World War II: Electronics and the U.S. Navy Radar, sonar, loran, and infrared techniques

Before 1920 it was known that radio waves produce echoes. The phenomenon was used to verify the existence of the ionosphere and to determine the height of its various layers by measuring the time required for a radio signal, reflected as an echo, to return to the ground. In the early 1930s, scientific researchers in the United States and in Europe demonstrated that radio signals striking ships and aircraft would produce radio echoes that could be used to pinpoint their location. But it was not until the advent of World War II in 1939 that the United States Navy, in co-operation with the other armed services and civilian scientific groups, began a concerted effort toward the development of electronic devices with significant applications in fire control, navigation, and search capabilities. The sneak attack by the Jpanese on Pearl Harbor greatly accelerated and intensified the Navy's R&D program.