North Warning System is Donovan Wylie's third and final book of photographs on the themes of vision and power in military architecture, and draws a close to his Tower Series. Surveying a radar station just inside the Canadian Arctic, Wylie examines the detection of invisible threats through unmanned observation posts in remote regions. The development of long-range bombers and missiles after the Second World War made Canada's arctic frontier vulnerable to attack from the air. This forced Canada and the United States to jointly construct a matrix of short and longrange radar stations in the 1950s. Known as the Distant Early Warning Line, these stations provided electronic observation and surveillance capability across Canada's northern frontier throughout the Cold War. In the 1990s, these stations were upgraded to form the North Warning System (NWS) which is increasingly active-as international maritime traffic develops throughout the north, so does military presence. In North Warning System, whiteness takes on the quality of a blank canvas, a metaphor for the sweep of history.