基于磁强计的微小卫星姿态确定

针对极轨微小卫星，提出了一种基于磁强计的改进EKF算法。通过对仅利用磁强计确定卫星姿态时的误差来源进行分析，并且对磁强计量测模型做出了改进，并进行了适当的简化。在不明显影响精度的情况下，使得计算量得到了显著降低，数学仿真表明算法是收敛的、有效的。该算法具有极大的工程实用意义，对于EKF方法在低成本微小卫星的成功应用是一个有益的探索。     

基于扩张状态观测器的航天器自适应姿态控制

针对航天器三轴姿态大角度机动时动力学特性耦合强烈的情况,同时考虑外界干扰及执行器的不确定性,根据扩张状态观测器的相关理论,提出了航天器姿态机动的一种自适应输出反馈控制策略。首先,采用动量矩定理和欧拉法建立了航天器的姿态动力学模型。然后,在此基础上,利用扩张状态观测器能够准确地获取航天器三轴间非线性耦合及其他未知的外界干扰信息的能力,设计了一种仅需要姿态角测量值的自适应输出反馈控制律,使航天器在大角度姿态机动的同时能够通过自适应律补偿控制力矩的输出偏差。仿真结果表明,在多种任务模式下,航天器都可以很好地完成姿态机动任务,从而验证了控制律的正确性和有效性。     

交会对接气浮仿真平台视觉辅助姿态确定

文章首先介绍了用于空间合作卫星最后逼近段交会对接任务的仿真平台,描述了文中使用的姿态运动学方程和视觉成像算法。为了充分利用陀螺仪和视觉系统进行姿态确定,采用传统的扩展卡尔曼滤波（EKF）对两种测量数据进行融合,实现对姿态和陀螺仪漂移的估计。为了克服EKF调节参数过多和计算过程需要求逆的问题,设计了一种新的非线性观测器。最后,通过在对接仿真平台上进行试验,对比验证了非线性滤波器的有效性以及实用性。     

单通道测量条件下DORIS实时定轨方法研究

重点研究了单通道测量条件下的DORIS实时定轨方法。推导了实时定轨的扩展卡尔曼滤波（EKF）公式,仿真生成了47个测站的观测数据文件,分析了单通道测量条件下EKF滤波器的收敛性及其精度。仿真实验表明,仅有单个测站的观测数据时,滤波难以收敛;但在不同测站的观测数据切换时,滤波收敛效果显著。对于轨道高度为800km的卫星,初轨各轴位置误差小于500m、速度误差小于5m/s时,滤波平均收敛时间约55min;若各项系统误差能准确建模,实时轨道的位置精度将优于40cm。     

跟飞编队卫星相对导航自适应EKF算法研究

针对跟踪星对目标星跟飞编队的相对导航问题,提出了基于自适应扩展卡尔曼滤波（EKF）的相对导航算法。以线性离散化的椭圆参考轨道相对动力学模型为导航状态方程．设计了虚拟测量量及相应的测量矩阵,避免了求解雅可比矩阵的复杂计算。为了适应构型尺寸变化引起的模型误差变化,提出了模型误差在线估计算法。仿真显示,算法具有较快的收敛速度和较高的估计精度。该研究成果可作为卫星编队应用的有益参考。     

The flyby anomaly: A case for strong gravitomagnetism?

In the last two decades an anomalous variation in the asymptotic velocity of spacecraft performing a flyby manoeuvre around Earth has been discovered through careful Doppler tracking and orbital analysis. No viable hypothesis for a conventional explanation of this effect has been proposed and its origin remains unexplained. In this paper we discuss a strong transversal component of the gravitomagnetic field as a possible source of the flyby anomaly. We show that the perturbations induced by such a field could fit the anomalies both in sign and order of magnitude. But, although the secular contributions to the Gravity Probe B experimental results and the Lense–Thirring effect in geodynamics satellites can be made null, the detailed orbital evolution is easily in conflict with such an enhanced gravitomagnetic effect.     

