By transmitting radio waves and listening for their echoes, a radar can detect objects day or night and in all kinds of weather. By concentrating the waves into a narrow beam, it can determine direction. And by measuring the transit time of the waves, it can measure range.

Having reviewed the basic radar concepts, we move on now to the practical consideration of their implementation. While there is an endless variety of radar designs, we can get a rough idea of what is involved by considering three generic radars. First is a radar of the sort used by the all-weather interceptors of the 1950s and 1960s, called simply a 'pulsed' radar. In different configurations, it still is used today. The second generic type is a far more capable one, called a 'pulse-doppler' radar. It is the kind used in the current generation of conventional fighter and attack aircraft. In various forms, it too has a variety of applications. The third generic type is a pulse-doppler radar tailored to meet the special requirements of stealth aircraft.

Having become acquainted with the basic radar principles and approaches to their implementation, in this chapter we'll briefly look at representative practical uses of airborne radar. Some of these - such as air-to-air collision avoidance, ice patrol, and search and rescue are primarily civil applications. Others - such as early warning and missile guidance - are military. Still others - such as storm avoidance and windshear warning - are both.

Since radio waves and alternating current (ac) signals are vital to all radar functions, any introduction to radar logically begins with them. Indeed, many radar concepts which at first glance may appear quite difficult are simple when viewed in the light of a rudimentary knowledge of radio waves and ac signals. In this chapter we will consider the nature of radio waves and their fundamental qualities.

One of the most powerful tools of the radar engineer - and certainly the simplest - is a graphic device called the phasor. Though no more than an arrow, the phasor is the key to a nonmathematical understanding of a great many seemingly esoteric concepts encountered in radar work: the formation of real and synthetic antenna beams, sidelobe reduction, the time-bandwidth product, the spectrum of a pulsed signal, and digital filtering, to name a few. Unless you are already skilled in the use of phasors, don't yield to the temptation to skip ahead to chapters 'about radar.' Having mastered the phasor, you will be able to unlock the secrets of many intrinsically simple physical concepts which otherwise you may find yourself struggling to understand. This chapter briefly describes the phasor. To demonstrate its application, the chapter goes on to use phasors to explain several basic concepts which are, themselves, essential to an understanding of material presented in later chapters.

This chapter presents the origin and conversion of the decibels. Decibels or dB, as it is called - is one of the most widely used tools of those who design and build radars. Decibels are commonly used to express gains and losses. Gain is output divided by input. Loss is input divided by output.

A primary consideration in the design of virtually every radar is the frequency of the transmitted radio waves - the radar's operating frequency. How close a radar may come to satisfying many of the requirements imposed on it - detection range, angular resolution, doppler performance, size, weight, cost, etc. - often hinges on the choice of radio frequency. This choice, in turn, has a major impact on many important aspects of the design and implementation of the radar. In this chapter, we will survey the broad span of radio frequencies used by radars and examine the factors which determine the optimum choice of frequency for particular applications.

In this chapter, the author discusses how the energy radiated by an antenna is distributed in angle and examine the salient characteristics of the radiation pattern - beamwidth, gain, and sidelobes. It was then shown how the sidelobes may be reduced; how fast, versatile beam positioning may be accomplished with electronic scanning; and how high angular resolution and angular measurement accuracy may be achieved. Finally, how the beam may be optimized for ground mapping is discussed.

Radars are of two general types: continuous wave-called CW-and pulsed. A CW radar transmits continuously and simultaneously listens for the reflected echoes. A pulsed radar, on the other hand, transmits its radio waves intermittently in short pulses, and listens for the echoes in the periods between transmissions. Pulsed radars fall into two categories: (1) those that sense doppler frequencies and (2) those that do not. The former have come to be called pulse-doppler radars; the latter, simply pulsed radars. Here, though, pulsed will be used in a general sense to refer to any radar that transmits pulses. In this chapter, the authors will consider the advantages of pulsed transmission, characteristics of the pulsed waveform, and effects of pulsed transmission on transmitted power and energy.

