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951.
A mathematical model and a theory for the phase and frequency transfer between N mutually synchronized oscillators is presented. Mathematical equations are given for evaluating the phase and frequency transfer accuracy achievable. The Fokker-Planck technique is applied and an approximation solution is obtained for the phase difference process between a pair of network oscillators. The case of the three-nodal network is analyzed and it is shown that path delays and network coupling give mean bias error variations and signal-to-noise ratio (SNR) fluctuations in the phase difference process. The analysis is then extended to an N-nodal network to show that the SNR fluctuations tend to stabilize while the mean bias error variations are averaged between nodes as N becomes larger. 相似文献
952.
With the advent of automatic dependent surveillance (ADS), a detailed probability model of aircraft cross-track deviations is required to determine the impact of ADS. A suitable causal probability model is presented where normal navigation, degradation, pilot blunder, and failure are each modeled by Gaussian density functions with mean and standard deviations defined by the physics of the event. The overall model is a weighted sum of the Gaussian error probabilities and is thus amenable to extrapolation. Overlap and encroachment probabilities are derived,and the effect of ADS on this is model determined. It is shown that by using a simple form of ADS separation standards can be reduced, and transmitting a figure of merit (FOM) providing information on failures and degradations can further reduce separation standards. The results suggest an improvement by a factor of two from current separation standards 相似文献
953.
Profeta J.A. III Vogt W.G. Mickle M.H. 《IEEE transactions on aerospace and electronic systems》1990,26(2):225-231
Discrete-time estimation and compensation are discussed as a solution to the problems encountered when disturbances are present and degrade the performance of continuous automatic control systems. The method described, under a mild set of conditions, allows the designer to locate the closed-loop system poles substantially anywhere and provide disturbance rejection as large as desired by increasing the dimensions of the disturbance estimator. This estimation/compensation scheme results in a gain characteristic, below some frequency (ωc ), with a slope of q ×20 dB/decade. The value of q and ωc can be chosen by the designer, within the physical limitations of the problem, so that the system error resulting from either deterministic or stochastic disturbances is sufficiently reduced. The method has been investigated for use with known linear, time-invariant systems 相似文献
954.
The capability of lead-acid batteries for supplying very high power for a short time is explored. The application of such a battery for accelerating a hypersonic plane is used to illustrate the requirements. A technique for analyzing batteries and controlling voltage for pulse loads is described. Evaluation of lead-acid batteries in production and voltage regulation by switching batteries in and out are covered. Alternatives to batteries, including superconducting magnetic energy storage, are discussed 相似文献
955.
J.G. Luhmann R.C. Elphic C.T. Russell L.H. Brace R.E. Hartle 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(10):17-21
Theoretical models of the ionosphere of Venus have been constructed in the past without due consideration of the fact that the ionosphere is sometimes magnetized. This paper examines some differences between the magnetized and unmagnetized dayside Venus ionosphere using the Pioneer Venus Orbiter Langmuir probe and magnetometer data. Particular attention is given to the evaluation of the altitude profiles of the thermal electron heating and comparison of the magnitude of the magnetic force(¯vׯB) ׯB with other forces in the ionosphere. Several examples illustrate how heating profiles are different in the magnetized ionosphere with effective heating below ~200 km altitude reduced by orders of magnitude compared to the field-free ionosphere. The force associated with the magnetic field is comparable to other forces in the magnetized ionosphere. The measured plasma density, electron temperature and magnetic field thus suggest that large-scale magnetic fields should be included in future ionosphere models. 相似文献
956.
Kwon W.H. Lee K.S. Kwon O.K. 《IEEE transactions on aerospace and electronic systems》1990,26(6):1011-1021
An optimal FIR (finite impulse response) filter and smoother is introduced for the time-varying state-space model. The suggested filter has an FIR structure and utilizes finite observation. It is shown that the impulse response of the optimal FIR filter can be obtained by a simple Riccati-type matrix differential equation. Especially for time-invariant systems, this FIR filter reduces to previously known simple forms. For implementation, a recursive form of the optimal FIR filter and smoother is derived by using adjoint variables, and computational algorithms are suggested. It is also shown by sensitivity analysis that the proposed optimal FIR filter alleviates potential divergence characteristics of the standard Kalman filter 相似文献
957.
