The side-looking radar installed in OKEAN provides a spatial resolution that is intermediate between these two extremes. OKEAN's sensors deliver images with a swath width of 460km, enabling the satellite to cover the entire NSR in the course of 11 orbits. In addition, the image's spatial resolution of 2km is capable of picking up leads in multi-year ice as wide as 500m, and multi-year ice floes as small as 250m (Bushuev, 1998). Finally, RADARSAT, Canada's first earth-resources remote-sensing satellite, offers a region of accessibility of up to 500km. This satellite is expected to provide images much wider than those made available by the other SAR systems.
* The relation between strength of backscattering and ice conditions
Active sensors, particularly SAR, offer not only high spatial resolution but also the possibility of specifying types of ice, such as first-year or multi-year ice. The images captured by active sensors are presented as variations in brightness, converted from the strength of backscattering (reflection) of microwaves radiated from a satellite. The strength of backscattering is a function of the roughness of the ice surface, snow accumulation on the ice cover, salinity of ice and (in the case of open sea) the state of the sea. In using microwave satellite images to discriminate sea ice from open sea and detect various types of ice, it is vital that the relationship between ice conditions and backscattering should be known functions. For this reason, research was conducted to compare satellite images with the results of observations on the surface; those findings appear in Figure 4.2-12 (WP-38). The Y-axis of the diagram shows intensity of backward scattering. As the diagram shows, however, the relationship between backscattering and type of ice is not a simple one-to-one mapping. A certain range of strength of backscattering roughly corresponds to each type of ice, and a given strength of backscattering may correspond to several ice conditions. In producing maps of ice conditions from satellite data, therefore, reference data such as temperature and wind-speed must be taken carefully into account, and the experience and expertise of the mapmakers is crucial.
Figure 4.2-12 Relation between the backscattering of the SAR system on the ERS-1 satellite and ice conditions (WP-38)
* Acquisition, processing and transmission of data
Sea ice conditions change constantly with the winds and tides. This means that, when using remote-sensing data on ice conditions to determine actual ship routes, the real-time ice data should instantaneously be delivered to the ship operators. In the experimental voyages conducted in the NSR, this point of view is incorporated into assessments of satellite data for route determinations. In a recent trial in the Kara Sea, image data from ERS-2 and RADARSAT were received by a land-based station in Tromse, processed, and transmitted via INMARSAT to the escorting icebreaker. The time taken from image capture by the satellite to delivery to the icebreaker was three to four hours (Pettersson, 1999);
