Study On Digital Monopulse Tracking System Based On Monopulse Technique And Adaptive Antenna

The monopulse concept first appeared in the 1940s. It is a technique for precise tracking, developed basing on two other tracking techniques, conical scan and lobe switching. Monopulse radars are able to provide all the lobes sensitive to angle error simultaneously, and compare each lobe's output in a single radar pulse. Therefore, they are free of errors due to pulse-to-pulse fluctuations in target echo strength, which are a principal source of error in lobe switching and conical scan. Traditional monopulse radars usually obtain simultaneous multi-lobes by the design of monopulse feeds, which require multiple feeds to be placed symmetrically about the focus on the focus plane of the antenna. In this way, the advantages of monopulse over the other angle-tracking techniques are obtained at the price of greater equipment complexity and cost.Smart antenna first appeared in the 1950s, a little later than monopulse. The original purpose to develop smart antenna was to effectively recognize and track targets in sophisticated electric-magnatic environment. Today, smart antenna is almost equivalent to adaptive antenna, which means utilizing adaptive methods to adjust receiving or transmitting features in order to enhance antenna performances. Among all the techniques in adaptive antenna, the DOA (Direction-of-Arrival) estimating and adaptive DBF (Digital Beam Forming) techniques, both based on array signal processing, dominate the development of adaptive antenna. Combined with array antenna techniques, DOA and adaptive DBF lead to digital antennas with super flexibility, accuracy and anti-interference ability. Not only the lobe direction of such an adaptive antenna can be changed by simply adjusting the weights to the array signals, but also zeros in the lobe can be formed at the directions of jams. Besides, as long as the digital signal processing power is sufficient, multiple digital lobes with different direction and zeros can be formed simultaneously by weighting the same array signal with different weighting coefficients in parallel.Monopulse needs multiple lobes simultaneously, and adaptive antenna can get multiple lobes simultaneously. Obviously, these two techniques make a very good combination, which leads to a digital monopulse tracking system. Through the use of adaptive antenna, complex and expensive monopulse feed design could be avoided, and all the advantages of array antenna and array signal processing could be utilized. More importantly, the digital monopulse system still has the ability of fast and precise angle-tracking.The purpose of this dissertation is to:â‘ Design the system structure of a digital monopulse system.â‘¡Analyze the parameters of a digital monopulse system, and study the characteristics of them by simulation.â‘¢Find some way to improve tracking performance of a digital monopulse system.The author's work includes:(1) First, the basic monopulse theory is studied, and a structure of the digital monopulse tracking system (DMTS) is developed. Then the constitution of the three main parts of the DMTS ( the array antenna, the transmitting part and the receiving part) are discussed.(2) In a DMTS, the monopulse technique is for precise target angle-tracking. Before that, target acquisition and simultaneous multi-lobe forming muse be done. So the initial target acquisition methods and digital monopulse beams forming methods are studied. As for the initial target acquisition, the DOA method of power spectrum peak searching is chosen, and the reason for choosing it rather than other methods is explained. Then traditional DOA methods and subspace-based DOA methods are discussed. As for monopulse beamforming, several optimum adaptive beamforming rules are introduced in brief.(3) Differences of error sources between a traditional monopulse system and a DMTS are compared. And advantages of digital techniques are discribed. Then a jam cancelling method based on DOA estimation as well as the structure to implement this method is presented. This method is designed specifically for a multi-target angle tracking DMTS. It utilizes the output of the DOA estimation in the DMTS to provide jam angle information for the monopulse beamforming parts, so that jam cancelling can be better done during the adaptive beamforming. Therefore, it is a very efficient method.(4) In order to study the relation between tracking performance and monopulse parameters of a DMTS, a simulation panel is constructed using Simulink. The structure of this panel is described. And problems encounted during the building of this panel as well as the solutions are introduced. This panel is very helpful not only during the study of relations between tracking performance and monopulse parameters of a DMTS, but also for further studies on the DMTS. (5) In the last, using this simulation panel, the monopulse parameters in a DMTS and the relations between them and the tracking performance of the DMTS are studied and analyzed. After that, a novel method to improve tracking performance of a DMTS is presented, which makes use of the flexibility of the inter-component-beam-angle (ICBA) in a DMTS to increase tracking accuracy. Simulation results show that, if a constant ICBA is used in monopulse tracking, then when the incoming signal to noise ratio (SNRi) remains steady, the tracking accuracy can't be improved, and when SNRi degrades, the accuracy will unavoidably drop. But if variable ICBA is used in a DMTS, then when SNRi remains steady, increasing the ICBA can improve tracking accuracy, and when SNRi degrades, increasing the ICBA can keep the decrease of accuracy to the minimum, or even hold the accuracy unchanged.