Generally, few things are of more fundamental concern to both designer and user alike than the maximum range at which a radar can detect targets. In this chapter, we will learn what determines that range. We will begin by tracking down the sources of the electrical background noise against which a target's echoes must ultimately be discerned and finding what can be done to minimize the noise. We will then trace the factors upon which the strength of the echoes depends and examine the detection process. Finally, we'll see how, by integrating the return from a great many transmitted pulses, a radar can pull the weak echoes of distant targets out of the noise. Since radio waves of the frequencies used by airborne radars travel essentially in straight lines, a target must be within the line of sight to be detected. Range may be further limited by clutter or man-made interference. Ultimately, it is determined by the signal-to-noise energy ratio.

In the last chapter, we learned that within the line of sight, in the absence of interference and competing ground return, detection range is ultimately determined by the ratio of the energy received from a target-the signal-to the energy of the background noise. We identified the principal factors which determine the signal and noise energies and became acquainted with the detection process. Building on that knowledge, in this chapter we will write a general equation for maximum detection range and analyze it to see how the individual factors we have identified influence the range. We will then narrow down to the special case of volume search. Finally, we will consider the statistical variation in detection range and see how it is accounted for.

By far the most widely used method of range measurement is pulse delay ranging. It is simple and can be extremely accurate. But since there is no direct way of telling for sure which transmitted pulse a received echo belongs to, the measurements are, to varying degrees, ambiguous. In this chapter, we will look at pulse delay ranging more closely - learn how target ranges are actually measured and consider the nature of the ambiguities. We will see how ambiguities may be avoided at low PRFs, and resolved at higher PRFs. We will then consider ambiguities of a secondary type, called 'ghosts,' and see how these may be eliminated. Finally, we will look briefly at how range is measured during single-target tracking.

Ideally, if we wanted both long detection range and fine range resolution, we would transmit extremely narrow pulses of exceptionally high peak power. But there are practical limits on the level of peak power one can use. To obtain long detection ranges at PRFs low enough for pulse delay ranging, fairly wide pulses must be transmitted. One solution to this dilemma is pulse compression. That is, transmit internally modulated pulses of sufficient width to provide the necessary average power at a reasonable level of peak power; then, 'compress' the received echoes by decoding their modulation. This chapter explains the two most common methods of coding - linear frequency modulation and binary phase modulation. It also briefly describes a third method, polyphase modulation.

With FM ranging, the time lag between transmission and reception is converted to a frequency shift. By measuring this shift, the range is determined. Typically, the transmitter frequency is changed at a constant rate. The change is continued over a considerable period of time so the frequency difference can be accurately measured.

In this chapter, the authors look at the Doppler shift more closely-first, in terms of the compression or expansion of wavelength and, second, in terms of the continuous shift of phase. It was then pointed out the factors which determine the Doppler frequencies of the return from both aircraft and the ground. Finally, the special case of the Doppler shift of a target's echoes as observed by a semiactive missile was considered.

Transmitting a radio frequency signal in pulses markedly changes in the signal's spectrum, as observed by the tuned circuit of a receiver. Whereas the spectrum of a continuous wave of constant wavelength consists of a single line, the spectrum of a single pulse of the same wavelength covers a band of frequencies and has a sin x/x shape. The width of the central lobe of this spectrum varies inversely with pulse width. If the pulses are as narrow as those used in many radars, the central lobe may be several megahertz wide.

This chapter gives the reasons. It begins by raising the fundamental question of exactly what is meant by the spectrum of a signal. This, as you'll see, is actually the crux of the matter. The chapter then explains the spectrum of a pulsed signal in two quite different ways: first, in terms of a conceptually simple but powerful analytical tool, called the Fourier series, and second, in terms of what physically takes place when a radio frequency signal passes through a lossless narrowband filter. For those readers who have some familiarity with calculus, the essence of both explanations is presented in more precise, mathematical terms - the Fourier transform - at the end of the chapter.

