一种适用于大频偏和低信噪比条件的频率估计器研究

为了实现对大频偏和低信噪比条件下信号载波频率快速估计,在比较现有常用估计器基础上,提出一种改进频率估计方案,可以将估计器估计范围提高到数据速率的一半而噪声性能接近MCRB下界。通过仿真和硬件实现,验证了改进频率估计器对大频偏和低信噪比的适用性。
     

Corotating Interaction Regions at High Latitudes

Ulysses observed a stable strong CIR from early 1992 through 1994 during its first journey into the southern hemisphere. After the rapid latitude scan in early 1995, Ulysses observed a weaker CIR from early 1996 to mid-1997 in the northern hemisphere as it traveled back to the ecliptic at the orbit of Jupiter. These two CIRs are the observational basis of the investigation into the latitudinal structure of CIRs. The first CIR was caused by an extension of the northern coronal hole into the southern hemisphere during declining solar activity, whereas the second CIR near solar minimum activity was caused by small warps in the streamer belt. The latitudinal structure is described through the presentation of three 26-day periods during the southern CIR. The first at ∼24°S shows the full plasma interaction region including fast and slow wind streams, the compressed shocked flows with embedded stream interface and heliospheric current sheet (HCS), and the forward and reverse shocks with associated accelerated ions and electrons. The second at 40°S exhibits only the reverse shock, accelerated particles, and the 26-day modulation of cosmic rays. The third at 60°S shows only the accelerated particles and modulated cosmic rays. The possible mechanisms for the access of the accelerated particles and the CIR-modulated cosmic rays to high latitudes above the plasma interaction region are presented. They include direct magnetic field connection across latitude due to stochastic field line weaving or to systematic weaving caused by solar differential rotation combined with non-radial expansion of the fast wind. Another possible mechanism is particle diffusion across the average magnetic field, which includes stochastic field line weaving. A constraint on connection to a distant portion of the CIR is energy loss in the solar wind, which is substantial for the relatively slow-moving accelerated ions. Finally, the weaker northern CIR is compared with the southern CIR. It is weak because the inclination of the streamer belt and HCS decreased as Ulysses traveled to lower latitudes so that the spacecraft remained at about the maximum latitudinal extent of the HCS. This revised version was published online in August 2006 with corrections to the Cover Date.     

Resonance transitions associated to weak capture in the restricted three-body problem

An interesting dynamics is studied in the restricted three-body problem where a particle abruptly transitions between resonance states, called a resonance hop. It occurs in a region about the secondary mass point which supports weak capture. This region, called a weak stability boundary, was recently proven to give rise to chaotic dynamics. Although it was numerically known that the resonance hop was associated with this boundary, this process was not well understood. In addition, the dynamical structure of the weak stability boundary has not been well understood. In this paper, we give a way to reveal the global structure of the weak stability boundary associated to resonance motions. This structure is shown to be surprisingly rich in resonant periodic motions interconnected by invariant manifolds. In this case, nearly all the motions are approximately resonant in nature where resonance hops can occur. The correlation dimension of orbits undergoing resonant motions, associated to the weak stability boundary, is also examined. The dynamics analyzed in the present paper is related to that studied by J. Marsden et al. under the perspective of Lyapunov orbits and the associated invariant manifolds. Applications are discussed.     

一种毫米波段圆锥共形低副瓣微带天线设计

设计了一种工作在毫米波段的1×8圆锥共形低副瓣微带天线阵列。分析了T型结构功分器的基本原理,采用底边馈电方式设计矩形微带贴片单元,利用该单元和并联馈电网络构成1×8等功分微带天线阵,根据T型结构功分器原理设计了一个服从切比雪夫分布的1×8圆锥共形低副瓣微带天线阵。采用基于有限积分算法(FI)的CST三维电磁仿真软件对两种天线阵进行了分析和对比,结果表明两者均工作在毫米波段,后者虽然实现了低副瓣性能,但是相对于前者,其增益稍有下降且波束宽度有所展宽。     

