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
of Contents]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 (if any) are hereby acknowledged.
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Radar and Loran
Vital electronic aids to navigation
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
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.
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
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.
As 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.
A long-persistence 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 receiver electronically.
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.
An 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 supervision.
Conventional 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
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 second.
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.
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.
Power requirements 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