RadarLab

Tracking radar systems are used to measure the target’s relative position in range, azimuth angle, elevation angle, and velocity. A typical tracking radar has a pencil beam to receive echoes from a single target and track the target in angle, range, and/or doppler.Its resolution cell defined by its antenna beamwidth, transmitter pulse length, and/or doppler bandwidth is usually small compared with that of a search radar and is used to exclude undesired echoes or signals from other targets, clutter, and countermeasures.The primary output of a tracking radar is the target location determined from the pointing angles of the beam and position of its range-tracking gates. The angle location is the data obtained from synchros or encoders on the antenna tracking axes shafts (or data from a beam positioning computer of an electronic-scan phased array radar). In some cases, tracking lag is measured by converting tracking-lag-error voltages from the tracking loops to units of angle.

The principal applications of tracking radar are weapon control and missile range instrumentation. In both applications a high degree of precision and an accurate prediction of the future position of the target are generally required. The earliest use of tracking radar was in gunfire control. The azimuth angle, the elevation angle, and the range to the target were measured, and from the rate of change of these parameters the velocity vector of the target was computed and its future position predicted.

Tracking techniques can be divided into range/velocity measurement and measurement. It is also customary to distinguish between continuous single-target tracking radars and multi-target track-while-scan (TWS) radars. Tracking radars utilize pencil beam (very narrow) antenna patterns. It is for this reason that a separate search radar is needed to facilitate target acquisition by the tracker. Still, the tracking radar has to search the volume where the target’s presence is suspected. For this purpose, tracking radars use special search patterns, such as helical, T.V. raster, cluster, and spiral patterns.

Range is the distance from the radar site to the target measured along the line of sight. A pulsed radar determines radar range, R, by measuring the time interval, t, between the transmitted and received signal: \begin{equation} R=\frac{ct} {2} \end{equation}

Target range is measured by estimating the round-trip delay of the transmitted pulses. The process of continuously estimating the range of a moving target is known as range tracking. Since the range to a moving target is changing with time, the range tracker must be constantly adjusted to keep the target locked in range. This can be accomplished using a split gate system, where two range gates (early and late) are utilized.

In early radars, the receive time was measured by observing the pulse return on a display, such as an A scope. Some later radars use automatic range measurement,using two contiguous range gates, called Early-late gates or split-gate tracker. When the energy in the two gates is equal, the crossover time between the gates is at the center of the received pulse. Many modern radars sample the received signal and determine the target range by fitting the sample data to a replica of the pulse. The range measurement of a target is obtained, in principle, by locating the center of the received pulse.The optimal estimation is achieved by passing the pulse received in the matched filter, calculating the derivative of the output signal and detecting the zero crossing instant.The derivative operation is usually replaced with an operation called Early-Late Gate.

The concept of split gate tracking is illustrated in Figure 2, where a sketch of a typical pulsed radar echo is shown in the figure. The early gate opens at the anticipated starting time of the radar echo and lasts for half its duration. The late gate opens at the center and closes at the end of the echo signal. For this purpose, good estimates of the echo duration and the pulse centertime must be reported to the range tracker so that the early and late gates can be placed properly at the start and center times of the expected echo. This reporting process is widely known as the “designation process.” The early gate produces positive voltage output while the late gate produces negative voltage output. The outputs of the early and late gates are subtracted, and the difference signal is fed into an integrator to generate an error signal. If both gates are placed properly in time, the integrator output will be equal to zero. Alternatively, when the gates are not timed properly, the integrator output is not zero, which gives an indication that the gates must be moved in time, left or right depending on the sign of the integrator output.

Angle tracking is concerned with generating continuous measurements of the target’s angular position in the azimuth and elevation coordinates. The accuracy of early generation angle tracking radars depended heavily on the size of the pencil beam employed. Most modern radar systems achieve very fine angular measurements by utilizing monopulse tracking techniques. Tracking radars use the angular deviation from the antenna main axis of the target within the beam to generate an error signal. This deviation is normally measured from the antenna’s main axis. The resultant error signal describes how much the target has deviated from the beam main axis. Then, the beam position is continuously changed in an attempt to produce a zero error signal. If the radar beam is normal to the target (maximum gain), then the target angular position would be the same as that of the beam. In practice, this is rarely the case. In order to be able to quickly achieve changing the beam position, the error signal needs to be a linear function of the deviation angle. It can be shown that this condition requires the beam’s axis to be squinted by some angle (squint angle) off the antenna’s main axis.

