Oboe Mk. III, which resisted German jamming efforts. Mk. III also allowed up to four aircraft to use a set of frequencies (stations) and allowed different styles of approach, not just the arc. Late war use By this point the Battle of the Ruhr was long over and the majority of the RAF's bombing efforts concentrated on targets that were too far into Germany to be visible to Oboe. H2S took on the primary role in this era. The D-Day invasions and subsequent breakout allowed this to be addressed by setting up new Oboe stations on the continent. Late in the war, Oboe was used to assist food drops to the Dutch still trapped under German occupation, as part of Operation Manna. Drop points were arranged with the Dutch Resistance and the food canisters were dropped within about of the aiming point using Oboe. Technical details Oboe used two stations at well-separated locations in England to transmit a signal to a Mosquito Pathfinder bomber carrying a radio transponder. The transponder re-transmitted the signals, which were then received by the two stations. The round-trip time of each signal gave the distance to the bomber. Each Oboe station used radio ranging to define a circle of specific radius. The intersection of the two circles pinpointed the target. The Mosquito flew along the circumference of the circle defined by one station, known as the "Cat", and dropped its load (either bombs or marking flares, depending on the mission) when it reached the intersection with the circle defined by another station, known as "Mouse". There was a network of Oboe stations over southern England and any of the stations could be operated as a Cat or a Mouse. The Mark I Oboe was derived from Chain Home Low technology, operating at upper-range VHF frequencies of 200 MHz (1.5 metres). The two stations emitted a series of pulses at a rate of about 133 per second. The pulse width could be made short or long so it was received by the aircraft as a Morse code dot or dash. The Cat station sent continuous dots if the aircraft was too close and continuous dashes if the aircraft was too far and from these the pilot could make course corrections. (The Germans used a similar method with Knickebein.) Various Morse letters could also be sent; for example, to notify an aircrew that their Mosquito was within