Research On Ground-based Wideband Distributed Coherent Aperture Radar

In order to increase detection range and improve measurement accuracy, enlarging the aperture of antenna is usually applied to radar. However, there are some inherent drawbacks in conventional large aperture radar, such as quite difficult to transport, bad in battlefield survivability, expensive to build, harsh requirement in manufacturing technology level, difficult for system maintenance. These drawbacks limit the application and development of conventional large aperture radar. In order to overcome these disadvantages, MIT Lincoln Laboratory proposed a new radar named distributed coherent aperture radar(DCAR), in which the signal fusion is applied to several smaller aperture radars to obtain the equivalent performance with conventional large aperture radar. To date, the research of DCAR has been in the initial stage and some key issues in realization of the new radar are still to be resolved.In this paper, the constitution and workflow of DCAR are analyzed. With regard to the key problems in realization of wideband full coherence, three methods are proposed respectively, that is the estimation method of coherent parameters based on orthogonal signal and coherent signal, the synchronization method based on self calibration and radiometric calibration, and the wideband full coherent technology based on stepped frequency signal. Considering the structure of DCAR with multi-unit radars, a suppression method of mainlobe interference is researched. The main contents and contributions can be summarized as follows:1. Because the ranges from different unit radars to target are different, there are corresponding time difference and phase difference between signals transmitted by unit radars at target. The time difference and phase difference are named coherent parameters in this paper. In order to realize full coherence of DCAR, all transmitting signals should be added coherently at target, thus the coherent parameters should be obtained accurately. The estimation method of coherent parameters is researched deeply in this paper. Firstly, the constitution and workflow of DCAR are analyzed, and the mathematic models of coherent parameters are developed according to the relative position relationship between unit radars and target. And then, corresponding to the workflow of DCAR, the estimation methods based on orthogonal signal and coherent signal are proposed respectively. To improve the estimation accuracy of coherent parameters, a method of orthogonal signals design is proposed based on modified cost function. The Cramer Rao Low Bound(CRLB) of coherent parameters corresponding to two estimation methods is derived, and simulations are carried out. At last, a filtering method of coherent parameters based on Kalman filter is proposed to track the coherent parameters of moving target, and the effectiveness of proposed method is verified by simulation results. Coherent parameters are the key parameters to realize full coherent technology, and the research of estimation method of coherent parameters provides the technology support for realization of full coherent technology.2. Time synchronization error and phase synchronization error lead to estimation bias in estimations of coherent parameters, and hence reduce the coherent performance of DCAR. With regard to the synchronization problems, a synchronization method based on self calibration and radiometric calibration is proposed in this paper. Firstly, the mathematic models of time synchronization error and phase synchronization error are developed according to analyze the source of synchronization error. The effect of synchronization errors to coherent performance is researched, and the requirement of synchronization accuracy is given at the cost of coherent performance degradation. And then, a scheme of same local oscillator source of all unit radars is designed. Then the synchronization errors are estimated by self calibration and radiometric calibration, and then the transmitting signals are adjusted by estimations of synchronization errors to realize system synchronization. Simulation results verify the effectiveness of proposed synchronization method. System synchronization is the foundation of realization of full coherent technology, and the research of synchronization method of time and phase lay a foundation for realization of full coherent technology.3. In order to meet the requirement of high range resolution in modern radar, it is needed to research the wideband DCAR. However, the time synchronization error will seriously reduce the coherent performance in wideband DCAR, and lead to the wideband full coherence unrealizable. In this paper, a robust full coherent technology based on stepped frequency(SF) signal is proposed to reduce the effect of time synchronization error. Firstly, based on the characteristic of instantaneous narrowband and synthetic wideband, the conventional linear frequency modulation(LFM) signal is proposed to be replaced by SF signal, and it would be increase the system robustness to time synchronization error with the high measurement performance of wideband system, and hence reduce the requirement of time synchronization accuracy. And then, in order to quantitatively analyze the coherent performance of DCAR, the definition of efficiency of cohere-on-transmit and efficiency of full coherence are given respectively. In real system the SNRs of two echoes in unit radars are not the same, and it would lead to the original calculation method of SNR in ideal full coherence inapplicable. To resolve this problem, a new calculation equation of SNR in ideal full coherence is proposed, and the corresponding efficiency of cohere-on-transmit and efficiency of full coherence are given. At last, in order to verify the theory of wideband full coherent technology based on SF signal, an experimental system of DCAR is designed and developed. The corresponding experiments are carried out by the experimental system, and good results are obtained. Hence, the effectiveness of proposed algorithm is also verified by experimental results besides by the simulation results. Wideband radar is the inexorable trend of radar future development, and the research of wideband full coherent technology provides the technology support for future development of DCAR.4. With regard to the mainlobe interference and considering the structure of DCAR with multi-unit radars, a suppression method of mainlobe interference based on large aperture auxiliary array is researched. With the high space resolution of large aperture, the mainlobe interference of main radar is canceled by interference of auxiliary array. Firstly, the geometry of array with uniform array spacing, including the array length, the array spacing and the antenna element number of array, is designed. And then, using the correlation between interference received by auxiliary array and interference received main radar, two adaptive beam forming suppression methods of mainlobe interference are proposed based on Minimum Mean Square Error(MMSE) rule and Maximum Signal to Interference plus Noise Ratio rule(MSINR), respectively. In the following, high grating lobes exist in the array pattern due to the sparse large aperture auxiliary array with uniform array spacing, and hence the array spacing is redesigned based on modified genetic algorithm under the constraints of minimum element spacing and total array length to reduce the grating lobes. At last, the radar echo data containing mainlobe interference is processed by the two suppression methods of mainlobe interference, and the simulation results verify the effectiveness of the proposed methods. The research on mainlobe interference suppression based on large aperture auxiliary array expanding the function and application area of DCAR.