To sort out the radar return from various objects according to doppler frequency, the receiver output is applied to a bank of narrowband filters. If sorting by range is also desired, a separate bank is provided for each range increment. The width of the passband of a narrowband filter is primarily determined by the filter's integration time but is increased by losses. So that return will not be lost when a target straddles two filters, the passbands are made to overlap. So that only one line of a target's spectrum will fall within the band of frequencies bracketed by the bank, the passband of the bank is made no greater than the PRF.

In the preceding chapter we saw how, for digital filtering, the radar returns are translated to video frequencies by a pair of synchronous detectors and sampled at precisely timed intervals. And we learned how the samples are converted to digital numbers. We were told that the numbers are then supplied to a computer (signal processor), which 'forms' a separate bank of doppler filters for each sampling interval (range gate). But little was said about the way in which the filters are formed. In this chapter, we will learn how that is done. After briefly reviewing what the stream of numbers supplied to the computer represents, we will derive the simple set of equations (algorithm) which the filter must repeatedly compute to form a filter - the discrete Fourier transform - and see how the required mathematical operations may be organized. Finally, we will briefly consider what can be done to reduce the sidelobes which invariably occur on either side of a filter's passband. The organization of a complete filter bank and the ingenious approach taken to minimizing the otherwise staggering computing load (the fast Fourier transform) are covered in the next chapter.

The FFT is an algorithm which vastly reduces the amount of processing necessary to form a bank of digital filters with the DFT. Its efficiency is achieved primarily by choosing the parameters of the bank so that they are harmonically related and consolidating the formation of the filters into a single multiple-step process. By making the number of filters, N, equal to a power of two and the number of samples summed equal to N, the processing is accomplished in log₂N steps. In the first step, each sample is algebraically summed with one of the other samples. In each succeeding step, certain phase rotations are performed, and each partial sum is algebraically summed with one of the other partial sums.The required phase rotations and pairing of the quantities to be summed in each step can readily be determined mathematically. The basic processing instruction for performing the individual partial summations - consisting of a phase rotation, a complex addition, and a complex subtraction - is called the FFT butterfly. The phase rotations themselves are performed the same way as in the DFT.

In many radar applications, knowing a target's present position (angle and range) relative to the radar is not enough. Often one must be able to predict the target's position at some future time. For that, we must also know the target's angular rate and its range rate. Range rate may be determined by one of two general methods. In the first, called range differentiation, the rate is computed on the basis of the change in the measured range with time. In the second and generally superior method, the radar measures the target's doppler frequency - which is directly proportional to the range rate. In this chapter, we will look at both methods briefly. We will then take stock of potential doppler ambiguities and see how they may be resolved.

In this chapter, the authors will consider what determines the amplitude of the ground return. The authors will then examine the doppler spectrum of each of the three categories of ground return, and the relationship of the composite spectrum to the doppler frequencies of target aircraft in representative situations. Finally, the authors will consider the problem of exceptionally strong sidelobe return reflected by certain objects on the ground.

In the last chapter, the sources of ground return and its acquaintance with its Doppler spectrum was surveyed. The profound effects of range and Doppler ambiguities on ground return was not considered. In this chapter, after briefly considering the dispersed nature of ground clutter, the effects of ambiguities on the range and Doppler profiles for a representative flight situation were examined and the problem of separating target echoes from clutter was shown.

In this chapter, after briefly examining the problem, we will be introduced to two highly effective techniques for detecting such targets. One is called Classical DPCA, for displaced phase center antenna. The other and newer technique are called notching or clutter nulling. We'll take up Classical DPCA first; then, notching; and, finally, a combination of the two. In closing we'll touch on the adaptation of these techniques to precise angle measurement.

Few parameters of a pulsed radar are more important than the PRF. This is particularly true of doppler radars. Other conditions remaining the same, the PRF determines to what extent the observed ranges and doppler frequencies will be ambiguous. That, in turn, determines the ability of the radar not only to measure range and closing rate directly, but to reject ground clutter. In situations where substantial amounts of clutter are encountered, the ability to reject clutter crucially affects the radar's detection capability. In this chapter we will survey the wide range of pulse repetition frequencies employed by airborne radars and see in what regions significant range and doppler ambiguities may occur. We will then take up the three basic categories of pulsed operation - low, medium, and high PRF - and learn what their relative merits are.

