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]." ;-)
May 1959 Popular Electronics
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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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Doppler Radar Charts
the AirlanesNew navigational system gives pilots instant indication of ground speed
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.
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
DETERMINING SPEEDSuch 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.
. 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)
. 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.
. 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.
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 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
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 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 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 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.
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.
. 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).
. 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.
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.
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.
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.
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.
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
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
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.