Monopulse Radar

The pseudo millimeter wave 22-29GHz ultra-wideband (UWB) is attractive for the applications in short-range (SR) automotive radar sensors in order to contribute to significant reduction of traffic fatalities. Although CMOS is suitable for the short-range radar since processing units can be implemented in the same chip with the UWB front-end building block, it is difficult to operate CMOS pulse generators at such a high frequency. To realize the pseudo-millimeter-wave band using CMOS, we have proposed a new pulse generator consisting of a series of delay cells and edge combiners with waveform shaping. As a result of measurement using 90nm CMOS technology, 1Gbps pulses are successfully generated with a power consumption of 1.4mW at a supply voltage of 0.9V. This result will be the key technology for a one-chip SR cost effective radar sensors.

In this paper, we introduce an adaptive phase-comparison monopulse technique for angle estimation of two targets in the same antenna beam. The proposed method determines a more suitable technique(between conventional phase comparison monopulse technique and Zheng's method) based on interference between two targets in Fourier domain. Consequently, regardless of the interference, angles of each individual target can be accurately estimated by means of the proposed method. In simulations, we assumed that two point targets with same velocity are located in the same antenna beam, and the accuracy improvement of the proposed method is verified by using several simulations.

In this paper, a monopulse sensor which determines a target location using amplitude-comparison monopulse technique is presented. This sensor can allow the missile to track the target when additional jamming signals are not presented. Then, we applied the spot noise jamming technique to the monopulse sensor. Based on the simulation results, we can effectively figure out the performances of the spot noise jamming technique for the monopulse sensor in various jamming scenarios.

This paper introduces the inverse synthetic aperture radar(ISAR) imaging technique for rear view target of an automobile, which uses both linear frequency modulation-frequency shift keying(LFM-FSK) waveform and monopulse tracking. LFM-FSK waveform consists of two sequential stepped frequency waveforms with some frequency offset, and thus, can be used to generate ISAR images of rear view target of an automobile . However, ISAR images can often be blurred due to non-uniform change rate of relative aspect angle between radar and target. In order to address this problem, one-dimensional(1-D) Lagrange interpolation technique in conjunction with angle information obtained from the monopulse tracking is applied to generate uniform data across the radar's aspect angle. Simulation results show that the proposed method can provide focused ISAR images.

For highly integrated imaging systems above 100 GHz, the complexity and chip area increases significantly with increasing number of channels. In addition, bulky dielectric lenses prevent applications in spatially restricted surroundings. The presented concept of an imaging monopulse radar with a mechanically flexible front end reduces the required chip area and allows the antenna to be placed in any desired position apart from the sensitive electronics. The radar system is based on a two-channel 160 GHz microwave monolithic integrated circuit (MMIC) feeding a flexible dielectric waveguide. Depending on the angle of the incidence signal, a sum mode (HE11 mode) and a difference mode (HE21 mode) are excited in the dielectric waveguide. MMIC and waveguide are connected by a self-aligning transition reducing the requirements for packaging accuracy. The required chip area of the transition is only 0.022λ 2 with a spacing between the on-chip antennas of λ/4. The measured ambiguity-free region between -18 • and 15 • is defined by the modified elliptical lens antenna focusing in the E-plane only. A mechanical bending of the flexible waveguide is possible down to a radius of at least 2 cm without affecting the angle estimation capability.

A novel synthesis method for holographic multi-feed antennas is presented to combine all sub-holograms into an angle-dependent shared holographic aperture. In order to find the global error minimum between the shared holographic aperture and all ideal sub-holograms, a non-pixel-based genetic optimization is used. For a more accurately implementation of the analytical impedance tensor an eigenvector approach taking all tensor components into account is introduced. The optimized holographic antenna has four feeds for an exemplary integration into a 2D-monopulse radar system and is realized on a fused silica wafer due to its lower loss at millimeter-wave frequencies compared to Teflon-based materials. The antenna prototype provides a measured gain of 23 dBi, a polarization purity of 26 dB and a side-lobe level of 20 dB for feed 1, 2 and 4 is reached across 76 GHz-81 GHz. The S-parameters are measured from 60 GHz to 90 GHz, and a reflection coefficient of -14 dB and an inter-port coupling <-38 dB are achieved. The measured 2D-monopulse patterns provide a FOV of 10 • , the sum beam has a gain of 26 dBi leading to an aperture efficiency of 45 % and the differential beams show a null depth of 30 dB. INDEX TERMS Holographic antenna, metasurface antenna, multi-feed antenna, multi-beam antenna, surface wave, leaky wave, monopulse radar, glass technology.