At low PRF, mainlobe clutter may largely be eliminated by offsetting the Doppler spectrum so the central line is at dc and passing the return through a clutter canceller and bank of Doppler filters. To keep the clutter in the canceller's rejection notches, the 'offset' must be varied with radar speed and antenna look angle.

In this chapter, we will take a closer look at medium PRF operation. We will see what must be done to separate targets from clutter and how the signal processing is performed. We will then take up the problems of rejecting ground moving targets, eliminating blind zones, minimizing sidelobe clutter, and rejecting sidelobe return from those targets on the ground which have exceptionally large radar cross sections.

Detection range, of course, increases with the ratio of the signal energy to the energy of the background noise and clutter. By employing a high duty factor, high PRF waveform, therefore, long detection ranges can be obtained against nose-aspect targets even in a clutter environment. However, where strong sidelobe clutter is encountered, detection ranges against low-closing-rate (tail-aspect) targets may be impaired because of range ambiguities. In this chapter, we will consider a high duty factor, high PRF waveform, see what must be done to separate targets from ground return, and learn how the signal processing is done. We'll then take up the problem of range measurement, eclipsing loss, and the steps which may be taken to improve performance against low-closing-rate targets.

In the preceding chapters, we became acquainted with various approaches to target detection. In this chapter, we'll take a closer look at the techniques for tracking the targets that are detected: the single-target track (STT) and track-while-scan (TWS) modes introduced in Chap. 2.

An increasingly important airborne radar application is making radar maps of sufficiently fine resolution that topographic features and objects on the ground can be recognized. In this chapter, the authors discuss how ground map resolution is defined and see what the optimum resolution is for various uses; then, review the approaches taken to providing it.

In this chapter, the authors examine the SAR principles more closely and become acquainted with the basic digital processing techniques. The authors see (1) how the equivalent of a long array antenna may be synthesized from returns gathered over a period of several seconds by a comparatively small real antenna, (2) how the array may be focused, (3) what determines the angular resolution of such an array, and (4) how the computing load can be reduced by processing the returns with doppler filtering techniques.

In the last chapter, we saw how SAR takes advantage of a radar's forward motion to synthesize a very long linear array from the returns received over a period of up to several seconds by a small real antenna. We learned how the array may be focused at virtually any desired range and how the immense amount of computing required for digital signal processing may be dramatically reduced through doppler filtering techniques. In this chapter, we will consider certain critical aspects of SAR design which, if not properly attended to, may seriously degrade the quality of the maps or perhaps even render them useless: selection of the optimum PRF, sidelobe reduction, compensation for phase errors resulting from deviation of the radar bearing aircraft from a perfectly straight constant - speed course - called motion compensation - and the minimization of other phase errors.

Operationally, SAR has several striking advantages. First, with a small physical antenna operating at wavelengths suitable for long-range mapping, SAR can provide azimuth resolutions as fine as a foot or so. Second, by increasing the length of the array in proportion to the range of the area to be mapped, the resolution can be made independent of range. Third, since the array is formed in the signal processor, the basic SAR technique can conveniently be adapted to a wide variety of operational requirements.

In this chapter, we will be introduced to the six basic types of countermeasures - chaff, noise jamming, false targets, gate stealers, angle deception, and decoys. We will see how each is used, and learn how it is implemented and what its limitations are. Chaff, the simplest of all ECM, can screen an entire raid from radars operating over a wide range of frequencies. But moving-target indication rejects chaff return.

In the previous chapter, we examined the principal types of electronic countermeasures (ECM). We learned how each type is implemented and what its limitations are. In this chapter, we will examine some of the important electronic counter-countermeasures (ECCM) which have been devised to exploit the limitations of ECM and so defeat them. We will begin by examining the conventional techniques for combating noise jamming, gate stealing, and angle deception. We will then look at some significant advanced ECCM developments which promise quantum jumps in a radar's ability to contend with severe noise jamming, as well as with various other ECM.