Sequential lobing is one of the first tracking techniques that was utilized by the early generation of radar systems. Sequential lobing is often referred to as lobe switching or sequential switching. It has a tracking accuracy that is limited by the pencil beam width used and by the noise caused by either mechanical or electronic switching mechanisms. However, it is very simple to implement. The pencil beam used in sequential lobing must be symmetrical (equal azimuth and elevation beam widths).

Tracking is achieved (in one coordinate) by continuously switching the pencil beam between two predetermined symmetrical positions around the antenna’s Line of Sight (LOS) axis. Hence, the name sequential lobing is adopted. The LOS is called the radar tracking axis, as illustrated in Figure 5. As the beam is switched between the two positions, the radar measures the returned signal levels. The difference between the two measured signal levels is used to compute the angular error signal. For example, when the target is tracked on the tracking axis, as the case in Figure 3(a), the voltage difference is zero and, hence, is also the error signal. However, when the target is off the tracking axis, as in Figure 3(b), a nonzero error signal is produced. The sign of the voltage difference determines the direction in which the antenna must be moved. Keep in mind, the goal here is to make the voltage difference be equal to zero.

In order to obtain the angular error in the orthogonal coordinate, two more switching positions are required for that coordinate. Thus, tracking in two coordinates can be accomplished by using a cluster of four antennas (two for each coordinate) or by a cluster of five antennas. In the latter case, the middle antenna is used to transmit, while the other four are used to receive.

Conical scanning is a system used in early radar units to improve their accuracy, as well as making it easier to steer the antenna properly to point at a target. Conical scanning is similar in concept to the earlier lobe switching concept used on some of the earliest radars, and many examples of lobe switching sets were modified in the field to conical scanning during World War II, notably the German Würzburg radar. Antenna guidance can be made entirely automatic, as in the American SCR-584. Potential failure modes and susceptibility to deception jamming led to the replacement of conical scan systems with monopulse radar sets.

Conical scanning addresses this problem by “moving” the radar beam slightly off center from the antenna's midline, and then rotating it. Given an example antenna that generates a beam of 2 degrees width – fairly typical – a conical scanning radar might move the beam 1.5 degrees to one side of the centerline by offsetting the feed slightly. The resulting pattern, at any one instant in time, covers the midline of the antenna for about 0.5 degrees, and 1.5 degrees to the side. By spinning the feed horn with a motor, the pattern becomes a cone centered on the midline, extending 3 degrees to the sides in our example Figure 4.

The key concept is that a target located at the midline point will generate a constant return no matter where the lobe is currently pointed, whereas if it is to one side it will generate a strong return when the lobe is pointed in that general direction and a weak one when pointing away. Additionally the portion covering the centerline is near the edge of the radar lobe, where sensitivity is falling off rapidly. An aircraft centered in the beam is in the area where even small motions will result in a noticeable change in return, growing much stronger along the direction the radar needs to move. The antenna control system is arranged to move the antenna in azimuth and elevation such that a constant return is obtained from the aircraft being tracked. Whilst use of the lobe alone might allow an operator to “hunt” for the strongest return and thus aim the antenna within a degree or so in that “maximum return” area at the center of the lobe, with conical scanning much smaller movements can be detected, and accuracies under 0.1 degree are possible.

Conical scan, where the antenna scans a small cone around the target position.Lobing technique was extended to continuous rotation of the beam around the target (conical scan) as in Figure 5. Angle-error-detection circuitry is provided to generate error voltage outputs proportional to the tracking error and with a phase or polarity to indicate the direction of errors. The error signal actuates a servosystem to drive the antenna in the proper direction to reduce the error to zero.Because of the rotation of the squinted beam and the target’s offset from the rotation axis, the amplitude of the echo signal will be modulated at a frequency equal to the beam rotation frequency.

