May 1959 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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 in sound form 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. This 1959
Popular Electronics magazine article attempts to explain Doppler radar
to the uninitiated. 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.
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 the runway.
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.
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 Marconi Company)
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.
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.
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
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
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.
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
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 panel.
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
air-speed, 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
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 off.
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 from limitations.
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 is involved.
"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
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 automatically.
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
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.
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 Co.
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, to-day'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
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 2, 2022
(updated from original post