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981.
Blanding W.R. Willett P.K. Bar-Shalom Y. Lynch R. 《IEEE transactions on aerospace and electronic systems》2008,44(1):201-216
The maximum likelihood probabilistic data association (ML-PDA) tracking algorithm is effective in tracking Very Low Observable targets (i.e., very low signal-to-noise ratio (SNR) targets in a high false alarm environment). However, the computational complexity associated with obtaining the track estimate in many cases has precluded its use in real-time scenarios. Previous ML-PDA implementations used a multi-pass grid (MPG) search to find the track estimate. Two alternate methods for finding the track estimate are presented-a genetic search and a newly developed directed subspace (DSS) search algorithm. Each algorithm is tested using active sonar scenarios in which an autonomous underwater vehicle searches for and tracks a target. Within each scenario, the problem parameters are varied to illustrate the relative performance of each search technique. Both the DSS search and the genetic algorithm are shown to be an order of magnitude more computationally efficient than the MPG search, making possible real-time implementation. In addition, the DSS search is shown to be the most effective technique at tracking a target at the lowest SNR levels-reliable tracking down to 5 dB (postprocessing SNR in a resolution cell) using a 5-frame sliding window is demonstrated, this being 6 dB better than the MPG search. 相似文献
982.
Gordon R. W. MacLean 《Space Policy》1995,11(2)
Since the first attempts of people to communicate over long distances the technology employed has not only become more complex in response to the amount of information that people have wanted to send but has, curiously enough, also exhibited cyclic tendencies in the course of the evolution to the present day. Now another change is occurring with the advent of ever-larger capacity fiber optic systems, and in some circles the thought is voiced ‘what of the future of satellite communications?’ This article addresses some of the issues at play in this environment and provides a perspective for consideration. 相似文献
983.
The purpose of this study was to quantify the gas exchange between plants growing in a Closed Environmental Life Support System (CELSS) and the metabolism of human subjects undergoing various levels of physical exercise, and subsequently determine the buffer characteristics in relation to the carbon exchange established for plants in this closed loop life support system. Two men (ages 42 and 45 yr) exercised on a cycle ergometer at three different work intensities, each on a separate day. The CELSS, a 113 m3 chamber, was sized to meet the needs of one human. The plants, consisting of 20 m2 of potato, provided oxygen to the human during an artificially lighted photosynthesis phase and the human provided CO2 to the plants. The average rates of exchange for the subjects were 0.88, 1.69, and 2.47 liters O2/min and 0.77, 1.47, and 2.21 liters CO2/min at approximately 25%, 50%, and 75% of their maximal aerobic capacity, respectively. The photosynthetic rate for the CELSS was 0.95 liters/ min. A balance between human CO2 production and plant utilization was noted at approximately the 50% VO2max level. The oxygen balance and changes were not within detectable limits of the CELSS instrumentation for the durations of these exercise exposures. If a CELSS environment is the methodology selected for long term spaceflight, it will be important to select plants that efficiently grow at the available light and nutrient levels while balancing the needs for the human crew at their levels of physical activity. 相似文献
984.
Global dependence on the space sector is increasing rapidly. However, the growing challenges outer space poses to international security have not, as yet, been effectively addressed in the United Nations Conference on Disarmament (CD) or in other multilateral fora. This paper briefly reviews the current state of affairs of international security and outer space and explores the challenges the international community faces in building international agreements for improved space security and global stability. It examines some of the hurdles that are faced in tackling space security issues are and provides some thinking on ways to overcome them. The paper concludes that there is a potential role for the disarmament community and the CD in particular. As potential conflicts in outer space emerge, having invested bodies and communities such as the CD and a strong cadre of disarmament diplomats and NGOs to act as an informed hub on space and disarmament issues is essential. 相似文献
985.
Rao N.N. Beckwith R.I. Ernst E.W. 《IEEE transactions on aerospace and electronic systems》1974,(6):765-769
Differential time-delay errors in HF hyperbolic position-fixing systems arising from ionospheric propagation time delays are estimated by employing a propagation prediction model for several path-pairs. A comparison of the prediction model results with experimental observations indicates that the differential time-delay errors can be reduced significantly by the use of the prediction model. 相似文献
986.