This paper presents a novel type of waveguide monopulse feeder, which has the concept of a space feed, suitable for liner phased array antenna. The design methods and measurement results of the 2-to-39 port feeder at X-band are described. Measurement results show that this feeder has characteristics of the required optimum 1-D monopulse power distribution, low insertion loss and good input match.

In the past years, the study of microstrip patch antennas has made significant progress because of their miniature size, low cost, compatibility, and ease of manufacture compared to traditional antennas. In this study, different microstrip patch antenna array configurations including 2-element, 4-element and 16-element were studied and designed in the range of frequencies of 0.25-0.55 THz. This study aims to enhance the gain and the radiation properties of these proposed antenna array configurations using air cylinder holes photonic band gap (PBG) substrate instead of the homogeneous substrate. Simulation was conducted with the help of CST Microwave Studio software for the different substrates. To obtain better radiation properties, the proposed antenna arrays have around 0.35 THz resonant frequencies where there exists a low atmospheric attenuation window in the terahertz band. The outcomes demonstrated that the studied antenna arrays based on the photonic band gap substrate provided additional performance in terms of the return loss, bandwidth, gain, directivity and beamwidth compared to their corresponding conventional antenna arrays counterparts based on the homogeneous substrate. Among all designed antenna array configurations, the 16-element antenna array based on the photonic band gap has successfully achieved impressive gain and directivity performance of 16.75 dB and 17.40 dBi, respectively. Finally, the proposed antenna arrays are highly promising candidates for wireless communications, imaging, sensing, medical diagnosis and threat detection.

Monopulse radars operate with simultaneous beams and are used for tracking purposes, relying only on hardware to provide a real-time indication of the target position. A monopulse array and its feeding network is implemented and tested, in the frequency of 5.4 GHz, based on patch antennas using dieletric to provide wide bandwidth and high gain. The system field styrofoam as pattern was evaluated in two different environments, a shielded room and an indoor area. Results agreed with simulation for either individual elements as well as the final integrated monopulse array.

An isolation technique, which does not require conventional circulators, is proposed for the realization of a simple and low-cost aperture-coupled circularly polarized antenna for application to fullduplex devices. The approach is based on the use of slotlines loops to provide surface current cancellation in specific regions of the antenna structure, leading to improved axial ratio and isolation between the ports in excess of 50 dB. Circular polarization is achieved by introducing a double-box hybrid coupler, which is optimized to obtain good matching and isolation of the quadrature signals. On this basis, both right-and lefthand circularly polarized beams are achieved by interchanging the transmitting and receiving antenna ports, enabling full-duplex operation and reconfigurability. While the antenna structure is designed for 2.45 GHz operation, one can take advantage of the proposed approach to tune the frequency of maximum isolation. Both single-element prototypes as well as a 2×2 array are fabricated and measured, showing good agreement with the simulations and validating the proposed isolation approach. The beam steering capabilities as well as the application to a Van Atta retrodirective antenna array and the possibilities of achieving delta and sum patterns for monopulse operation are also reported. The proposed full-duplex antenna can also represent an excellent solution for narrowband wireless power transmission systems.

In the work, the study of the achievable accuracy of determining the coordinates of the radiating in the radio range of spacecraft using a three‑element amplitude correlation direction finder has been performed. The proposed scheme makes it possible to select single and group low‑power complex signals and simultaneously determine the bearings of several sources, the signals of which can be separated in time‑frequency‑code space. As an example, the radiation pattern of a three‑element antenna system with an orthogonal arrangement of partial diagrams with respect to the equal‑signal direction is considered. The results of statistical modeling of the correlation processing algorithm obtained for a three‑channel monopulse system with a 3.7° pattern width and estimates of the potential accuracy of determining the bearing are presented. Simulation results are given to evaluate the instrumental accuracy of the direction finder. The proposed method of direction finding provides the ability ...

