Radar Techniques Using Array Antennas
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This book gives an introduction to the possibilities of radar technology based on active array antennas, giving examples of modern practical systems. There are many valuable lessons presented for designers of future high standard multifunction radar systems for military and civil applications. The book will appeal to graduate level engineers, researchers, and managers in the field of radar, aviation and space technology.
Inspec keywords: radar signal processing; array signal processing; object detection; military radar; energy management systems; radar clutter; beam steering; synthetic aperture radar; decision making; active antenna arrays
Other keywords: moving target detection; adaptive clutter suppression; sequential detection; array signal processing; electronic steerable antennas; target imaging; military applications; adaptive digital beamforming; superresolution; radar techniques; SAR; multifunction radar systems; radar decision planning; pulse compression; energy management; adaptive monopulse; active array antennas; radar decision making; civil applications
Subjects: Antenna arrays; Signal processing theory; Radar equipment, systems and applications; Computer vision and image processing techniques; Military radar, detection and tracking systems; Signal processing and detection; Optical, image and video signal processing
- Book DOI: 10.1049/PBRA010E
- Chapter DOI: 10.1049/PBRA010E
- ISBN : 9780852967980
- e-ISBN: 9781849191432
- Page count: 488
- Format: PDF
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Front Matter
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1 Introduction
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Central to future radar technology will be the application of array antennas, and this book is dedicated to the application of these antennas which offer great flexibility for system design in combination with suitable control and signal processing. The design work can be imagined roughly in the following way: on the basis of a required radar function we look for a signal and data processing concept with the aid of statistical signal theory or filter theory. For a developed concept the efficiency is examined analytically or by simulation studies. The antenna concept, suitable to the required function, can then be developed using basic knowledge about the antenna array. Within this book we will find this implicit procedure at several points. Statistical signal theory forms an important fundamental tool, and is presented as simply as possible and limited to the most important aspects. In addition, we need some basic relations for the most important characteristics of array antennas. Then we have to choose between the available array-antenna architectures.
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2 Signal representation and mathematical tools
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In this book we will consider antenna arrays which consist of many individual antenna elements, and therefore a large number of signals, one for each element, has to be processed at the same time. The signals can be assigned to locations on the antenna, forming discrete sampling of the spatial wave field for transmitting or receiving. Signals transmitted or received with a radar system also have to be represented as a function of time. Signal samples are formed in the spacial and temporal dimension for digital signal processing with signal processors or computers and for recording for later analysis.
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3 Statistical signal theory
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In this chapter we summarised the basic tools for considering problems of signal processing from an engineering viewpoint. After the introduction of basic statistics we discussed the likelihood ratio test for deriving optimal processing rules for signal or target detection. Some important examples have been treated.
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4 Array antennas
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In this chapter we will discuss the most important and well-known relations and parameters of an array antenna which are relevant for the conception of a radar system. Possible implementations for different applications will be summarised. A set of antennas or an array of antenna elements is distributed on a metal ground plane, preferably on a regular grid.
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5 Beamforming
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Beamforming with all antenna elements of an array for transmitting or receiving means reproducing a desired beam pattern as closely as possible. Generally, a narrow main beam with high gain at the centre in the desired direction combined with low sidelobes for all other directions is required. The fundamental procedure for beamforming is weighting the individual signals of all antenna elements and then summing all these weighted signals.
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6 Sampling and digitisation of signals
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At the output of beamforming and the receiver channel, with a bandpass filter at the intermediate frequency IF, the received signals are available for further processing. They are at this stage continuous analogue signals. For digital processing we need sampled values of the orthogonal components / and Q as described in chapter 2 with equation 2.1. In this chapter we will discuss the necessary sampling rate and methods to derive the orthogonal components / and Q. For the interested readers and for completeness we start here with an explanation of the term analytical signal. This material is discussed in more detail within several text books, e.g. Reference 1.
