May 1958 Popular Electronics
Table of Contents
People old and young enjoy waxing nostalgic about and learning some of the history
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Just as today's generation of engineering students grew up with
and are totally accustomed to and proficient at using computers,
smartphones, positioning devices, CAE software, and various
combinations of the aforementioned, so have the latest cadre
of pilots grown up with GPS and electronic flight charts and
planners in the cockpit. The difference is that whereas engineering
students are not still required to learn to use a slide rule
and a drafting table to earn an engineering degree, pilots are
still required to learn to navigate using primitive (not meant
derisively) instruments and ground-based navaids to earn a pilot's
license. That's not a bad thing, though, because whereas if
your graphing, 2500-function calculator quits working, the only
thing at risk is your test score if you happen to be taking
an exam. However, if your electronic navigation fails while
in a limited visibility environment or in controlled airspace,
you had better be able to do some seat-of-the-pants flying or
you could be in deep doo-doo. This 1958 article from Popular
Electronics presents the newfangled TACAN (TACtical Air Navigation)
and Loran (LOng RAnge Navigation) systems recently introduced
(at the time) by the CAA (Civil Aeronautics Authority), which
is now the FAA (Federal Aeronautics Administration). It was
to dead reckoning navigation what the HP-35 calculator was to
the slide rule.
Finding Your Way in Space
By Brooks Currey, Jr.
It's a long haul from the old sea dog to today's jet
There was a time when a well-moistened forefinger was a man's
only navigation instrument. The cool side told him the wind
direction-and thus, his course. Today, man relies on the thin
metal "forefingers" jutting from high-speed high-altitude aircraft
to keep him informed on position, direction, velocity and other
data so necessary to flight. And satellites circle some 300
to 400 miles above the earth - spring-loaded "forefingers" busily
transmitting spatial information back to us.
The evolution of navigation from an uncertain art to a specialized
science has been long and arduous. Its basic principles have
always existed - they only awaited discovery. By necessity,
navigation has always tagged along in the wake of mathematical
and astronomical development. The ancient Greeks, we know, taught
that the world was round and that any position on its surface
could be determined by latitude and longitude. Basic principles,
therefore, did not mystify man so much as the instruments used
to verify them.
Direction by Radio.
The development of electronic aids to navigation begins within
our present century - in the early 1900's when Marconi's wireless
made radio direction-finding the first electronic aid.
Early shipboard direction finders were simple loop antennas,
with a tunable receiver (100 to 1800 kc.), a set of earphones,
and an azimuth indicator. To operate, the navigator simply tuned
in a radio station of known position, rotated the loop to minimum
gain, and then read the relative bearing on the azimuth indicator.
By using several such stations - the more the better - and drawing
the bearings on a chart, the ship's position was located at
the intersection of the bearings.
As refinements were made on radio direction-finding equipment,
the speed and range of aircraft were steadily increased. Aerial
navigation amplified existing problems of navigation, and introduced
many new ones. For instance, the time allowed to compute position
decreased in direct proportion to the increasing flight speed.
A part of this new problem was solved by the low-frequency radio
Now standard for nearly all airplanes, radio range equipment
makes use of a network of ground stations and a receiver in
the airplane. In operation, four radio beams of approximately
3° width are transmitted along the CAA (Civil Aeronautics Authority)
airways - intercontinental "super-highways" 10 miles wide which
are divided into 1000' altitude levels.
Block diagram shows what's behind the "beam"
that pilots fly on the CAA network of airways.
Basically, the ground station has two pairs of transmitting
antennas, each matched pair being placed at diagonal corners
of a square. One pair transmits "A" (dit dah), the other pair
"N" (dah dit) , The signals are transmitted in a figure-eight
pattern. A and N signals overlap to provide equal signal intensity
along the four 3° beams.
The pilot tunes his receiver to the proper station frequency,
between 200 and 400 kc., and listens for the station's call
letters, e.g., LGA for La Guardia Field, New York. Once identified,
the pilot hears either A or N in keyed intervals. (The N signal
is always assigned to the quadrant containing true North to
minimize confusion.) If the pilot hears an N, he knows he is
off the beam, and he turns left or right. In so doing, he notes
that the original N grows into a steady tone where the dah dit
and dit dah overlap. When the pilot cannot distinguish A or
N, he is "on the beam."
Radar and Racon. The big impetus to electronic
aids came during World War II, with the advent of radar. Using
narrow beams of microwaves of 1 to 12 cm. in wavelength, radar
measures the time it takes an energy pulse to travel out, echo
off an obstruction and return. One mechanization of this effect
is Racon (RAdar beaCON), which provides the air navigator with
both distance and bearing information on a standard PPI (Plan
Position Indicator) scope.
Simplified diagram below shows how the Racon
(RAdar beaCON) operates. Note use of scope.
Airborne Racon equipment includes a primary radar operating
on a frequency in the 200- to 10,000-mc. frequency range. The
ground beacon consists of a secondary radar containing a receiver,
time-delay unit and transmitter.
In operation, the navigator "interrogates" the ground beacon
with a pulse from his radar. This triggers a coded pulse from
the beacon which is transmitted in all directions. The navigator
observes the beacon response on a PPI scope in much the same
manner as he observes targets.
