4.8 A Method for Measuring the Ice Thickness 
  
In combination with the XIIth International Congress in Ottawa, 
1972, the author of this report had the opportunity to visit the 
National Research Council (NRC) for the purpose of Ice-studies, 
among other interesting things, methods for measuring the ice 
thickness. 
The Canadians then preferably used a specially constructed radio- 
meter, so-called radio spectrometer, for simultaneously trans- 
mission on two frequencies and measuring the temperature of the 
reflected radiation (in Kelvin-degrees). 
The frequencies being chosen so that the transmitted energy in one 
case was reflected from the upper side of the ice, in the other from 
the water, which is the boundary surface of the under-side of the 
ice. 
As an example of suitable frequencies the wavelengths 9 cm and 
4.5 cm respectively were mentioned. 
An accuracy of 0.1?K is possible. 
5 A COMPREHENSIVE REMOTE SENSING EXPERIMENT ON SEA ICE, PERFORMED 
  
IN THE BAY OF BOTHNIA, IN MARCH 1975 
  
In the first half of the current decade, the total volume of cargo 
passing the frontiers of Sweden amounted to around 100 million 
tons/year. Of this volume 90 7 was seaborne. It is evident that 
under these conditions it is of vital importance to keep the ports 
open all the year round. In Fig 9 it is shown that the Bay and Sea 
of Bothnia are covered with ice during normal ice winters. During 
severe ice winters, however, the entire Baltic area, including the 
Oresund Channel as well as the Kattegat and parts of the Skagerrak, 
are ice-covered. In the 20-years period of 1955 - 1975, no less than 
6 winters have been classified as severe winters. 
A prerequisite for economical round-the-year shipping is availability 
to the Ice-breaking Service and shipping of adequate ice information