Multi-loop retrodirective cross-eye jammers offer the possibility of both simplifying system implementation and improving system performance. However, the signals for each jammer loop propagate along different paths, leading to potentially detrimental effects on system performance. Static and dynamic compensation for pathlength differences are introduced and analysed. Of the two, static compensation is simpler but is only effective for limited engagement geometries. Dynamic compensation is more general but requires an accurate estimate of the engagement geometry.

The effect of path-length differences on multiloop retrodirective cross-eye jammers is evaluated. It is shown that such jammers may act as beacons, and the conditions under which this occurs are investigated for two-loop jammers. The sensitivity of the two-loop crosseye gain to path-length differences is also studied and is found to be small for small path-length differences, but to increase rapidly. The effect of the two-loop crosseye jammer parameters on path-length effects is also considered. Index Terms-Cross-eye jamming, electronic warfare (EW), electronic countermeasures (ECM), and radar countermeasures.

Multi-loop retrodirective cross-eye jammers offer the possibility of both simplifying system implementation and improving system performance. However, the signals for each jammer loop propagate along different paths, leading to potentially detrimental effects on system performance. Static and dynamic compensation for pathlength differences are introduced and analysed. Of the two, static compensation is simpler but is only effective for limited engagement geometries. Dynamic compensation is more general but requires an accurate estimate of the engagement geometry.

The effect of path-length differences on multiloop retrodirective cross-eye jammers is evaluated. It is shown that such jammers may act as beacons, and the conditions under which this occurs are investigated for two-loop jammers. The sensitivity of the two-loop crosseye gain to path-length differences is also studied and is found to be small for small path-length differences, but to increase rapidly. The effect of the two-loop crosseye jammer parameters on path-length effects is also considered. Index Terms-Cross-eye jamming, electronic warfare (EW), electronic countermeasures (ECM), and radar countermeasures.

Experimental measurements that accurately simulate the effect of a retrodirective cross-eye jammer on a monopulse radar are described. The accuracy of a recently published extended analysis of retrodirective crosseye jamming and the limitations of the conventional phase-front analysis of cross-eye jamming are illustrated by the experiments.

Certain radar applications may require maintaining the difference pattern slope and twin beam shape, while main null is scanning in the presence of mutual coupling. It is also desirable in monopulse radar applications to be able to generate acceptable sum and difference patterns using single simplified feed structure. This paper focuses on these problems and provides a solution based on Intelligent 𝑧-Plane Boundary Condition-Particle Swarm Optimiser (IzBC-PSO) to compensate for the difference pattern degradation while scanning a small coupled array (𝑁 = 8). In second case, a simplified feed is proposed that only requires phase flip for 50% of elements to produce sum and difference patterns for the monopulse array, consisting of isotropic elements.

Designing reflector antennas to minimize the effects of spillover and feed blockage requires optimization of accurate numerical models. A FEKO [1] model of a horn-fed parabolic reflector is optimized for maximum directivity using a genetic algorithm (GA).

Traditional monopulse radar cannot resolve two closely spaced targets present in one resolution cell (range and Doppler) by means of the monopulse ratio. This study presents a closed-form solution to resolve the directions of arrival of two unresolved targets using a single snapshot of four independent channels in phase comparison monopulse radar. If both targets have the same elevation or same azimuth direction, the proposed scheme cannot estimate their directions. To estimate the direction of such targets, an extra antenna is required. The impact of input noise power, the targets’ direction, and phase difference of the targets’ signal on the accuracy of angle estimation are also explained. The numerical simulation result validates the effectiveness of the presented scheme.

An extended and rigorous analysis of retrodirective cross-eye jamming in a radar system scenario is presented. This analysis removes the approximations that limit the validity of other analyses of cross-eye jamming. These results imply that under certain conditions, a monopulse radar system can be more easily deceived than suggested by conventional cross-eye analyses. Furthermore, the cross-eye jammer antenna patterns do not affect the induced monopulse error.

The matching required between the two directions through a retrodirective cross-eye jammer is considered using both the traditional phase-front analysis and an extended analysis. The design parameters to achieve a specified tracking error are derived and an optimal design is proposed. The results for the extended analysis show that the tolerances required to induce large angular errors in a monopulse radar are not as strict as the traditional analysis suggests.

