A simplified approach to uncertainty quantification for orbits in impulsive deflection scenarios

For the majority of near-Earth Asteroid (NEA) impact scenarios, optimal deflection strategies use a massive impactor or a nuclear explosive, either of which produce an impulsive change to the orbit of the object. However, uncertainties regarding the object composition and the efficiency of the deflection event lead to a non-negligible uncertainty in the deflection delta-velocity. Propagating this uncertainty through the resulting orbit will create a positional uncertainty envelope at the original impact epoch. We calculate a simplified analytic evolution for impulsively deflected NEAs and perform a full propagation of uncertainties that is nonlinear in the deflection delta-velocity vector. This provides an understanding of both the optimal deflection velocities needed for a given scenario, as well as the resulting positional uncertainty and corresponding residual impact probability. Confidence of a successful deflection attempt as a function of launch opportunities is also discussed for a specific case.     

Achievable debris orbit prediction accuracy using laser ranging data from a single station

Earlier studies have shown that an orbit prediction accuracy of 20 arc sec ground station pointing error for 1–2 day predictions was achievable for low Earth orbit (LEO) debris using two passes of debris laser ranging (DLR) data from a single station, separated by about 24 h. The accuracy was determined by comparing the predicted orbits with subsequent tracking data from the same station. This accuracy statement might be over-optimistic for other parts of orbit far away from the station. This paper presents the achievable orbit prediction accuracy using satellite laser ranging (SLR) data of Starlette and Larets under a similar data scenario as that of DLR. The SLR data is corrupted with random errors of 1 m standard deviation so that its accuracy is similar to that of DLR data. The accurate ILRS Consolidated Prediction Format orbits are used as reference to compute the orbit prediction errors. The study demonstrates that accuracy of 20 arc sec for 1–2 day predictions is achievable.     

Storm-time atmospheric density modeling using neural networks and its application in orbit propagation

Upper atmospheric densities during geomagnetic storms are usually poorly estimated due to a lack of clear understanding of coupling mechanisms between the thermosphere and magnetosphere. Consequently, the orbit determination and propagation for low-Earth-orbit objects during geomagnetic storms have large uncertainties. Artificial neural networks are often used to identify nonlinear systems in the absence of rigorous theory. In the present study, an attempt has been made to model the storm-time atmospheric density using neural networks. Considering the debate over the representative of geomagnetic storm effect, i.e. the geomagnetic indices ap and Dst, three neural network models (NNM) are developed with ap, Dst and a combination of ap and Dst respectively. The density data used for training the NNMs are derived from the measurements of the satellites CHAMP and GRACE. The NNMs are evaluated by looking at: (a) the mean residuals and the standard deviations with respect to the density data that are not used in training process, and (b) the accuracy of reconstructing the orbits of selected objects during storms employing each model. This empirical modeling technique and the comparisons with the models NRLMSIS-00 and Jacchia-Bowman 2008 reveal (1) the capability of neural networks to model the relationship between solar and geomagnetic activities, and density variations; and (2) the merits and demerits of ap and Dst when it comes to characterizing density variations during storms.     

Optimal two-impulse rendezvous with terminal tangent burn considering the trajectory constraints

The two-impulse orbital rendezvous problem with a terminal tangent burn between coplanar elliptical orbits is studied by considering a lower bound on perigee radius and an upper bound on apogee radius for the transfer orbit. This problem requires that two spacecraft arrive at the rendezvous point with the same arrival flight-path angle after the same flight time. The admissible range of the final true anomaly that meets the perigee and apogee constraints is obtained in closed form. The revolution number of the transfer orbit is expressed as a function of the true anomaly and the revolution numbers of the initial and final orbits. All the feasible solutions are obtained with a bound on the revolution number of the final orbit. Then, the minimum-fuel one is determined by comparing their costs. Finally, two numerical examples are provided to obtain all the feasible solutions for given initial impulse points and the optimal solution with the initial coasting arc. Numerical results show that the minimum-fuel solution for the terminal tangent burn rendezvous is close to that for the cotangent rendezvous when the rendezvous time is long enough; however, the cotangent rendezvous may not exist when the rendezvous time is short.     

非晶态薄带冷却铜辊用内反馈静压轴承数值模拟

由于内反馈式静压轴承具有高回转精度、大承载能力和高油膜刚度等特性,固将其引入到非晶态薄带冷却铜辊支承结构中。为了获取冷却铜辊用内反馈静压轴承的承载能力特性,利用流体力学软件建立不同偏心距下的内反馈式静压轴承油膜模型,研究不同进油压力下随偏心距变化时轴承油膜压力场分布情况并计算出其承载能力。结果表明：在同等进油压力下,油膜承载能力随偏心距的增大而增大;在同等偏心距下,油膜承载能力随进油压力的增大而增大。最终通过与工作现场实验数据比对分析,当进油压力分别为3.0 MPa和3.5 MPa时,数值模拟载荷值与实际载荷值的相对误差分别为5.3%和5.4%,证明了数值模拟结果的可靠性。     

磁暴下的LEO卫星精密轨道确定

对低轨卫星（LEO）,大气阻尼摄动是主要的定轨误差源.尤其在发生磁暴时,求解一个大气阻尼因子的定轨方法已不能充分吸收大气密度计算不准所造成的定轨误差,因而在标校统一S波段（USB）的测量系统差和随机差时往往计算失真.本文提出了一种求解折线型Cd因子的新方法,克服了动力学模型不准所带来的定轨误差,通过与独立的GPS数据比较,定轨精度有明显提高,同时给出的测量系统差和随机差更加真实可信.     

一种有效提高CE-1卫星卸载前后精密星历衔接精度的定轨策略

主要讨论了动量轮卸载对嫦娥一号卫星绕月轨道的影响以及这种影响对其精密星历衔接段精度的影响,分析了不同轨道改进策略下的精密星历衔接弧段的精度,认为使用2~3d的弧段进行轨道改进及卸载计算能得到较高精度的搭接星历,并通过使用实测数据进行定轨改进计算进行了验证,结果表明,该策略可以使星历衔接精度在沿迹方向(T)提高至优于100m量级。     

Improved kHz-SLR tracking techniques and orbit quality analysis for LEO missions

Satellite gravity field missions such as CHAMP, GRACE and GOCE are designed as low Earth orbiting spacecraft (LEO) with orbit heights of about 250–500 km. The challenging mission objectives require a very precise knowledge of the satellite orbit position in space. For these missions precise orbit information is typically provided by GPS satellite-to-satellite tracking (SST) observations supported by satellite laser ranging (SLR).     

对数螺旋线非开普勒轨道的可行性分析

提出了基于形状的非开普勒轨道设计方法,并分析了对数螺旋线轨道的应用可行性。 首先在假定轨道形状的前提下,通过变量代换推导出了在极坐标系下一般曲线用于轨道设计 的基本方程;然后分析了对数螺旋线表达式中的关键参数与航迹角的关系,得到了相应轨道 的基本性质;最后对待定参数的对数螺旋线轨道应用的可行性进行了推导和分析,结果显示 采用对数螺旋线设计非开普勒轨道是可行的,但仅适用于从椭圆初始轨道实现转移的情况。