The "blind landing" apparatus at Malmi Airport in the 1930's - top-notch technology of its time

Named after the original manufacturer, "Lorenz" was from 1934 on the instrument approach system used in German commercial air traffic and also became widely used elsewhere in the 1930's. It operated at ~30 MHz frequency and its transmitter antenna beamed a radio signal along the line of the runway through an antenna reflected at both sides. The system continuously switched between the two reflectors in such a manner that the pilot of an approaching aircraft would hear from his receiver 1150 Hz Morse code dots when he was to the left of the centerline and Morse code dashes when he was to the right of the centerline. On the centerline, the two transmissions would blend together into a continuous signal. The range of the beam was about 20-30 km at 500 W transmission power.

An advertisement in Aero 9/1937. The text says "This is how the blind landing apparatus installed by Telefunken ensures the safety of air traffic at Helsinki land airport under unfavourable weather conditions."

 
The widely used German system was found to be the most appropriate also for the land airport of Helsinki which was under construction in the mid-1930's. The Telefunken system installed on two crossing runways of Malmi Airport was top-notch technology of its time and provided distance and glide path information as well as directions. From a pilot's point of view it was similar to today's ILS system (which was, incidentally, also developed already in the late 1930's in the U.S.) In addition to the audible radio signals, the vertical hand on the "blind landing" instrument on a well-equipped aircraft would tell the pilot which way to turn to get to the beam centerline, and the horizontal hand would show either the approximate distance from the airfield or the position of the aircraft in relation to the correct glide path, depending on the position of a selector switch.

The system also had a weak (about 5 W) upwards-transmitting front marker and main marker beacon about 2300-3300 m and 300 m away from the airfield. The beacons could be heard on the radio and they also showed on the "blind landing" instrument as red and green lights as the aircraft passed them. These beacons, both transmitting at 7.9 m wavelength, were distinguished by the tone and the content of their signals. The front marker beacon transmitted Morse code dashes at 1700 Hz and the main marker beacon transmitted Morse code dots at 700 Hz.

A magnification of the system scheme (in original German)

A magnification of the system scheme (in English). The numbers associated with the beacons are wavelengths.