Your RF Cafe
May 1959 Popular Electronics[Table 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 are hereby acknowledged. See all articles from Popular Electronics.
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]." ;-)
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.
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 system.
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 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 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.
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.
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, 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