Study On Relevant Techniques Of Microwave Sparse Array Synthetic Impulse And Aperture Radar

Great advantages of the Synthetic Impulse and Aperture Radar (SIAR) technique have been shown by its successful applications in meterwave band and ground wave band. In this dissertation the SIAR technique is extended to microwave band to improve the radar's range and angle resolution and a novel microwave sparse array SIAR is proposed. In this novel radar system, which is also called Multi-Carrier-Frequency (MCF) Multiple-Input Multiple-Output (MIMO) radar, the transmit array consists of sparse subarray, the receive array consists of dense subarray and the orthogonality of the transmitted signals are realized by frequency diversity and spatial diversity. Focusing on the relevant theories of MCF MIMO radar and the key techniques to implemente microwave sparse array SIAR, this dissertation deals mainly with the following contents:1. The transmit signal model and receive signal model of SIAR are constructed. The multiple dimensional Ambiguity Functions (AF) of SIAR with the array aperture are derived based on the theory of matched filter and the steering vector of SIAR shows that the range, angle and doppler frequency are coupled each other. The AF's for far-field targets and narrowband waveforms are simplified using the orthogonal characteristics between the transmit signals of MIMO radar. And it can be concluded that the range resolution are determined not only by the bandwidth of the transmit signal but also the bandwidth among the transmit antennas. Take Uniform Linear Arrays (ULA) for example, the characteristics of AF and the resolution of LFM signal using multiple carrier frequency are analyzed in the case of monostatic and bistatic radar. The range and angle resolution varies with the target's position for the bistatic MIMO radar. These results and conclusions can be used for the reference of parameter selection and signal processing of SIAR.2. On the basis of array model of MIMO radar, a general array model of SIAR is constructed and a realization sheme of microwave SIAR with sparse subarray set for the transmitted array and dense subarray set for the received array is proposed, in which the grating lobes of the sparse array locate at the nulls of subarray exactly by reasonable selections of the number of subarray and the space between subarrays. Then a modified Genetic Algorithm (GA) with integral code, combined selection and double cross operation is employed to optimize subarray position of transmit antenna arrays to decrease the pattern's sidelobe level for a fixed subarray number and a constant array aperture. The simulation results show that the modified GA converges rapidly with a maximum relative Peak Sidelobe Level (PSL) of -31.1dB, which satisfies the project well.3. The signal preprocessing method based on digital dechirp processing is given as well as some key problems, e.g. the design of Channel Separation Filter (CSF), the range resolution analysis, the precision analysis of velocity compensation and the decision of coherent integration number etc. Two impulse synthetic methods, which are respectively IDFT Coherent Synthesis method (short for ICS method) and Spatial Domain Synthetic Bandwidth method (short for SDSB method), are proposed in the dissertation by applying IDFT coherent synthesis method and synthetic bandwidth method of stepped frequency signals to SIAR. The ICS method, using the idea of obtaining the corse image first and subsequently the precise image, can obtain the High Range Resolution Profile (HRRP) at the angle where the target is, while the spurious peaks appear when synthesizing a large range scene. The excuse of spurious peaks is analyzed and then a novel signal processing method, SDSB method, is proposed. The multi-carrier-frequency LFM returns transmitted diversely in space are first deramped by the dechirp processing and then the individual channel signals after the channel separation are combined into an LFM signal with a larger bandwidth by time shift in sequence. Finally the IFFT processing is applied to the concatenated signal to achieve HRR. The SDSB method can be implemented easily, avoid the spurious peaks effectively without extra computation and immune to the target movement.4. An equivalent Nt×Nr array is formed after the signal preprocessing in sparse array SIAR, where Nt and Nr are respectively the transmit and receive antenna number, and the equivalent steering vector is the kronecker product of transmit steering vector and receive steering vector in the case of narrow band signal. The super resolution of range and angle is realized by adopting the Spatial-Temporal MUSIC (MUltiple SIgnal Classification), which is based on the coupling of range and angle and can improve the precision of range and angle estimation when there are many targets present. The Cramer-Rao Bound (CRB) of range and angle estimation are derived. Further more, the variations of estimation variance of range and angle with Signal to Noise Ratio (SNR), snap number and direction of arrival are studied and simulated.5. The gain and phase error model of SIAR is presented and the corresponding calibration method by estimating the combined gain and phase errorΓv of the equivalent array after the signal preprocessing is proposed based on the coupling characteristics of gain and phase error of the transmit and receive array. Two estimation methods, Subspace Fitting (SF) Method and Maximum Likelihood (ML) Method, are proposed to estimate the gain and phase error when there is an auxiliary source. The Cramer-Rao Bound(CRB) of gain and phase error estimation are derived, how the methods'performance relates to SNR and snap number, and the influence of the angle error of assistant source on the estimation performance are studied and simulated, which validate the gain and phase error calibration methods. Additionally, the entire estimation ofΓv can also calibrate the error by digital sampling, truncation and filtering etc in the process of signal preprocessing.6. The Signal to Interference plus Noise Ratio (SINR) and the characteristics of the direct-path wave are discussed, which shows that the direct-path wave can be used to obtain the time synchronization and carrier frequency synchronization. Three methods, which are binary envelope detection method, extended slipping window accumulation method and correlative detection method, are proposed to obtain the time synchronization of LFM pulse signal. The computational complexity and performance of the three methods are analyzed and compared. The time synchronization errorΔt which is caused by ADC sampling and its influence are analyzed. Estimation ofΔt can be summed up as high-accuracy frequency estimation in a small extent and three estimation methods, which are zoom FFT method, auto-correlation function method and MUSIC method, are introduced and their performance are compared and simulated. The carrier frequency difference, which may exist in bistatic radar, can be obtained by coherent integration of direct-path wave. Finally the flow of the time and carrier frequency synchronization and the carrier frequency tracking method are summarized.