Dec. 1931 / Jan. 1932 Short Wave Craft
Wax nostalgic about and learn from the history of early electronics. See articles
from Short Wave Craft,
published 1930 - 1936. All copyrights hereby acknowledged.
Landing Systems (ILS) has been around since the early 1930s, as made apparent by
this article in Short Wave Craft magazine. Frequencies, circuits, and infrastructure
equipment have evolved over the years, but fundamentally, landing an aircraft (airplane,
helicopter, dirigible) under 'blind' flying conditions has not changed. Two precision
beams - one in elevation and one in azimuth - broadcast by ground-based installations
are detected by airborne receivers and relative positions are displayed for the
pilot's use in navigation. ILS does not help the pilot fly the aircraft; it only
leads him to the runway threshold. In the past couple decades, space-based Global
Positioning System (GPS) equipment has increasingly been used to replace ground-based
microwave systems; however, in order to achieve necessary positional precision and
fast-enough update rates, supplementary ground-based stations are needed to provide
Planes Can Land in Fog! Thanks to Short Waves
Airplanes have been repeatedly landed while "flying blind" by the aid of this
new short wave "landing beam", which has been developed by Uncle Sam's radio experts.
This means that all airports will shortly be made "visible", even in a fog, thanks
to short waves.
By Lieut. Myron F. Eddy*
U. S. N. Retired
Plan view, showing airport and how landing beam short wave signal
is picked up and used to guide plane in landing safely.
No matter what altitude the plane has when entering zone of the
Landing Beam, the "Landing Indicator" will immediately show whether the plane should
commence gliding by reading "Too High" or "Too Low".
Oscillator circuit of Landing Beam Transmitter.
Simplified dials used to indicate landing conditions to pilot
in plane; how close he is to the ground and if he is "off" or "on" the course.
Diagram showing elements of landing beam receiver and indicator
carried on plane.
For the past three years the Bureau of Standards have been working on the short-wave
landing beam for airplanes. Now they have it. The beam makes blind landings possible.
The why and wherefore is most interesting. In the first place you realize that
the blind flying of planes has been going on for some time. All the U.S. airways
have radio-beacons that a pilot can follow from one airport to another, but suppose
that he can't see an airport - how is he to land? The short-wave landing beam is
the answer. And here is how it works.
The regular airways radio beacon transmitter is located at one side of the airport
- not on the airport. There is also another (marker) beacon, projecting a beam a
short distance along the other side of the airport. But the beam transmitter is
put right on the airport - right at the end of the landing runway. The pilot follows
the airways beacon to the airport where he wishes to land. He knows when he passes
over this beacon, because its signals change. Therefore, upon passing the beacon,
the pilot turns left to the marker beam, straightens out on it, turns sharp left
again and so hits the landing beam. He then turns left once more and follows the
beam to a landing. It is as simple as "no right turns" and "one-way traffic" - sometimes
The value of this system is obvious. It is literally a "life-saver". The landing
beam .may be set at the airport in the direction of prevailing winds or any other,
safe landing direction. It may be made steep, for a short approach, as in mountainous
localities, or more gradual when a longer approach is desired. It can 'be, picked
up at any altitude between 500 arid 5,000 feet. No manipulation on the part of the
pilot is required since the tubing is fixed. He merely switches in his "landing-beam"
receiver at the proper time. This receiver is visual and is mounted on the flight
If he keeps the needle on this instrument in the middle by controlling the plane's
elevators, he will come in at a normal landing glide and level off without ever
seeing the field. His altimeter will tell him when to cut the throttle completely;
although he could hit with it partly on and do nothing worse than bounce.
Landing Beam Operates on 3 Meters
So much for the system; now about the apparatus. Remember that there must be
control, or guidance, over three dimensions. For this reason a beam (called a "localizing
beam") must be directed along the runway to form a lane, which will keep the pilot
from swerving off the runway to right or left, while gliding down the landing beam.
Also, the landing beam itself, although projected as a straight line (tangent to
the horizontal runway) must guide the plane down a curved path, which becomes flatter
and flatter near the ground end. This path is actually the line of equal intensity
and is always below the axis of the projected beam.
The landing-beam transmitter operates on about 3 meters. A directive antenna
system is, of course, required; this consists of a number of short doublet horizontal
antennas, which run perpendicular to the main supporting structure, which in itself
is not high enough to be a flight obstruction. This whole outfit can be tilted and
locked into position to give any desired angle above the horizon to the projected
The transmitter projecting the localizing beam along the runway operates on about
1,200 meters. It is a 200-watt set and uses small loop antennas. When signals from
this beacon are picked up, the landing beam receiver aboard the plane is immediately
There is a horizontal doublet type antenna about five feet long on the plane,
employed for the short-wave landing receiver. The regular airways beacon receiver,
either visual or aural, takes care of the airways beacon, the airport boundary marker
beacon along the runway.
The boundary beacon consists of a 50-watt transmitter feeding a loop-type antenna.
Modulation of the radiated wave to the desired frequency, 1,000 cycles per second,
is obtained by supplying the transmitting tube with a plate voltage of 1,000 cycles
frequency. On the airplane, earphones connected in series with the main course indicator
are employed with a special filter circuit. The boundary line is defined by a zero,
as the plane passes over the boundary line; then an increasing signal as he comes
within the landing-field area,
On the airplane, the signal current in the output circuit of the special shortwave
receiving set employed to receive the landing beam is first rectified and then passed
through a landing beam "indicator". This "indicator" is mounted on the instrument
board of the plane.
To help in maneuvering onto the field a "rough distance indicator" has been developed
which is also automatic. It works like the ordinary tuning meter, found on any receiver
which has automatic volume control, and constantly indicates to the pilot the approximate
distance of the plane from the localizing-beam transmitter. This transmitter is
usually just back of the landing beam transmitter: and so the pilot usually "sets
her down" just before the rough distance indicator registers zero - whether he feels
like it or not! Otherwise he piles up on the transmitter antennas - if he does not
see them at the last instant.
Posted September 23, 2020(original 2/18/2015)