Interference Suppression For Space-borne SAR And Precise Imaging Of Space Object

Space-borne surveillance radar is an important part of the all-in-one system for ground-sea-air-space strategic early warning and monitoring. Based on a variety of technical means and operating modes such as synthetic aperture radar(SAR) imaging, inverse synthetic aperture radar(ISAR) imaging and moving target indicator, space-borne surveillance radar have the capability of ground and sea surveillance as well as space surveillance. The observation platform of space-borne surveillance radar runs on the outer space orbit, which overcomes the limit of the earth curvature. Consequently, space-borne surveillance radar has the advantages of global coverage, all-time capability, free access to the hostile areas and outstanding anti-destroy ability. Furthermore, space-borne surveillance radar can offer continuous monitoring of any hot spots around the world and sufficient pre-warning time for strategic defense. Thus, space-borne surveillance radar can effectively make up for the disadvantages of the traditional ground-based, sea-based and air-borne surveillance systems and has been widely used in both civil and military areas.The space-borne surveillance radar system can be divided into earth surveillance module and space surveillance module. For the earth surveillance module, the electromagnetic environment is relatively complicated in the radiation area of space-borne surveillance radar, including the unintentional interference and a variety of artificial narrow-band and wide-band interferences for electronic countermeasures. Due to the wide-band characteristic of the echo signal, the earth surveillance module is easy to be affected by above interference sources. For the space surveillance module, due to the limited power-aperture-product and relatively long operating range in space targets observation, the echo signal-to-noise ratio(SNR) declines. This brings serious jump errors and drift errors in range alignment, causing harmful effect on ISAR imaging quality and leading to difficulty for image interpretation and target identification as well.This dissertation addresses the practical problem in the earth surveillance module and the space surveillance module of space-borne surveillance radar system, such as SAR interference suppression in complicated electromagnetic environment and ISAR range alignment method under low SNR. The main work can be summarized as follows:1. The basic operating modes of space-borne surveillance radar system as well as their characteristics are introduced. Taking broadside strip-map mode as an example, the basic echo signal model of earth surveillance module is analyzed. The possible types of interferences in earth surveillance module as well as their signal model and basic characteristics are summarized. Taking account the characteristics and tasks of space-borne surveillance radar, the tentative operating model of space surveillance module is given. Taking linear motion model as an example, the basic echo signal model of space surveillance module is given. According to the slant distance expansion equation of one scattering point, the major problems in space surveillance module and their inducements are analyzed.2. Based on the time variant signal model of narrow-band interference(NBI) and taking a full consideration about the time variant characteristics, the problem of time variant NBI suppression is thorough researched. Under the condition of time variant NBI, the limitation of training sample size causes great performance degradation of the conventional NBI suppression algorithms. A novel RFI suppression method using iterative adaptive approach(IAA) and orthogonal subspace projection(OSP) method is proposed for SAR. Dispensing with parametric search and model order estimation, the proposed method estimates the NBI power spectrum adaptively and iteratively, utilizing few training samples and filtering the NBI based on the OSP method. Both the simulation and experimental results are provided to illustrate the performance of the proposed method.3. Under the condition of wide band interference(WBI) with the characterist ics of good time-frequency concentration but high non-stationarity, the limited time width of the window function in short-time Fourier transform(STFT) causes limited instantaneous frequency(IF) resolution and leads to great performance degradation of the conventional WBI suppression algorithm based on time-frequency filtering(TFF) method. A novel WBI suppression method using IAA and OSP method is proposed for SAR. Dispensing with parametric search and model order estimation, the proposed method improves the IF resolution in STFT by means of the IAA method and filters the WBI based on the OSP method, meanwhile, obtains time-frequency distribution(TFD) with 2-D high resolution and no cross-terms. Both the simulation and experimental results are provided to illustrate the performance of the proposed method.4. Under the condition of low SNR, the correlation of neighboring returns decreases and leads to great performance degradation of the conventional range alignment methods. An iterative range alignment method based on iterative cleaning and weighted least squares(ICWLS) is proposed for ISAR imaging. Based on the smoothly moving characteristics of the orbit targets, the range migration of the echo envelopes are modeled as a polynomial and estimated iteratively with the weighted matrix. Thus the range alignment is realized by the estimation of the motion parameters of the moving target. Besides, a cleaning operation is proposed to eliminate the jump errors. Thus the range alignment precise is greatly improved by the proposed method. However, the weighted matrix is approximated as a diagonal matrix based on the error punishing principle, which is not the statistically optimal form. Therefore, in order to further optimize the range alignment performance, another iterative range alignment method based on optimized weighted fitting(Iterative Optimized Weighted Fitting, IOWF) is proposed, in which the coefficients of the above mentioned polynomial are evaluated by optimized weighted fitting. The comparison with the traditional methods shows that the proposed method can effectively restrain the transmission of the jump errors and relieve the decline of the parameter estimation precision caused by the drift errors, especially for the target under the low signal-to-noise ratio(SNR) background.