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The ODAR (Omni Directional Airborne Radar) was also tested at 
different altitudes. At low altitudes a relatively detailed PPI- 
image was obtained, on which several ice characteristics could 
be recognized. With increasing altitude the large scale structure 
of the ice appeared. At an altitude of 1,500 m, the ODAR could 
overview the total Bay of Bothnia and give a rough indication of 
the ice extension. 
The ship's radar had a much better resolution than the FLAR and 
the ODAR. It gave a detailed map of some of the ice parameters 
close to the ship, i.e. within a radius of 3 km. Heavy ridges 
could be detected up to a distance of 5 km from the ship. 
5.1.3 Radar altimeter 
A helicopter-borne radar altimeter operating at 5 GHz was tested. 
The most useful results were obtained over areas with high ice 
ridges. 
Tests showed that the radar altimeter could differentiate between 
ice and open water. Thin layers of water on the ice gave, however, 
the same signal as open water. 
As the instrument is nonscanning, information was only obtained 
along the flight track and the ice concetration is therefore not 
easily obtained. 
The radar altimeter gave a clear indication of its capability to 
map ice roughness, and from the registrations it seems possible 
to obtain information on the vertical dimensions as well as the 
aeral coverage of the deformed ice along the flight path. 
5.1.4 Microwave radiometer 
  
A passive microwave radiometer with two frequencies, 0.6 GHz and 4.7 GHz, 
was tested. 
The resultas showed that the low frequency radiometer is a possible 
means for determining ice thickness provided the salinity of the 
ice is very low.