Radar and Loran
July 1959 Popular
old farts like me remember anything about LORAN (LOng RAnge Navigation).
My familiarity with it came not from boat navigation, but from airplane
navigation. Before LORAN became totally obsolete due to GPS (phased
out in U.S. and Canada in 2010), the transmitter stations were commonly
tuned in in order to obtain positional fixes via triangulation.
Whilst taking flying lessons at Lee Airport, in Edgewater, Maryland,
the ground instructor included it in the lessons, and even the FAA
Private Pilot exams had a question or two on LORAN. The el cheapo
Piper Colts that I flew were luck to have a VOR receiver in it,
so I never actually used LORAN. This article touches on the principles
of both radar and loran.
July 1959 Popular Electronics
People old and young enjoy waxing nostalgic about and learning some of the history
of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights
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Radar and Loran
Vital electronic aids to navigation
Clark E. Jackson
MORE than a decade, the navigator has had electronic eyes which
penetrate fog and darkness, seek out hidden dangers, and chart the
surroundings for him. Radar (Radio Detection and Ranging) was a
secret word during the early part of World War II and has been credited
by many as having been most instrumental in bringing victory to
the Allies in Europe and the Pacific. Loran (Long Range Navigation)
was also developed during World War II as a rapid and simple way
of getting accurate navigational fixes. Today more and more standard
navigational charts have loran lines, making it possible to transform
a loran receiver reading directly into a graphic navigational fix.
Ceiling radar installation saves space and allows shorter lead-in
cables to antenna atop wheelhouse, (RCA photo)
Lightweight. low-cost radar equipment can be installed on a
28-footer without placing too much of a burden on the ship's
battery. (Bendix-Pacific photo)
Loran equipment, excellent for long range navigation. (Edo Corp.
A crude form of "radar" was in use
before electronic radar came into being. From the beginning of time,
navigators have used audio signals to determine their distance off
steep shores hidden in fog, by sounding a short blast with a whistle
or horn and then listening for the echo. Knowing the speed of sound
to be approximately 1100 feet per second, if it took five seconds
for an echo to return, the shore or cliff was known to be about
a mile away.
A radar unit transmits short bursts of microwave
radio energy (between 1000 and 30,000 megacycles) which are reflected
by solid objects. Distance is determined by the length of time it
takes a radar pulse to hit a target and return. Since radio waves
travel at the speed of light - 186,000 miles per second-the time
from the instant a radar pulse is transmitted to the instant the
echo returns is' measured in microseconds (millionths of a second).
In a typical radar unit, more than a thousand pulses per
second are transmitted in rapid succession as if fired from a machine
gun. The pulses are spaced far enough apart in time to allow an
echo to return before the next pulse is transmitted. The time required
for a pulse to travel to a target and return as an echo is measured
on a cathode-ray tube, similar in many respects to a TV picture
tube. The echoes appear on the round screen as glowing "blips."
Antenna Requirements. Radar pulses should travel in a narrow
beam in order to allow the detection of small objects and the determination
of direction as well as distance. A modified form of parabolic reflector,
the so-called "dish" antenna, is used to focus microwave signals
into a narrow beam. The antenna is rotated 3600 horizontally so
it can scan in all directions, and the antenna rotator motor is
synchronized with another motor at the radar screen.
the antenna rotates, the trace line on the radar screen sweeps around
with it. This is done by rotating the magnetic sweep coils around
the neck of the cathode-ray tube. The sweep starts at the center
of the round tube face and rotates in step with the antenna. It
looks something like a windshield wiper. Blips appear along the
trace whenever a radar echo is received.
picture tube is used, so that the echoes remain on the screen until
the sweep has rotated a full revolution and sometimes longer. As
the radar beam rotates, a map of the surrounding area is "painted"
on the screen. Large areas, such as the shoreline and islands, appear
as large lighted masses. Boats and other small objects appear as
small blips. Lighter hash or clutter may appear near the center
of "home" point on the screen due to "sea return," a mass of echoes
from the surface of the water.
Although marine radar equipment
sometimes has a range up to 40 miles, maximum range is limited
by the height of the antenna above water. Radar is a line-of-sight
device, and as the antenna height is decreased, the effective range
is reduced. While it might be interesting to see objects 20 miles
away, objects within a range of eight miles are generally of more
concern to the navigator.
Typical Installation. A marine
radar installation consists of a transmitter, receiver, antenna,
monitor, and power supply. One antenna serves both transmitter and
receiver since the transmitter is turned on for only a microsecond
or so and then pauses to allow the receiver to intercept the echo.
The antenna is automatically switched from the transmitter to the
Use of radar on pleasure craft
is limited by space, the capacity of the electric power source,
and the owner's pocketbook. Prices for radar units start at about
$1500. Power requirements are heavy, running from 300 watts to more
than 1500 watts, which means that small boats equipped with 12-
volt batteries may not have enough reserve electrical power for
radar equipment. An auxiliary generator driven by a small engine
can be used if a boat does not have 32- or 115-volt batteries.
FCC radio station license is required and the radar equipment must
qualify for FCC type acceptance. It must be installed and serviced
under the supervision of a person holding a suitable FCC operator's
license endorsed for radar. The master of a vessel, however, may
use a licensed radar set without being required to have an operator's
license. No one else may use the radar except under his personal
navigational devices such as radio direction finders are being supplemented
on military and commercial vessels by loran, with its longer range
than a conventional radio direction finder, no loop to rotate, and
greater accuracy. Like radar, loran employs pulses, but operates
at much lower frequencies (1750-1950 kc.). Loran, however, does
not require a transmitter on board ship.
an "electronic stop watch," loran measures the difference in the
time required for radio signals to arrive from different synchronized
radio stations. Since radio waves travel at constant velocity, the
traveling time of a signal is proportional to the distance between
transmitter and receiver.
Chain of Operation. A loran chain
consists of two or three radio stations, a "master" and one or two
"slaves." These transmitters are located from 200 and 400 miles
apart and transmit short pulses simultaneously or offset by a precise
time interval. The pulses are repeated between 25 and 35 times per
The difference in arrival time of signals from a
group of loran stations is measured by a loran unit aboard ship.
To get a "fix," it is necessary to take readings on two pairs of
loran stations or a single three-station loran chain. The ship is
located at the point where the hyperbolic curves or distance lines
on the loran chart intersect.
Daytime range of loran is
around 700 miles over water and about 450 miles at night, using
the direct ground wave from the loran stations. At night, the reflected
sky wave permits use at distances up to 1400 miles, but the results
are less accurate. Error in line position of up to ±3 nautical miles
and in fixing of up to ±10 nautical miles is apt to occur in using
night sky waves at extreme distances.
for loran are more modest than for radar (as little as 135 watts),
but the cost of the equipment is about the same, starting at around
$1500. For reliable fixes in all kinds of weather, loran is of great
importance to the boatman who ventures offshore beyond the range
of radar or direction finders.