共查询到20条相似文献,搜索用时 15 毫秒
1.
Vignes D. Acuña M.H. Connerney J.E.P. Crider D.H. Rème H. Mazelle C. 《Space Science Reviews》2004,111(1-2):223-231
We report observations of magnetic fields amplitude, which consist of a series of individual spikes in the Martian atmosphere. A minimum variance analysis shows that these spikes form twisted cylindrical filaments. These small diameter magnetic filaments are commonly called magnetic flux ropes. We examine the global characteristics of magnetic flux ropes, which are observed on 5% of the elliptical orbits of Mars Global Surveyor. Flux ropes are more often observed in Venus' atmosphere (70% of the orbits). In this paper we report some of the global characteristics of the flux ropes identified in the Martian atmosphere. No flux ropes are observed in the southern hemisphere of Mars. Most of them occur at high solar zenith angles, close to the terminator plane, and at high latitude with altitudes below 400 km. The orientation of the flux ropes appears random while in the case of Venus the orientation is more horizontal near the terminator for altitudes greater than 200 km. We have identified fewer flux ropes for SZA between 40 to 60 deg and for SZA lower than 20 deg, like in the case of Venus (Elphic and Russell, 1983b). Statistically, Mars' ionosphere with SZA range between 40circ to 60circ is less magnetized than near the subsolar point. As the Martian ionosphere is quite often magnetized by the magnetic components of the crustal field, this crustal magnetic field seems to inhibit the flux ropes formation in the southern hemisphere. However, some orbits without crustal magnetic field, called magnetic cavities, were observed without flux ropes. So the flux ropes formation process seems to be uppressed by another factor, like the solar wind dynamic pressure for Venus (Krymskii and Breus, 1988). 相似文献
2.
Considerable experience accumulated during the past decade in strategies for processing GPS data from ground-based geodetic
receivers. First experience on the use of GPS observations from spaceborne receivers for orbit determination of satellites
on low altitude orbits was gained with the launch of TOPEX/POSEIDON ten years ago. The launch of the CHAMP satellite in July
2000 stimulated a number of activities worldwide on improving the strategies and algorithms for orbit determination for Low
Earth Orbiters (LEOs) using the GPS.
Similar strategies as for ground-based receivers are applied to data from spaceborne GPS receivers to determine high precision
orbits. Zero- and double-differencing techniques are applied to obtain kinematic and/or reduced-dynamic orbits with an accuracy
which is today at the decimeter level. Further developments in modeling and processing strategies will continuously improve
the quality of GPS-derived LEO orbits in the near future. A significant improvement can be expected from fixing double-difference
phase ambiguities to integer numbers. Particular studies focus on the impact of a combined processing of LEO and GPS orbits
on the quality of orbits and the reference frame realization.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
3.
We find the forms of the orbits in a self-consistent galactic model generated by a N-body simulation of the collapse of a protogalaxy. The model represents a stationary elliptical galaxy of type E5, which is approximately axisymmetric around its longest axis. The orbits are of three main types, box orbits (including box-like orbits), tube orbits and chaotic orbits. The box or box-like and tube orbits are represented by closed invariant curves on a Poincaré surface of section. The forms of the orbits and of the invariant curves can be explained by a third integral of motion I, that is given by the Giorgilli (1979) computer program. The nonresonant form of the third integral explains the box orbits, while a resonant form of this integral explains both the box orbits and the 1:1 tube orbits. The N-body model gives the distribution of velocities F, which is an exponential of the third integral. 相似文献
4.
探讨了把定义在区域[-l,l]上的任意可积函数展成Fourier级数的各种形式并示以典型的算例。结果表明函数各种以无穷Fourier级数表达的展式是等价的。然而,以有限和形式表达的各种展式的精度有差别。因此,当把一个函数展成Fourier级数时,既要考虑可能的展开形式,又要考虑展开式的精度,力求二者之间的最佳选择。 相似文献
5.