The amplitude of the modulation depends on angular distance between the target direction and the rotation axis. žThe location of the target in two angle coordinates determines the phase of the conical scan modulation relative to conical scan beam rotation. Continuous beam scanning is accomplished by mechanically moving the feed of an antenna since the antenna beam will move off axis as the feed is moved off the focal point. The feed is typically moved in a circular path around the focal point, causing a corresponding movement of the antenna beam in a circular path around the target. A typical block diagram is shown in Figure 6. A range tracking system is included which automatically follows the target in range, with range gates that turn on the radar receiver only during the time when the echo is expected from the target under track. Range gating excludes undesired targets and noise. The system also includes an automatic gain control (AGC) necessary to maintain constant angle sensitivity (volts of error-detector output per degree of error) independent of the amplitude of the echo signal. This provides the constant gain in the angle-tracking loops necessary for stable angle tracking.

AGC has the purpose of maintaining constant angle error sensitivity in spite of amplitude fluctuations or changes of the echo signal due to change in range. It is also important for avoiding saturation by large signals which could cause the loss of the scanning modulation and the accompanying error signal. Monopulse Elaboration

The radar video output contains the angle-tracking-error information in the envelope of the pulses, as shown in Figure 7. The percentage modulation is proportional to the angle-tracking error, and the phase of the envelope function relative to the beam-scanning position contains direction information. Angle tracking-error detection (error demodulation) is accomplished by a pair of phase detectors using a reference input from the scan motor. The phase detectors perform essentially as dot-product devices with sine-wave reference signals at the frequency of scan and of proper phases to obtain elevation error from one and azimuth error from the other.

A major parameter in a conical-scan radar is the size of the circle to be scanned relative to the beamwidth. Figure 8 shows a circle representing the 3 dB contour of the beam at one position of its scan. The half-power beamwidth is QB. The dashed circle represents the path described by the center of the beam as it is scanned. The radius of the dashed circle is β, the offset angle.

Amplitude comparison monopulse tracking is similar to lobing in the sense that four squinted beams are required to measure the target’s angular position. The difference is that the four beams are generated simultaneously rather than sequentially. For this purpose, a special antenna feed is utilized such that the four beams are produced using a single pulse, hence the name monopulse.Finally, in sequential and conical lobing variations in the radar echoes degrade the tracking accuracy; however, this is not a problem for monopulse techniques since a single pulse is used to produce the error signals. Monopulse tracking radars can employ both antenna reflectors as well as phased array antennas.

Figure 9 shows a typical monopulse antenna pattern. The four beams A, B, C, and D represent the four conical scan beam positions. Four feeds, mainly horns, are used to produce the monopulse antenna pattern. Amplitude monopulse processing requires that the four signals have the same phase and different amplitudes.

A good way to explain the concept of amplitude monopulse technique is to represent the target echo signal by a circle centered at the antenna’s tracking axis, as illustrated by Figure 10(a), where the four quadrants represent the four beams. In this case, the four horns receive an equal amount of energy, which indicates that the target is located on the antenna’s tracking axis. However, when the target is off the tracking axis (Figure. 10(b-d)), an unbalance of energy occurs in the different beams.

This unbalance of energy is used to generate an error signal that drives the servo-control system. Monopulse processing consists of computing a sum Σ and two difference Δ (azimuth and elevation) antenna patterns. Then by dividing a Σ channel voltage by the Δ channel voltage, the angle of the signal can be determined.

Monopulse comparator is a part of receive antenna. Therefore, insertion loss is paramount importance, since it impacts the system NF. Size is important, since it is integrated to the phased array. Microstrip is preferred, although waveguide is not too bad. The radar continuously compares the amplitudes and phases of all beam returns to sense the amount of target displacement off the tracking axis. It is critical that the phases of the four signals be constant in both transmit and receive modes. For this purpose, either digital networks or microwave comparator circuitry are utilized.

An amplitude-comparison monopulse feed is designed to sense any lateral displacement of the spot from the center of the focal plane. A monopulse feed using the four-horn square, for example, would be centered at the focal point. It provides a symmetry so that when the spot is centered equal energy falls on each of the four horns. However, if the target moves off axis, causing the spot to shift, there is an unbalance of energy in the horns. The radar senses the target displacement by comparing the amplitude of the echo signal excited in each of the horns. This is accomplished by use of microwave hybrids to subtract outputs of pairs of horns, providing a sensitive device that gives signal output when there is an unbalance caused by the target being off axis. The RF circuitry for a conventional four-horn square (Figure 11) subtracts the output of the left pair from the output of the right pair to sense any unbalance in the azimuth direction. It also subtracts the output of the top pair from the output of the bottom pair to sense any unbalance in the elevation direction.