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7 Pulse compression with polyphase codes
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Pulse compression with sufficient Doppler tolerance may be achieved using polyphase codes derived from linear and nonlinear frequency modulation. Low sidelobes in range and Doppler are required, especially for the radar search function. These may be achieved by an LFM-derived phase code together with Hamming weighting or by applying a phase code from nonlinear frequency modulation. A loss in resolution has to be taken into account in both cases. For a discrete and known Doppler frequency with an expanded and mismatched filter vector a sidelobe reduction is possible. The compression is then achieved without a loss in resolution. A set up for the expanded filter vector results in a least-square minimisation for all range elements. This version may be useful for search in clutter or for target tracking. Complementary phase codes show increasing sidelobes and responses from unambiguous range from targets with Doppler-shifted echoes. Pulse eclipsing and the corresponding blind ranges can be tolerated to a certain extent in the search mode. Reduced resolution and signal peak power can be sufficient for target detection at shorter ranges. Range resolution may be improved by a factor of about four by oversampling and a least-mean-square filter, if the subpulse matched filter is not strictly bandlimiting.
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8 Detection of targets by a pulse series
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Generally, a series of echo signals is the basis for target detection. An arbitrary length of this series is applicable by using the beam agility, which enables adaptation of the transmit energy to target strength and of the required Doppler resolution. The step scan operation of a phased array provides an important difference when compared with mechanical scanning radars with respect to the detection procedure and clutter suppression. Clutter suppression may be divided between strong fixed clutter and varying Doppler-shifted clutter, the first of which may be suppressed by a simple recursive filter. Using post processing the latter can be arranged advantageously, in an adaptive manner, after the filter bank. Sea clutter suppression can be improved by a special multiplex operation of the beam, resulting in elongated dwell times in each beam position. For a longer series of pulses, a possible Doppler shift during the dwell time has to be considered. The usual filter bank shows a degraded detection performance because the target energy is distributed into several filters. An autocorrelation estimation test will give a more robust performance in this case. The detection performance is evaluated and compared on the basis of simulation studies.
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9 Sequential detection
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This chapter presents sequential detection applied for the search mode of phased-array radars. A range dependent weighting has to be used to combine the test function from range cells for forming one test function for the respective beam position. Then the highest gain, expressed in SNR saving, is achieved at maximum range.
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10 Adaptive beamforming for jammer suppression
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An adaptive array, based on an active receiving array with subarrays, results in an efficient countermeasure against the threat of multiple jammers. This possibility represents a major advantage and a motivation for implementing an active phased-array radar. The adaptive technique is superior to a low sidelobe antenna in the case of multiple jammers. If there are K jammers then from theory at least K + 1 subarrays with receiving channels are necessary to create notches in the antenna pattern for the suppression of the jammer signals. In practice, to counter some inaccuracies of the receiving antenna, it is recommended to have at least 2K subarrays. Some algorithms for computing the adapted weighting vector, to form the beam from the subarray outputs, are discussed. For this adaptation algorithm the loaded sample matrix inversion is efficient and recommended. Projection methods based on eigenvector matrices describing the jammer and signal space offer a jammer suppression independent of their power. Their difficulty is the separation between these spaces, especially for high jammer power and channel inaccuracies. The lean matrix-inversion algorithm needs the lowest number of computations and is especially recommended for a higher number of jammers. For the learning phase only about 2K snapshots of the jammer signals are then necessary. By some first experimental results the adaptive principles are demonstrated and theoretical expectations are confirmed.
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11 Monopulse direction estimation
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After the detection of targets in the search mode there follows target location with range and direction estimation which is as precise as possible. This procedure may in principle be performed with the same receiving data as that already used for detection. Multifunction radar systems with phased arrays may apply an additional acquisition mode after target detection to confirm the first target detection and thereby cancel to a large extent false alarms produced by noise and interference. This radar task may be performed with increased transmit energy compared to that used for the search mode, because there are relatively fewer acquisition orders. Because of the higher SNR for the acquisition mode the location accuracy will be improved. The waveform will be chosen for improved range resolution. After target acquisition the location function will also be applied to the target tracking process.
