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941.
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944.
Lee-Anne McKinnell Oyapo Chimidza Pierre Cilliers 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
The International Reference Ionosphere (IRI) parameters B0 and B1 provide a representation of the thickness and shape, respectively, of the F2 layer of the bottomside ionosphere. These parameters can be derived from electron density profiles that are determined from vertical incidence ionograms. This paper aims to illustrate the variability of these parameters for a single mid latitude station and demonstrate the ability of the Neural Network (NN) modeling technique for developing a predictive model for these parameters. Grahamstown, South Africa (33.3°S, 26.5°E) was chosen as the mid latitude station used in this study and the B0 and B1 parameters for an 11 year period were determined from electron density profiles recorded at that station with a University of Massachusetts Lowell Center for Atmospheric Research (UMLCAR) Digisonde. A preliminary single station NN model was then developed using the Grahamstown data from 1996 to 2005 as a training database, and input parameters known to affect the behaviour of the F2 layer, such as day number, hour, solar and magnetic indices. An analysis of the diurnal, seasonal and solar variations of these parameters was undertaken for the years 2000, 2005 and 2006 using hourly monthly median values. Comparisons between the values derived from measured data and those predicted using the two available IRI-2001 methods (IRI tables and Gulyaeva, T. Progress in ionospheric informatics based on electron density profile analysis of ionograms. Adv. Space Res. 7(6), 39–48, 1987.) and the newly developed NN model are also shown in this paper. The preliminary NN model showed that it is feasible to use the NN technique to develop a prediction tool for the IRI thickness and shape parameters and first results from this model reveal that for the mid latitude location used in this study the NN model provides a more accurate prediction than the current IRI model options. 相似文献
945.
C. Pardini L. Anselmo 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
On 11 January 2007, the People’s Republic of China conducted a successful anti-satellite test against one of their defunct polar-orbiting weather satellites. The target satellite, called Fengyun-1C, had a mass of 880 kg and was orbiting at an altitude of about 863 km when the collision occurred. Struck by a direct-ascent interceptor at a speed of 9.36 km/s, the satellite disintegrated, spreading the cataloged fragments between 200 and 4000 km, with the highest concentration near the breakup height. By the end of April 2008, 2377 pieces of debris, including the original payload remnant, had officially been cataloged by the US Space Surveillance Network. Of these, nearly 1% had reentered the Earth’s atmosphere. This deliberate act is the largest debris-generating event on record, and its consequences will adversely affect circumterrestrial space for many years. 相似文献
946.
对航天器交会寻的段不同的机动模式,考虑相对导航最远联络距离、最晚联络开始时间以及最早轨道圆化机动时间等约束条件,阐述绝对导航向相对导航的过渡阶段(即介于调相段与寻的段之间的漂移段)的设计问题,包括漂移段终点与相对导航联络点位置的确定以及漂移段起点位置偏差分析,建立了漂移段起点标称位置与调相段机动速度偏差的关系式,给出漂移段起点标称位置范围及调整方法。 相似文献
947.
正压漏孔校准装置的设计 总被引:1,自引:0,他引:1
正压漏孔校准装置是一种校准气体漏率的计量标准设备 ,可采用定容法和定量气体动态比较法对正压漏孔校准 ,校准范围为 10 2~ 5× 10 - 5 Pa· L /s。该文介绍正压漏孔校准装置的校准原理 ,设计方法 ,参数选取。 相似文献
948.
一种微波及毫米波滤波器的精确设计方法 总被引:3,自引:0,他引:3
介绍了一种微波及毫米波滤波器精确设计方法:由滤波器的原型参数,导出滤波器的广义阻抗变换系数;采用模匹配法或耦合积分方程技术等数值算法精确计算各个耦合结的S参数,进而转换成广义阻抗变换系数;通过插值,获得满足原型要求的滤波器各个耦合结的结构尺寸以及谐振腔长度,将结构参数代入模匹配程序、HFSS或CST软件进行分析,分析结果与预期值一致。设计了26GHz矩形波导带通滤波器,模匹配法和HFSS以及CST软件的仿真结果、测试数据与设计要求一致,说明该种设计方法是有效的。 相似文献
949.
Alexandre Belli Nikita P. Zelensky Frank G. Lemoine Douglas S. Chinn 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2021,67(3):930-944
Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) is a tracking technique based on a one-way ground to space Doppler link. For Low Earth Orbit (LEO) satellites, DORIS shows a robust capability in terms of data coverage and availability, due to a wide and well-distributed ground network, where data are made available by the International Doris Service (IDS). However, systematic errors remain in the DORIS data, such as instabilities of the on-board clock due to radiation encountered in space, which limit the accurate determination of station positions.The DORIS on-board clock frequency stability is degraded by the increased radiation found in the region of the South Atlantic Anomaly (SAA) and has been shown to degrade station position estimation. This paper introduces a new model correction to the DORIS data for the frequency of the Jason-2 Ultra Stable Oscillator (USO), derived from the Time Transfer by Laser Link (T2L2) experiment (Belli and Exertier, 2018). We show that a multi-satellite DORIS solution including this T2L2-corrected data applied to the frequency modelling for The DORIS data, improves the estimation of station coordinates. We show the tie residuals with respect to collocated GPS stations are improved by several millimeters. We also demonstrate that the 117-day (Jason-2) draconitic signal in the geophysical parameters is reduced, implying that the origin of this signal is not just solar radiation pressure mis-modeling, but also radiation-induced clock perturbations on the Jason-2 DORIS Ultra-Stable-Oscillator (USO). Finally we demonstrate through comparisons with the International Earth Rotations and Reference Systems Service (IERS) C04 series for Earth Orientation Parameters (EOP), that the estimation of EOP is improved in both a Jason-2 DORIS-only and a multi-satellite DORIS solution for EOP. 相似文献
950.
采用纳秒脉冲激光器对6061铝合金焊后氧化物进行激光清洗实验。首先分析焊斑清洗前和清洗后的表面形貌和元素组成,探明焊后氧化物组成成分。其次再利用前期实验所获得不同工艺参数对清洗质量的影响规律,设计正交实验,划定各因素区间范围,分析参数对清洗质量的影响权重。利用正交实验法分析得到:影响表面轮廓线峰谷值的参数主次顺序为激光功率>脉冲频率>扫描速度>脉冲宽度;影响氧元素质量百分比的参数主次顺序为脉冲宽度>扫描速度>激光功率>脉冲频率。通过综合平衡法最终获得优化工艺参数组:扫描速度、激光功率、脉冲频率、脉冲宽度分别为2 950 mm/s、141 W、29 kHz、330 ns。优化后可确保铝合金焊后氧化物清洗彻底、无基材损伤、清洗效率高。 相似文献