Research On Techniques Of Long Time Coherent Integration Detection For Air Weak Moving Target

With stealth technique and aircraft development, lots of far-range, low observable and highly maneuvering air weak targets appear. They share a lot in common that the targets’ SCR/SNR is too low to detect whether in time domain or in frequency domain, which results in poor detection performance of tradition signal processing algorithms. Therefore, how to realize the detection of these targets is a research hotspot in radar field. Long time coherent integration technique is an effective method for the detection of weak targets, i.e., increaing the collected target’s echo energy via a longer integration time. However, because of the targets’ high speed motion and maneuvering motion, the range migration, Doppler migration and beam migration of weak targets’ echo are inevitable during the long time observation. If the echoes distributed in different range cells, in different Doppler channels and in different beams could not be collected effectively, it is difficult to improve the detection performance. This dissertation focus on the compensation algorithm for range, Doppler and beam migrations of weak target detecting. The main works can be summarized as follows:1. A compensation algorithm for Doppler migration is proposed in this dissertation. When a radar works in a high pulse repetition frequency mode, the constant radial acceleration target’s echoes will concentrate in a single range cell after pulse compression. The algorithm compensate the target’s Doppler migration in range-Doppler frequency domain by constructing a phase compensation function in Doppler frequency domain via searching target’s acceleration. After the Doppler compensation, the output of the coherent integration will concentrate in a single Doppler channel which will help to improve the detection performance of weak targets. The simulation examples show the detection performance and the receiver operating characteristic will improve via adoping the algorithm.2. To increase the resident time in a single range cell of the targets with a constant radial velocity, an axis rotation(AR) transform for range migration compensation is proposed. According to the characteristic of echo distribution, AR transform the two-dimensional range migration compensation in fast time-slow time domain into an one-dimensional searching for the correct axis rotation angle in fast time-slow time domain. The target’s echoes in different range cells could be concentrated in a single range cell via AR with a low computation complexity. Subsequently, MTD(Moving Target Detection) could be performed. Accordingly, the motion parameters and the number of targets could be estimated. Moreover, the transform error, the variation of the Doppler frequency and resolution with axis roation angle are analyzed. Then, an improved axis rotation MTD(IAR-MTD) algorithm is proposed to rectify the drawbacks mentioned above. The simulation examples show, for weak targets with constant radial velocity, the detection range and the detection performance of radar will be improved via IAR-MTD. Compared with MTD, Radon Fourier transform and Keystone transform, IAR-MTD have a better integration performance and a less computation complexity.3. For the weak targets with a constant radial acceleration, an improved axis rotation fractional Fourier transform(IAR-FRFT) algorithm is proposed to compensate the range/Doppler migration and realize the coherent integration simultaneously. IAR-FRFT could eliminate the linear range migration and alleviate the nonlinear range migration via IAR, and realize the coherent integration via FRFT. The coherent integration time and the freedom-of-degree for detecting multi-target are increased via a novel concept, i.e., an equivalent Doppler frequency. The simulation examples show, compared with twice second-order keystone transform, IAR-FRFT has a lower computational complexity because FRFT can be realized by Fast Fourier transform; compared with FRFT, IAR-FRFT has a longer coherent integration time, a better integration gain and a better mutli-target detection performance. For the maneuvering weak target, an improved axis rotation discrete chirp-Fourier transform(IAR-DCFT) algorithm and a multi-range-cell IAR-DCFT(MR-IAR-DCFT) are proposed respectively. MR-IAR-DCFT is the generalization of IAR-DCFT. When the target’s echoes could be concentrated in a single range cell after IAR, IAR-DCFT could coherently integrate well. When the targets’ echoes still distribute in some neighbor range cells after IAR, MR-IAR-DCFT should be adopted via a sliding window operation. Thus, MR-IAR-DCFT could compensate the range/Doppler migration and realize the coherent integration effectively. The simulation examples show that IAR-DCFT is the optimal estimator and detector; IAR-DCFT and MR-IAR-DCFT could increase CIT and improve the coherent integration gain effectively, and could detect multi-target via several freedom-of-degree, i.e., target’s initial range, initial velocity, equivalent velocity, initial acceleration and jerk.4. To detect the weak targets with beam migration, a novel tri-dimensional time model(i.e. fast time, slow time, and beam time) and a novel tri-dimensional signal model which based on the time model are given firstly. Subsequently, to estimate the target’s tangency velocity, we extend the concept of Doppler frequency to tangency Doppler frequency. The tangency Doppler frequency may be a potential way to estimate the target’s tangency velocity. Then, according to the presented models, we propose two multi-beam associated(MBA) coherent integration algorithms(CIA) based on time-shared multi-beam(TSMB) and space-shared multi-beam(SSMB), respectively. MBACIA-TSMB and MBACIA-SSMB could both eliminate beam migration via associating multi beams and realize coherent integration via discrete Fourier transform. The simulation examples show that the detection performance of MBACIA-SSMB is better than that of MBACIA-TSMB. But, what is sacrificed is the increasing of system complexity. So, in some applications, a hybrid scheme based on the two algorithms may be a good compromise between system complexity and detection performance. Moreover, the capability of estimating target’s radial velocity and tangency velocity is testified. Although the precision of tangency Doppler frequency is lower than that of Doppler frequency, a potential way of estimating tangency velocity via long coherent integration is presented.