The Arctic Ocean covers an area of roughly 10 million km2. The terrain, in reality an ocean of ice, varies in altitude from sea level to pressure ridges and floes up to 60 m high, in which extensive leads of open water may well be encountered. In the short summer period the temperature can rise significantly above freezing point but during winter the air temperature will drop to levels well below —48°C. During the winter, high winds will buffet radio antenna installations and ice storms render an aerial array virtually useless. In the Arctic basin, periods of mist and fog frequently occur, and during these it will be impossible to determine bearing or position by conventional navigation methods. During any extended journey beyond the land mass surrounding the Arctic Ocean it is important, therefore, to establish communication with base headquarters at an early stage and at regular intervals thereafter to ensure adequate re-supply, provide a homing aid for the re-supply aircraft and to call for assistance or instructions in the event of an emergency.