Principles of Modern Radar - Part I
June 1960 Radio-Electronics

June 1960 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Radar, as with a lot of (probably most of) technology that once was the purview of military, government, university, and corporate entities due to size, complexity, and cost, now is integrated into many consumer products. Boaters and even private aviation pilots have benefitted from radar for decades, at ever increasing levels of capability and often decreasing cost. Your car is one of the biggest beneficiaries of modern radar if it has back-up sensors, lane keeper-inner, low visibility assistance, collision avoidance, and other functions. A mix of short-range, medium-range, and long-range radar, in conjunction with lidar and sonar, is utilized to implement the big scheme dubbed Advanced Driver Assistance Systems (ADAS). Self-driving cars are utterly dependent on all these technologies. This Principle of Modern Radar article form a 1960 issue of Radio-Electronics magazine is a good summary on where the transition to commercialization began.

Principles of Modern Radar - Part II was published in the July 1960 issue.

Principles of Modern Radar

Part I - The first section of this two-part series describes the operation of pulse-modulated and frequency-modulated radar, and shows some of the ways of displaying data

By Jordan McQuay

Modern radar has advanced far beyond the ponderous equipment of short range and limited accuracy that emerged as one of the major technical achievements of World War II.

Through continuous improvement, the range of modern radar has been increased to global and even outer-space distances. Accuracy has been infinitely improved. And the usefulness has been expanded - with many new military as well as commercial applications.

Originally developed as a military weapon, radar was a closely guarded secret until the closing days of World War II. With dissemination of technical data to the radio-electronics industry, came extensive development and continuous improvement. Today, radar has achieved advanced technical status. Because of its inherent nature and operating characteristics, it is primarily a military adjunct to national defense. But it also has many peaceful uses.

For navigation at sea, small radars (Fig. 1) are installed on pleasure boats and small commercial vessels. Larger setups are used on ocean-going liners and transports (Fig. 2).

For air navigation and flight safety, radar is used aboard aircraft as altimeters and to map areas over which the plane is flying. Ground-based radar is used to control flights of aircraft (Fig. 3).

For storm detection, radar can locate and portray heavily charged clouds associated with hurricanes, thunderheads and other weather disturbances (Fig. 4).

Military radar is used for short- and long-range surveillance, artillery fire control, mortar locating, aerial navigation, electronic mapping and many other purposes.

Whatever the application, the basic principles of all types of radar equipment are deeply rooted in radio and electronics.

Accuracy has been improved through increased use of microwaves - often in the millimeter part of the spectrum - through development of electron tubes and circuitry capable of handling such wavelengths. Advanced types of cathode-ray tubes now permit pictures of extreme definition and accuracy. Many of the important new developments in high-speed automatic data-processing equipment have been applied directly to modern radar.

Radar in general

A radar is a composite radio-electronic apparatus that detects and locates objects and targets - such as aircraft, ships, buildings, mountains, terrain, and even people - at various distances and with incredible accuracy, even when darkness, fog or clouds make the targets invisible.

A radar consists essentially of a microwave transmitter, an antenna system, a microwave receiver, a timer or synchronizer and an indicator-all working with microsecond precision.

Rf energy is broadcast in any desired direction. When it strikes an object or target, a minute portion of the RF energy is reflected and returns to the radar within a few thousandths of a second.

Knowing the speed of RF energy (186,000 miles per second) and measuring time differences between the transmitted energy and any reflected echoes, these data are translated electronically into direct distance or range - from the radar to each object or target. For example, an echo is picked up 100 μsec after the RF energy pulse is transmitted. In that length of time, the RF energy has traveled 18.6 miles. This means that the object is half that distance or 9.3 miles away. (The distance is halved because the signal has to travel both ways - to the object and back.)

The angular direction or azimuth of a target is determined by the physical position of the movable antenna system.

Through accurate measurements of range, altitude, and azimuth, the exact location of any target can be determined by electronic geometry - whether the target is in space, or on the ground or sea.

Data obtained directly or through electronic processing are displayed continuously on an indicator. This is usually a cathode-ray tube, called a scope. Any of several different kinds of scopes (Fig. 5), may be used, depending upon the type of data to be displayed-range, altitude, azimuth or other.

The A-scope is widely used - but provides only range data. The scan is a single horizontal line across the scope screen, which is calibrated in feet, yards or miles. The beginning of the trace first shows the main pulse radiated by the radar transmitter. Then, various objects and targets appear along the trace at distances corresponding to their range from the radar. Since the antenna beam is highly directive, the maximum-strength signal appears as the brightest glow along the trace of an A-scope when the antenna is pointing directly at the target - providing a physical indication of its azimuth.

The B-scope plots range against azimuth and is used primarily for radar on aircraft. There is a vertical sweep, and the position of the sweep is aligned with the azimuth position of the antenna - which usually scans a region up to 90° on either side of the aircraft, dead ahead. Reflected signals from targets appear on the B-scope as small glowing spots.