Thomas W. Baberg 《Aerospace Science and Technology》2001,5(8):495-504
The early long-range-jets were operated by five cockpit crew members: Captain, Copilot, Flight Engineer, Navigator and Radio-Operator. Nowadays even the big jets are flown by only two pilots. This development could only be achieved by introducing new technologies to the airline industry and automating aircraft systems. Due to the scientific progress made in the last decades today's aircraft are most reliable – a fact which is clearly reflected by accident statistics. Technical failures became more and more unlikely and the lack of manpower on the flight deck was more than compensated. However, starting in the late 80s a number of accidents occurred to highly automated aircraft, which alarmed accident investigators worldwide. In most of the cases automation was a factor or at least one link in the accident chain of events. The Federal Aviation Administration (FAA) launched a study on “The Interfaces Between Flight Crews and Modern Flight Deck Systems” and came up with numerous findings. In the following article I like to discuss the topic ‘Man-Machine-Interface in modern Transport Systems’ from an Aviation Safety Perspective. 相似文献
987.
Cellular signal processing in multi-, as well as in unicellular organisms, has to rely on fundamentally similar mechanisms. Free-living single cells often use the gravity vector for their spatial orientation (gravitaxis) and show distinct gravisensitivities. In this investigation the gravisensitive giant ameboid cell Physarum polycephalum (Myxomycetes, acellular slime molds) is used. Its gravitaxis and the modulation of its intrinsic rhythmic contraction activity by gravity was demonstrated in 180 °turn experiments and in simulated, as well as in actual, near-weightlessness studies (fast-rotating clinostat; Spacelab D1, IML-1). The stimulus perception was addressed in an IML-2 experiment, which provided information on the gravireceptor itself by the determination of the cell's acceleration-sensitivity threshold. Ground-based experiments designed to elucidate the subsequent steps in signal transduction leading to a motor response, suggest that an acceleration stimulus induces changes in the level of second messenger, adenosine 3',5'-cyclic monophosphate (cAMP), indicating also that the acceleration-stimulus signal transduction chain of Physarum uses an ubiquitous second messenger pathway. 相似文献
988.
Thomas W. Becker 《Space Policy》1991,7(1)
The discoveries and knowledge gained from space exploration and technology development are valuable scientific education tools which are not being adequately used in the classroom. Moreover, the increasing application of space technology to everyday life and industry requires a work force educated to be able to work productively in this field and to take advantage of all it has to offer. This article provides a survey of what is presently being done in the field of space education in Europe and the USA. Although encouraging, it is not nearly enough to meet the needs of 21st century society, principally because current efforts lack government or industry support. The author argues that this state of affairs must change, and proposed an international symposium as a first step towards this goal. 相似文献
989.
Ginter S. Gisler G. Hanks J. Havenhill D. Robinson W. Spina L. 《Aerospace and Electronic Systems Magazine, IEEE》1998,13(5):27-32
Flywheel Energy Storage Systems represent an exciting alternative to traditional battery storage systems used to power satellites during periods of eclipse. The increasing demand for reliable communication and data access is driving explosive growth in the number of satellite systems being developed as well as their performance requirements. Power-on orbit is the key to this performance, and batteries are becoming increasingly unattractive as an energy storage media. Flywheel systems offer very attractive characteristics for both energy storage, in terms of energy density and the number of charge/discharge cycles, and the important side benefit of spacecraft attitude control 相似文献
990.
W. Wang A.G. Burns S. Solomon T.L. Killeen 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2005,36(12):2486-2491
Current first-principles global models of the coupled thermosphere–ionosphere (T–I) system use grids that are too coarse to simulate the mesoscale and small-scale structures that occur in this complex system. These small-scale and mesoscale structures have a great effect on global-scale neutral and plasma distributions and have important consequences for daily space weather. In this paper, we present a new first-principles, high-resolution, T–I nested grid (TING) polar cap model that incorporates multiple nesting levels and two-way interaction. The TING model simulation of the electron densities and temperatures demonstrates the importance of high spatial resolution. It is found that both the mid-latitude electron density trough and its associated dawn electron temperature peak are more pronounced and structured in the nested grid than in the coarse grid. Using the TING model to simulate ionospheric F2 region electron density variations with geomagnetic activity and universal time (UT) is also discussed. 相似文献