《中国航空学报》2023,36(3):335-356
Distant Retrograde Orbits (DROs) in the Earth-Moon system have great potential to support varieties of missions due to the favorable stability and orbital positions. Thus, the close relative motion on DROs should be analyzed to design formations to assist or extend the DRO missions. However, as the reference DROs are obtained through numerical methods, the close relative motions on DROs are non-analytical, which severely limits the design of relative trajectories. In this paper, a novel approach is proposed to construct the analytical solution of bounded close relative motion on DROs. The linear dynamics of relative motion on DRO is established at first. The preliminary forms of the general solutions are obtained based on the Floquet theory. And the general solutions are classified as different modes depending on their periodic components. A new parameterization is applied to each mode, which allows us to explore the geometries of quasi-periodic modes in detail. In each mode, the solutions are integrated as a uniform expression and their periodic components are expanded as truncated Fourier series. In this way, the analytical bounded relative motion on DRO is obtained. Based on the analytical expression, the characteristics of different modes are comprehensively analyzed. The natural periodic mode is always located on the single side of the target spacecraft on DRO and is appropriate to be the parking orbits of the rendezvous and docking. On the basis of quasi-periodic modes, quasi-elliptical fly-around relative trajectories are designed with the assistance of only two impulses per period. The fly-around formation can support observations to targets on DRO from multiple viewing angles. And the fly-around formation is validated in a more practical ephemeris model. 相似文献
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柔性转子-挤压油膜阻尼器系统中非协调涡动轨道的实验研究 总被引:2,自引:0,他引:2
本文以实验为基础分析了在柔性转子——挤压油膜阻尼器系统中出现非协调涡动轨道的可能性及规律性;研究了系统参数(如油膜的径向间隙比、承载长度、不平衡量及定心弹簧刚度等)对非协调涡动轨道的影响以及非协调涡动轨道与转子振动之间的关系。 相似文献
7.
胡铁乔 《中国民航学院学报》2006,24(4):32-35
分析了51系列单片机在KeilC开发环境中64位数据的显示问题。51系列单片机为8位,其常用的开发环境KeilC最大支持32的整数,因此无法显示64位的整数。对此作了分析,提出了两种解决办法:计算法和查表法。结合实例,对其原理和实现过程作了详细的分析,并用C语言实现了这两种算法,给出了源程序和流程图. 相似文献
8.
Aiming at a 1-cm Orbit for Low Earth Orbiters: Reduced-Dynamic and Kinematic Precise Orbit Determination 总被引:1,自引:0,他引:1
The computation of high-accuracy orbits is a prerequisite for the success of Low Earth Orbiter (LEO) missions such as CHAMP,
GRACE and GOCE. The mission objectives of these satellites cannot be reached without computing orbits with an accuracy at
the few cm level. Such a level of accuracy might be achieved with the techniques of reduced-dynamic and kinematic precise
orbit determination (POD) assuming continuous Satellite-to-Satellite Tracking (SST) by the Global Positioning System (GPS).
Both techniques have reached a high level of maturity and have been successfully applied to missions in the past, for example
to TOPEX/POSEIDON (T/P), leading to (sub-)decimeter orbit accuracy. New LEO gravity missions are (to be) equipped with advanced
GPS receivers promising to provide very high quality SST observations thereby opening the possibility for computing cm-level
accuracy orbits. The computation of orbits at this accuracy level does not only require high-quality GPS receivers, but also
advanced and demanding observation preprocessing and correction algorithms. Moreover, sophisticated parameter estimation schemes
need to be adapted and extended to allow the computation of such orbits. Finally, reliable methods need to be employed for
assessing the orbit quality and providing feedback to the different processing steps in the orbit computation process.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
9.
A method for calculating the harmonic components of the currents and voltages in a parallel-loaded resonant converter using frequency-domain techniques is presented. The converter is divided into an inverter section and a rectifier section. A harmonic model is developed for the resonant converter in which the rectifier section is treated as a voltage-dependent current sink. All voltages and currents in this model are represented by a Fourier series. The unknown coefficients in all Fourier series are calculated by using the harmonic model and Kirchhoff's laws. Because of the nonlinear nature of the rectifier section, an iterative technique must be utilized to find the unknown Fourier coefficients 相似文献
10.
This paper presents a new three-dimensional (3-D) near-field inverse synthetic aperture radar (ISAR) imaging technique. A 3-D ISAR image can be obtained by processing coherently the backscattered fields as a function of the frequency and two rotation angles about axes which are mutually orthogonal. Most of the existing ISAR algorithms are based on the Fourier transform and as such can tolerate only small amounts of wavefront curvature. Wavefront curvature must be taken into account when imaging an object in the near-field. Near-field ISAR imaging of large objects using a direct Fourier inversion may result in images which are increasingly unfocused at points which are more distant from the center of rotation. An algorithm based on an azimuth convolution between a near-field focusing function and the frequency domain backscattered fields is discussed. This convolution is efficiently implemented by using fast Fourier transform (FFT) techniques. Furthermore, in order to further alleviate the computational load of the algorithm, the discrete Fourier transform (DFT) of the focusing function is evaluated by means of the stationary phase method. Experimental results show that this technique is precise and virtually impulse invariant 相似文献
11.
