May 1959 Popular Electronics
Table of Contents
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Doppler radar is familiar to most people these days mainly because
of the weather reports available online and on television reports.
Not many actually understand the principle behind it, though.
A handful can tell you that it is the frequency shift phenomenon
that occurs when a train goes by while blowing its horn. Almost
none could say why or how it is useful in detecting storm systems
or for tracking aircraft. Having worked as an air traffic control
radar technician in the USAF, and then having done the RF and
analog system circuit design for a prototype Doppler weather
radar as an engineer, I have had a lot of exposure, but I am
by no means an expert. All I can say is, "It's rad[ar]."
Doppler Radar Charts the Airlanes
New navigational system gives pilots instant indication of
ground speed and location
The jet airliner strains against its wheel brakes at the
end of one of International Airport's busy runways, its engines
building up power for the New York-to-Paris hop. Waiting for
his control tower clearance, the captain scans the dials of
a special instrument assembly. Among other things, they tell
him his present longitude and latitude and the number of miles
he must fly to reach Paris. Hearing the tower controller clear
him for take-off, he releases the brakes and catapults down
Once airborne, the captain sets his course by compass and
heads out to sea. For the next six or seven hours, he listens
for no radio beacons, and there is no navigator to calculate
the plane's position. Instead the captain keeps checking that
special instrument grouping. It tells him exactly where he is
at all times, exactly what path he is making over the faceless
ocean, thousands of feet below. It tells him exactly how many
miles he has to go before he lets down at Paris. It even tells
him whether he's riding a tailwind or bucking a headwind.
With no other guide, he brings the plane down through a curtain
of clouds at the end of his journey, within five miles of the
Paris airport. Had he been using conventional navigating techniques,
he would have considered himself doing well to come within 25
miles of his destination.
DETERMINING SPEED. Signal is beamed at ground
ahead of plane. Reflected signal is then received. Ground speed
is a function of shift between frequencies of beamed and received
signals, together with depression angle. Measurement of reflected
signal's Doppler shift gives ground speed. (Diagram at right
and diagrams on following page through the courtesy of the Canadian
Such an incident is not far from becoming commonplace in
transoceanic and transcontinental airline flying. It is already
an ordinary occurrence in military navigation. The equipment
that makes such spectacular accuracy possible is the Doppler
radar navigation system.
HOMING PIGEON. Thanks to Doppler radar, Navy
flyers find their way home to their carrier. Here, pilot of
A3D bomber adjusts Ryan Aeronautical unit. Instruments show
latitude, longitude, ground speed, drift, etc.
Doppler radar provides exact ground speed and angle-of-drift
information which is continuously fed into a computer previously
primed with basic position and distance data. The computer digests
this information and the results of the computer's cerebration
appear as meter readings. Everything a pilot needs to know for
pin-point accuracy is contained on one easily read instrument
SAMPLE LAYOUT. Arrangement of assembly designed
by Laboratory for Electronics, Inc., for use in a jet plane.
Combined antenna-transceiver-computer package is mounted in
plane's belly, while ground-speed and drift-angle indicator
(circular dial) and control panel are in cockpit. Control panel
indicates plane's exact longitude and latitude.
Ocean of Air Currents. Before Doppler radar was developed, a
flyer had no way of knowing his exact ground speed and angle
of drift. He did know his approximate airspeed, which is literally
the speed of the air moving past his airplane. If the air were
dead calm, an airspeed indication would give him a reasonably
good idea of how fast he was actually going. But the air is
never completely still. It is really an ocean of gas with currents
flowing in many different directions at varying speeds. It can
change speed and direction in an instant.
Let's say, for example, that a plane flies through a 50-mile-an-hour
headwind. The airspeed indicator reads 300 miles an hour. Actually,
though, the plane is traveling at a ground speed of only 250
miles an hour. Now suppose the wind suddenly slacks off to 10
miles an hour. The airspeed indicator will still show 300 miles
an hour, because this is the speed at which 'the plane continues
to fly through the surrounding air. But, in reality, it is now
going over the ground at 290 miles an hour. The pilot has no
way of knowing that he's picked up ground speed unless he later
times himself between two check-points.
