2003 Wings Publishing
Leader Cables
The Loth Instrument Navigation System
During World War I, a leader-cable system for guiding ships into harbour during foggy weather was developed by Lt. Arthur W. Loth, an engineering officer in the French Navy. In the early 1920s, Loth developed a similar system for guidance of aircraft.The Loth system was based on a cable through which electrical alternating current at 600 Hz frequency was passed to create a magnetic field around the cable, this field being detectable at considerable distances from the cable.
In the aircraft version, three panels of insulated copper wire were installed on the airplane at right angles to one another, so they were aligned with the pitch, roll and yaw axes. These panels were connected to a receiver via a switching device so the aircraft's navigator could listen-in on any two of the three panels to determine the position of the airplane relative to a Loth leader cable encircling the airport. Using this system, a navigator could determine the correct course to approach the cable at right angles and establish when the cable was crossed.
Tests were conducted in 1922 at Chartres (just outside of Paris in northwest France). An electrical cable was mounted on existing telegraph poles. Horizontal angle detection was made from 10,000 feet (3048m) altitude, lateral and longitudinal vertical angle detection from 8000 feet (2438m) and the detection of all three angles from 6000 feet (1829m).
Since barometric altimeters do not measure height above ground, the possible height-measuring capability of several of the leader-cable systems was of particular significance. However, accuracy of such altitude information demanded optimum operating conditions, invariability of the magnetic-field intensities at different installations and virtually no distortion of the magnetic field.
Alternative instruments investigated in the late 1920s included a sonic altimeter, based on the time taken by sound to strike the ground and return to the airplane; a capacity altimeter, which measured the distance from the ground by detecting the change in electrical capacity between two plates on the aircraft as it approached the ground, and a radio altimeter, which used the direct reflection of radio waves.
The Loth system was further developed and in 1931 the first demonstrations was held for the U.S. Army Air Corps at Wright Field near Dayton, Ohio using concentric energized cables that could be detected five miles (eight km) from the airport.
I recently found an article discussing this experiment in Aviation, December 1931. You can read this article HERE.
2003 Wings Publishing