V
Early Radar and Navigational Aids
GEE
The first significant radio navigational aid used by Bomber Command entered operational service from August 1942, although it had been experimented with twelve months earlier. Radio transmissions from three ground based positions emitted a precisely timed signal which was received via an oscilloscope positioned in the navigators position. The device in the aircraft was nicknamed the ‘Goon Box’ because it was so easy to use.
On receiving the transmission, the navigator could accurately plot the position of the aircraft to within a mile over Germany and approximately 165 yards when over Britain. The further away from the radio transmitters which were all based in Britain before 1944, the less accurate the device became. Its maximum effective range was 350 miles but beyond that the system still set the aircraft on a fairly accurate course to the target. The main shortcoming of the device was that the Germans did not take long to work out how to jam it and, in a very short space of time, its use beyond the Dutch coast was limited. However, its use for crews returning home was initially more appreciated than its ability to find the target. This factor alone compensated for any of its failings.
H2S
H2S was a10 cm radar fitted below the rear fuselage which transmitted a fan shaped beam. The beam was very broad in the vertical but narrow in the horizontal and rotated around the vertical axis to scan the ground below the aircraft. The signal from the radar bounced back to the aircraft from the ground and the time it took to return was proportional to the distance the ground was from the aircraft. A cathode ray tube (CRT) display would register the return as a dot relative to the position of the aircraft.
Depending on the type of terrain the aircraft was flying a different response would be received by the aircraft. When over water, the H2S beam was reflected away from the aircraft so nothing would appear on the CRT. However, when over land, a larger proportion of the emitted energy from the radar would bounce back and appear as dots on the CRT. When over a dense built up area, a larger amount of energy would return to the aircraft to display a higher proportion of dots. The return was not affected by the weather making dense cloud cover irrelevant.
Crews became very skilled at interpreting the picture that H2S presented them and its introduction from January 1943 in the Stirling and Halifax gave Bomber Command yet another advantage. H2S was not perfect though, although if a target was coastal, the lack of a return from water made ports in particular clearly stand out on the CRT. Rivers and lakes were also good points of reference and H2S was good for large built up targets such as factories.
Although H2S was used until the end of the Second World War, examples of the device inevitably fell into enemy hands and, to counter it, the Germans developed the FuG (Funkgerät) 350 Naxos radar detector. Fitted into enemy night-fighters, Naxos could home onto any bomber using its H2S.
OBOE
Oboe was the codename given to the ground-controlled blind bombing device which was incredibly accurate for the day. The device could operate at heights up to 30,000ft and speeds in excess of 300mph but could still achieve an operational error of just 300 yards; this distance reduced dramatically at lower altitudes.
Developed by 109 Squadron, it was initially trialled using a pair of pressurised Wellington VIs although it would be forever associated during the Second World War with the high-performance Mosquito.
The bombing device was controlled by two ground stations; one was codenamed ‘Cat’ and was based in Norfolk, the other, ‘Mouse’, was positioned in Kent. The two stations could accurately place an aircraft directly on track to the target and inform the aircraft when to drop its TIs, bombs or both. While Oboe was not expert in measuring direction, it could measure range with an accuracy that allowed the attacking aircraft to fly on a curved track at a constant distance from the ground station. This was achieved by signals transmitted from the ground station to the aircraft, which were then amplified and sent back to ground station. Using both ‘Cat’ and ‘Mouse’ the aircraft’s distance was quickly worked out by plotting where both ground stations’ signals intersected.
The sortie began at first with the aircraft starting the arced track over the North Sea and flying on a southerly heading towards the target. The pilot then flew the route keeping ‘Cat’ at a constant range, which was the same range to the target; effectively flying a circle around the Norfolk-based ground station. The pilot knew whether he was on track on or not by a tone in his earphones. If he was closer to ‘Cat’, he would receive dots and if he was further away, dashes were received; a continuous tone meant that the arc was being followed accurately.
As the aircraft closed on the target, ‘Mouse’, which was already recording the approach speed and height, started to emit warning signals. A signal would be sent when the aircraft was three minutes from the target, followed by a final signal which would inform the navigator to release the TIs onto the target. Once the navigator had pressed the bomb release, the aircraft’s transmitter was instantly cut, at which point the two ground stations would know the exact time of release.
As with devices that rely on signals, there were some disadvantages, but these were few, purely because of the excellent performance of the Mosquito, which was the main aircraft to use Oboe. German night-fighter crews could home in on Oboe, but interceptions were rare and casualties even rarer.
MONICA
This was a warning device for the benefit of the rear gunner. The device worked by radiating signals from the rear of the bomber which were only reflected back when another aircraft was behind it. Each echo made a ‘pip’ sound into the intercom and as the enemy aircraft closed on the bomber the ‘pips’ sounded closer together.
The initial obvious disadvantage was that Monica did not distinguish between friend or foe nor did it give any indication of whether the enemy fighter was high, low, left or right! The former disadvantage turned Monica into more trouble than it was worth when bombers began flying in tightly packed formations or streams. The device would then continually pick up all of the friendly aircraft around it driving the crew mad with a continuous solid tone.
When the Luftwaffe began to introduce its version of AI radar, SN2, once again Monica proved more of a hindrance as the signals it was emitted were easily picked up by the enemy fighter which would home in on the bomber.
First used operationally on June 16/17, 1943, the device was later modified to Mk IIIA standard which entailed using a visual indicator rather than acoustic. The same disadvantages were still in place and the device was generally turned off.
As the availability of H2S began to spread through Bomber Command, a new enemy warning device was introduced to supersede Monica. At first given the code name Mousetrap, the device had to be renamed Fishpond not long after as the former was already in use.
Fishpond used the same basic theory of reflecting echoes as Monica but received the returned information via CRT instead. Designed by the physicist and radio astronomer Sir Bernard Lovell (1913-2012), Fishpond made use of the H2S which up until then only displayed signals at the height the aircraft was flying. Fishpond, however, then exploited the base not used, that of the radiations between terra firma and the aircraft.
Making use of the H2S scanner, any echo from an enemy aircraft was projected onto a separate tube. On the display, the ground return around the edge looked like a halo while the centre looked like a black infinite black hole. All airborne objects between the ground and the aircraft were displayed on the tube in their exact position.
The advantages over Monica were that the ‘pip’ was replaced by a silent visual blip on the screen and the bearing and distance of the enemy aircraft could be worked out. Even if several blips were on the screen, any that were moving towards the bomber were most likely to be night-fighters. There was also the added advantage that mid-air collisions were reduced as the crews now knew where the rest of the bombers were.
The main disadvantage was that the device was geared to a fighter approaching from below which, at the time, especially when schräge Musik was introduced, was generally where the enemy would attack. The device was eventually rendered as useless as Monica when the Luftwaffe introduced Naxos which homed in on any signal emitted by either the H2S or Fishpond.
BOOZER
First used on November 13/14, 1942 by 7 Squadron Stirlings during an attack on Genoa, the device warned aircraft when ground defences were taking a particular interest in them. A red light came on in the Wireless Operator’s compartment when searchlights or guns were vectoring the bomber. By receiving this early warning the pilot could take evasive action before being coned or shot up by predicted flak.
Fully introduced in early 1943, the rearward-looking aerial was fitted in the tail of the bomber. It was particularly good at giving crews notice of when their aircraft was being monitored by ground-based Würzburg radar, and by FuG 202/212 transmissions from a night-fighter’s AI radar.