Lorenz (navigation)

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 equipment", or LFF.

Lorenz used a single radio transmitter at 38 MHz and three antennas placed in a line parallel to the end of the runway. 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 decending 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.

The basic LFF system was later used by the Luftwaffe as night-bombing navigation system known as X-Gerät ("X-device"). 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.

See also

* Battle of the beams
* radio navigation