The Figure 11 comparator is the circuitry which performs the addition and subtraction of the feedhorn outputs to obtain the monopulse sum and difference signals. It is illustrated with hybrid-T or magic-T waveguide devices. These are fourport devices which, in basic form, have the inputs and outputs located at right angles to each other. However, the magic Ts have been developed in convenient “folded” configurations for very compact comparator packages.The subtracter outputs are called difference signals, which are zero when the target is on axis, increasing in amplitude with increasing displacement of the target from the antenna axis. The difference signals also change 180° in phase from one side of center to the other. The sum of all four horn outputs provides a reference signal to allow angle-tracking sensitivity (volts per degree error) even though the target echo signal varies over a large dynamic range. AGC is necessary to keep the gain of the angle-tracking loops constant for stable automatic angle tracking.

A simplified monopulse radar block diagram is shown in Figure 12. The sum channel is used for both transmit and receive. In the receiving mode the sum channel provides the phase reference for the other two difference channels. Range measurements can also be obtained from the sum channel. In order to illustrate how the sum and difference antenna patterns are formed, we will assume $\frac{sinϕ} {ϕ}$ a single element antenna pattern and squint angle $ϕ_{0}$ . The sum signal in one coordinate (azimuth or elevation) is then given by

\begin{equation} ∑(ϕ) = \frac{sin(ϕ – ϕ_{0})} {(ϕ – ϕ_{0})} + \frac{sin(ϕ + ϕ_{0})} {(ϕ + ϕ_{0})} \end{equation}

and a difference signal in the same coordinate is

\begin{equation} Δ(ϕ) = \frac{sin(ϕ – ϕ_{0})} {(ϕ – ϕ_{0})} - \frac{sin(ϕ + ϕ_{0})} {(ϕ + ϕ_{0})} \end{equation}

The sum signal, elevation difference signal, and azimuth difference signal are each converted to intermediate frequency (IF), using a common local oscillator to maintain relative phase at IF. The IF sum-signal output is detected and provides the video input to the range tracker. The range tracker determines the time of arrival of the desired target echo and provides gate pulses which turn on portions of the radar receiver only during the brief period when the desired target echo is expected. The gated video is used to generate the dc voltage proportional to the magnitude of the ∑ signal or |∑| for the AGC of all three IF amplifier channels. The AGC maintains constant angle-tracking sensitivity (volts per degree error) even though the target echo signal varies over a large dynamic range by controlling gain or dividing by |∑|. AGC is necessary to keep the gain of the angle-tracking loops constant for stable automatic angle tracking.

The three-channel amplitude-comparison monopulse tracking radar is the most commonly used monopulse system. The three signals may sometimes be combined in other ways to allow use of a two-channel or even a single-channel IF system.

Phase comparison monopulse is similar to amplitude comparison monopulse in the sense that the target angular coordinates are extracted from one sum and two difference channels. The main difference is that the four signals produced in amplitude comparison monopulse will have similar phases but different amplitudes; however, in phase comparison monopulse the signals have the same amplitude and different phases. Phase comparison monopulse tracking radars use a minimum of a two-element array antenna for each coordinate (azimuth and elevation), as illustrated in Figure 13. A phase error signal (for each coordinate) is computed from the phase difference between the signals generated in the antenna elements.

A second monopulse technique is the use of multiple antennas with overlapping (non squinted) beams pointed at the target. Interpolating target angles within the beam is accomplished, as shown in Figure 13, by comparing the phase of the signals from the antennas (for simplicity a single-coordinate tracker is described). If the target were on the antenna boresight axis, the outputs of each individual aperture would be in phase. As the target moves off axis in either direction, there is a change in relative phase. The amplitudes of the signals in each aperture are the same so that the output of the angle error phase detector is determined by the relative phase only. The phase detector circuit is adjusted with a 90° phase shift in one channel to give zero output when the target is on axis and an output increasing with increasing angular displacement of the target with a polarity corresponding to the direction of error.

