非等间距显式算法在重力波非线性传播数值模拟中的初步应用

采用3阶TVD型显式时间积分方法,建立了2维可压缩大气中重力波非线性传播的非等间距显式数值模式.对小振幅重力波传播过程的模拟结果表明,该模式能够很好地再现小振幅重力波的传播过程,并能保持能量守恒关系和各扰动量之间的相位关系,与线性重力波理论预测的结果吻合很好.对有限振幅重力波的模拟结果表明,与格点数相同的等间距网格中的模拟结果相比,采用非等间距网格能够以更高的分辨率模拟重力波的不稳定传播直到破碎的整个过程;在非等间距网格中的模拟结果与在加密一倍的等间距网格中的模拟结果相同,说明了用非等间距网格模拟重力波的饱和与破碎过程是正确的,并且采用非等间距网格能够大大减少计算量.      

重力卫星星间高精度测距技术研究

卫卫跟踪(SST)技术是当前地球重力场测量最有价值和应用前景的方法之一.高精度星间测距系统是低低卫卫跟踪(SST-ll)重力卫星的关键有效载荷.GRACE卫星携带的K波段测距系统(KBR K Band Ranging System)是一微米量级的测距系统,通过处理高精度的星间距离和距离变化率数据,可以恢复出地球重力场.GRACE后续计划又提出了一种更高精度的激光干涉测距系统.在研究KBR及激光干涉测距系统测量原理的基础上,提出了一种KBR系统的基本结构,详细分析了两种测距系统的关键技术及国内目前的研究水平,提出了我国开展星间测距系统研究的一些建议.      

Two-maneuver indirect contingency return from a low lunar orbit

The problem of contingency return from the low lunar orbit is studied. A novel two-maneuver indirect return strategy is proposed. By effectively using the Earth’s gravity to change the orbital plane of the transfer orbit, the second maneuver in the well-known three-maneuver return strategy can be removed, so the total delta-v is reduced. Compared with the single-maneuver direct return, our strategy has the advantage in that the re-entry epoch for the minimum delta-v cost can be advanced in time, with a minimum delta-v value similar to that of the direct return. The most obvious difference between our strategy and the traditional single- or multiple- maneuver strategies is that the complete transfer orbit is a patch between a two-body conic orbit and a three-body orbit instead of two conic orbits. Our strategy can serve as a useful option for contingency return from a low lunar orbit, especially when the delta-v constraint is stringent for a direct return and the contingency epoch is far away from the return window.     

High precision subsurface structural mapping of the Trompsburg complex (South Africa) from gravity and magnetic data

Trompsburg Complex, South Africa, is obscured by the Karoo sediments, and it is not observed on the surface. Knowledge of the boundaries of the subsurface geological bodies in the region is of main interest in many applications of the Earth Sciences. There are many methods developed to delineate boundaries of subsurface geological sources such as the curvature-based method and parabola-based methods. In this study, we applied an improved approach based on parabolic curve fitting to the gradient amplitudes of gravity and magnetic data for extracting geological structures of the Trompsburg Complex. The results showed ring structures with a diameter of about 50 km, that include granitic rocks in the central part of the Trompsburg Complex. The results also demonstrated the existence of boundaries of the other structures in the south-eastern region of the Complex, which were not determined in previous studies. The result provides new information for a better understanding of the structural and tectonic features of the Trompsburg Complex. The success of this study suggests that the improved technique based on parabolic curve, is helpful in outlining the edge locations of subsurface geologic structures.     

Novel orbit-attitude combination mode for solar power satellites to reduce mass and fuel

Solar power satellite receives great attention because it can release the energy crisis and environmental problems in the future. However, the launch and maintenance costs are tremendous due to the large system mass and large fuel consumption to counteract space perturbations. To reduce mass and fuel, a novel quasi-Sun-pointing attitude in Sun-frozen orbit is proposed. The Sun-frozen orbit has a nonzero eccentricity vector that always points towards the Sun. The quasi-Sun-pointing attitude is a periodic solution of the Sun-pointing attitude angle. Although about 3 % electricity must be given up because of the variation of Sun-pointing attitude angle, little control action is required to deal with the solar radiation pressure and gravity-gradient torque. The algorithm to obtain initial conditions is proposed. The influences of system parameters and structural flexibilities are studied. Simulation results reveal that the quasi-Sun-pointing attitude in Sun-frozen orbit dramatically reduce fuel consumption, the dry mass, and complexity of the control system. In addition, structural vibration is hardly induced by the gravity-gradient torque. Thus, the bending stiffness as well as the mass of the supporting structure can be reduced.     

Autonomous navigation for lunar final approach based on gravity gradient measurements

Lunar final approach navigation is critical for pin-point lunar landing in future missions. This study investigates the use of lunar gravity gradient measurements for autonomous navigation of a lunar probe during the final approach phase. As the spacecraft approaches the Moon, the strength of gravity gradient signals improves. A spaceborne gravity gradiometer can precisely measure local gravity gradients, and the latest lunar gravity model GL1500E is used to provide reference values. The employed truncation degree and order of the gravity model are increased stepwise considering the decreasing altitude of the spacecraft in order to reach a compromise between computational costs and model accuracy. An iterative Kalman filter is developed for coupled orbit and attitude estimation using gravity gradient measurements and attitude quaternions obtained from star sensors. A simulated spacecraft with a gradiometer noise level of 0.01 E is considered. Simulation results show that the spacecraft’s position converges rapidly and achieves an accuracy of less than 100 m at the last epoch.