Drift is the second great problem in aviation navigation.
Suppose an airplane is pointed due north and flying at a fair
clip. Now suppose a strong wind is blowing from the west. Obviously,
the wind will tend to push the plane sideways. Thus, the plane's
true course over the earth will be roughly northeast. The difference
between the true course and the direction in which the plane
is heading is the angle of drift.
If a pilot or navigator knows the exact direction and speed
of the wind, he can compute his ground speed and path - or track
across the earth with some accuracy. But when either the speed
or the direction of the wind changes, his calculations are thrown
DETERMINING DRIFT. In zero position (diagram
at top), twin radar beams straddle plane's nose, one aimed to
the left and one to the right. When wind causes plane to move
in direction different from heading (direction in which nose
is pointed), Doppler frequency shift of right beam is greater
than that of left beam and antenna swings until frequency shifts
are equal again (diagram underneath top diagram).
Older Systems. For years we've had a number of radio and
radar aids to help pilots on over-water flights or in conditions
of poor land visibility. They are great helps, but they suffer
There are many radio ranging and beacon devices for overland
flying. A radio beacon serves as a check-point, but it is useless
unless a plane flies over or very near it. The various ranges
tell whether a plane is on or off course - provided the course
and range coincide - and give some idea of the degree of error.
But, even when a range is available, a certain amount of calculating
"Loran" is one of the most widely used over-water navigation
systems. It depends on a number of transmitters scattered around
the world which send out arc-shaped signals. A plane receives
these signals as distinctive blips on a radar-type scope. With
the help of special charts, the intersecting blips from neighboring
Loran transmitters are interpreted by a trained navigator. It
is possible for the navigator to locate his plane on an intersection
and determine the direction of flight. By timing the flying
time from one intersection to another, he can also compute his
true surface speed.
This procedure takes time, obviously, time in which errors
can pile up-particularly at today's jet speeds. Correcting an
error takes time, too. And whenever the wind changes, the navigator
must start from scratch. On the other hand, with a Doppler computer,
the pilot always knows his true location and direction, and
how fast he's really going. He can make a correction instantly,
and if the plane is on autopilot, the correction will be made
Frequency Changes. Doppler radar is based on an 1842 discovery
by Christian Johann Doppler, an Austrian physicist. In essence,
Herr Doppler found that the pitch of a given sound is relative
to the movement of its source with respect to an observer.
Imagine that you are standing by a railroad track listening
to the whistle of an approaching train. If the speed of the
train is constant, the pitch of the whistle will seem higher
to you than it does to a passenger on the train. As the train
passes by, you'll hear a sudden drop in frequency. That's because
the sound waves are "stretched" when the locomotive moves away
from you. In a similar manner, when the train was coming towards
you, they were compressed (and raised in frequency).
This same phenomenon occurs with radio waves. If we put a
radar set in an airplane and beam it at the ground ahead as
we fly, the faster we fly, the higher will be the frequency
of the signal reflected from the ground. If we beam a signal
at the ground behind us, an increase in the plane's speed makes
the returning signal drop to a lower frequency.
Unlike conventional radar systems, Doppler radar doesn't
measure the time a transmitted signal takes to bounce back.
Instead it measures the frequency shift between the transmitted
signal and the reflected signal.
In actual practice, at least two radar beams are used. A
simple Doppler system has a dual antenna sending out two beams,
one forward and to the left, the other forward and to the right.
A servo motor turns the antenna assembly automatically.
Let's say a plane is heading due north, but because of a
crosswind, it is actually moving northwest. The frequency shift
of the left-hand beam will be greater than that of the right-hand
beam, since it is aimed more nearly in the actual direction
of the plane's movement. Instantly, the computer will command
the servo motor to turn the antenna until the frequency shift
for each beam is the same. The beams are now straddling the
desired flight path.
