the ice surface above the waterline into ice-thickness data, will under-estimate the true thickness of the ice.
ESA therefore goes to great lengths to ensure that CryoSat’s data are as accurate as possible. To understand and correct for any errors that could creep in, large-scale expeditions to the polar regions have to be undertaken to collect information on snow and ice properties.
These ventures were needed before launch as an essential input to the software that transforms CryoSat’s measurements into ice-thickness maps, and after launch to make direct comparisons between ground and satellite measurements.
The video below, shot during the 2011 validation campaign in Greenland, provides a glimpse into what this involves.
ESA has organised a number of these huge logistical undertakings in the polar regions. They often involve taking simultaneous measurements from the air and on the ground by various groups of scientists.
On the ground, sophisticated equipment 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 very similar to that on CryoSat. These airborne data are then compared to the measurements taken on the ground and to the actual satellite measurements to fully understand and correct for sources of error.
All this ground work represents an essential part of the mission so that he best possible trend in ice-thickness change over time can be derived.
Since 2003 a number of campaigns have been carried out in the Arctic. Typically, these campaigns take place in either the spring or the early autumn, seasons that correspond to periods of maximum or minimum snow cover. Coordinated measurements taken by a radar altimeter called ASIRAS (Airborne Synthetic Aperture and Interferometric Radar Altimeter) on an aircraft, which simulates the SIRAL radar altimeter on-board CryoSat-2, are compared with measurements taken by scientists on the ground.
Campaign activities to date
During the course of 2002, two campaigns were successfully carried out. The USA-ESA LaRA (Laser and Radar Altimeter) campaign, obtained for the first time ever simultaneous data from a laser and radar altimeter over Greenland ice sheets and Arctic sea ice areas. The ESAG (European Survey of Arctic Gravity) campaign focused mainly on collecting gravity data in the polar region to support satellite gravity missions such as GOCE. However, the addition of a laser altimeter also made it possible to obtain valuable topographical profiles for typical sea ice conditions east of Greenland.
In April 2003 the first CRYOVEX (CRYOsat Validation EXperiment) campaign was carried out in the Arctic. The main aim of this campaign was to collect simultaneous, collocated airborne laser and radar-altimeter data, and helicopter-borne ice-thickness data for the assessment of ice-thickness retrieval errors.
In March 2004 the ASIRAS (Airborne Synthetic Aperture and Interferometric Radar Altimeter System) instrument, which simulates CryoSat measurements from an aircraft, was successfully tested over Svalbard, Norway. The aim of the campaign was to acquire the first data sets from the instrument over snow and ice to make sure that it could be operated in adverse weather conditions.
In June 2004 ground experiments were carried out on the main ice sheets in the north of Canada, Greenland and Norway. In addition, an aircraft carried out surveys over each of the in-situ sites using both the ESA radar-altimeter ASIRAS, to simulate CryoSat measurements, and a laser scanner to support the interpretation of the radar measurements. Additional laser measurements were also taken from another aircraft.
By September 2004, the last of the first set of CRYOVEX campaigns had been completed. Three ground teams (one on Devon Island in the Canadian Arctic and two on the massive central ice-sheet in Greenland) participated in the campaigns. The main aim of the experiment was to collect data on the properties of snow and ice, which would have later enabled scientists to interpret the large-scale variations in ice thickness over time measured by the CryoSat mission.
In March 2005 the Bay of Bothnia campaign was carried out in the Baltic Sea. The campaign scrutinised issues relating to sea ice. The experiment examined the possible sources of error that could arise in the sea-ice thickness maps that will be generated using data from ESA's ice mission CryoSat-2.
Following the loss of the original CryoSat satellite in October 2005 and the decision to build a replacement mission CryoSat-2, campaign activites remain as important as ever - therefore, validation campaigns in the Arctic continue to be carried out.
Last update: 12 September 2011