periodically sent from a ground station, and on reception, the transponder would send out an answering pulse after a known delay. A ground operator used an oscilloscope to measure the time between broadcast and reception and deduced the range in a fashion similar to conventional radar systems. He then radioed this information to the bomber by voice, telling them when to release their bombs. The downside to the beam-type system of navigation is that the beams cannot be focused perfectly and in practice are fan-shaped, growing wider with increasing distance from the broadcaster. This means they have decreasing accuracy with increasing range. Measurements of distance are dependent only on the accuracy of the equipment, and are independent of range. This means their accuracy is a fixed percentage of the measurement, and so is linear with range. It is possible to use two such measurements to provide a location fix, but such systems are generally difficult to use, as they require two range measurements to be made in quick succession, while the aircraft is moving. Oboe The Air Ministry developed a distance-measuring system known as Oboe which first started reaching the Pathfinder Force in late 1941 and was used experimentally in 1942. Oboe avoided the problems with two distance measurements by using only one at a time. Before the mission, the distance from one of the Oboe stations to the target was measured and an arc of that radius drawn on a conventional navigation chart. For instance, for an attack on a target in Düsseldorf, the distance between the Oboe station near Walmer and the target would be about ; an arc with a radius around the station would be drawn, passing through Düsseldorf. Now the "range" of the bombs being dropped would be calculated, the distance between the point where the bombs are released and the point that they hit. For missions around altitude, range is typically on the order of for a high-speed aircraft like the de Havilland Mosquito. The planners would calculate the place along the arc where the bombs would need to be dropped to hit the target. This calculation, carried out on the ground, could be as time-consuming as required, allowing for the consideration of winds, atmospheric pressure, even the tiny centrifugal force generated by the aircraft following the radius curve. During the mission, the bomber crew would fly to one end of the arc using