Consider Figure 13 since the angle α is equal to $ϕ + \frac{π}{2}$, it follows that

\begin{equation} {R_1}^2 = R^2 + (\frac{d}{2}^2) - 2\frac{d}{2}Rcos(ϕ + \frac{π}{2}) \end{equation}

\begin{equation} =R^2 + \frac{d^2}{4} - dRsinϕ \end{equation}

and since d « R we can use the binomial series expansion to get

\begin{equation} {R_1} ≅ R(1 + (\frac{d}{2R})sinϕ) \end{equation}

Similarly,

\begin{equation} {R_2} ≅ R(1 - (\frac{d}{2R})sinϕ) \end{equation}

The phase difference between the two elements is then given by

\begin{equation} ϕ = \frac{2π}{λ}(R1 – R2) = \frac{2π}{λ}dsinϕ \end{equation}

where λ is the wavelength. The phase difference ϕ is used to determine the angular target location.Note that if ϕ=0, then the target would be on the antenna’s main axis. The problem with this phase comparison monopulse technique is that it is quite difficult to maintain a stable measurement of the off boresight angle ϕ, which causes serious performance degradation. This problem can be overcome by implementing a phase comparison monopulse system as illustrated in Figure 14.

The (single coordinate) sum and difference signals are, respectively, given by

\begin{equation} Δ(ϕ) = {s_1} - {s_2} \end{equation}

where the ${s_1}$ and ${s_2}$ are the signals in the two elements. Now, since and have similar amplitude ${s_1}$ and ${s_2}$ are different in phase by ϕ, we can write

\begin{equation} {s_1} = {s_2}e^{-jϕ} \end{equation}

It follows that

\begin{equation} Δ(ϕ) = {S_2}(1 - e^{-jφ}) \end{equation}

\begin{equation} Σ(ϕ) = {S_2}(1 + e^{-jφ}) \end{equation}

The phase error signal is computed from the ratio $\frac{Δ}{Σ}$. More precisely,

\begin{equation} \frac{Δ}{Σ}$= \frac{1-e^{-jφ}}{1 + e^{-jφ}} = jtan(\frac{ϕ}{2}) \end{equation}

which is purely imaginary. The modulus of the error signal is then given by

\begin{equation} \frac{|Δ|}{|Σ|} = tan(\frac{ϕ}{2}) \end{equation}

This kind of phase comparison monopulse tracker is often called the half-angle tracker.

Figure 15 shows the antenna and receiver for one angular-coordinate tracking by phase-comparison monopulse. Any phase shifts occurring in the mixer and IF amplifier stages cause a shift in the boresight of the system. The disadvantages of phase-comparison monopulse compared with amplitude-comparison monopulse are the relative difficulty in maintaining a highly stable boresight and the difficulty in providing the desired antenna illumination taper for both sum and difference signals. The longer paths from the antenna outputs to the comparator circuitry make the phase-comparison system more susceptible to boresight change due to mechanical loading or sag, differential heating, etc.

In the simultaneous-lobing method of angle estimation, two or more radar receive beams are simultaneously formed by the antenna and processed in parallel receive channels. A single transmit beam covers the angular region to be simultaneously processed on receive. Stacked beams, monopulse, and phase interferometry are all examples of the use of simultaneous lobing for target elevation angle estimation. While very different in implementation for a radar system, the fundamental accuracies of these techniques are all analyzed in a similar fashion. Because the receive beams in this technique are formed and processed simultaneously, the relative phase of the return between receive channels can, if desired, be used to aid the angle extraction accuracy. If it is used, the process is termed phase-coherent or simply coherent, and a close match in phase between receive channels must be maintained.

Monopulse: In general, the term monopulse refers to a radar technique to estimate the angle of arrival of a target echo resulting from a single-pulse transmission by using the amplitude and/or phase samples of the echo in a pair of simultaneously formed receive beams. Historically the term has been associated with the simultaneous generation and processing of a sum receive beam and a difference, or delta, receive beam. These beams are so named because of the early and still common method used to form them, i.e., by adding and subtracting, respectively, the two halves of the antenna aperture. While this method is a relatively inexpensive way to produce a sum-difference beam pair, it is not necessarily the best way from a performance standpoint. Furthermore, it is unnecessarily constraining in many phased array applications, especially where the feeds account for a small fraction of the cost of the total radar. Typically a sum beam may be designed for good detectability and sidelobes. The delta beam is then optimized for accuracy performance, perhaps with other constraints. The defining characteristic of a sum beam is that it has approximately even symmetry about the beam boresight, while a delta beam has approximately odd symmetry about the same boresight. Without loss of generality, the delta beam may be assumed to be adjusted or calibrated to be in phase with the sum beam, in the sense that the ratio of the two patterns is real and odd about the beam boresight versus angle of arrival.

