一种低信噪比下PN码的快速捕获技术

直扩系统中,为了硬件实现的简便,PN码的捕获一般都采用串行搜索的方法.但当PN码很长时,由于搜索相位点很多,串行搜索法将使捕获时间过长.并且当信噪比较低时,会有较高的虚警概率.这些都会使系统的捕获性能降低.提出一种基于匹配滤波器的快速捕获算法,改善了在极低信噪比下的性能.降低了平均捕获时间.     

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白噪声Kalman滤波模型在GPS单向授时中的应用

全球定位系统(GPS)的应用越来越广泛,尤其是高精度的星载原子钟使得授时的精度得到了很大的提高。针对GPS授时过程中接收机受到各种噪声的影响,利用Kalman滤波原理,建立状态方程和观测方程,对噪声进行分类并加以讨论。运用Kalman滤波原理对接收机钟差数据进行分析,计算结果表明Kalman滤波可提高GPS单向授时精度。     

δC and δN biogeographic trends in rocky intertidal communities along the coast of South Africa: Evidence of strong environmental signatures

Ecosystem dynamics driven by top-down controls have been well documented in rocky intertidal communities, while the effects of bottom-up influences are comparatively poorly understood. We hypothesized that large-scale signatures of the physical environment may be identifiable along the South African coastline as it is subject to two very different current systems (Benguela and Agulhas Currents) that profoundly influence primary production and thus both food type and availability. Through stable isotope analysis, we examined biogeographic patterns in multiple trophic levels at four sites along a 1400-km stretch of South African coastline and investigated the dietary role of macroalgal-derived organic carbon in rocky intertidal communities. The general positioning of trophic groups was comparable across all sites, with animals from the same trophic levels grouping together and with a δ¹⁵N fractionation of 1–2‰ between levels. The species found at all sites demonstrated east–west δ¹⁵N enrichment, presumably reflecting a biogeographic shift in nitrogen sources linked to upwelling on the west coast. Filter-feeders gave particularly clear results. Using discriminant analysis, mussels could be categorized into four geographic groups based on carbon and nitrogen signatures: east coast, southeast coast, south-west coast and west coast. Barnacles and polychaetes showed similar geographic groupings to mussels, but with shifts in actual values (1‰ depletion in δ¹³C and 3‰ enrichment in δ¹⁵N relative to mussels). This suggests that fractionation varies between species within a trophic level.     

Adsorption of extracellular proteases in seawater onto filters during size fractionation

In aquatic environments extracellular enzymes are bound to microbial cells or exist in a free and adsorbed state. Various filters have been used to fractionate these enzymatic activities, but enzymes may be readily adsorbed onto some materials, and such adsorption can induce errors in the estimation of enzymatic activity. In this study we examined three filters to determine the most suitable filter for fractionation when estimating proteolytic enzyme activity in seawater. We found that the polycarbonate Nuclepore membrane, widely used for size fractionation because of its pore-size accuracy, was the most favorable for this purpose, even though it adsorbed slightly more enzymes than the low-protein-binding polyethersulfone membrane. We also found that trypsin-and chymotrypsin-type enzymes were more easily adsorbed than aminopeptidases.     

Cobalt, manganese, and iron near the Hawaiian Islands: A potential concentrating mechanism for cobalt within a cyclonic eddy and implications for the hybrid-type trace metals

The vertical distributions of cobalt, iron, and manganese in the water column were studied during the E-Flux Program (E-Flux II and III), which focused on the biogeochemistry of cold-core cyclonic eddies that form in the lee of the Hawaiian Islands. During E-Flux II (January 2005) and E-Flux III (March 2005), 17 stations were sampled for cobalt (n=147), all of which demonstrated nutrient-like depletion in surface waters. During E-Flux III, two depth profiles collected from within a mesoscale cold-core eddy, Cyclone Opal, revealed small distinct maxima in cobalt at 100 m depth and a larger inventory of cobalt within the eddy. We hypothesize that this was due to a cobalt concentrating effect within the eddy, where upwelled cobalt was subsequently associated with sinking particulate organic carbon (POC) via biological activity and was released at a depth coincident with nearly complete POC remineralization [Benitez-Nelson, C., Bidigare, R.R., Dickey, T.D., Landry, M.R., Leonard, C.L., Brown, S.L., Nencioli, F., Rii, Y.M., Maiti, K., Becker, J.W., Bibby, T.S., Black, W., Cai, W.J., Carlson, C.A., Chen, F., Kuwahara, V.S., Mahaffey, C., McAndrew, P.M., Quay, P.D., Rappe, M.S., Selph, K.E., Simmons, M.P., Yang, E.J., 2007. Mesoscale eddies drive increased silica export in the subtropical Pacific Ocean. Science 316, 1017–1020]. There is also evidence for the formation of a correlation between cobalt and soluble reactive phosphorus during E-Flux III relative to the E-Flux II cruise that we suggest is due to increased productivity, implying a minimum threshold of primary production below which cobalt–phosphate coupling does not occur. Dissolved iron was measured in E-Flux II and found in somewhat elevated concentrations (0.5 nM) in surface waters relative to the iron depleted waters of the surrounding Pacific [Fitzwater, S.E., Coale, K.H., Gordon, M.R., Johnson, K.S., Ondrusek, M.E., 1996. Iron deficiency and phytoplankton growth in the equatorial Pacific. Deep-Sea Research II 43 (4–6), 995–1015], possibly due to island effects associated with the iron-rich volcanic soil from the Hawaiian Islands and/or anthropogenic inputs. Distinct depth maxima in total dissolved cobalt were observed at 400–600 m depth, suggestive of the release of metals from the shelf area of comparable depth that surrounds these islands.     

基于卡尔曼滤波的INS／USBL水下导航系统模型研究

简述惯性导航技术(INS)和超短基线定位技术(USBL),并分析了各自的优缺点.为了满足深海作业对导航定位高精度、高数据更新率的技术需求,提出基于卡尔曼滤波技术(Kalman Filter)实现INS和USBL的融合集成,研究了系统模型,进行了水下定位试验,给出了误差分析结果,证明了模型的可靠性.     

基于序列非线性滤波SAR影像水体自动提取

由于合成孔径雷达（SAR）图像中固有斑点噪声的强烈影响,某些对于光学图像有很好效果的目标提取区域分割方法,但是对于SAR图像来说,效果不很好,针对水体目标的亮度及形状分布特征,不作阈值分割处理,而采用序列非线性滤波处理方法,可以快速有效地实现SAR影像水体目标的自动提取识别。     

分数阶微积分在医学图像处理中的应用

简要介绍了分数阶微积分和医学图像的概念,然后从分数阶微分基本定义出发,得到了可以作用于二维医学图像的分数阶微分掩模,掩模可以根据对图像的需求进行增强,最后通过实验证明,这个方法可以有效完成对医学图像的处理,并且弥补了传统方法不能连续改变处理效果的缺点,是一种简单可行并且效果较好的图像增强方法。     

一种新ⅡR分数阶微分滤波器

从频域角度出发,分析了几种基于典型微分算子的ⅡR分数阶微分滤波器的数字实现,提出一种频率响应更接近理想微分的新算子,在此基础上运用连续分数扩充方法实现了分数阶微分滤波器的设计,并详细推导基于新算子的ⅡR分数阶微分滤波器的数字实现。实验证明,基于新算子的ⅡR分数阶微分滤波器频率响应整体上优于Tustin和Simpson等其他ⅡR分数阶滤波器。