Navigation Aids (NavAids)
At the outbreak of the war the majority of navigation was done by Dead Reckoning sometimes backed up by rudimentary Radio Direction Finding (RDF). There was also astronavigation which was not that accurate and also took time for the “sights” to be taken and the ensuing calculations to be made. As the war progressed radio based navigation and airborne radar was developed and deployed across the RAF and specifically in Bomber Command. A summary of the main navigation aids is described below.
Radio Direction Finding (RDF)
The basic principles of direction finding by radio were well established before the war. If you transmitted on a known frequency from your aircraft, a base station on the ground would be able to determine your bearing from them. If another base station located elsewhere also picks up your transmissions then another bearing can be determined and where these cross is your position. This position can therefore be determined by the base station operators on the ground and then be transmitted to the aircraft. The procedure can be repeated to enable an aircraft to be directed to an aerodrome.
This was the basis of the “Darky” system deployed to assist aircraft returning to the UK who needed help in finding an airfield to land at (typically in the ‘Dark’ hence the name).
Gee
This system used Radio Base stations in the UK (and later stations deployed in France after D-Day) and equipment on board the aircraft to display timing differences of the signals received from the base stations to determine the aircraft’s position.
It was introduced operationally in March 1942 and since it was based on line of sight, at the height the Bomber Command planes were flying at meant its range was up to around 350 miles. Accuracy depended on the navigator’s use of the equipment and did decrease with the distance from the radio stations but would be usually in the range of 1 to 5 miles. Three radio base stations were used, a Master station (called A) and two Slave stations (B and C).
Navigators used special charts to plot the timing differences from A & B signals and A & C signals to determine their “fix” or location. Unfortunately the Germans discovered Gee very early on in its lifecycle and therefore jammed GEE signals so GEE was only useful outbound across the North Sea and then inbound to assist in finding one’s home airfield.
A big advantage of GEE was that it was a passive system, meaning the aircraft only received signals and didn’t transmit any so Germans could not ‘home in’ on the aircraft.
The Gee system - fitted above the Navigator's Table
H2S
H2S was the world’s first airborne ground scanning radar system introduced in February 1943. It allowed the identification of features on the ground for navigation and to enable “blind” bombing. It was particularly good at picking up water features such as estuaries, lakes, rivers and canals. It allowed attacks outside the range of radio navigation aids like Gee or Oboe (see below). The range could be adjusted with a maximum range of 100nm.
However, as it was an active system (i.e. transmitting), German night fighters could ‘home in’ on it. From January 1944 H2S transmissions could be detected by German night fighters fitted with the “Naxos” system which enabled them to locate the bomber stream from around 20 miles away. Despite this Bomber Command carried on with the use of it, since on balance, the advantages of using it outweighed the disadvantages (i.e. the potential increase in the risk of being shot down).
The Germans also deployed a ground based detection system ‘Korfu’ which allowed them to constantly plot the bomber stream from take-off to landing.
Fishpond Screen shown on far left of Wireless Operator's Position
H2S Fishpond
As well as ground returns anything in the way of the H2S beam would result in a return and from this was borne a system modification to display any other aircraft detected below, such as German night fighters. In essence, the centre part of the return was sent to a new monitor based at the Wireless Operator’s position and was called ‘Fishpond’.
It was first deployed in October 1943 and then progressively rolled out to all aircraft in Bomber Command. It had a range of five miles and the night fighter's bearing from the bomber and its range could be shown to allow the wireless operator to call for immediate evasive flight manoeuvres - typically the 'corkscrew'.
Note:
There is some debate about the origins of the name H2S. In one version as the developers of the system apparently thought that Lord Cherwell, who was Scientific Advisor to Churchill, wasn’t keen on it and so development work slowed. When he asked about progress of the radar he was appalled by the delays and said the delays “stink” So the developers called it H2S after Hydrogen Sulphide gas (the rotten eggs smelling gas). When he later asked why it was called H2S the developers had to quickly come up with an alternative reason and settled on Home Sweet Home.
An alternative story is that Lord Cherwell was frustrated by the complexities and delays associated with it and said “Call it H2S – it stinks”.
Oboe
Oboe coupled radio base stations with radio transponder technology in the aircraft (a transponder on picking up a specific signal retransmits it).
The guidance system used two well separated stations to track the aircraft. The tracking stations used radio ranging to define circles, the radius of which was the distance from the station to the target and with two circles intersecting at the target.
An aircraft transponder transmitted signals back to the ground stations with operators there assessing the time it took for the signal to return to work out the distance between the station and the aircraft.
A “Cat” station was used to adjust the aircraft's flight path and a “Mouse” Station used to indicate when the bomber was close to the target and then when the bomber was over the release point.
Oboe Blind Bombing System
Oboe was an active system so this meant that the aircraft was transmitting so potentially allowing the Germans to ‘home in’ on the transmitted signals.
A limitation was that it could only control a single aircraft at a time so this meant it was principally used by a Pathfinder aircraft for target marking allowing blind bombing in conditions where the ground could not be seen. It required very accurate flying by the pilot to stay “on the beam” but resulted in a very, very accurate system for pinpointing targets. The Germans referred to the path of Oboe equipped aircraft as a ‘Boomerang’ as they saw a noticeable Arc on their radar displays.
Gee-H
Gee-H was a combination of Gee and Oboe systems performing much the same job as Oboe and at much the same range. Its big advantage was that it could be operated by up to around 70-80 aircraft at a time. ‘Gee-H’ was the world’s first ‘hyperbolic navigation system’, in which the aircraft would transmit a radio pulse with the system then measuring the time difference between replying signals from various ground stations.
The system was not quite as accurate as Oboe but still delivered an accuracy of around 150 yards with a range of about 300 miles. The accuracy decreased a little because now, rather than ground station operators monitoring the aircraft’s position, it was the Navigator in the aircraft doing this and he used a smaller oscilloscope than the ground station operator had been using with Oboe (the navigator used his existing Gee navigation system).
This meant that the navigator could use his Gee system for ‘normal’ navigation, then switch to the Gee-H mode for blind bombing. He would then see two traces on his screen, the upper one effectively monitoring his track to the target (it showed any timing differences to show if the aircraft was on the pre-planned arc of flight over the target) and the lower one decreasing as he neared the bomb release point. Gee-H was much harder to jam as each aircraft selected its own timing sequence for its pulses. This was also the reason why the system could support more aircraft up to a theoretical maximum of 100 but more practically up to 80.
In Jim’s case his squadron, XV squadron was part of 3 Group in Bomber Command, and from June 1944 all Lancaster aircraft in this Group were fitted with Gee-H. This allowed 3 Group to be independent of the Pathfinder force as well as to develop their own target market techniques. Therefore when Jim says his aircraft bombed on ‘Special Equipment’, it is highly likely he is referring to Gee-H, although this could also refer to the use of H2S radar.
As the RAF gained more air superiority in the skies over German and were able to undertake daylight bombing, aircraft fitted with Gee-H were usually marked with two horizontal yellow stripes on the tailfins allowing other aircraft to formate on them and bomb when the leader bombed.