The wideband microstrip antenna is a development of the purpose of this study is to design a wideband antenna at a frequency of 1030 MHz-1090 MHz which will be applied to the Monopulse Secondary Surveillance Radar (MSSR) antenna. The study uses a single microstrip antenna using the DGS (Defected Ground Structure) method and the DMS (Defected Microstrip Structure) method, which can increase the antenna bandwidth. From the results of the element simulation, adding the DGS (Defected Ground Structure), and DMS (Defected Microstrip Structure) structures to the single antenna obtained a result of 2.83 dB.

Ice-thickness measurements are needed to calculate fluxes through fast-flowing outlet glaciers in Greenland, Alaska, Patagonia and Antarctica. However, relatively high attenuation of radio waves by dielectric absorption and volume scattering from englacial water hampers detection of the bed through warm deep ice. In the past we have had success measuring ice thickness of temperate glaciers using a ground-based monopulse radar system operating at low frequencies (2 MHz). Here we adapt the same system to operate from an airplane. Test flights over Bering Glacier, Alaska, USA, detected the bed through ice up to 1250m thick. Flights across the Seward–Malaspina Glacier system, Alaska, resolved the ice thickness of Malaspina Glacier, but strong hyperbolic-shaped returns obscured the bed echo through the Seward throat. It is likely that this clutter in the signal was caused by off-nadir returns from chaotic surface crevasses that are ubiquitous in the throat region.

This paper studies the performance of modern guided systems in presence of a proposed deception jamming technique. The proposed jammer uses the intercepted signal of the guided system to deceive the instantaneous amplitude comparison (IAC) monopulse radar, which is used in the most modern guided system. The performance of the modern guided systems in the presence of jamming for a typical case of radar guided missile is discussed for the following guidance methods: proportional navigation, pure pursuit, and constant bearing. It is found that, the guided system that uses pure pursuit guidance method is the most affected by the proposed jamming technique (largest miss tracking distance). Constant bearing guidance method is the lowest method affected by the proposed jamming technique (lowest miss tracking distance). Proportional navigation, that is the most used guidance method, is moderate affected by the proposed jamming technique. This effect is enough to confuse the guided system and miss track the target.

The problem of synthesizing "optimal" sum and difference patterns subject to arbitrary sidelobe bounds by means of a simple feeding network is dealt with. It is shown how a hybrid optimization procedure properly acting on both clustering into subarrays and excitations of these latter greatly improves performances of known feeding architectures with respect to existing procedures. Results of the proposed technique, which significantly outperform existing methods, renew the interest of these simple feeding architectures, and emphasize the effectiveness of this kind of hybrid procedures exploiting the convexity of the problem with respect to a part of the unknowns.

Ice-thickness measurements are needed to calculate fluxes through fast-flowing outlet glaciers in Greenland, Alaska, Patagonia and Antarctica. However, relatively high attenuation of radio waves by dielectric absorption and volume scattering from englacial water hampers detection of the bed through warm deep ice. In the past we have had success measuring ice thickness of temperate glaciers using a ground-based monopulse radar system operating at low frequencies (2 MHz). Here we adapt the same system to operate from an airplane. Test flights over Bering Glacier, Alaska, USA, detected the bed through ice up to 1250m thick. Flights across the Seward–Malaspina Glacier system, Alaska, resolved the ice thickness of Malaspina Glacier, but strong hyperbolic-shaped returns obscured the bed echo through the Seward throat. It is likely that this clutter in the signal was caused by off-nadir returns from chaotic surface crevasses that are ubiquitous in the throat region.

This paper studies the performance of modern guided systems in presence of a proposed deception jamming technique. The proposed jammer uses the intercepted signal of the guided system to deceive the instantaneous amplitude comparison (IAC) monopulse radar, which is used in the most modern guided system. The performance of the modern guided systems in the presence of jamming for a typical case of radar guided missile is discussed for the following guidance methods: proportional navigation, pure pursuit, and constant bearing. It is found that, the guided system that uses pure pursuit guidance method is the most affected by the proposed jamming technique (largest miss tracking distance). Constant bearing guidance method is the lowest method affected by the proposed jamming technique (lowest miss tracking distance). Proportional navigation, that is the most used guidance method, is moderate affected by the proposed jamming technique. This effect is enough to confuse the guided system and miss track the target.