The C-scope plots elevation against azimuth and is also used aboard aircraft. The display is similar to a graph, with the vertical center of the scope representing a direction dead ahead.

The J-scope presents the same type of range information on a circular trace that the A-scone presents on a horizontal trace. Advantage of the J-scope over the A-scope is that the J-scope has a longer trace for a given range. This allows more accurate determination of distance between the radar and a target. Reflected signals appear as outward deflections of the circular trace of the J-scope.

The PPI-scope - Plan Position Indicator - is an electronic polar map. At the center of the scope screen is the main pulse, representing the location of the radar. A circular scale around the screen is calibrated in degrees of azimuth with respect to the location of the radar. The distance from the center of the screen to the illuminated spot of a target is the range to that target. The scan of a PPI-scope is a trace extending from the center of the tube to the outer edge of the screen. This trace is rotated around the circumference of the A-scope synchronously with the physical rotation of the antenna system. Objects or targets within range of the radar appear on the screen as bright spots. Utilizing a long-persistence screen, the PPI-scope thus provides a polar map of the area within range of the radar.

For all kinds of scopes, target data are displayed almost instantly. If a target moves or changes its position, the scope display will also change.

The effective range of any radar depends upon its ability to distinguish between reflected energy from objects and targets, and other interfering "noise" signals and disturbances which may be present. Although the radar transmitter may radiate as much as several kilowatts of power, only a part reaches the target and a still smaller amount is reflected to the radar receiver. Reflected signals may be as weak as 1μv upon reaching the antenna system.

To economize on transmitted power and to provide extremely accurate target reporting, RF energy is concentrated in a very narrow beam by the antenna system. This is done with various kinds of parabolic reflectors associated with the active elements of the antenna. For low-power short-range radar, a small-size antenna is sufficient. For long ranges, the antenna is sometimes very large (see head photo).

The entire antenna system is constructed to move or rotate so that the narrow beam of RF energy can probe a large arc across the region or area being searched or surveyed.

Depending on its design, a radar can detect and locate targets at ranges of a few thousand feet or yards - or thousands of miles.

The primary qualification of any radar is its ability to measure accurately all reflected signals in terms of range or distance. There are three basic methods of measurement corresponding to the three basic kinds of modern equipment: (1) pulse-modulated or PM radar, (2) frequency-modulated or FM radar and (3) pulse-Doppler or PD radar.

Pulse-Modulated Radars

PM radar is widely used for detecting and locating air and sea targets. Range is measured in terms of the time required for a pulse of RF energy to travel to a target and return to the radar.

For short ranges, radars are physically small and compact (Fig. 1). Larger PM radars locate targets as far as the horizon (Fig. 6). Even larger ones are used to track missiles and satellites at distances of thousands of miles (Fig. 7).

All PM radars are basically similar(Fig. 8). A microwave transmitter - usually a magnetron - broadcasts extremely short-duration pulses of amplitude-modulated RF energy at regular intervals. The number of such pulses per second is called the prf (pulse recurrence frequency) and is controlled by the timer of the radar. The prf is usually a fixed value of several thousand pulses per second. The duration of each RF pulse is extremely short - only a few microseconds. After radiating each pulse, the transmitter is switched off to allow the reception of echoes from targets within range of the PM radar. After this interval, the transmitter is turned on again and broadcasts a brief RF pulse. The process is repeated over and over again, according to the prf of the radar.

Pulse constants - the prf and the pulse width - determine the maximum and minimum range limits as well as the range accuracy and resolution of a PM radar.

Enough time must be allowed between the transmitted pulses for echoes to return from targets within range of the radar. Otherwise the reception of echoes from more distant targets will be obscured by succeeding pulses of RF transmission. This necessary time interval fixes the highest possible value of prf.

When the antenna system is turned or rotated at a constant speed (usually about 10 or 20 rpm), the beam of pulsed RF energy strikes a target for a relatively short time. During this time, enough RF pulses must be transmitted to assure receiving sufficient echoes to produce an indication on the radar scope. Thus, the rotational speed of the antenna plus the persistence of the scope screen determines the lowest possible value of prf.

The minimum range at which targets can be detected is determined primarily by the duration of each transmitted pulse. If a target is so close to the radar that its echo is returned before the transmitter is switched off, reception of such echoes will be impossible.

A duplexer, or TR switch, permits use of a single antenna for both transmission of RF pulses and reception of echoes. When the transmitter is operating, the duplexer blocks off the microwave receiver. When the transmitter is not functioning, all incoming echo signals are fed from the antenna directly to the receiver.

A radar receiver is a superheterodyne tuned to the same frequency as the transmitter (several thousand megacycles). It is highly sensitive to the weak echo signals, which are often less than a microvolt. These signals are mixed with the signal from a local oscillator (usually a klystron) to provide an if signal of 30 or 60 mc for broadband if amplification. After detection, the pure video signals are amplified and then applied to an indicator or scope. Most radar receivers also include automatic circuits for local oscillator frequency control and for gain control of incoming echo signals.