准确计算自旋稳定卫星姿态参数是正确实施姿态控制的前提。本文基于最小二乘(LS)理论,建立了在线计算卫星姿态参数的一组具有容错功能的递推LS算法,简要论证了该算法具备良好的统计最优性质。 相似文献
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In many Fourier imaging applications, the presence of unaccounted for amplitude or phase errors in the Fourier domain data can lead to a degraded system impulse response and high sidelobes in the image domain. Historically, many methods for data-driven correction of these effects have been proposed, and numerical optimization of nonquadratic, p-norm image quality metrics has recently emerged as a robust solution. This paper presents a tutorial examination of the sources of image sidelobes in Fourier imaging applications, and studies the effectiveness of p-norm regularization algorithms under various experimental conditions. Several observations are made, including comments on robustness to noise and methods for tapered window design and energy-constrained sparse aperture imaging. Image examples are presented as experimental validation. 相似文献
14.
Two novel automatic frequency tracking loops 总被引:3,自引:0,他引:3
Two automatic-frequency-control loops are introduced and analyzed in detail. The algorithms are generalizations of the well-known cross-product automatic-frequency-control loop with improved performance. The first estimator uses running overlapping discrete Fourier transforms (DFTs) to create a discriminator curve proportional to the frequency estimation error, whereas the second one preprocesses the received data and then uses an extended Kalman filter to estimate the input frequency. The algorithms are tested by computer simulations in a low carrier-to-noise-ratio (CNR) and highly dynamic environment. The algorithms are suboptimum tracking schemes with a larger frequency error variance compared to an optimum strategy, but they offer simplicity of mechanization and a CNR with a very low operating threshold 相似文献
15.
Robert B. Decker 《Space Science Reviews》1988,48(3-4):195-262
We review the basic techniques and results of numerical codes used to model the acceleration of charged particles at oblique, fast-mode, collisionless shocks. The emphasis is upon models in which accelerated particles (ions) are treated as test particles, and particle dynamics is calculated by numerically integrating along exact phase-space orbits. We first review the case where ions are sufficiently energetic so that the shock can be approximated by a planar discontinuity, and where the electromagnetic fields on both sides of the shock are defined at the outset of each computer run. When the fields are uniform and static, particles are accelerated by the scatter-free drift acceleration process at a single shock encounter. We review the characteristics of scatter-free drift acceleration by considering how an incident particle distribution is modified by interacting with a shock. Next we discuss drift acceleration when magnetic fluctuations are introduced on both sides of the shock, and compare these results with those obtained under scatter-free conditions. We describe the modeling of multiple shock encounters, discuss specific applications, and compare the model predictions with theory. Finally, we review some recent numerical simulations that illustrate the importance of shock structure to both the ion injection process and to the acceleration of ions to high energies at quasi-perpendicular shocks. 相似文献
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Joint time-frequency transform for radar range-Doppler imaging 总被引:5,自引:0,他引:5
Conventional radar imaging uses the Fourier transform to retrieve Doppler information. However, due to the complex motion of a target, the Doppler frequency shifts are actually time-varying. By using the Fourier transform, the Doppler spectrum becomes smeared and the image is blurred. Without resorting to sophisticated motion compensation algorithms, the image blurring problem can be resolved with the joint time-frequency transform. High-resolution time-frequency transforms are investigated, and examples of applications to radar imaging of single and multiple targets with complex motion are given 相似文献
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依据DFF(离散傅立叶变换),FFT(快速傅立叶变换)、加窗等数字信号处理理论,以及复指数函数的相关性质,并结合目前国内航空发动机整机振动信号处理现状,提出了基于FFr的航空发动机整机振动分量跟踪监视方法.该方法首先将时域加速度振动信号分解为多个简谐振动合成结果,然后根据系统输人的转子转速信号,跟踪计算转子基频激起的简... 相似文献