Avionics News May 2013 - 62

The Westinghouse BC-11-A was part of the first system of electronics
specifically designed for airplanes.

THE HISTORY OF AVIONICS
Continued from page 61

the elapsed time between the identification signal and peak
signal strength; thus, determining the radial from the station.
Eventually, this reverse RDF technology evolved into the
VHF omnirange system. One advantage of reverse RDF is
the radial or bearing is relative to the ground station instead
of to the airplane. As a result, pilots can estimate and correct
for winds aloft more easily.
During the 1930s, engineers developed a variety of
beam systems. Generically speaking, beam systems broadcast two directional, narrow and overlapping signals. The
signals can be differentiated because each is modulated
differently.
In the late 1930s, Germany developed two-beam systems, the Lorenz and the Sonne. In the U.S., engineers
developed the four-course radio range system. This system
was used both before and after World War II. A few of the
four-course range systems were still in use during the 1980s.
To navigate using the four-course range, pilots would listen to
the audio received. One beam would be modulated with a Morse
code A, or dot dash. The other beam would be modulated with
a Morse code N, or dash dot. When the pilot was on course, the
dots from one letter would be overlapped with the dashes from
the other letter, producing a steady tone. Although instruments
were available to give a visual indication of whether or not the
aircraft was on the beam, most systems relied on the pilot’s ears.
During World War II, Germany was using the Lorenz system
as an instrument landing system. Essentially, the localizer and
glideslope in use today is an evolution of this system.
Marker beacon systems developed along with the range stations. The marker would be placed along airways to denote or
mark certain locations. These were known as fan or airway
markers. The marker beacon survives today as part of the ILS.
World War II and Radar
In the same manner World War I drove the development of
airborne communication radios, World War II drove the development of several technologies. The beam systems mentioned
above were refined in the years immediately before and during
World War II. This war also stimulated development of radar
and transponders.
In 1936, the British were able to use radio waves to detect
an aircraft flying at 15,000 feet at a range of 60 miles. As the
British developed this technology in advance of World War II,
it became more secret.
During these pre-war years, both the British and the Germans
were working to improve radar technology. Both were aware
of the development of the magnetron by Richard Hull in 1920.
The Germans chose not to use magnetrons in their World War
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avionics news

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2013

II radars due to the device’s frequency instability. This problem
didn’t deter the British, and they continued to develop magnetron-based radar technology.
At one point, due to fears of imminent German invasion, the
British packed up all the secret radar and magnetron research into
a steamer trunk and placed it on a ship bound for the U.S.
The Allies surged ahead in radar technology so much that when
the Axis recovered a radar-equipped P-38, they couldn’t believe
the device within was a radar. Instead, they thought the Allies
had planted the device for them to find as part of a psychological
warfare effort.
Along with radar came identification, friend or foe (IFF). An
active system, the IFF receives the radar signals and responds
with a code, originally 2-digit, which identifies the aircraft as
friendly. Eventually, this system developed into secondary-surveillance radar and ATC transponders of today.
Post-War Navigation
Although bits and pieces of automatic direction finders, such
as goniometers and crossed-loop antennas had been around
since before 1910, the radio compass didn’t evolve into the
automatic direction finder until after World War II. Bill Lear’s
ADF 12 became very popular in general aviation aircraft. The
ADF was relatively inexpensive, and pilots could use them to
navigate to nondirectional beacons and to AM radio stations.
Outer compass locators and a few NDBs still exist today, and
many aircraft are still equipped with an ADF.
Just after World War II, the U.S. government began implementation of the VHF omnirange (VOR). This technology took
a reverse RDF technology and placed it in the VHF band. The
first VOR-defined airway stretched from New York City to
Chicago. Since pilots no longer had to listen to a steady tone
and the bearing or radial was relative to the station instead of

Avionics News May 2013

Table of Contents for the Digital Edition of Avionics News May 2013