The Doppler navigator computer then "takes out its slide
rule" and calculates the difference between the planned flight
path and the plane's actual heading and shows this difference
on an indicator as the drift angle. At the same time, the frequency
shift of the beams is measured and converted into a reading
of true ground speed.
In some systems, the antenna does not move, and a computer
determines drift angle by comparing the returning signals of
the two beams. This complicates the electronics but cuts down
antenna size and eliminates moving parts. In other rigs, such
as the Janus System (named after the Greek god who could look
forward and backward simultaneously), up to four beams may be
used, two aimed forward and two behind.
Instead of comparing the reflected signal to the transmitted
signal, the latter type of device usually compares the forward
signal returns to those from the diagonally opposite beams.
One of the big advantages of the four-beam system is that it
is unaffected by the airplane's rolling and pitching. It also
permits the use of a less accurately calibrated transmitter,
since a change in transmitter frequency has little effect.
Military Uses. The introduction of Doppler radar navigators
is generally credited to General Precision Laboratory, Inc.
This company test-flew the first Doppler gear back in 1948.
By 1954, it was in quantity production for the U.S. Air Force.
A variation of the first Doppler system was put into production
for the Royal Air Force by Marconi's Wireless Telegraph Co.,
Ltd., in England. In Canada, a corporate affiliate of the British
firm, Canadian Marconi Co., began supplying the Royal Canadian
Air Force with its own version of the Doppler system.
HURRICANE HUNTER. Doppler equipment installed
in this B-47 by General Precision Labs enables it to find the
eye of a hurricane and determine its exact speed.
The U.S. Navy got into the act, too, and after breaking ground,
retained Ryan Aeronautical Co. to continue development of its
own system. Laboratory for Electronics, Inc., came out with
several systems, one particularly suitable for helicopters.
Other manufacturers include Collins Radio Co. and General Electric
A prime reason why Doppler radar navigators are popular with
the military is that they require no ground installation, which
naturally would not be available in enemy territory.
Until fairly recently, the military kept Doppler radar devices
all to itself. But in 1957 the security wraps were removed,
and various manufacturers began to offer commercial versions
geared to the needs of civil aviation.
Commercial Applications. The first commercial purchase of
Doppler equipment was made recently by Pan-American World Airways
from Canadian Marconi Co. Six systems were ordered, to be installed
in Pan-American's six-plane fleet of Boeing 707 jet clippers.
By the time you read this, all of the jetliners will probably
have the new systems aboard.
Other transoceanic airlines overseas are considering the
purchase of Doppler equipment. British Overseas Airways Corp.
has already piled up over 150,000 miles flight-testing the
British Marconi system, and Air France is also evaluating it.
Airliners which are equipped with Doppler radar have several
advantages over airliners using other types of navigation systems.
Doppler-equipped airliners can sniff out favorable jet streams
and latch onto them for free rides. They can also avoid speed-killing
headwinds the same way. Combined with the ability to fly undeviatingly
along the shortest possible route, this wind-sniffing talent
spells much quicker flights and substantial fuel economy. It's
been estimated that a Doppler navigation system can cut fuel
consumption by at least 15%.
Still another dividend is offered by Doppler radar. It will
allow pilots to report their exact position, flight path and
speed to air traffic controllers. This means a much smaller
likelihood of mid-air collisions, today's number one flying
headache. Pilots will further appreciate Doppler radar since
a deluxe Doppler navigational computer can be hooked to an autopilot
- a plane so equipped will virtually navigate itself to any
place on the globe without any hands on the controls.
With its purchase of the Canadian Marconi equipment, Pan-American
World Airways has opened a new chapter in the story of aerial
navigation. Other carriers are bound to follow the example as
they replace their current propeller-driven planes with jet
types. Most of these jetliners will have built-in provision
for Doppler navigation systems.
It may not be long before you can take any airliner, secure
in the knowledge that Doppler radar will help you get to your
destination more quickly and safely than ever before.
Posted September 21, 2011