A series of ambitious in-situ observation campaigns that took place in the Arctic are enabling the validation and calibration of Copernicus Sentinel-3 sea ice data, and helping prepare for future polar altimetry missions, such as the CRISTAL Copernicus Expansion Mission.
These monitoring activities also served as testbed for new methods and technologies.
The campaigns formed part of the ESA and EU-backed St3TART (Sentinel-3 Topography Mission Assessment through Reference Techniques) project, which is now drawing to a close after delivering two years of impressive results.
A key aim of St3TART is to produce Fiducial Reference Measurements (FRM) for validation of the Copernicus Sentinel-3 Surface Topography Mission (STM) measurements of inland waters, land ice, and sea ice.
The use of altimetry to measure sea ice thickness is a relatively new application that builds on the success of ESA’ CryoSat mission. It poses unique challenges for calibration and validation activities.
This is because ice thickness is not directly perceivable by altimetry, which can only measure the part of the ice that protrudes above the water, known as the freeboard. To convert altimetry measurements into ice thickness estimates, many other parameters such as snow depth as well as water, ice and snow densities are needed.
As a result, in-situ observations and space-borne measurements are of a different nature, which makes them difficult to compare directly.
In addition, pack ice is typically inaccessible and highly dynamic, preventing the use of fixed monitoring equipment.
A diverse range of observation techniques
As part of St3TART – which is implemented by NOVELTIS – monitoring campaigns addressed these challenges by employing a range of monitoring techniques and by implementing statistical approaches to counter the inevitable spatial and temporal differences between satellite measurements and local reference measurements.
Led by the Technical University of Denmark (DTU), the St3TART sea ice team organised two campaigns.
The first – called the ESA St3TART spring campaign – used new and tested sensors affixed to piloted aircraft, uncrewed drones, and autonomous drifting buoys.
The second – called the DESIR project – was executed from the Le Commandant Charcot exploration cruise ship and aimed to further test the drone setup in a moving environment.
These campaigns served as a testbed for a variety of novel measurement techniques, such as T-buoys and drones, as well as delivering baseline measurements for calibration and validation activities.
It also illustrated the challenges associated with completed field campaigns in the Arctic environment.
Testing drone monitoring systems
The use of drones for measuring sea ice has great potential but there many issues to overcome to reduce costs, and increase the flexibility, reliability and sustainability of monitoring missions.
During a campaign in Greenland, which was part of the ESA ST3TART 2022 spring campaign, the team addressed these challenges by deploying a drone equipped with vorteX.io lidar to collect sea ice measurements.
The team demonstrated the capability of the lidar-equipped drone system to be deployed in Arctic conditions with temperatures as low as -13°C, and showed that the lidar was able to collect valuable and accurate measurements.
In addition, precise GNSS positioning coordinates were also computed to support the drone measurements, and the team confirmed the value an embedded camera to take pictures of the surface.
The use of drones to measure sea ice was further explored as part of the DESIR project, which gave the team the opportunity to test drone deployment from a moving ship.
Snow is major source of errors for satellite altimetry and, as a result, accurate in-situ measurements of snow – as well as sea ice thickness and freeboard – are required for satellite data validation.
As part of St3TART, an Ice-T buoy developed by LOCEAN was deployed that provides real-time measurements of ice thickness, thermal profiles of ice and snow, atmospheric pressure, air temperature, ice drift and horizontal current, temperature, and salinity at the base of the ice.
During St3TART, the snow-measuring capability of the Ice-T buoy was improved through the integration of a pair of miniature Frequency-Modulated Continuous Wave radar sensors operating at 120 GHz and 24 GHz, respectively.
The buoy was deployed below a Copernicus Sentinel-3B track and the airborne team under flew this track, as well as a Copernicus Sentinel-3A track, to gather complementary measurements of sea ice.
As a result, the team provided a unique collection of Copernicus Sentinel-3, airborne and in situ measurements, demonstrating the strength of combining different FRMs for Copernicus Sentinel-3 data validation.
The sea ice campaigns completed as part of the St3TART project were successful in testing and exploiting new technologies for sea ice measurements, and the FRMs produced are essential for the calibration and validation of Copernicus Sentinel-3 STM land data products.
Planned for launch in 2028, the CRISTAL mission will carry, for the first time on a polar mission, a dual-frequency radar altimeter and microwave radiometer, to measure and monitor sea ice thickness, overlying snow depth and ice-sheet elevations.
The upcoming CIMR mission, planned for launched in 2028, will carry a wide-swath conically-scanning multi-frequency microwave radiometer to provide observations of sea-surface temperature, sea-ice concentration and sea-surface salinity.
It will also observe a wide range of other sea-ice parameters. CIMR responds to high-priority requirements from key Arctic user communities.