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273.
R. Musci T. SchildknechtT. Flohrer G. Beutler 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(7):1071-1076
A new population of uncatalogued objects in geosynchronous Earth orbits (GEO), with a mean motion of about 1 rev/day and eccentricities up to 0.6, has been identified recently. The first observations of this new type of objects were acquired in the framework of the European Space Agency’s (ESA) search for space debris in GEO and the geostationary transfer orbit (GTO) using the ESA 1-m telescope on Tenerife. Earlier studies have postulated that the perturbations due to the solar radiation pressure can lead to such large eccentricities for GEO objects with a high area-to-mass ratio (A/M). The simulations showed that the eccentricities of GEO objects with large A/M exhibit periodic variations with periods of about one year and amplitudes depending on the value of A/M. The findings of these studies could be confirmed by observations from the ESA 1-m telescope on Tenerife. 相似文献
274.
Design and Application of Discrete Sliding Mode Control with RBF Network-based Switching Law 总被引:3,自引:0,他引:3
This article proposes a novel approach combining exponential-reaching-law-based equivalent control law with radial basis function (RBF) network-based switching law to strengthen the sliding mode control (SMC) tracking capacity for systems with uncertainties and disturbances. First, SMC discrete equivalent control law is designed on the basis of the nominal model of the system and the adaptive exponential reaching law, and subsequently, stability of the algorithm is analyzed. Second, RBF network is used to form the switching law in a direct and online manner with sliding-mode-related inputs and by approximating evaluation function; and the method to adjust its parameters is devised. Finally, comparable experiments are carried out to verify the application of the proposed approach to an inner-axis driven by a direct current (DC) torque motor through extra-low speed servo for a high precision flight simulator, and the axis works under deteriorating conditions such as periodically fluctuating torques of motor, nonlinear friction, and time-varying model parameters. The results show that the combined SMC can effectively improve the servo performance, for instance, to a stable 0.000 08 (°)/s speed response, the tracking error would be within 0.000 08° in 98% of operating times. Moreover, the hybrid nature of the approach imparts the RBF network the features of removing offline training and ease to set initial parameters. 相似文献
275.
L. Anselmo C. Pardini 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
A large set of simulations, including all the relevant perturbations, was carried out to investigate the long-term dynamical evolution of fictitious high area-to-mass ratio (A/M) objects released, with a negligible velocity variation, in each of the six orbital planes used by Global Positioning System (GPS) satellites. As with similar objects discovered in near synchronous trajectories, long lifetime orbits, with mean motions of about 2 revolutions per day, were found possible for debris characterized by extremely high area-to-mass ratios. Often the lifetime exceeds 100 years up to A/M ∼ 45 m2/kg, decreasing rapidly to a few months above such a threshold. However, the details of the evolution, which are conditioned by the complex interplay of solar radiation pressure and geopotential plus luni-solar resonances, depend on the initial conditions. Different behaviors are thus possible. In any case, objects like those discovered in synchronous orbits, with A/M as high as 20–40 m2/kg, could also survive in this orbital regime, with semi-major axes close to the semi-synchronous values, with maximum eccentricities between 0.3 and 0.7, and with significant orbit pole precessions (faster and wider for increasing values of A/M), leading to inclinations between 30° and more than 90°. 相似文献
276.
A.L. Mishev J.N. Stamenov I.I. Angelov N.O. Ahababian I.N. Kirov E.S. Malamova 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
We review the main activities carried out at Moussala peak (2925 m above sea level, 42°11′N, 23°35′E) station in Bulgaria, connected with cosmic ray investigations during the last five decades. Several important results obtained at the station are reported. The detector design and corresponding methodological studies of the presently operational devices are shown as well, precisely the Cherenkov light telescope, lead free neutron monitor and muon telescope. The scientific potential of the existing complex is discussed. 相似文献
277.
高速加工(HSM)具有许多优点,被广泛应用在航空产品的数控加工中。但是使用高速加工技术,不仅要有适合高速加工的设备,选择适合进行高速加工的刀具,更为重要的是选择合理的编程策略。任何编程策略的不合理都会造成严重的后果。所以数控编程一直是高速加工的重点和难点。以UG软件为研究对象,作者提出了实现高速加工的数控编程的策略,并使用高速铣雕设备对生成的NG代码进行了加工,加工结果验证了NG代码是正确的、合理的。说明了采用合理的加工策略,使用UG软件可以实现高速加工的编程。 相似文献
278.
279.
T.K. Yeoman H.C. Scoffield D.M. Wright L.J. Baddeley A.N. Vasilyev N.V. Semenova 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
A brief review is provided of recent progress in understanding the ionospheric Alfvén resonator (IAR) at high latitude. Firstly, naturally occurring resonances of the IAR as detected by pulsation magnetometers in the auroral zone at Sodankylä and in the polar cap at Barentsburg are considered. The characteristics of the IAR in the two regions are broadly similar, although the effects of solar illumination are less clear at the higher latitudes. Secondly we review recent attempts to stimulate the IAR through high-power radio frequency experiments both in the auroral zone at Tromsø with the European Incoherent SCATter (EISCAT) heater, and within the polar cap at Longyearbyen with the Space Plasma Exploration by Active Radar (SPEAR) facility. In the auroral zone at, Tromsø the stimulated IAR has been observed by ground-based magnetometers, and through electron acceleration observed on the FAST spacecraft. At SPEAR in the polar cap, the stimulated IAR has been investigated, with ground magnetometers, with the first results indicative of a positive detection. 相似文献
280.
Yu. G. Kutsenko 《Russian Aeronautics (Iz VUZ)》2009,52(2):208-213
A modification of the Zimont turbulent combustion model is presented; the modification makes it possible to describe processes in diffusion and homogeneous flame fronts as well as to take into account the influence of different factors that cause flame destabilization. The model is based on the equation for a reaction completeness extent (completeness of combustion). The model realization to simulate the premixed combustion is described and the modification of the turbulent combustion model is applied to simulate a process of flameout downstream of the flameholder. 相似文献