从地球到日-火系平动点轨道的转移

平动点轨道特殊的空间位置及动力学特征,使其在深空探测中具有重要的应用。以日-火系平动点轨道(Lissajous与Halo轨道)任务为目标,结合平动点轨道的不变流形理论,研究了小推力转移问题。首先给出了圆型限制性三体动力学模型下平动点附近不变流形(稳定和不稳定流形)高阶分析解以及相应的计算实例。接着以流形分析解为基础,建立了初始小推力轨道优化模型,并利用改进的协作进化算法求解初始小推力轨道。最后将初始轨道离散,采用多点打靶法将最优控制问题转化为参数优化问题,并用序列二次规划方法(SQP)求解。仿真结果证明轨道设计方法的有效性。     

Space radiation environment in low earth orbit during influences from solar and geomagnetic events in December 2006

In low earth orbit, the SAA region is the dominant contributor to both proton environment and electron environment from the standpoint of radiation dose for spacecraft lifetime. However, the polar region and the horn region are sometimes strongly disturbed due to large solar and geomagnetic events. During large disturbances, enhancements in proton flux are measured in the polar region, which gives temporary more severe space radiation environment than that given in the SAA region. On the other hand, enhancements in electron flux are measured mainly in the horn region corresponding to the outer radiation belt, which are likely sources of high-energy electrons in the inner radiation belt. These short-term disturbances have another radiation hazard to spacecraft such as single event and electrostatic discharge.     

一种有电压补偿电路的高性能低压CFOA及其小信号分析

提出了一种新型的电流反馈型运算放大器,该电路所具有的电压补偿电路克服了以往基于CMOS的CFOA两个输入端静态条件下不平衡的缺点,NMOS与PMOS互补的输入结构提高了电路的信噪比。采用0.6μm工艺参数(低阈值),1.5V电源电压,利用HSPICE进行了仿真,获得了4.95mW的功耗,4.5 kΩ的输入电阻,53.5°的相位裕度以及与增益无关的带宽和极大的转换速率。     

Semi-global leader-following consensus-based formation flight of unmanned aerial vehicles

In this paper, we investigate a formation control problem of multi-agent systems(specifically a group of unmanned aerial vehicles) based on a semi-global leader-following consensus approach with both the leader and the followers subject to input saturation. Utilizing the low gain feedback design technique, a distributed static control protocol and a distributed adaptive control protocol are constructed. The former solves the problem under an assumption that the communication network is undirecte...     

乌鲁木齐机场近30年低能见度天气特征分析

应用乌鲁木齐机场30年(1981-2010)的观测资料,分析了该机场低能见度天气特征和变化趋势.结果表明:低能见度日数增长趋势明显;造成机场低能见度的主要天气是雾、浓烟和沙尘暴;95％的低能见度天气主要出现在冬半年,仅有5％的低能见度天气出现在夏半年;近年来大雾和浓烟日数明显增长,沙尘暴日数却明显降低,新疆地区气候向暖湿化的转化可能是出现这种变化的主要原因.     

CFRP/钛合金叠层构件低温钻削的轴向力与成孔质量研究

CFRP和钛合金叠层构件以其优异的性能广泛应用于航空、航天、汽车等领域。二者由于材料加工特性的差异,需要采用不同的钻削工艺参数才能获得满意的加工质量。为保证孔同轴度和加工效率,通常采用同一参数一体钻孔。一体钻孔会导致孔的界面烧伤、复材表面划伤、孔径一致性差等问题。为解决上述问题,提出一种低温钻孔工艺,通过钻削试验,研究低温条件下CFRP/钛合金叠层构件钻孔的轴向力、孔径一致性和表面质量。试验表明,低温条件下CFRP/钛合金叠层构件钻孔的轴向力增加,孔径一致性和表面质量均得到改善,证明了低温下CFRP/钛合金叠层构件一体钻孔的可行性。