Dynamic change of ERPs related to selective attention to signals from left and right visual field during head-down tilt

To study further the effect of head-down tilt (HDT) on slow positive potential in the event-related potentials (ERPs), the temporal and spatial features of visual ERPs changes during 2 hour HDT (-10 degrees) were compared with that during HUT (+20 degrees) in 15 normal subjects. The stimuli were consisted of two color LED flashes appeared randomly in left or right visual field (LVF or RVF) with same probability. The subjects were asked to make switch response to target signals (T) differentially: switching to left for T in LVF and to right for T in RVF, ignoring non-target signals(N). Five sets of tests were made during HUT and HDT. ERPs were obtained from 9 locations on scalp. The mean value of the ERPs in the period from 0.32-0.55 s was taken as the amplitude of slow positive potential(P400). The main results were as follows. 1) The mean amplitude of P400 decreased during HDT which was more significant at the 2nd, 3rd and 5th set of tests; 2) spatially, the reduction of mean P400 amplitude during HDT was more significant for signals from RVF and was more significant at posterior and central brain regions than that on frontal locations. As that the positive potential probably reflects the active inhibition activity in the brain during attention process, these data provide further evidence showing that the higher brain function was affected by the simulated weightlessness and that this effect was not only transient but also with interesting spatial characteristics.     

Bone loss during long term space flight is prevented by the application of a short term impulsive mechanical stimulus

In long term space flight, the mechanical forces applied to the skeleton are substantially reduced and are altered in character. This reduced skeletal loading results in a reduction in bone mass. Exercise techniques currently used in space can maintain muscle mass but the mechanical stimulus provided by this exercise does not prevent bone loss. By applying an external impulsive load for a short period each day, which is intended to mimic the heel strike transient, to the lower limb of an astronaut during a long term space flight (5 months), this study tests the hypothesis that the bone cells can be activated by an appropriate external mechanical stimulus to maintain bone mass throughout prolonged periods of weightlessness. A mechanical loading device was developed to produce a loading of the os-calcis similar to that observed during the heel strike transient. The device is activated by the astronaut to provide a transient load to the heel of one leg whilst providing an equivalent exercising load to the other leg. During the EUROMIR95 mission on the MIR space station, an astronaut used this device for a short period daily throughout the duration of the mission. Pre- and post-flight measurements of bone mineral density (BMD) of the os-calcis and femoral neck of the astronaut were made to determine the efficacy of the device in preventing loss of bone mineral during the mission. On the os-calcis which received the mechanical stimulus, BMD was maintained throughout the period of the flight, while it was reduced by up to 7% on the os-calcis which received no stimulus. Post-flight, BMD in both the stimulated and non-stimulated os-calcis reduces, the extent of this reduction however is less in the stimulated os-calcis. For the femoral neck, the mechanical stimulation does not produce a positive effect.     

静磁波振荡器的现状

当前在增加带宽和提高工作频率方面,要求不断提高现代雷达相通讯系统的性能,这就需要研究关于模拟信号处理的新技术和新工艺。实际上,用于上述目的的声表面波(SAW)器件,已经得到广泛应用,成效显著;但是对于要求中心频率高于1GHz的雷达和通讯系统,声表面波器件在制造上已经接近实际工艺的物理极限。本文介绍一种新工艺,能得到更高频率。这种工艺是以在外延铁淦氧膜(类似钇铁柘榴石[YIG])中传播的静磁波(MSW)为基础的。静磁波用于器件,使它能在0.5到30GHz微波中心频率上提供高达2.2GHz瞬时带宽。这种静磁波信号处理工艺,是在声表面波采用的横向滤波概念的基础上建立的,已经广泛研究了十年之久。本文首先研究静磁波的物理性质和极限,讨论它与声表面波的异同点。其次,讨论静磁波谐振器,并介绍静磁波振荡器的理论及现状。     

一种自动化高精度相位测量系统

一种采用了多种已有技术、具有多种特点的全自动测量系统已研制成功。这个系统为双混频时差技术的扩展而保持了它的重要特点:间歇时间为零,能进行相位差绝对测量,精度很高,能测量频率相同的振荡器并根据使用人选择的时间进行测量。对某一套设计参数来说,理论分辨度为0。2ps,测量噪声为2ps(有效值)并可在所选时间的0.1s内进行测量。由于增加了定标器以消除过去实验中发生的周期模糊(CyCle ambiguify),双混频时差技术得到了发展。在这方面,系统的功能相当于一个分频器加上钟,对设备中被测的每一个器件的恒定相位延迟进行贮存。自动化是以ANSI/IEEE—583(CAMAC)接口标准为基础的。每个测量通道包括一个混频器,过零检波器、定标器和时间间隔计数器。一个两倍宽度的标准卡马克(CAMAC)组件中安装有四个通道。这个组件安装在一个标准的CAMAC箱内,备有控制器以便与多种计算机以及与任何IEEE—488兼容的装置相对接。现有两个系统已工作数月,一个系统每天工作24小时,从NBS时标的15个钟取得数据,另一个系统在实验室的短期实验中使用。     

