Monopulse Radar System Research Articles

Monopulse time‐modulated antenna array using iterative Fourier technique

SummaryThe iterative Fourier technique (IFT) is applied to design monopulse radar tracking system using time‐modulated linear antenna array (TMLA). IFT is used to modify the time sequences of the radio‐frequency switches connected to the antennas to generate sum and difference patterns with suppressed interference‐plus‐noise signal while constraining the power losses associated with side‐band radiation (SBR). The time modulation allows quick and precise control of the excitation distribution, which increases the tracking accuracy. To obtain best angle sensitivity, the article computes the tradeoff among side‐lobe level (SLL), half power beam width, directivity, and dynamic efficiency. The approach is proved successful to handle several design constraints through adaption in radiation domain as well as in aperture domain. Authors illustrate additional representative TMLA examples where the time sequences of the selected elements are prefixed to enhance the dynamic efficiency and directivity for different SLL and side‐band level (SBL). Application of IFT saves the computational cost significantly with respect to existing state‐of‐the‐art optimization techniques. Introduction of the symmetric switching sequences simplifies the feed network greatly. The proposed design solution is validated with a fabricated TMLA prototype consisting of two time‐switched printed dipoles with microstrip via‐hole balun at 2.45 GHz. Copyright © 2016 John Wiley & Sons, Ltd.

Difference beam aided target detection in monopulse radar

The classical detection step in a monopulse radar system is based on the sum beam only, the performance of which is not optimal when target is not at the beam center. Target detection aided by the difference beam can improve the performance at this case. However, the existing difference beam aided target detectors have the problem of performance deterioration at the beam center, which has limited their application in real systems. To solve this problem, two detectors are proposed in this paper. Assuming the monopulse ratio is known, a generalized likelihood ratio test (GLRT) detector is derived, which can be used when targeting information on target direction is available. A practical dual-stage detector is proposed for the case that the monopulse ratio is unknown. Simulation results show that performances of the proposed detectors are superior to that of the classical detector.

Cluster Analysis for the Synthesis of Subarrayed Monopulse Antennas

The design of monopulse radar systems requires the synthesis of both a sum pattern and a difference pattern, which is quite flexible and hard to be optimized for subarrayed antennas. In this study, innovative approaches to synthesize the optimal sum and difference patterns independently for subarrayed monopulse arrays are presented. By exploiting the relationship of the subarray partition and optimal excitations, the synthesis problem is formulated as the clustering of a set induced by the optimal excitations. Thus, the optimal subarray partition is transformed to a clustering problem. A clustering method is presented to solve the array synthesis with uniform excitation at element level. Furthermore, an improved approach is proposed to synthesize low sidelobe patterns. When compared with several classical approaches, various numerical experiments validate the effectiveness and potentiality of the proposed methods.

Empirical mode decomposition-based monopulse processor for enhanced radar tracking in the presence of high-power interference

Monopulse radars are used to track targets that appear in the look direction beam width. The distortion of the tracking information produced when high power interference signals (jamming) are introduced through the radar antenna can cause severe erroneous outcomes from the monopulse tracking processor. This leads to errors in the target tracking angles that may cause target mistracking. New monopulse radar structures that utilizes an empirical mode decomposition (EMD) based denoising processor are presented in this paper. EMD filtering is carried out for the complex radar chirp signal with subsequent detrending, thresholding, and denoising processes. These processes are used to decrease the noise level in the radar processed data to improve the signal to noise ratio. The performance enhancement using the monopulse radar tracking system with EMD based filtering is included using the standard deviation angle estimation error (STDAE) for different jamming scenarios and different target SNRs. STDAE shows that the new EMD based system works well in the case of main beam interference and side lobe interference for the conventional processor and, in the case of the spatial adaptive processor, is effective in the case of main lobe interference.

Monopulse MIMO Radar for Target Tracking

We propose a multiple input multiple output (MIMO) radar system with widely separated antennas that employs monopulse processing at each of the receivers. We use Capon beamforming to generate the two beams required for the monopulse processing. We also propose an algorithm for tracking a moving target using this system. This algorithm is simple and practical to implement. It efficiently combines the information present in the local estimates of the receivers. Since most modern tracking radars already use monopulse processing at the receiver, the proposed system does not need much additional hardware to be put to use. We simulated a realistic radar-target scenario to demonstrate that the spatial diversity offered by the use of multiple widely separated antennas gives significant improvement in performance when compared with conventional single input single output (SISO) monopulse radar systems. We also show that the proposed algorithm keeps track of rapidly maneuvering airborne and ground targets under hostile conditions like jamming.

Extended Analysis of Retrodirective Cross-Eye Jamming

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.

Development of A Monopulse Radar System

The first part of this paper provides background on radar developments at the General Electric (GE) Company in Schenectady, New York. The object (using available literature and patents) is to show there was an early and continuing effort to develop microwave radar at GE. The author was a participant in this radar program during World War II and in particular during the development of a monopulse radar. No attempt is made in this paper to establish a priority for the work reported.

A low-frequency ice-penetrating radar system adapted for use from an airplane: test results from Bering and Malaspina Glaciers, Alaska, USA

AbstractIce-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.

