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941.
Early in 1969 the U. S. Air Force placed into synchronous orbit the largest communications satellite built to date. This vehicle, the tactical communications satellite (TACSAT), together with a variety of ground terminals, is designed to test experimentally and develop tactical communications concepts for all military services. This paper describes the spacecraft design focusing on the communications repeater. Measured performance characteristics affecting communications utilization of the spacecraft are presented.  相似文献   
942.
The phenomenon of light flashes (LF) in eyes for people in space has been investigated onboard Mir. Data on particles hitting the eye have been collected with the SilEye detectors, and correlated with human observations. It is found that a nucleus in the radiation environment of Mir has roughly a 1% probability to cause an LF, whereas the proton probability is almost three orders of magnitude less. As a function of LET, the LF probability increases above 10 keV/micrometer, reaching about 5% at around 50 keV/micrometer.  相似文献   
943.
基于哈希链表和时间链表的HTTP代理缓存机制的实现   总被引:4,自引:0,他引:4  
随着互联网的快速发展,越来越多的用户需要访问Internet资源,局域网人的众多用户如何能够通过一条Internet连接进行快速的网络浏览?这涉及到WEB代理服务器的核心部分--HTTP缓存系统。HTTP代理服务器的缓存系统将用户浏览的网页保存在代理服务器中,一旦有用户浏览相同的网页并且该网页还没有更新,则代理服务器直接将网页传递给用户,无需重新下载。因此,缓存明显加快了浏览速度。本文首先介绍了代理服务器的缓存原理,然后对缓存机制的具体功能进行了划分,最后给出基于哈希链表和时间链表的缓存系统的实现方法。  相似文献   
944.
945.
    
Verification of arms control treaties is essential to provide adequate international confidence in treaty compliance. This Viewpoint underlines the responsibility of non-superpower countries to become more closely involved in space surveillance of treaties to which they are parties. In Canada, Paxsat research has focused on two potential applications of space-based remote sensing to multi-lateral arms control verification. The necessary technology is readily available in non-superpower countries for the Paxsat concept to be put into operation.  相似文献   
946.
    
Harris PR 《Space Policy》1989,5(2):147-154
Space exploration is now moving beyond the stage when technical development dominated research. As thinking shifts towards the problems of long-term colonization of the Solar System, the biological and behavioural sciences must make major contributions. This article outlines some of the issues which must be addressed, such as group behaviour and dynamics in space flight, the environment created at space stations and other outposts, recruitment and training of suitable spacefarers, and the planning and governing of space settlements. Ten dimensions of the human aspect of space habitation are described and an indication given of how they may form the basis for a new taxonomy of space planning, operations and management.  相似文献   
947.
Langmuir waves and turbulence resulting from an electron beam-plasma instability play a fundamental role in the generation of solar radio bursts. We report recent theoretical advances in nonlinear dynamics of Langmuir waves. First, starting from the generalized Zakharov equations, we study the parametric excitation of solar radio bursts at the fundamental plasma frequency driven by a pair of oppositely propagating Langmuir waves with different wave amplitudes. Next, we briefly discuss the emergence of chaos in the Zakharov equations. We point out that chaos can lead to turbulence in the source regions of solar radio emissions. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
948.
    
It is a known fact that ionosphere is the largest and the least predictable among the sources of error limiting the reliability and accuracy of Global Navigation Satellite Systems (GNSS) and its regional augmentation systems like Satellite Based Augmentation System (SBAS) in a safety-of-life application. The situation becomes worse in the Equatorial Ionization Anomaly (EIA) region, where the daytime ionization distribution is modified by the fountain effect that develops a crest of electron density at around ±15° to ±20° of the magnetic equator and a trough at the magnetic equator during the local noon hours. Related to this phenomenon is the appearance of ionosphere irregularities and plasma bubbles after local sunset. These may degrade further the quality of service obtained from the GNSS/SBAS system of the said periods. Considering the present operational augmentation systems, the accuracy and integrity of the ionosphere corrections estimate decreases as the level of disturbances increases. In order to provide a correct ionosphere correction to the user of GNSS operating in African EIA region and meet the integrity requirements, a certified ionosphere correction model that accurately characterizes EIA gradient with the full capacity to over-bound the residual error will be needed. An irregularities detector and a decorrelation adaptor are essential in an algorithm usable for African sub-Saharan SBAS operation. The algorithm should be able to cater to the equatorial plasma vertical drifts, diurnal and seasonal variability of the ionosphere electron density and also should take into account the large spatial and temporal gradients in the region. This study presents the assessment of the ionosphere threat model with single and multi-layer algorithm, using modified planar fit and Kriging approaches.  相似文献   
949.
  总被引:4,自引:0,他引:4  
Matched filtering (MF) of phase history data is a mathematically ideal but computationally expensive approach to bistatic synthetic aperture radar (SAR) image formation. Fast backprojection algorithms (BPAs) for image formation have recently been shown to give improved O(N/sup 2/ log/sub 2/N) performance. An O(N/sup 2/ log/sub 2/N) bistatic polar format algorithm (PFA) based on a bistatic far-field assumption is derived. This algorithm is a generalization of the popular PFA for monostatic SAR image formation and is highly amenable to implementation with existing monostatic image formation processors. Limits on the size of an imaged scene, analogous to those in monostatic systems, are derived for the bistatic PFA.  相似文献   
950.
A space diversity method of receiving and processing ILS localizer information has been developed and flight tested. Multiple lateral ly separated antennas are used which sense the ILS signal on and around the approach path. Combining these signals suppresses ILS beam distortion produced by multipath signal interference and provides much improved guidance information to the landing aircraft control system.  相似文献   
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