This paper studies the performance of modern guided systems in presence of a proposed deception jamming technique. The proposed jammer uses the intercepted signal of the guided system to deceive the instantaneous amplitude comparison (IAC) monopulse radar, which is used in the most modern guided system. The performance of the modern guided systems in the presence of jamming for a typical case of radar guided missile is discussed for the following guidance methods: proportional navigation, pure pursuit, and constant bearing. It is found that, the guided system that uses pure pursuit guidance method is the most affected by the proposed jamming technique (largest miss tracking distance). Constant bearing guidance method is the lowest method affected by the proposed jamming technique (lowest miss tracking distance). Proportional navigation, that is the most used guidance method, is moderate affected by the proposed jamming technique. This effect is enough to confuse the guided system and miss track the target.

This paper presents three approaches to improving the efficiency of an array aperture by interleaving two arrays in the same aperture area. The interleaved arrays have aperiodic spacings that are integer multiples of a set minimum spacing and are optimized to reduce the maximum sidelobe level. Fully and partially interleaved sum arrays operating at the same frequencies are demonstrated as well as interleaved sum and difference arrays for a monopulse system. A genetic algorithm is used to optimize arrays of isotropic point sources as well as arrays of dipoles modeled using the method of moments. Narrow beamwidths are possible while avoiding high sidelobes. The available aperture area is efficiently used.

Multiplayer games are an important and popular game mode for networked players. Since games are played by a diverse audience, it is important to scale the difficulty, or challenge, according to the skill level of the players. However, current approaches to real-time challenge balancing (RCB) in games are only applicable to single-player scenarios. In multiplayer scenarios, players with different skill levels may be present in the same area, and hence adjusting the game difficulty to match the skill of one player may affect the other players in an undesirable way. To address this problem, we have previously developed a new approach based on distributed constraint optimization, which achieves the optimal challenge level for multiple players in real-time. The main contribution of this paper is an experiment that was performed with our new multiplayer real-time challenge balancing method applied to eco-driving. The results of the experiment suggest the effectiveness of RCB.

Multiplayer games are an important and popular game mode for networked players. Since games are played by a diverse audience, it is important to scale the difficulty, or challenge, according to the skill level of the players. However, current approaches to real-time challenge balancing (RCB) in games are only applicable to single-player scenarios. In multiplayer scenarios, players with different skill levels may be present in the same area, and hence adjusting the game difficulty to match the skill of one player may affect the other players in an undesirable way. To address this problem, we have previously developed a new approach based on distributed constraint optimization, which achieves the optimal challenge level for multiple players in real-time. The main contribution of this paper is an experiment that was performed with our new multiplayer real-time challenge balancing method applied to eco-driving. The results of the experiment suggest the effectiveness of RCB.

A monopulse searching and tracking radar antenna array with a large number of radiating elements requires a simple and efficient design of the feeding network. In this paper, an effective and versatile method for jointly optimizing the sum and difference patterns using the genetic algorithm is proposed. Moreover, the array feeding network is simplified by attaching a single common weight to each of its elements. The optimal sum pattern with the desired constraints is first generated by independently optimizing amplitude weights of the array elements. The suboptimal difference pattern is then obtained by introducing a phase displacement π to half of the array elements under the condition of sharing some sided elements weights of the sum mode. The sharing percentage is controlled by the designer, such that the best performance can be met. The remaining uncommon weights of the difference mode represent the number of degrees of freedom which create a compromise difference pattern. Simul...

In monopulse radar antennas, the synthesizing process of the sum and difference patterns must be fast enough to achieve good tracking of the targets. At the same time, the feed networks of such antennas must be as simple as possible for efficient implementation. To achieve these two goals, an iterative fast Fourier transform (FFT) algorithm is used to synthesize sum and difference patterns with the main focus on obtaining a maximum allowable sharing percentage in the element excitations. The synthesizing process involves iterative calculations of FFT and its inverse transformations; that is, starting from an initial excitation, the successive improved radiation pattern and its corresponding modified element excitations can be found repeatedly until the required radiation pattern is reached. Here, the constraints are incorporated in both the array factor domain and the element excitation domain. By enforcing some constraints on the element excitations during the synthesizing process,...

