顾及多路径误差改正的GNSS大坝形变监测研究

在GPS短基线相对定位中,可以根据多路径误差周期约为1天的重复性特点建立误差模型,采用坐标域或观测值域滤波的方法进行多路径误差改正.北斗系统由GEO、IGSO和MEO三种卫星类型组成,其中MEO卫星类型接近7天的轨道周期与GEO和IGSO约为1天的轨道周期差异较大,因此对北斗观测值进行多路径误差改正时只能采用观测值域滤波方法.本文以某水库大坝形变监测网为例,对GPS和BDS观测值分别建立多路径误差模型,根据各自卫星轨道周期进行多路径误差改正,结果表明经改正后大坝形变监测精度有较大提高.     

Terminal area trajectory planning using the energy-tube concept for reusable launch vehicles

This paper concerns a terminal area energy management (TAEM) guidance system for a winged re-entry vehicle and describes the use of the energy-tube concept in a planning and estimation algorithm. In this paper, the focus is on the analysis of the energy management capabilities during changes in the initial conditions. The planning algorithm calculates the best heading alignment cylinder (HAC) position, based on the initial state at the TAEM interface. A cross-section of the energy tube contains all combinations of altitude and velocity from which it is possible to reach the runway. Ideally, the re-entry vehicle enters the terminal area in the ‘middle’ of the cross-section such that it has sufficient capabilities to react to off-nominal conditions that would require more or less energy dissipation. Each HAC position has a particular energy-tube cross-section. By shifting the HAC position, an optimal HAC can be found such that the initial state is situated in the middle of the cross-section. The planning algorithm uses a nominal longitudinal strategy, which is situated in between the maximum-dive and maximum-range capabilities, to calculate the optimal HAC position. However, in some cases the nominal longitudinal strategy is insufficient to reach the runway. Hence, in these cases, deviations from the nominal longitudinal strategy (nominal energy dissipation) are required during the actual flight. These deviations are calculated by the estimation algorithm and the magnitude of the deviation is based on the current position in the energy tube.     

Analysis and experiments for delay compensation in attitude control of flexible spacecraft

Space vehicles are often characterized by highly flexible appendages, with low natural frequencies which can generate coupling phenomena during orbital maneuvering. The stability and delay margins of the controlled system are deeply affected by the presence of bodies with different elastic properties, assembled to form a complex multibody system. As a consequence, unstable behavior can arise. In this paper the problem is first faced from a numerical point of view, developing accurate multibody mathematical models, as well as relevant navigation and control algorithms. One of the main causes of instability is identified with the unavoidable presence of time delays in the GNC loop. A strategy to compensate for these delays is elaborated and tested using the simulation tool, and finally validated by means of a free floating platform, replicating the flexible spacecraft attitude dynamics (single axis rotation). The platform is equipped with thrusters commanded according to the on–off modulation of the Linear Quadratic Regulator (LQR) control law. The LQR is based on the estimate of the full state vector, i.e. including both rigid – attitude – and elastic variables, that is possible thanks to the on line measurement of the flexible displacements, realized by processing the images acquired by a dedicated camera. The accurate mathematical model of the system and the rigid and elastic measurements enable a prediction of the state, so that the control is evaluated taking the predicted state relevant to a delayed time into account. Both the simulations and the experimental campaign demonstrate that by compensating in this way the time delay, the instability is eliminated, and the maneuver is performed accurately.     

Validation of attitude/deformation sensing techniques for space flexible manipulators

This paper proposes an approach that makes use of two different techniques to sense and identify both the rigid attitude motion and the flexible dynamics of a manipulator. With the first technique, an accurate attitude motion determination, based on the use of a global navigation satellite systems (GNSS) signals, is performed. For this purpose, some antennas are placed on the manipulator in order to obtain the observable phase of the GNSS signal. The second technique, based on the use of accelerometer sensors, is used in order to identify the dynamic signature during the motion of the flexible link. Specifically, the modal parameters are estimated using data recorded from accelerometers, conveniently placed on the structure, by means of an output-only based approach. The developed algorithms used for both the attitude estimation and the output-only modal analysis are validated by experimental activities carried out on an in-house testbed representing a two flexible arm manipulator.     

