electronic navigation systems, post-war term for what in the Second World War were known as radio navigation systems. The term embraced all radio and
radar systems used to assist with general navigation and/or target location and attack. Although these systems were used predominantly by aircraft, the same or similar systems were sometimes fitted to ships.
For the crews of bombers flying over land by day and through clear skies, navigation using ground features was a relatively simple matter. But if they had to fly over long stretches of sea, or above cloud, or at night, accurate navigation was considerably more difficult. Hence the need for electronic navigation aids to assist crews to find and bomb their targets. Accurate bombing requires no more care than extremely accurate navigation, so anything that assists the one also assists the other.
The 1930s had seen a rapid development in radio navigational techniques, spurred by the need for airlines to maintain schedules despite changeable weather. At the beginning of the Second World War most bombers were fitted with radio direction finders, which gave bearings of radio beacons on the ground. For flights to a point in the vicinity of the beacon, i.e. by an aircraft returning to base after a mission, the method was adequate and it would continue in use throughout the war. But the radio bearings became steadily less accurate as the distance from the beacon increased, so simple direction finding was of little use when trying to navigate to targets more than 320 km. (200 mi.) from the beacon.
Something better was needed, and the Germans were the first to develop it. Before the war the Luftwaffe developed Knickebein, a device similar in operation to the radio range system employed by airliners to navigate between special ground beacons. The ground transmitter radiated a so-called
Lorenz beam, which in fact comprised two overlapping beams with Morse dots transmitted in one and Morse dashes in the other; where the beams overlapped the dots and the dashes interlocked to produce a steady note lane much finer than either of the beams that produced it. Knickebein employed a huge directional aerial nearly 100 m. (315 ft.) across and 30 m. (100 ft.) high, supported on bogies running on a circular railway track to enable it to be aligned accurately on the target. The device produced a steady note lane only 0.3° wide and operated on one of three frequencies—30, 31.5, or 33.3 MHz. The signals were picked up using the standard airfield approach receiver fitted to all Luftwaffe twin-engined bombers and its use required no specialized training for the crew apart from normal instrument flying.
When using Knickebein to attack a target, the bomber crew flew along the steady note lane from one transmitter, and released the
bombs as they passed through the steady note lane from a second transmitter which crossed the first at the bomb release point. Using transmitters situated in France, the Netherlands, and Norway, the Luftwaffe could align two or more beams over any target in the UK. With the system their bombers could attack with an accuracy of about 1 km. (0.6 mi.) without the crews needing to see their target. Knickebein worked well enough against area targets, until the RAF got to hear about it and jammed it to the point of uselessness (see
electronic warfare).
All German multi-engined bombers could use Knickebein. In addition, for use by a specially trained night precision bombing
Gruppe (established at about 30 aircraft), the Luftwaffe developed a more accurate beam bombing system codenamed
X-Gerät (X-apparatus).
X-Gerät operated on frequencies in the 66 to 74 MHz band and employed Lorenz beams similar to those used by Knickebein, but there were four instead of two: one beam to mark the approach to the target, and three cross-beams (see Map 29). Used in conjunction with a special clock,
X-Gerät would release the bombs automatically when the aircraft reached the previously computed bomb release point. When the RAF learned of
X-Gerät, it built and deployed jammers designed to counter the system.
The third of the German beam systems, the
Y-Gerät, employed only a single ground station. The bomber approached its target by flying along a complex beam, and to measure the distance along the beam the ground station transmitted a separate ranging signal which the aircraft picked up and re-radiated. Operators at the ground station were then able to compute the position of the aircraft with considerable accuracy and, when it reached the bomb release point, they ordered the crew to release the bombs. So long as it could operate without hindrance the
Y-Gerät was very effective, but once the method of operation was known to the British it proved relatively easy to jam.
It is easy to make light of the German beam systems, and their failure in the face of counter-measures. But it should be remembered that for the first two and a half years of the war they were far in advance of anything in service in any other nation. The significance of the German systems falls into relief if we consider how badly the RAF Bomber Command performed without such aids. If the accuracy of the German night raids on the UK during the period was mediocre, that of the RAF on Germany was miserable. In mid-1941,
photographic reconnaissance revealed that only one in three of RAF crews placed their bombs within 8 km. (5 mi.) of the aiming point (see
Butt report). For the Ruhr area, which was almost continually enshrouded in industrial haze, the figure was one in ten. As Churchill chided his chief of air staff: ‘It is an awful thought that perhaps three quarters of our bombs go astray…If we could make it half and half we should virtually have doubled our bombing power.’
The revelations provided a mighty impetus for the development of electronic navigational systems in the UK. The first to go into service, early in 1942, was GEE. This system employed three ground transmitters working in concert, to radiate a complex chain of pulses in a predetermined order. Using a special receiver, the aircraft navigator could measure the differences in the time of arrival of the various pulses. He then read off his position from a special map. Because the distances from the transmitters were much greater, GEE was not so accurate over Germany as Knickebein had been over the UK. Nevertheless there was a marked improvement in bombing accuracy—until the Germans showed that they too could play the jamming game.