Effective employment of both ECM and ECCM depends on the ability of (a) ELINT to determine the capabilities of the radars of potential hostile forces, (b) the ESM system to determine the electronic order of battle, and (c) the ability of the RWRs in the individual aircraft to detect the RF emissions of any enemy system that threatens the aircraft, identify the sources of the emissions, and determine optimum responses.

Electronically steered array antennas, ESAs, have been employed in surface based radars since the 1950s. But, because of their greater complexity and cost, they have been slow to replace mechanically steered antennas in airborne applications. However, with the advent of aircraft of extraordinarily low radar cross section and the pressing need for extreme beam agility, in recent years avionics designers have given the ESA more attention than virtually any other 'advanced' radar concept. In this chapter, we will briefly review the ESA concept, become acquainted with the two basic types of ESAs, and take stock of the ESA's many compelling advantages, as well as a couple of significant limitations.

This chapter begins by discussing those design considerations common to both passive and active ESAs. It then takes up the considerations pertaining primarily to passive ESAs and, finally, those pertaining solely to active ESAs.

Viewed nose-on, a typical fighter aircraft has a radar cross section (RCS) on the order of one square meter. A similarly viewed low observable aircraft may have an RCS of only 0.01 square meter. Unless special RCS reduction measures are employed, even a comparatively small planar array antenna can have an RCS of up to several thousand square meters when viewed from a broadside direction! Since an aircraft's radome is transparent to radio waves, if stealth is required, steps must be taken to reduce the RCS of the installed antenna. In this chapter, we will be introduced to the sources of reflections from a planar array antenna, learn what can be done to reduce or render them harmless, and see why these steps are facilitated in an ESA. We will then take up the problem of avoiding so-called Bragg lobes, which are retrodirectively reflected at certain angles off broadside if the radiator spacing is too large compared to the radar's operating wavelength. Finally, we will very briefly consider the critically important validation of an antenna's predicted RCS.

The advent of active ESAs, the emergence of low RCS aircraft, and the growing threat of electronic countermeasures have given impetus to advanced work in several key areas of radar technology. This chapter, presents some significant developments spawned by that work: Innovative approaches to multiple-frequency operation-for reducing vulnerability to countermeasures and avoiding detection by the enemy; Advanced signal integration and detection techniques -for small target detection; Bistatic modes of radar operation - for increasing survivability and for circumventing the limitation on power-aperture product imposed by a tactical aircraft's small size; Space-time adaptive processing - for efficiently rejecting external noise and jamming and compensating for the motion - induced clutter spread with which long range surveillance radars must contend; True-time-delay beam steering - a technique still in its infancy which promises to broaden the instantaneous bandwidth of an active ESA sufficiently to enable simultaneous shared use of the same antenna for radar, electronic warfare, and communications; interferometric SAR - for making accurate high-resolution topographic maps.

To enable the resolution of multiple targets at long ranges and to increase detection sensitivity against low-closing-rate targets, a number of new waveforms have been developed. In this chapter the authors will take up three of these: Range-gated high PRF: Pulse burst: Monopulse doppler. The authors will also briefly consider a new search-while-track mode, which takes advantage of the ESA's extreme beam agility. The authors will introduce to a mode-management software architecture for flexibly allocating the radar's resources and ensuring prompt response to high priority requirements in complex tactical situations.

Low probability of intercept (LPI) is the term used for there being a low probability that a radar's emissions will be usefully detected by an intercept receiver in another aircraft or on the ground. For the air battle of the future, LPI is essential. In conventional aircraft the most important need for LPI is to avoid electronic countermeasures. In low observable aircraft, LPI additionally enhances the element of surprise and denies the enemy use of radar intercept queuing of its fighters. In aircraft of both types, LPI prevents successful attacks by antiradiation missiles. In this chapter, we will review the generic types of intercept receivers and see what strategies may be used to defeat them. We'll then take up specific design features which may be incorporated in a radar to ensure a low probability of intercept. Finally, we'll very briefly assess the cost of LPI and consider possible future trends in LPI design.