Folded Reflectarray를 이용한 고이득 94 GHz 모노펄스 안테나

본 논문은 94GHz를 중심 주파수로 하며, 모노펄스 형식의 추적 레이다 시스템에 사용되는 folded reflectarray 안테나를 제안한다. 추적 레이다는 대부분 목표물로부터 반사되어 돌아온 반사 신호들을 비교하여 목표물을 찾으며, 모노펄스 형식에서는 이러한 신호들의 비교를 동시에 수행한다. 본 논문에서 제안한 모노펄스 형식의 folded reflectarray 안테나는 polarizing grid, reflectarray, 다중 모드 feed horn, 그리고 도파관으로 제작된 comparator로 이루어져 있다. 제작된 안테나는 다중 모드 feed horn과 comparator의 모노펄스 급전부에 의해 sum 모드, azimuth 모드, 그리고 elevation모드, 이렇게 세 개의 방사 패턴 형식을 나타낼 수 있다. 측정 결과, sum 패턴에서는 36dB, azimuth 패턴에서는 33.5dB, 그리고 elevation 패턴에서는 27.2 dB의 이득을 확인할 수 있었다 This paper proposes a high-gain monopulse antenna using the folded reflectarray for a monopulse target-tracking radar systems designed at the center frequency of 94 GHz. In target-tracking radar systems, the angle of arrival of the incoming wave Is determined by comparing the signal received on two or more non-coincident antenna patterns. This is the physical basis of most target-tracking techniques and the comparison is made simultaneously in a monopulse radar systems. In this paper, the antenna consists of polarizing grid, reflectarray, multimode feed horn, and comparator implemented by wavguide. The antenna is able to have three radiation patterns by using the monopulse feed systems assembled by multimode feed horn and comparator. The antenna demonstrates maximum gains 36dB, 33.5dB and 27.2dB at sum mode, azimuth mode, and evevation mode respectively.

Monopulse radar based on spatiotemporal correlation of stochastic signals

The theory and technique of angle-of-arrival (AOA) estimation using random-noise or other stochastic transmit waveforms is addressed. The additional uncertainties induced by signal itself and the statistical complexity of the received signals result in major challenges. The statistical properties of the random-noise interferometer and monopulse radar system are studied and compared theoretically using an approximation method. Furthermore, a random-noise coherent correlation receiver (CCR) architecture is proposed. The concept of mean monopulse characteristic curve (MMCC) is introduced. Experimental results using an X-band random-noise monopulse radar system validate the theoretical predictions of random-noise monopulse characteristics and suggest potential applications such as surveillance, imaging, and maneuvering target tracking.

New‐configuration wideband single‐axis and dual‐axis planar monopulse comparators

AbstractNovel configurations of single‐axis and two‐axis planar monopulse comparators are presented, providing a very close amplitude and phase matching over a wide bandwidth with an application in broadband monopulse radar systems. Unlike the conventional monopulse comparators, the present unique design features have offered a very good return loss as well as amplitude and phase imbalance of ± 0.1 dB and ± 1.5° over a large bandwidth (>15%) in L band, for two‐axis monopulse comparators. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 427–430, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10341

Unresolved Rayleigh target detection using monopulse measurements

When the returns from two or more targets interfere (i.e., the signals are not resolved in the frequency or time domains) in a monopulse radar system, the direction-of-arrival (DOA) estimate indicated by the monopulse ratio can wander far beyond the angular separation of the targets. Generalized maximum likelihood (GML) detection of the presence of unresolved Rayleigh targets is developed with probability density functions (pdfs) conditioned on the measured amplitude of the target echoes. The Neyman-Pearson detection algorithm uses both the in-phase and quadrature portions of the monopulse ratio and requires no a priori knowledge of the signal-to-noise ratio (SNR) or DOA of either target. Receiver operating characteristic (ROC) curves are given along with simulation results that illustrate the performance and application of the algorithm.

Nonbiased geometric centroid for high resolution radar angle tracking

An efficient algorithm is presented for tracking the true and nonbiased geometric centre of an array of scatterers on a single target. This algorithm is specifically designed for the monopulse radar angle tracking system with stepped frequency pulse train. The crosscorrelation function of the sum and difference channels normalised by the autocorrelation of the sum channel is used to calculate the geometric centre of each uniquely separated pair of scatterers. Dividing by the autocorrelation function eliminates the effects of different targets radar cross-sections (RCS), however, the zero separation term of the error function (DC term) is still biased toward the bigger scatterer. The nonzero terms (AC terms) are the down range distance separations between scatterers and are independent of their RCS. By simply dropping the zero separation term, and averaging the remaining terms together, the aimpoint becomes the unbiased geometric centre of the array.

