Oboe was a British aerial blind bombing targeting system in World War II, based on radio transponder technology. Using triangulation to determine relative location, the system consisted of a pair of radio transmitters on the ground, which sent signals which were received and retransmitted by a transponder in the aircraftt (typically a De Havilland Mosquito or Avro Lancaster). By comparing the time each signal took to reach the aircraft, its location could be calculated to a fair degree. In this way, an aircraft could be directed blindly over a specific target, all of which were pre-calculated and kept on file. Prior to a mission, a circle was drawn around the primary Oboe transmitter so that it passed over the selected target, with the primary transmitter in the exact center, and the radius of the circle noted. The transponder equipped bomber (or bombers, one at a time) would then attempt to fly along the circumference of this circle towards the target. By keeping careful track of the range between the transponder and transmitter, the Oboe operator in England would use the equipment to see if the bomber strayed from the path of the circle, and give the pilot instructions on how to regain it. While the primary transmitter could tell that the aircraft was on the circle, it was impossible to tell at what point it was. For this, the range from the secondary transmitter was also measured, and a circle drawn around it; the bomber would be at the point where the two circles intersected. By repeatedly doing this, the progress of the bomber could be tracked, and when it reached the point where the primary line passed over the target, a coded signal was sent alerting the crew to drop their bombs (they would typically also be alerted to waypoints and when the target was near) The path was only wide, allowing for much greater accuracy than other systems like Gee. The curved path of the aircraft was quite evident to German radar operators, who came to call the system "Boomerang" after the arc segment left on their displays as the aircraft appeared and disappeared out of range.