Having read of the many advanced radar techniques in the offing, processor architecture may seem of little import. But the fact is that most of the advanced capabilities of airborne radars to date have only been made practical by substantial increases in digital processing throughput. In the 1970s, multimode operation was made possible in fighters by replacing the hardwired FFT processor with a programmable signal processor (PSP) having a throughput of around 130 MOPS. In the 1980s, the addition of real-time SAR was made possible by quadrupling processing throughput. In the 1990s, the active ESA and other advanced capabilities of the F-22 were made possible by again quadrupling throughput. Vastly higher throughputs will be needed to make practical some of the advanced radar capabilities currently envisioned. Spread spectrum, for example, is highly desirable for both ECCM and LPI. Yet, even a 500 MHz instantaneous bandwidth will require 500,000 MOPS. In this chapter, we'll examine the key architectural features of the late 1990s-era processors: parallel processing, high throughput density, efficient modular design, fault tolerance, and integrated processing. We'll then take stock of a few technology advances which promise substantial throughput increases in the future.

This book section discussed the different radar system for reconnaisance and surveilance like the APS-145 which is the latest version of the Airborne Early Warning radar for the US Navy's carrier based E-2C Hawkeye. Early versions of the aircraft went into service in 1963. Since then, it and the radar have undergone numerous upgrades. The APY-2 is the radar for the U.S. Air Force E-3 Airborne Early Warning and Control System (AWACS). From an operational altitude of 30,000 ft, the radar can detect low altitude and sea-surface targets out to 215 nmi, coaltitude targets out to 430 nmi, and targets beyond the horizon at still greater ranges. Joint STARS is a long-range, long-endurance, air-to-ground surveillance and battle management system carried aboard the U.S. Air Force E-8C aircraft. Operating at altitudes up to 42,000 feet, the system's high-power pulse-doppler radar is capable of looking deep behind hostile borders from a stand-off position and monitoring fixed and moving targets with a combination of high-resolution SAR mapping and moving-target indication (MTI) vehicle detection and tracking.

The APG-77 is a multimode pulse-doppler radar meeting the air dominance and precision ground attack requirements of the F-22 stealth dual-role fighter. It may be armed with six AMRAAM missiles or two AMRAAMs plus two 1,000 pound GBU-33 glide bombs, two sidewinder IR missiles, and one 20mm multi-barrel cannon-all of which are carried internally for low RCS. Four external stations are also available to carry additional weapons or fuel tanks. At present, very little can be said at an unclassified level about the radar other than that it employs an active ESA, that it incorporates extensive LPI features, and that its signal and data processing requirements are met by a common integrated processor (CIP). The active ESA provides the frequency agility, low radar cross-section, and wide bandwidth required for the fighter's air dominance mission.

The APQ-181 is the multimode pulse-Doppler radar for the B-2 longrange stealth bomber. It employs a low-RCS passive ESA antenna and incorporates advanced LPI features. Except for that and the fact that, like the APQ-164, it gives the aircraft the autonomous ability to navigate safely around hazards and use them to mask defensive systems, very little can yet be said about the radar at an unclassified level.

Longbow is a fast-reaction, low exposure, high-resolution, millimeterwave fire-control radar designed for the AH-D Apache attack helicopter. The radar also provides obstacle warning to alert the pilot to navigation hazards, including man-made structures, towers, etc.

This chapter presents the implementation of synthetic aperture radar in military aircrafts.

The RDR-4B is a pulse-doppler forward-looking weather radar, operating in nearly every type of commercial transport aircraft. Besides the ability to penetrate weather systems and accurately map rainfall and turbulence, it meets all FAA requirements for stand-alone windshear detection. At altitude, the radar provides a detailed ground-clutter-free color weather display of a ±40° forward sector out to a selectable maximum range of up to 320 nmi. Whenever the absolute altitude is less than 2,300 feet, the windshear detection mode is automatically activated on alternate antenna scans.