Analysis of interference effects on monopulse radars

The complete numerical analysis of angle-tracking performance of a coherent amplitude-comparison monopulse radar system in a two-target situation is described. This system, equipped with automatic gain control (AGC) and phase-locked loop (PLL), was originally invented for improving the target-tracking performance in multiple-target situation. The general behavior depends on Doppler separation of the targets relative to the bandwidths of AGC and PLL. The performance of this system has previously been roughly analyzed by using linear approximation for four extreme cases: wideband and narrowband AGC in combination with wideband and narrowband PLL. In this study, the author performs nonlinear analysis of the same system for all bandwidths of AGC and PLL, because Doppler separation varies over a wide range in actual tracker, and therefore it is indispensable to know the total system behavior. A special numerical technique called the Galerkin method is used. As a result of this analysis, the extensive target-tracking performance for all intermediate bandwidths of AGC and PLL has been clarified. The result is useful for actual target tracker design. >

Influence of an interfering second target on a certain amplitude‐comparison monopulse radar system

AbstractThis paper examines angular tracking performance in an amplitude‐comparison non‐pulse tracking radar employing both AGC and phase synchronization (PLL) for the case where an interfering target as well as the main target are present within the beam of the tracking radar. the interfering target causes generation of an incorrect error output signal and the antenna boresight cannot be held on the main target. the effects of the interfering target are quite complex as they are dependent on the difference in Doppler frequencies of the two targets and on the bandwidths of the AGC and PLL systems. In this paper we calculate the error output using both the harmonic balance method and Galerkin's method for an arbitrary PLL bandwidth in combination with a narrowband AGC and a wideband AGC. From these results, we find that when the PLL has a comparatively narrow bandwidth the approximate analysis using the harmonic balance method gives sufficiently accurate results for practical application.

Maximum-likelihood estimation of the angular position and extent of a target

The maximum-likelihood estimates of the angular position and angular extent of a target relative to the boresight of a monopulse radar system is derived, and the statistics of the estimates are determined. It is shown that the moments of the estimates do not exist when the thermal noise power in the receiver is nonzero. However, conditional moments are determined which do exist, as well as conditional density and/or distribution functions.

Analysis and Design of Antennas for Air Traffic Collision Avoidance Systems

The analysis and design procedure of an antenna for a CW Doppler radar system being developed for pilot warning of midair collision hazards is presented. The antenna consists of two vertical arrays of half-wavelength dipoles mounted near a circular conducting cylinder. Each vertical array is composed of three vertical dipoles. Each array provides relatively uniform illumination (2-3 dB) in the forward 180 angular segment of the horizontal plane and approximately + 10-150 coverage in the vertical plane. The antenna could be used in a two-mode operation, either in a standard monopulse radar system (sum and difference amplitude patterns) or in a system where amplitude and phase are the measurable quantities.

Simulation of the Feed Patterns due to Four Horns Used in Sum and Difference Comparison Monopulse Radar from Single Horn Radiation Pattern

The simulation of the radiation patterns due to four pyramidal horns, from the radiation pattern of a single pyramidal horn for the sum and difference comparison type monopulse radar system is the prime concern of this paper. The simulation is done to facilitate the optimization of slope and linearity of the difference mode patterns in the two principal planes and automatic optimization of the boresight axis characteristics is observed. The specific value of the squint angle of the individual horn axis from the boresight axis for the optimum slope and linearity is determined through simulation at various squint angles. The practical results obtained with the excitation of the system from trimode four-channel monopulse bridge and the simulated results are compared.

Gain- and phase-matched low-noise TWT amplifiers

This paper will be presented in two parts. The first presents the latest advances in precision gain and phase-matched amplifiers. A set of three C-Band low-noise traveling-wave amplifiers has been gain and phase matched to stringent combination of electrical and environmental speeifications for a new mono-pulse radar system. No previous C-Band amplifiers of any kind have attained such a low noise figure (about 5 db at room temperature) with a power output of 20 milliwatts. In addition, these units are required to remain extremely gain and phase stable over a wide temperature range, to be gain and phase matched, and meet numerous other specifications sueh as low VSWR, fine grain, spurious PM, tube-to-tube interaction, and low AM-PM conversion. These results, which are important for designers of sensitive mono-pulse and phase array radar receivers with wide dynamic range, are a significant advance with respect to those reported by the author two years ago. At that time, the best gain stability achivable over a wide temperature range was ±1 db. The best phase stability consistently achieved was ±15°.

Design of Monopulse Antenna Difference Patterns with Low Sidelobes*

The flexibility of modern monopulse radar antenna systems makes possible the independent optimization of sum and difference patterns. The two parameter difference pattern, developed here for the circular aperature antenna, is designed to have nearly equal sidelobes similar to those of the Taylor sum pattern. The difference pattern is asymptotic to a model difference pattern which has the greatest slope (angle sensitivity) for a given sidelobe level. The model function is unrealizable because it has uniform sidelobes which are infinite in extent. The two parameter difference pattern is realizable and is expressed in a Fourier — Bessel series of N terms in a manner similar to Taylor's treatment of the sum pattern. The other parameter, A, controls sidelobe level. Comprehensive tables of the Fourier — Bessel coefficients are given for both the circular aperture series and the difference pattern series. Directivity and angle sensitivity are investigated and found to have maximum values that decrease as sidelobe level decreases. The monopulse system performance using the asymptotic difference pattern and the Taylor sum pattern compares favorably with a maximum likelihood angle estimation system. Development of a line source difference pattern is presented in the appendix.