A convex optimization program is presented for wideband arrays. Constraints are imposed on the frequency variation of excitation coefficients to ensure that the optimal solution can be realized in a wideband active electronically scanned array (AESA). AESA implementation with true time delays (TTDs) and phase shifters are handled separately. We also discuss the general case of combining TTDs and phase shifters. Contrary to single-frequency optimization, the wideband optimization method presented here ensures that the computed excitation is optimal over a specified bandwidth. It is shown that there is a trade-off between instantaneous bandwidth and sidelobe level. The proposed method works for both narrow and wideband arrays, as illustrated with examples. In addition to regular arrays, the method is also applicable to monopulse arrays. The optimization program is implemented in terms of embedded element patterns to account and compensate for mutual coupling, radome and platform effects.

This paper studies the performance of modern guided systems in presence of a proposed deception jamming technique. The proposed jammer uses the intercepted signal of the guided system to deceive the instantaneous amplitude comparison (IAC) monopulse radar, which is used in the most modern guided system. The performance of the modern guided systems in the presence of jamming for a typical case of radar guided missile is discussed for the following guidance methods: proportional navigation, pure pursuit, and constant bearing. It is found that, the guided system that uses pure pursuit guidance method is the most affected by the proposed jamming technique (largest miss tracking distance). Constant bearing guidance method is the lowest method affected by the proposed jamming technique (lowest miss tracking distance). Proportional navigation, that is the most used guidance method, is moderate affected by the proposed jamming technique. This effect is enough to confuse the guided system and miss track the target.

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A planar antenna is presented as part of a Signal Identification System working at 1.06 GHz. The antenna presents a monopulse radiation pattern. Likewise, a scanning of the main beam direction is achieved in a ±45° azimuth coverage. A limited 3 dB amplitude drop of the received signal at the edges of the angular scanning area is only permitted. Furthermore, a broad beamwidth is required in the single element of the array. A rectangular linearly polarized double stacked patch has been then designed, with a significant size reduction in the nonresonant length, to increase the azimuth beamwidth of the array single element. The whole antenna structure is composed of 18 elements, which have been distributed in six columns of three patches per column. The feeding network of the array has been developed in microstrip technology, with small individual circuits connected by coaxial cables, to reduce the global losses of the system. The designed compact antenna and the transmitting and receiv...

The zooming and scanning capabilities of a Gregorian confocal dual reflector antenna are described. The basic antenna configuration consists of two oppositely facing paraboloidal reflectors sharing a common focal point. A planar feed array is used to illuminate the subreflector allowing the antenna to scan its beam. The resulting quadratic aberrations can be compensated by active mechanical deformation of the subreflector surface, which is based on translation, rotation and focal length adjustment. In order to reduce the complexity of the mechanical deformation, least squares fit paraboloids are defined to approximate the optimal correction surface. These best fit paraboloids considerably reduce scanning losses and pattern degradation. This work also introduces two different zooming techniques for the Gregorian confocal dual reflector antenna: the first consists of introducing a controlled quadratic path error to the main reflector aperture; and the second is based on reducing the size of the radiating aperture of the feeding array.

A novel design for a simple and compact monopulse feed is presented for illuminating a deep reflector from the primary focal point, e.g. a rotationally symmetric single-reflector parabolic antenna, with a diameter-to-focal-distance ratio of around 0.4. The proposed feed is designed as a multimode circular cavity (cup) antenna. The elliptical patch excites a left-hand-circularly polarised TE 11 mode for the sum channel. The monopole in the centre of the patch excites the orthogonal, linearly polarised TM 01 mode for the difference channel. Two prototypes for such a feed were developed and fabricated, where the first was a proof-of-concept device and the second was a computer simulation-optimised version. Both the simulations and the measurements of the optimised prototype are presented.

Motivated by the need to cope with different jamming scenarios in modern electronic warfare, an enhanced processing architecture is proposed for phased array radar by reversing the conventional order of adaptive beamforming and monopulse beamforming. Specifically, each row or column is utilized as the identical subarray for adaption and multiple degrees of freedom are exploited. The full-aperture monopulse beamforming is, then, performed along column and row. Without any prior knowledge, the mainlobe and/or sidelobe jammings can be canceled simultaneously, whereas the angle estimation accuracy is well preserved. Simulation results illustrate the efficiency of this technique with a rectangular array. Additionally, evaluation shows the subarraybased scheme maintains the high performance of both jamming cancelation and angle estimation at a low cost.

