Lorenz beam
The Lorenz beam blind landing system was an air radio navigation system in use from the latter 1930s. Prior to the World War II the Germans had deployed the Lorenz blind-landing aid at many airports and equipped most of their bombers with the radio equipment needed to use it. The name refers to the company that produced the system; Lorenz referred to it simply as the Ultrakurzwellen-Landefunkfeuer, German for "ultra-short-wave landing radio beacon", or LFF. Prior to the war the Lorenz equipment was installed for civil airports outside of Germany [1]
Description
The blind approach navigation system was developed starting in 1932 by Dr. E. Kramar of the Lorentz AG company.[2] It was adopted by Lufthansa in 1934 and sold around the world.[3] The Lorentz company was founded in 1880 by Carl Lorenz and is now part of ITT.
Lorenz used a single radio transmitter at 38 MHz and three antennas placed in a line parallel to the end of the runway.The Lorenz beam obtained a sharper beam than could be created by an aerial array by having two lobes of signal.[4]
The center antenna was always powered, while the other two were switched on and off in turn. With two of the antennas powered, the presence of the third resulted in a "kidney" shaped broadcast pattern centered on one of the two "side" antennas. The broadcast was switched so that the left antenna, as seen looking toward the runway on approach, was turned on only briefly, sending a series of "dots" 1/8th of a second long repeating once a second. When the left antenna was switched off the signal was sent to the right antenna instead, broadcasting a series of 7/8th second long "dashes". The signal could be detected for some distance off the end of the runway, as much as 30 km.
An aircraft approaching the runway would tune his radio to the broadcast frequency and listen for the signal. If they heard a series of dots, they knew they were off the runway centerline to the left (the dot-sector) and had to turn to the right to line up with the runway. If they were off to the right, they would hear a series of dashes instead (the dash-sector), and turned left. Key to the easy operation of the system was an area in the middle where the two signals overlapped, where the dots of the one signal "filled in" the dashes of the other, resulting in a steady tone known as the equi-signal. By adjusting their path until they heard the equi-signal, the pilot could align their aircraft with the runway for landing.
Two small radio beacons were also used with Lorenz, one 300 m off the end of runway, the HEZ, and another 3 km away, the VEZ, also broadcast on 38 MHz and modulated at 1700 and 700 Hz, respectively. These signals were broadcast directly upward, and would be heard briefly as the aircraft flew over them. To approach the runway, the aircraft would fly to a published altitude and then use the main directional signals to line up with the runway and started flying toward it. When they flew over the VEZ they would start descending on a standard glide slope, continuing to land or abort at the HEZ depending on whether or not they could see the runway.
Lorenz could fly a plane down a straight line with relatively high accuracy, enough so that the aircraft could then find the runway visually in all but the worst conditions. However it required a fairly constant monitoring of the radio by the pilot, who would often also be tasked with talking to the local control tower. In order to ease the workload, Lorenz later introduced a cockpit indicator that could listen to the signals and display the direction to the runway centerline as an arrow telling the pilot which direction to turn. The indicator also included neon lamps to indicate when the aircraft crossed over the marker beacons.
Later derivatives of the system had signals of equal length in the pattern left-right-silence, to operate a visual indicator in the cabin.
The Lorenz system was similar to the Diamond-Dunmore equi-signal radio guidance system, developed by the US Bureau of Standards in the early 1930s. [5]
Use for blind bombing
In the Second World War the Lorenz beam principle was used by the German Luftwaffe as the basis of a number of blind bombing aids, notably 'Knickebein' ('crooked leg') and the 'X-Gerät' ('X-Apparatus'), in their bombing offensive against English cities during the winter of 1940/41. X-Gerät was very similar to LFF, modifying it only slightly to be more highly directional and work over much longer distance. Using the same frequencies allowed their bombers to use the already-installed LFF receivers, although a second receiver was needed in order to pinpoint a single location.
These systems involved cross-beams of the same characteristics but on a different frequency, which would both enable the pilot to calculate his speed (from the time between crossing the Fore Cross Signal and crossing the Main Cross Signal), and indicate when he should drop his payload. The calculation was performed by a mechanical computer.
Lorenz modified this system to create the Viktoria/Hawaii lateral guidance system for the V-2 rocket.
Allied jamming effort
When the Allies discovered the existence of the 'Knickebein' system, they rapidly jammed it, however the 'X-Gerät' was not successfully jammed for quite some time. A later innovation by the Germans was the 'Baedeker' or 'Taub' modification, which used supersonic modulation. This was so quickly jammed that the Germans practically gave up on the use of beam-bombing systems, with the exception of the 'FuGe 25A', which operated for a short time towards the end of Operation Steinbock, known as the "Baby Blitz".
A further operational drawback of the system was that bombers had to follow a fixed course between the beam transmitter station and the target; once the beam had been detected, defensive measures were made more effective by knowledge of the course.[6]
Technical considerations
The reason why the Lorenz beam principle was necessary, with its overlapping beams, was because the sharpness of a beam increases approximately logarithmically with the length of the aerial array with which it is generated; a law of diminishing returns operates, such that to attain the sharpness achieved by the Lorenz system with a single beam (approximately 1 mile wide over a range of two hundred miles), an array of prohibitive size would be required.
See also
- Battle of the beams
- Radio navigation
- Experimental Radio Station Eberswalde
- Instrument landing system
- SCR-277
References
- ^ For example, Australia, according to http://www.airwaysmuseum.com/Lorenz%20system%20article.htm
- ^ http://www.radarworld.org/flightnav.pdf
- ^ Louis Brown, A Radar History of World War II: Technical and Military ImperativesCRC Press, 1999 ISBN 0750306599, page 113
- ^ R. V. Jones, Most Secret War: British Scientific Intelligence 1939-1945, Hodder and Stoughton, 1979 ISBN 0 340 24169 1 Chapter 11 The Crooked Leg
- ^ http://www.pnas.org/content/16/11/678.full.pdf
- ^ Jean-Denis G. G. Lepage,Aircraft of the Luftwaffe 1935-1945: An Illustrated History, McFarland, 2009,ISBN 0786439378, page 60