Radar is an electronic instrument used to detect and locate moving or fixed objects. Its development during the 1930s became focused on guarding against surprise attacks as British and German forces prepared for war. Before the second World War began in September 1939, the British had built a defensive chain of radar stations along the east and south coasts of England. Germany had a similar system. By 1940 airborne radar was possible, used not only for defence but also to show bombers their targets in enemy territory. Radar has many non-military uses, including navigation at sea, helping pilots to land safely, weather forecasters to keep track of approaching storms, and scientists to study other planets.