XANES analysis of organic residues produced from the UV irradiation of astrophysical ice analogs

Organic residues formed in the laboratory from the ultraviolet (UV) photo-irradiation or ion bombardment of astrophysical ice analogs have been extensively studied for the last 15 years with a broad suite of techniques, including infrared (IR) and UV spectroscopies, as well as mass spectrometry. Analyses of these materials show that they consist of complex mixtures of organic compounds stable at room temperature, mostly soluble, that have not been fully characterized. However, the hydrolysis products of these residues have been partly identified using chromatography techniques, which indicate that they contain molecular precursors of prebiotic interest such as amino acids, nitrile-bearing compounds, and amphiphilic compounds. In this study, we present the first X-ray absorption near-edge structure (XANES) spectroscopy measurements of three organic residues made from the UV irradiation of ices having different starting compositions. XANES spectra confirm the presence of different chemical functions in these residues, and indicate that they are rich in nitrogen- and oxygen-bearing species. These data can be compared with XANES measurements of extraterrestrial materials. Finally, this study also shows how soft X rays can alter the chemical composition of samples.     

Sailing with solar and planetary radiation pressure

Literature on solar sailing has thus far mostly considered solar radiation pressure (SRP) as the only contribution to sail force. However, considering a sail in a planetary mission scenario, a new contribution can be added. Since the planet itself emits radiation, this generates a radial planetary radiation pressure (PRP) that is also exerted on the sail. Hence, this work studies the combined effects of both SRP and PRP on a sail for two case studies, i.e. Earth and Venus. In proximity of the Earth, the effect of PRP can be significant under specific conditions. Around Venus, instead, PRP is by far the dominating contribution. These combined effects have been studied for single- and double-sided reflective coating and including eclipse. Results show potential increase in the net acceleration and a change in the optimal attitude to maximise the acceleration in a given direction. Moreover, an increasing semi-major axis manoeuvre is shown with and without PRP, to quantify the difference on a real-case scenario.     

Analysis of the solar sail deformation based on the point cloud method

The deformation of the solar-sail membrane is an important factor for causing inaccuracies in the solar-sail missions. This paper describes the solar sail under deformation by using a new modelling technique based on point cloud and triangular mesh generation. Two types of deformation, stemming from wrinkling and billowing, are modelled. The changes in the solar radiation pressure force and the moment caused by deformation are calculated and compared to the ideal non-deformed case. The heliocentric spiral trajectory and the orbital angular momentum reversal trajectory are taken as examples to quantify the influence of the deformation from an orbit point of view. Additionally, point cloud simplification, based on the normal vector and bounding box, is utilized to simplify the original deformed-sail model. It involves a reasonable reduction and renewal of the points in the model considering the variation of surface curvature. The simplification and its modelling accuracy are numerically investigated as well as computational efficiency.     

颤振抑制中的测量点和压电驱动器的分布

对于一个具有分布式压电驱动器的模型，设计了单输入／单输出（SISO），单输入／多输出（SIMO），多输入／多输出（MIMO）颤振主动抑制控制律，并进行了风洞试验验证。在此基础上，研究了测量点和压电驱动器的分布。     

海洋环境与疲劳载荷联合作用下喷丸超高强度钢损伤机制

舰载机起落架结构既要遭受海洋大气、海上盐雾和海浪飞溅等严酷海洋环境的侵蚀作用,又要承受较大的弹射起飞/拦阻着舰载荷,在海洋环境与疲劳载荷联合作用下超高强度钢起落架结构承载能力显著劣化,对使用安全构成严重挑战。针对超高强度钢喷丸和未喷丸两种试验件,基于舰载机服役的海洋环境,开展了腐蚀+疲劳交替试验和预腐蚀疲劳试验研究,得到了疲劳寿命变化规律,通过粗糙度、晶粒度、显微硬度、残余应力和疲劳断口分析,揭示了喷丸对疲劳寿命增强的作用机制、腐蚀+疲劳损伤交替作用机制和预腐蚀疲劳损伤作用机制。结果表明,喷丸强化后疲劳寿命平均增幅为93.1%;对于喷丸试验件,深度约为20 μm的轻微点蚀,导致疲劳寿命衰减幅度达到30%;喷丸强化与腐蚀两者之间存在着此消彼长的竞争机制;腐蚀+疲劳交替作用损伤机制对该型超高强度钢造成的疲劳寿命衰减要比预腐蚀疲劳损伤机制严重得多,加速腐蚀试验时间相同的条件下前者疲劳寿命为后者的47%~54%。