Transmitter and receiver are synchronized by a timer, which may be physically associated with the transmitter, receiver or indicator. The timer consists essentially of a controlled master oscillator, which establishes the prf and provides trigger voltages for other elements of a PM radar.

At the indicator, echo signals are measured in terms of the time interval between the transmission of an RF pulse and the reception of its reflected echo. This gives a direct, visual indication of range to an object or target. There is usually some "ground clutter" at ranges close to the radar, and there is usually an intense signal marking the main pulse of the transmitter; but this "interference" is fixed and at very close range.

The azimuth or direction of a target is determined by the angular position of the antenna system.

When the elevation or altitude of a target is required, such data are computed electronically from known information about the range and azimuth.

Improved types of radars have antenna systems that do not move physically. The pulsed RF beam scans vertically and horizontally by electronic means, and the antenna is stationary. This is sometimes called frequency scanning.

Very-long-range PM radar recently developed utilizes ionospheric scatter to detect and locate targets at ranges of several thousand miles. RF pulses - at about 30 mc - are beamed directly at the ionosphere, and bounce back and forth between the earth and reflecting layers until they strike a target in space, such as a missile or aircraft. Echoes return via the same scatter path, eventually reaching the receiver of the PM radar (Fig. 9).

Frequency-Modulated Radars

Instead of transmitting pulses of RF energy, an FM radar transmits a continuous wave varied rapidly and below a reference frequency.

Reflected waves (from objects or targets) arrive at the radar receiver some time after transmission, and thus have a slightly different frequency from that of the RF energy being broadcast at the moment of reception.

A comparison between the transmitted wave and the reflected wave at any instant provides a difference frequency. Knowing the speed of RF energy and knowing the rate at which the transmitted frequency is varied, electronic measurement of the difference frequency is a means of accurately measuring the range to a reflecting object or target.

Azimuth or direction of a target is determined conventionally, according to the physical position of the antenna system with respect to the target.

When either the FM radar or a target is moving with respect to the other, the frequency of the reflected wave will also be subject to the Doppler effect. This effect causes a further change in the frequency of the reflected wave, and provides a means of determining the speed or movement of the radar with respect to the target, or of the target with respect to the FM radar.

The FM radar uses most of the units already described for the PM type, and the block diagram of Fig. 8 - with slight modifications - could be used to picture an FM radar. The local oscillator in the FM type is connected to the detector as well as the mixer, and the amplifier need not be a video type, since no sharp pulses are handled. As will be explained below, two antennas are used, and the microwave transmitter has a direct connection to the mixer instead of the connection between timer and indicator of the PM radar.

A microwave transmitter - usually a magnetron operates continuously at a reference frequency of 500 to 1,000 mc or higher. A timer controls variations in this frequency. Unlike a PM radar, FM radar is not required to handle a high peak power output during transmission of RF energy.

Since an FM radar transmits and receives signals simultaneously, separate antennas are used for transmission and reception. When an FM radar is used as an altimeter, an antenna is located on each wing of the aircraft.

Low-reactance (large-diameter) half-wave dipoles are widely used for antennas at about 500 mc. Four- and five-element Yagi arrays have also been used. At operating frequencies above 1,000 mc, parabolic-cylinder reflectors are used with half-wave dipoles. Also used are slot type antennas which, for airborne installations, are mounted flush with the outer surface of an aircraft.

Reflected waves from the receiving antenna are fed to the mixer, which also accepts an attenuated signal directly from the microwave transmitter. The mixer is a balanced detector, composed of triodes (for 500-mc operation) or crystal diodes and waveguide circuits (for 1,000 mc and higher).

By combining these two input frequencies, the mixer produces a difference frequency, which is a verying low-frequency, or if signal. This signal passes through several if linear amplifiers. After detection, the signal is amplified and analyzed. Changes in the frequency of this signal represent measurements of range to an object or target. Some form of spectrum analyzer is used to differentiate between signals received simultaneously from several targets.

Indicators for FM radar may be any kind of scope (Fig. 5). When the radar is used as an altimeter, meters are used to simplify reading by pilots (Fig. 10). As an altimeter, the range or distance to ground - the plane's actual altitude - is measured.

Unlike PM radar, there is no realistic minimum range for FM radar. In practice, an airborne altimeter can determine distances as short as a few feet. Additionally, FM radar provides greater range precision and is more compact than the PM types.

The basic block diagram shows in Fig. 8 omits much of the ingenious and complicated circuitry included in an effort to distinguish between target signals and "noise" interference. These techniques are associated with such high-sounding terminology as "information theory" and "correlation of data." Despite these advances and improvements, there is still great difficulty in telling one target signal from another and, particularly, in distinguishing moving targets. This has led to the development of a new and advanced kind of equipment known as the pulse-Doppler or PD radar, which will be discussed in the next part of this article. To Be Continued

Posted July 6, 2023