Early in 1943 the RAF introduced its first radar precision bombing system, called Oboe (see Figure) because its pulses sounded similar to the musical instrument. This device exploited the fact that radar can measure the range of an aircraft with considerable accuracy (though it is somewhat less accurate in measuring bearing). The system employed two ground stations. One tracked the aircraft as it flew along an arc of constant range running through the target, and passed correction signals if the aircraft deviated from this arc. Meanwhile the second station measured the range along the arc, and when the aircraft reached the previously-computed bomb release point the release signal was broadcast. The aircraft carried a pair of repeater-transmitters which amplified the range and track signals before returning them to the ground stations; these airborne repeater-transmitters were small and light and they fitted easily into a small aircraft. Under operational conditions Oboe was extremely accurate. Its main disadvantage was that the curvature of the earth limited its use to the area within about 450 km. (280 mi.) of each of the ground transmitters, which, of course, had to be located in friendly territory. A further problem was that a pair of ground transmitter-receivers could control only one bomber at a time during its bombing run and that might last up to ten minutes. Nevertheless, an arc of 450 km. (280 mi.) from the east coast of England took in all of the important Ruhr industrial area in Germany and that was where the device came into its own. Oboe was fitted to Mosquito pathfinder aircraft which marked the targets for the main force of bombers. The Germans found the device difficult to jam, especially in its later short-wavelength versions and they were so impressed with it that they copied the principle for their Egon system which was used to attack targets in the UK during the early months of 1944.
The British GEE-H and the American Shoran systems were essentially similar to Oboe, but their transmissions were initiated from, and the computations were made in, the bomber. The ground stations had only to repeat back the pulses and could thus handle scores of aircraft simultaneously.
All these electronic aids employed ground transmitters, which meant they were unusable at distances of more than 450 km. (280 mi.) from the furthest transmitter. For bombers to attack accurately at greater distances, a system was required that was independent of the ground stations. Early in 1943 the development of the
cavity magnetron enabled the RAF to introduce such a system into service: the
H2S centimetric radar. H2S was, for its time, an extremely advanced type of radar which scanned the ground beneath the aircraft. The returning echoes came strongest from built-up or mountainous areas, less strongly from open countryside and least strongly from areas of water. By displaying the echo signals on a cathode-ray tube it was, therefore, possible to produce a fairly good representation of the terrain which could be compared with a map. Initially the size and weight of the H2S installation limited its application to heavy bombers. Effectively the maximum range of the device was the same as the maximum radius of action of the aircraft carrying it. Bombing accuracy depended on the nature of the terrain around the target. Coastal targets usually produced very distinctive echoes on the radar. Other targets, especially those inland and surrounded by broken terrain, produced less distinct echoes and were more difficult to find.
H2S in its Mark I, II, and III versions served the RAF until the end of the war. A few were delivered to the USAAF where they were fitted into B17s and B24s. Later the USAAF produced its own version of the radar, the H2X, which was used in very large numbers for bombing through cloud. In 1944 the B29 bomber appeared fitted with the APS-20, the most advanced ground mapping radar of the period. The Germans developed their own, but too late for it to be used operationally.
No survey of navigation devices is complete without mention of the German Sonne system which became operational in 1943. The Sonne ground station radiated a complex fan of beams on frequencies in the 300 KHz band, and gave bearings of unprecedented accuracy out to a distance of about 1,600 km. (1,000 mi.). The beauty of the system was that an aircraft or ship needed only a standard communications receiver to receive the signals. The radio operator tuned in to the appropriate Sonne station, counted the number of dots or dashes heard, and referred these to a special map to read off the bearing. The cross of two such bearings produced a ‘fix’.
The Luftwaffe set up Sonne stations along the Atlantic coast of Europe, from Norway down to the west coast of Spain, and on the Eastern Front. Once the method of operation of the system was known to RAF intelligence, its accuracy and simplicity made a profound impression. Over the
Western Approaches the device was far more effective than any comparable system available to the Allies. The RAF produced copies of a captured German Sonne map and, renamed Consol, the system was used by aircraft of Coastal Command. In fact it was of more use to the Allies than to the Luftwaffe, for by that stage of the war there was relatively little German air activity over the coastal areas. After the war the original German transmitters set up at Brest in France, at Stavanger in Norway, and at Lugo and Seville in Spain were kept in operation, and a captured transmitter was set up at Bushmills in Northern Ireland. Two transmitters were set up in the USSR, as were two more in the USA operating on a slightly altered system. The system continued in use for many years after the war, clear proof of the soundness of the original Sonne concept. See also
Eureka (2) and
strategic air offensives.
Alfred Price
Bibliography
Price, A. , Aircraft versus Submarine (London, 1973).
—— Instruments of Darkness (London, 1967).