Monopulse techniques are classified according to the manner in which the incident radiation is sensed, i.e., according to antenna and beamforming techniques, and independently according to how the various beams and channels are subsequently processed and combined to produce a target angle estimate. Amplitude comparison monopulse and phase comparison monopulse are categories of antenna-beamforming sensing techniques. In amplitude comparison monopulse, the antenna-beamformer generates a pair of sum and difference beams which, without loss of generality, may be assumed to be in phase, in the sense that their ratio is real. In phase comparison monopulse, two or more antennas or sets of radiating-receiving elements, physically separated in the elevation dimension, are used to generate two beams which have ideally identical patterns except for a phase difference which depends on the angle of incidence of the received target echo. Each of these techniques may be converted to the other, either in concept through mathematical sums and differences or physically through the use of passive RF hybrid combining devices. The fundamental accuracy performance of a phase comparison monopulse system is identical to that of an amplitude comparison monopulse system converted by this method, and vice versa. Therefore, the fundamental accuracy performance is addressed here from the conceptual viewpoint of amplitude comparison monopulse.

There are a variety of ways to implement monopulse processing on a sumdifference beam pair, depicted functionally in Figure 16, some of which have a substantial impact on the fundamental monopulse accuracy performance. In each of these implementations, returns from a single transmission are received in simultaneously formed sum and difference beams and processed coherently. In the full-vector monopulse of Figure 16.a, two complex (I, Q) samples are fully utilized to calculate a complex monopulse ratio statistic. This calculated statistic, the measured monopulse ratio, provides the basis for a computer table lookup of the target angle of arrival relative to the null in the delta beam. The computer lookup function is simply a tabulated version of the assumed monopulse ratio consisting of the assumed delta beam antenna pattern to that of the assumed sum beam versus angle off-beam boresight. The tabulated monopulse ratio is inverted in the lookup process by entering the table with the measured monopulse ratio and finding the corresponding off-boresight angle. The full-vector monopulse processing in Figure 16.b differs somewhat from that in Figure 16.a, in that after low-noise amplification to establish the system noise figure, an RF quadrature hybrid device is used to combine the delta and sum beam signals 90° out of phase, i.e., as Σ + jΔ. The purpose of this combining in the difference channel is to bring the signal strength in the difference channel to approximately the same amplitude at that in the sum channel. This causes unavoidable receiver nonlinearities to have nearly the same effect in the two channels, resulting in less degradation in accuracy performance attributable to receive-string nonlinearities. In the absence of nonlinearities, the two techniques in Fig. 16.a and b are mathematically identical because

\begin{equation} Im(\frac {Σ + jΔ} {Σ}) = Im|\frac{Δ}{Σ}|cosϕ) \end{equation}

Hence, they both provide the fundamental accuracy performance of full-vector monopulse processing, given by

\begin{equation} rmse = \frac{||{W_Δ} - f{W_Σ}||}{|f|(2x)^{\frac{1}{2}}} \end{equation}

The fundamental accuracy performance of amplitude-only monopulse processing is degraded at boresight by the probability of incorrect phase detection, i.e., the probability of deciding that the target is below boresight when it is actually above, or vice versa. This probability is 0.5 at beam boresight, which results in boresight fundamental accuracy which is a factor of 2 worse than that of full vector monopulse. At off-boresight angles, the phase detection error probability depends on the signal-to-noise ratio. At angles far from the beam boresight, the signal-to-noise ratio diminishes, causing the error probability again to approach 0.5. A minimum-error probability-maximum-accuracy condition is reached for intermediate angles.

The last monopulse implementation illustrated (Figure 16) is termed phase only monopulse. This processing is to be distinguished from the technique of phase interferometry, which has also been called by some authors phase comparison monopulse. In Figure 16.d, RF or IF hybrids are used to combine the sum and delta channels in quadrature, i.e., with a 90° phase shift. An accurate phase detector then detects the phase difference between the two channels. The underlying principle is that this phase difference will be in one-to-one correspondence with the delta-to-sum ratio, as illustrated in the vector diagram accompanying Figure. 16.d. In phase-only monopulse, off-boresight accuracy is sacrificed to gain the benefit of identical amplitude signals in the two receiver-processor channels.