Background and objectives
Since many years, radar data from satellites are used for the mapping of sea ice because radar data can provide images even when the area is covered by clouds or the sun is below the horizon.Ice coverage and ice concentration are some of the parameters obtained from the satellite images. The images that are used are usually only from one frequency and with one polarisation. Recently new satellites have been launched that make it possible to get radar data from three different frequencies and four different polarisations. This should improve the possibilities for accurate characterization of the ice cover. The main objective of this project was to evaluate how multi-polarisation radar data from the Japanese Advanced Land Observing Satellite (ALOS), the European ENVISAT, the German satellite TerraSAR-X and the Canadian RADARSAT-2 can improve classification of ice types, determination of sea ice concentration and detection of ice ridges and to give recommendations to the Swedish ice service.
The project has focused on sea ice in the northern part of the Baltic Sea, which is an area with a high level of winter traffic. In order to cover different ice and weather conditions, over 120 satellite images were acquired between February and April in 2007, 2008 and 2009.
To be able to validate the results from the satellite images it was important to have good field data from the studied sea ice areas. Information about location and structure of the ice edge, ice ridges and new ice was collected from a helicopter in order to cover a large area. Two cameras were used, one Nikon digital SLR camera and one thermal infrared camera from FLIR. During three years and 37 flight hours with a helicopter, approximately 5500 Nikon images and 37000 FLIR images were collected. Helicopter observations were complemented by measurements on the ice.
The left image shows measurement of the height of an ice ridge east of Piteå 2008-03-19. The
right image shows a photograph of the ridge taken from a helicopter. The black lines indicate a
distance of 50 m.
Results and conclusions
In the satellite images that are used by the ice service, wind often gives strong radar signals from open water. In some cases this makes it difficult to see the difference between sea-ice and open water. This problem can be reduced if radar images with two polarisations are used. This is also important for the estimation of ice concentration.
RADARSAT-2 images of ice conditions in the northern part of the Baltic Sea 2009-04-24. Both
images are taken at the same time and show the difference between the HH-polarisation (left) and
the HV-polarisation (right). In the HV image the darkest areas are open water. In the HH image
the signature from open water is affected by wind and is highly variable.
One objective of the project has been to compare radar data from different frequencies. Results from the project show that radar data at low frequencies, in this case from the ALOS satellite, have some advantages compared to data at higher frequencies, like the ENVISAT and RADARSAT-2 data that is used by the Swedish ice service today:
• Data from ALOS show stronger signatures from ice ridges and deformed ice.
• Data from ALOS seems to give better contrast between ice types during warm weather conditions with wet snow on the ice.
• Data from ALOS give better contrast between land and sea-ice.
In this project it has also been important to evaluate which data could be used operationally and to give recommendations to the Swedish ice service. To make operational ice maps it is important to have access to images that cover large areas frequently, preferably once or twice per day. Depending on ice drift and freezing/melting, an image that is more than one day old might not be useful anymore. Radar satellite images can usually be ordered either with high resolution for a small area or with low resolution for a large area. The ice services prefer the low resolution images, commonly called ScanSAR images. The images should also be available in near real time, preferably within a few hours after the satellite image was acquired.
ScanSAR images from ALOS fulfil most of the criteria for operational ice charting. One image can cover the whole northern part of the Baltic Sea and can give the same information as images from ENVISAT or RADARSAT-2, but the data availability might be limited. ScanSAR images from ALOS can cover data gaps and give additional information and will be an important complement to images from ENVISAT and RADARSAT-2.
On the left side a section of an ALOS ScanSAR image from 2007-02-12 and on the right side the
same area from an ENVISAT ScanSAR image from 2007-02-13.
Images from TerraSAR-X have higher resolution than images from any of the other satellites in this study. These images could be used for observations of smaller areas like harbours, lakes, rivers, channels or islands, but do not give the spatial coverage that is required for ice charting of large areas like the Baltic Sea.
Currently the only satellite that can provide ScanSAR images with more than one polarisation is RADARSAT-2. This option has not yet been used by the Swedish ice service. One of the recommendations from the project is that the production of ice maps should be adapted to handle RADARSAT-2 data with two polarisations.
Leif Eriksson, Chalmers University of Technology, Dept. Earth and Space Sciences
Karin Borenäs, Swedish Meteorological and Hydrological Institute, Research Department