Acceleration tracking control for a spinning glide guided projectile with multiple disturbances

A novel acceleration tracking controller is proposed in this paper, for a Spinning Glide Guided Projectile (SGGP) subject to cross-coupling dynamics, external disturbances, and parametric uncertainties. The cross-coupled dynamics for the SGGP are formulated with mismatched and matched uncertainties, and then divided into acceleration and angular rate subsystems via the hierarchical principle. By exploiting the structural property of the SGGP, model-assisted Extended State Observers (ESOs) are designed to estimate online the lumped disturbances in the acceleration and angular rate dynamics. To achieve a rapid response and a strong robustness, integral sliding mode control laws and sigmoid-function-based tracking differentiators are integrated into the ESO-based Trajectory Linearization Control (TLC) framework. It is proven that the acceleration tracking controller can guarantee the ultimate boundedness of the signals in the closed-loop system and make the tracking errors arbitrarily small. The superiority and effectiveness of the proposed control scheme in its decoupling ability, accurate acceleration tracking performance and anti-disturbance capability are validated through comparisons and extensive simulations.     

Data-driven model-free adaptive attitude control of partially constrained combined spacecraft with external disturbances and input saturation

This study presents an improved data-driven Model-Free Adaptive Control(MFAC)strategy for attitude stabilization of a partially constrained combined spacecraft with external disturbances and input saturation. First, a novel dynamic linearization data model for the partially constrained combined spacecraft with external disturbances is established. The generalized disturbances composed of external disturbances and dynamic linearization errors are then reconstructed by a Discrete Extended State Observer(DESO). With the dynamic linearization data model and reconstructed information, a DESO-MFAC strategy for the combined spacecraft is proposed based only on input and output data. Next, the input saturation is overcome by introducing an antiwindup compensator. Finally, numerical simulations are carried out to demonstrate the effectiveness and feasibility of the proposed controller when the dynamic properties of the partially constrained combined spacecraft are completely unknown.     

Extending multipath hemispherical model to account for time-varying receiver code biases

Multipath effects on code observables account for one of the major error sources in high-accuracy Global Positioning System (GPS)-based positioning, atmosphere sounding and timing applications. The multipath hemispherical model (MHM) represents one of the most widely used methods of mitigating code multipath effects by taking advantage of their spatial repeatability. The use of MHM usually assumes that the receiver code biases (RCBs) are time-invariant; however, this assumption is not always valid, as RCBs and linear combinations thereof (differential code biases, for instance) have long been found to be time-varying over a period of one day. In this contribution, we propose an extended multipath hemispherical model (EMHM) that is capable of mitigating the code multipath effects in the presence of time-varying RCBs. Consequently, the proposed EMHM has two advantages. First, the EMHM gives rise to code multipath corrections with improved reliability because it addresses the intraday variability of RCBs. Second, more interestingly, the EMHM allows easy and effective calibration of short-term temporal variations, if any, in the RCB on each frequency. These advantages are hopeful to benefit GPS code-related applications.     

Design of on-board calibration methods for a digital sun sensor based on Levenberg–Marquardt algorithm and Kalman filters

Digital sun sensor is one of the most important sensors used in the Attitude Determination System (ADS) of the satellite. Due to the harsh environmental conditions that exist in the space, various distortions may occur in the sun sensor optical system that lead to the reduced accuracy of this equipment. So, it is necessary to recalibrate the optical parameters of the aforementioned sensors. For this purpose, first a novel attitude independent error model is proposed for the SS-411 sun sensor that includes the central point of the CCD array, installation error, filter thickness and sensor misalignment. So, the mutual interfaces between the sensor parameters are considered in the developed model. In order to extract the sensor parameters, a nonlinear optimization technique called the Levenberg–Marquardt is applied to the developed model as a batch algorithm. In addition, the Extended Kalman Filter (EKF) and the Unscented Kalman Filter (UKF) have been utilized as sequential strategies. It will be shown that by considering a worst case of variation amount for sensor parameters, an accuracy improvement of about 17° is achieved by the developed calibration algorithms. Comparison between the developed algorithms represents that UKF has higher accuracy, shorter time convergence but higher computational load.