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12 Superresolution in angle
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Superresolution is offered by an active receiving array and special signal-processing procedures as an additional and new capability. Targets at the same range cell and within the beamwidth can be resolved to a limit of about 0.2 Θβ. The necessary SNR will be available by approaching target formations. The procedure for superresolution is derived from likelihood-estimation theory. The resulting parametric target model fitting algorithm, combined with stochastic approximation, is recommended. It is applicable for coherent target signals. One prominent application is resolving the multipath problem, especially for locating low-flying targets above the sea.
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13 Space-time adaptive processing
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To detect slowly moving targets, which always have their spectrum near the clutter spectrum, the ditch of the processing gain function, necessary for clutter suppression, must be as narrow as possible, matched to the clutter spectral width. This is best accomplished by STAP processing with sufficient degrees of freedom. These spatial and temporal degrees of freedom can be provided for future airborne phased-array radars by an adequate number of subarrays or super-subarrays and an FIR filter in the time domain. The final Doppler filter bank then follows.
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14 Synthetic aperture radar with active phased arrays
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This chapter discusses synthetic aperture radar with active phased arrays.
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15 Inverse synthetic aperture radar (ISAR)
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Imaging of tracked moving targets is possible by applying the principles of synthetic aperture. The relative motion between the target and the fixed radar system creates this aperture. The necessary rotation of the target with respect to the line of sight amounts to only few degrees. The movement of the target has to be known exactly for creating this aperture by processing; target tracking alone is not sufficient. Additional focusing techniques must be applied, and frequency and phase estimation from the Wigner Ville distribution seems to be an advantageous procedure. A one-dimensional image in the cross-range dimension is thus achieved in a plane which is orthogonal to the rotation axis. A two-dimensional image requires additionally a high range resolution. This can be achieved by a synthetic high bandwidth by applying a frequency-stepped pulse series instead of one pulse. If a target shows motions around several rotation axes, for example a ship, two dimensional images are also possible.
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16 Target classification
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Classification and possibly identification of observed radar targets is a natural requirement for radar systems applied for air or maritime traffic control and defence. In all cases without target information, for example provided by secondary surveillance radar (SSR) or with identification of friend or foe (IFF), the radar echo signal has to be evaluated. For air surveillance it is of great importance to distinguish as fast as possible between targets of interest, such as airplanes, and false targets like birds or flocks of birds. By this distinction the formation and processing of false tracks, thereby wasting computer and radar time and power is avoided.
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17 Experimental phased-array system ELRA
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The ELRA system was developed with active array antennas, signal and data processing subsystems for the demonstration of a multifunction radar system. The antenna components have been developed and built within the labs of FGAN-FFM electronics department (EL). During all phases this project has stimulated theoretical studies, especially in the field of sequential detection and array processing.
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18 Floodlight radar concept (OLPI)
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Because a surveillance radar must transmit continually, it is easy to detect by reconaissance and ESM and therefore vulnerable to jamming and anti-radar missiles. In contrast, a tracking radar is only activated to support defence actions after the detection of penetrating targets. The intention of this chapter is to discuss a radar operational concept which permits minimum vulnerability against these threats. It is therefore especially suitable for a medium-range search function. This concept could in principle be implemented by a special radar system or it may also be applied to the search mode of a multifunction phased-array radar.
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19 System and parameter considerations
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This chapter discuss some aspects of the choice of some main systems' parameters and their relationships for multifunction operation. We consider the observation of the air space to detect, locate and track flying targets, the classical task of a surveillance radar. The aim is to establish a track for each target as early as possible. We have discussed in several preceding chapters the arbitrary movement of the agile beam, which is generally a pencil beam. There is the desire to use this freedom in a most intelligent and effective way, which means to achieve reliable tracking of incoming targets with adequate location accuracy. The tracks should be established at a maximum range with a minimum of mean power. Thereafter, the tracks should be maintained with an adequate tracking rate and power.
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Back Matter
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