To differentiate between Racon signals and target echoes,
the beacon signals are coded as a series of pips as detected
by the PPI scope. Thus, bearings to the beacon can be taken,
and distance measured. Effective range of Racon operation is
limited only by the horizon or line-of-sight distance.
A more specialized system is DME (Distance Measuring Equipment),
though bearings to the ground beacon are not given. Fundamentally,
the ground equipment for DME is like that used for Racon. The
airborne equipment, however, differs in that the distance is
shown on a dial indicator instead of a PPI scope. Because this
indicator is susceptible to beacon interrogation pulses by other
aircraft, the airborne equipment contains a sweep-search circuit
in addition to a tracking circuit. In operation, the airborne
transmitter sends out a 936- to 986-mc. beam. A separate receiver
antenna picks up the beacon and any other transmissions.
Airborne VOR. Should the navigator wish to determine course
direction and not distance, he can use one of several omnirange
systems: low-frequency, v.h.f. or u.h.f. The omnirange equipment
provides the navigator with accurate courses either off or on
With the VOR system (V.h.f. Omnidirectional Range), the navigator
or pilot selects a station from a chart published by the CAA.
He next tunes in the frequency of the selected station by means
of a dial, and checks the coded or voice call of the station
with that given on the chart. The magnetic bearing of the station
from the aircraft is set into the system by means of a selector
wheel; the bearing of the station is thereafter retained at
all times until changed to a new station.
Two other very essential parts of the airborne VOR system
are the "left-right" and the "to-from" indicators. Once the
magnetic bearing of the station has been selected, the pilot-navigator
checks the "to-from" indicator to determine if his aircraft
is flying toward or away from that station. The "right-left"
indicator then guides him to the station by means of "flying
to the needle." If the needle points to the left of the correct
course, the pilot should turn left until it centers once more;
if it points to the right, he turns in that direction.
Loran. Another major electronic aid to navigation emerging
from World War II was Loran (LOng RAnge Navigation). Though
primarily associated with sea rovers, Loran is also used successfully
for aerial navigation. It is the only method that does not rely
on dead reckoning to compute position but rather on the hyperbolic
functions of analytic geometry.
Plotting a ship's position by Loran makes
use of the intersection of two hyperbolas, which are obtained
by tuning in two pairs of stations.
Assume that we are standing some distance from two mountains.
We find that mountain A is 100 miles from us, and B is 150 miles
away. The difference in distance is 50 miles. Now we can move
so that the difference in distances always remains 50 miles,
but only if we move in a hyperbolic path. This is the basic
method used in Loran, the major addition being that there must
be at least two pairs of "mountains." By using two pairs, two
hyperbolas result, and the point at which they intersect is
the ship's position.
In standard Loran, a pair of ground transmitters sends out
pulses at the rate of either 25 or 33 1/3 per second. Antenna
output is about 100 kw. at frequencies between 1700 and 2000
kc. Another nearby pair of stations, on the same frequency,
provides the navigator with the second hyperbola.
Aboard ship, the navigator has a conventional superheterodyne
receiver with four broad channels which are fixed-tuned. The
navigator selects any pair of stations, tunes in and reads the
time difference between the two signals on a cathode-ray tube.
He selects at least one other pair and repeats his computations.
The intersection of the two hyperbolas is then found on a specially
gridded Loran chart. A good navigator can obtain a fix in less
than five minutes.
Range of Loran navigation varies from 700 miles during the
day to twice that at night; reflection of waves from the ionospheric
layer in the evening gives this range boost. Ground waves, of
course, are primarily used because of their accuracy, though
tables have been prepared to take into account any sky wave
reflections during nighttime operation.
Tacan. Last and latest on the list of electronic aids to
navigation is all-weather Tacan (TACtical Air Navigation). Tacan
operates in the 1000-mc. band with 126 clear-frequency, two-way
channels available, each channel being spaced 1 mc, apart. In
the 1025- to 1150-mc. band, 126 frequencies are available for
air-to-ground transmission; for ground-to-air transmission,
63 frequencies are available within the 962- to 1024-mc. band,
and 63 more are in the 1151- to 1212-mc. band.
Tacan, shown atop the mast of the new supercarrier
"U.S.S. Forrestal," and u.h.f. radio by Federal Telecommunication
Labs (left circle) play important role in America's air-sea
In operation, the plane transmitter sends a distance interrogation.
This pulse is retransmitted by the beacon, and electronic measurement
of the elapsed time interval is converted to distance in miles.
Azimuth bearings are determined by measuring the phase difference
of a periodic transmission of a main and auxiliary reference
burst from the beacon. Identification of the station is made
by keyed Morse characters at regular intervals.
Flying laboratory of Federal Telecommunication
Labs flight-tests Tacan under all conditions.
We've come a long way in advancing the science of navigation
to a safe, dependable means of traveling from here to there.
There's always the chance that a tube can blow, or an amplifier
can malfunction, and throw the whole system out. But we'll have
to admit that it beats holding up a wet forefinger to the wind.
Illustration of how the system works.
Posted March 13, 2013