H D Pross M Kost J Kiefer 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(10):125-130
The influence of microgravity on the repair of radiation induced genetic damage in a temperature-conditional repair mutant of the yeast Saccharomyces cerevisiae (rad 54-3) was investigated onboard the IML-1 mission (January 22nd-30th 1992, STS-42). Cells were irradiated before the flight, incubated under microgravity at the permissive (22 degrees C) and restrictive (36 degrees C) temperature and afterwards tested for survival. The results suggest that repair may be reduced under microgravity. 相似文献
958.
对超声速流中准稳定三波结构进行了研究,该结构中主激波以一定的速度D沿超声速流运动,它不但引起相应的反射激波运动而且会使激波后气流中的一系列气动力参数发生变化,包括总压、总焓和熵等。本研究给出了这些量与入射激波强度之间的关系。在一定的速度D的情况下,给出了三波结构的力学相容性条件是可以被破坏的。 相似文献
959.
Mende S.B. Frey H.U. Immel T.J. Gerard J.-C. Hubert B. Fuselier S.A. 《Space Science Reviews》2003,109(1-4):211-254
The IMAGE spacecraft carries three FUV photon imagers, the Wideband Imaging Camera (WIC) and two channels, SI-12 and SI-13, of the Spectrographic Imager. These provide simultaneous global images, which can be interpreted in terms of the precipitating particle types (protons and electrons) and their energies. IMAGE FUV is the first space-borne global imager that can provide instantaneous global images of the proton precipitation. At times a bright auroral spot, rich in proton precipitation, is observed on the dayside, several degrees poleward of the auroral zone. The spot was identified as the footprint of the merging region of the cusp that is located on lobe field lines when IMF Bz was northward. This identification was based on compelling statistical evidence showing that the appearance and location of the spot is consistent with the IMF Bz and By directions. The intensity of the spot is well correlated with the solar wind dynamic pressure and it was found that the direct entry of solar wind particles could account for the intensity of the observed spot without the need for any additional acceleration. Another discovery was the observation of dayside sub-auroral proton arcs. These arcs were observed in the midday to afternoon MLT sector. Conjugate satellite observations showed that these arcs were generated by pure proton precipitation. Nightside auroras and their relationship to substorm phases were studied through single case studies and in a superimposed epoch analysis. It was found that generally there is substantial proton precipitation prior to substorms and the proton intensity only doubles at substorm onset while the electron auroral brightness increases on average by a factor of 5 and sometimes by as much as a factor of 10. Substorm onset occurs in the central region of the pre-existing proton precipitation. Assuming that nightside protons are precipitating from a quasi-stable ring current at its outer regions where the field lines are distorted by neutral sheet currents we can associate the onset location with this region of closed but distorted field lines relatively close to the earth. Our results also show that protons are present in the initial poleward substorm expansion however later they are over taken by the electrons. We also find that the intensity of the substorms as quantified by the intensity of the post onset electron precipitation is correlated with the intensity of the proton precipitation prior to the substorms, highlighting the role of the pre-existing near earth plasma in the production of the next substorm. 相似文献
960.
O'Neill M.J. McDanal A.J. Piszczor M.F. George P.J. Edwards D.L. Brandhorst H.W. Eskenazi M.I. Botke M.M. Jaster P.M. 《Aerospace and Electronic Systems Magazine, IEEE》2003,18(1):3-9
At IECEC 2001, this team presented a paper on the new stretched lens array (SLA), including its evolution from the successful SCARLET array on the NASA/JPL Deep Space 1 spacecraft. Since that conference, the SLA team has made significant advances in SLA technology, including component-level improvements, array-level optimization, space environment exposure testing, and prototype hardware fabrication and evaluation. This paper describes the evolved version of the SLA, highlighting recent improvements in the lens, solar cell, photovoltaic receiver, rigid panel structure, and complete solar array wing. In addition to excellent durability in the space environment, the near-term SLA will provide outstanding wing-level performance parameters: 180 W/kg specific power; 300 W/m/sup 2/ power density; 300 V operational voltage; 85% savings in cell area (cm/sup 2//W) and cell-related cost ($/W) compared to planar arrays; 9 kW/m/sup 3/ stowed power at launch. 相似文献