Propagation velocity uncertainty on GPR SAR processing

To detect buried landmines, Planning Systems Incorporated (PSI) has developed a ground-penetrating synthetic aperture radar (GPSAR) system. Since the electromagnetic wave propagation velocity in the soil depends on many factors, velocity uncertainty is inevitable. However, we have observed that, unlike conventional airborne/spaceborne synthetic aperture radar (SAR) systems, the PSI GPSAR system is very robust against the velocity uncertainty under mild conditions. Theoretical analysis is provided to explain this observation. Although our discussion is based on the PSI GPSAR system, it applies to other GPR-based (ground-penetrating radar) landmine detection systems as well.     

Basic performance of BeiDou-2 navigation satellite system used in LEO satellites precise orbit determination

The visibility for low earth orbit（LEO） satellites provided by the BeiDou-2 system is analyzed and compared with the global positioning system（GPS）. In addition, the spaceborne receivers＇ observations are simulated by the BeiDou satellites broadcast ephemeris and LEO satellites orbits. The precise orbit determination（POD） results show that the along-track component accuracy is much better over the service area than the non-service area, while the accuracy of the other two directions keeps at the same level over different areas. However, the 3-dimensional（3D） accuracy over the two areas shows almost no difference. Only taking into consideration the observation noise and navigation satellite ephemeris errors, the 3D accuracy of the POD is about30 cm. As for the precise relative orbit determination（PROD）, the 3D accuracy is much better over the eastern hemisphere than that of the western hemisphere. The baseline length accuracy is 3.4 mm over the service area, and it is still better than 1 cm over the non-service area. This paper demonstrates that the BeiDou regional constellation could provide global service to LEO satellites for the POD and the PROD. Finally, the benefit of geostationary earth orbit（GEO） satellites is illustrated for POD.     

TOPOLOGY OF CONFIGURATION SPACE FOR A THREE-BODY PROBLEM

考虑了限制在球面Ｓ２上运动的三个质点的力学系统，该系统的构型空间是Ｓ２上的一个纤维丛。本文证明了这个丛的第１陈省身示性类等于－２ｃ１（γ），其中γ是复射影空间ＣＰ１＝Ｓ２上的典范线丛。这个结果蕴涵该构型空间是Ｓ２上的非平凡丛。由此又算出了它的上同调群与同伦群。此外还讨论了上述拓扑性质对于诸如平衡位置的数目，周期轨道及约化相空间等涉及系统大范围性状的影响。     

STUDY ON FAST TRANSVERSAL FILTER (FTF)ALGORITHM FOR ACTIVE CONTROL OF STRUCTURAL RESPONSE (ACSR) OF HELICOPTER

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Energy absorption features of 3-D braided rectangular composite under different strain rates compressive loading

There are huge potential applications of 3-D braided composite in aerospace engineering because of the non-delamination feature of the composite under impact loading. This paper presents the analysis of energy absorption features of 3-D braided composite under compression with different strain rates. The 3-D 4-step rectangular braided composite coupons were tested on a material tester MTS 810.23 and a split Hopkinson pressure bar (SHPB) apparatus to obtain out-of-plane and in-plane compression stress vs. strain curves at quasi-static and high strain rate state. The failure modes and energy absorption features of the 3-D braided composite under different strain rates were analyzed both in time domain and frequency domain. The energy absorbed by the 3-D braided composite increases with the strain rate. From fast Fourier transform (FFT) analysis of compression stress vs. time histories, the power of energy absorption of the 3-D braided composite increases with strain rate and mostly concentrate on the high frequency region. While for quasi-static compression, the power distributes in very narrow frequency region and also is less than that in high strain rates. This feature corresponds to the different damage and energy absorption mechanisms of the 3-D braided composite under quasi-static and high strain rate compression.     

基于两级优化策略的导弹可变翼优化设计

针对多任务、可变翼导弹的外形优化设计需求,提出了一种两级优化方法,研究变形弹翼对导弹总质量和飞行性能的影响。在给定的任务和基准方案下,采用遗传算法和模式搜索法混合优化策略,完成了弹翼优化。结果显示,优化后的导弹发射质量和燃料消耗分别减小了5.46%和15.25%。     

A simplified data assimilation method for reconstructing time-series MODIS NDVI data

The Normalized Difference Vegetation Index (NDVI) is an important vegetation index, widely applied in research on global environmental and climatic change. However, noise induced by cloud contamination and atmospheric variability impedes the analysis and application of NDVI data. In this work, a simplified data assimilation method is proposed to reconstruct high-quality time-series MODIS NDVI data. We extracted 16-Day L3 Global 1 km SIN Grid NDVI data sets for western China from MODIS vegetation index (VI) products (MOD13A2) for the period 2003–2006. NDVI data in the first three years (2003–2005) were used to generate the background field of NDVI based on a simple three-point smoothing technique, which captures annual features of vegetation change. NDVI data for 2006 were used to test our method. For every time step, the quality assurance (QA) flags of the MODIS VI products were adopted to empirically determine the weight between the background field and NDVI observations. Ultimately, more reliable NDVI data can be produced. The results indicate that the newly developed method is robust and effective in reconstructing high-quality MODIS NDVI time-series.     

基于光分配网络ODN的数据中心设计

城市光纤化将大力推进城市系统物联化和互联化的发展,民用航空机场一般采用光纤入户(FTTH)作为光网覆盖的解决方案.要建设高效率低成本的FTTH光缆网络,其组网理念、产品选择以及施工方式需综合考虑.以杭州萧山国际机场国际航站楼综合布线系统为背景,给出了光分配网络ODN(optical distribution network)的设计规划、选材和施工等关注的重点,对光分配网络建设进行了研究.     

Cross-layer design of LT codes and LDPC codes for satellite multimedia broadcast/multicast services

According to large coverage of satellites, there are various channel states in a satellite broadcasting network. In order to introduce an efficient rateless transmission method to satellite multimedia broadcasting/multicast services with finite-length packets, a cross-layer packet transmission method is proposed with Luby transform （LT） codes for efficiency in the network layer and low density parity check （LDPC） codes for reliability in the physical layer jointly. The codewords generated from an LT encoder are divided into finite-length packets, which are encoded by an LDPC encoder subsequently. Based on noise and fading effects of satellite channels, the LT packets received from an LDPC decoder either have no error or are marked as erased, which can be mod- eled as a binary erasure channels （BECs）. By theoretical analysis on LT parameters and LDPC parameters, the relationships between LDPC code rates in the physical layer and LT codes word lengths in the network layer are investigated. With tradeoffs between the LT codes word lengths and the LDPC code rates, optimized cross-layer solutions are achieved with a binary search algorithm. Verified by simulations, the proposed solution for cross-layer parameters design can provide the best transmission mode according to satellite states, so as to improve throughput performance for satellite multimedia transmission.     

无穷观问题的研究(Ⅱ)——从Hausdorff的直觉和Poincaré的名言到Brouwer剧场现象

本组系列论文 ( )～ ( )从数学和认识论的角度系统地研究了无穷观问题的历史发展和现状 ,确立了无穷观背景世界的三分法原则 ,指出了两种无穷观相互排斥的局限性 ,形成了统一两种无穷于同一框架中的基本观点 ,并建立了一个统一实无限与潜无限于同一框架中的公理集合论系统 APAS。