This paper describes a 4-channel monopulse radar system with an additional delta-delta beam. Schemes for generation of this beam are considered for rectangular array as well as for circular array. This additional degree-of-freedom can be used to provide a secondary set of monopulse ratios. A scheme is derived where the two sets of monopulse ratios are combined before processing. The two sets of monopulse ratios complement each other and as a result, angle estimation accuracy is improved with mitigation of beam-shape loss. Current surveillance and tracking radars employ monopulse processing for detection and angle estimation of a target. Conventional monopulse processing involves one beam in transmit and multiple simultaneous beams on receive. Typically a sum beam without any tapering is employed in the transmit array for full power operation. A uniform weighting will have transmit antenna pattern narrowest beamwidth but higher sidelobes. On receive two or more beams are formed for target detection and angle estimation, i.e. the sum beam, the delta-azimuth beam and the delta elevation beam. The sum beam is used for surveillance search and target detection. Once a target is detected, the ratio of the delta-azimuth beam over the sum beam is used for azimuth angle estimation, and the ratio of the delta-elevation beam over the sum beam is used for elevation angle estimation. This approach for angle estimation is computationally efficient as it requires only the computation of the monopulse ratios and table look-ups for the angles. Received beams are typically tapered for sidelobe control leading to wider received beamwidth. Taylor weighting [1, 2, 9] is used for the sum beam and Bayliss weighting [2, 3] is used for the difference beams. Conventional monopulse radars form antenna beams through analog beamforming. Modern digital beamforming (DBF) radars may form monopulse beams through digital processing. DBF radars have more flexibility and capability in terms of number of beams and types of beams that they can generate. The flexibility and capability can be exploited for canceling jammers, high-resolution direction finding, signal separation and improvement of angle estimation. These capabilities can be characterized by degrees-of-freedom (DOFs) which are related to the number of signals the radar transmits and the number of digital receivers in the radar system. Next generation radars have multiple DOFs in transmit which is related to the number of exciters on the array. Next generation multiple-input multiple-output (MIMO) digital radar with multiple digital exciters and receiver supports multiple simultaneous transmit and multiple received beams.

Conventional methods of tracking a target using monopulse radar are severely limited in the presence of multipath, particularly at lowelevation angles. A technique using recursive eigendecomposition together with frequency-agile waveforms for tracking low-elevation targets in multipath has been developed. Frequency agility decorrelates the coherence of the direct and specular components on a pulse-to-pulse basis. This frequency agility also provides some robustness to the severe signal cancellation that arises when the direct and specular components are 180" out of phase. The proposed high-resolution, eigenstructure-based algorithm for solving the low-angle tracking problem can be implemented in three steps: (i) updating the covariance matrix, (ii) updating the eigenvalue decomposition of the covariance matrix, and (iii) updating the angle estimates by searching for the peaks of the updated MUSIC spectrum, or solving the zeros derived by the minimum-norm polynomial coeffcient. In addition, a phased-array implementation using three orthogonal simultaneous beams placed at half null-beamwidth apart has been developed. The angles can be determined by explicit computation or by a calibration curve in a lookup table such as in monopulse processing for angle estimation using two simultaneous beams.

Most present-day radar systems use monopulse techniques to extract angular measurements of subbeam accuracy. The familiar "monopulse ratio" is a very effective means to derive the angle of a single target within a radar beam. For the simultaneous estimation of the angles of two closely-spaced targets, a modification on the monopulse ratio was derived in [7], while [21] presented a maximum likelihood (ML) technique via numerical search. In this paper it is shown that the ML solution can in fact be found explicitly, and the numerical search of [21] is unnecessary. However, the ML solution requires the signal to noise ratio (SNR) for each target to be known, and hence we generalize it so it requires only the relative SNR. Several versions of expectation maximization (EM) joint angle estimators are also derived, these differing in the degree to which prior information on SNR and on beam pattern are assumed. The performances of the different direction-of-arrival (DOA) estimators for unresolved targets are studied via Monte Carlo, and it is found that most have similar performance: this is remarkable since the use of prior information (SNR, relative SNR, beam pattern) varies widely between them. There is, however, considerable performance variability as a function of the two targets' off-boresight angles. A simple combined technique that fuses the results from different approaches is thus proposed, and it performs well uniformly.