A working radar system was only developed during World War 2 in Britain. Each night during the war, German fighter planes flew over the English Channel and dropped bombs on London. The British didn't know when the planes were coming. The British military realized they needed an advantage to beat the Germans. They asked Watson-Watt to help. By 1940, they had a set of 51 radar stations along the coastline of England. The radar stations bounced radio waves into the air. The waves would hit the approaching German plane and bounce back to the station. This let the British calculate the distance. Once the British knew that German planes were coming, they would send their own planes into the air to fight them. The Germans did not have radar nor know about the British invention until the end of the war. Being able to spot enemy planes before they were within sight was a huge advantage. Without radar, many historians argue that Britain could have easily lost the war.