首次从理论上阐明石英谐振器在室温、低温下1/f噪声和Q值的关系

石英晶体振荡器的极限稳定度是由时域噪声本底相对应的石英谐振器频率噪声决定的。只有减小电子噪声的起伏,才能测量谐振器本身的谐振频率起伏。使用π型传输网络和相位平衡电桥,能够将激励源的相位起伏抑制50-60dB。此测量系统曾在室温下测量了大量的成对谐振器,发现1/f 频率噪声电平和 Q 值之间的相互关系遵循1/Q~4规律。为了将谐振器频率起伏的测量扩展到极低温,研究了一种能测不同谐振器对的新测量系统。因此,在4K 和1K 温度下测得了1/f 频率噪声,并观察到随着 Q 值的增加而噪声大大减小。现在有一种理论能解释1/Q~4规律。这就是根据声波衰减和速度变化引起的三声子相互作用过程来解释。声波衰减和速度变化这两种现象均可用热声子的张弛时间来加以说明。     

利用转发的GPS信号测定弹道

本文研究了这样一种试验飞行器弹道测定技术的性能,即GPS信号不在飞行器上处理,而经转发器中继到一个或多个远方接收站,随后再作信号和数据处理。在这种场合,弹道测定性能将取决于转发器的设计方案,接收站设备配备和布局,以及用于弹道估计的技术。这里研究的基本处理技术使用沿 NAVSTAR-转发器-接收站路径上的距离和或者距离差测量值。理论上讲距离和处理技术的性能一般较好,总不会比距离差处理技术的性能差。本文给出了计算结果,结果表明在转发方案中,这两种处理技术之间的性能差别可能会相当大。成功地开发这种性能差别将取决于系统设计方案,特别取决于转发器延时和接收机定时误差的控制和消除,而且还必须使接收站的布局相对于试验飞行器轨道的相对几何最佳化。     

用于航天器姿态确定的卡尔曼滤波

一、引言这篇报告回顾了过去二十年中卡尔曼滤波在姿态确定中的应用及发展,这一回顾不是面面俱到,而只限于适合于配有三轴陀螺仪及姿态传感器的航天器姿态的算法。我们认为这一种航天器系统是最适合于使用卡尔曼滤波的。卡尔曼滤波器使用了系统随时间变化的动态模型以及这样一种传感器测量模型,即基于当前以及以前的测量值利用线性估计器而获得系统状态的最高精度估计的测量模型。因此,从     

用于动态精密定位的GPS多卜勒处理

在本文中我们证明了巧妙地利用从C/A码得到的所有信息,可以获得高于报道的P码精度,而且成本低廉。可是这一性能只限于低动态、低人为干扰环境,即适用于典型的民用场合。P码的优势主要在于对付高动态(喷气飞机和导弹)和人为干扰环境。本文给出了在静态和低动态情况下,利用来自TRIMBLE 4000接收机的码相位、载波相位、多卜勒和多卜勒积分测量值所得定位结果。可以看到,特别在低动态情况下,可以获得高于10cm的相对(差分)定位精度。经短时间的起始校准和跟踪之后,每秒可得多组定位解。这种精度的提高来源于用积分多卜勒辅助码相位测量值。每秒的定位(极坐标或笛卡尔坐标)基于一组独立的完整测量数据,不需要假设的模型、内插和外推。     

美国航天飞机的通信与测控

航天飞机是具有空间飞行器和飞机特性,可重复使用的一种空地运输工具,它是美国八十年代初开始试飞的新型载人航天任务。整个任务期间,要经历动态环境差异极大的发射入轨、在轨飞行、会合对接和返回着陆等阶段,因而对其的通信与测控也显得十分复杂。本文说明了这些特点,并分阶段介绍了航天飞机任务对通信测控的要求,以及实现这些要求所采用的各种通信、测控和导航手段。最后简要地介绍了参与航天飞机通信测控的现有和未来的各系统以及有关的各航天中心。