A Concept of Position Fixing by a Passive Mode Navigation Satellite System

A position fix in a passive mode using satellites usually necessitates an expensive computer or lengthy hand calculation. This is the largest drawback of passive navigation and it would be more desirable if the user could find his position by a mere glance at a chart and table, as one uses Loran. The first step toward this goal is to use a synchronous satellite because it simplifies the problem. The next step is to find the position of the user by a Loran type of chart, which is universal, and correct this apparent position by looking at a special table which is made according to the amount of perturbation of both the satellite and the user's position. An example of the position fix along the route between Yokohama to Hawaii is shown. The concept can be extended to orbiting satellites due to the rules which govern the motion of satellites, if the fix accuracy is in the order of 2 to 5 miles. This method should be more accurate than the common sextant and more practical due to the fact that the satellite can be used at any time and in any weather. As a total system, it will be better than Omega because it could provide additional navigation information such as communication or traffic control by using satellites.     

Quick position determination using 1 or 2 LEO satellites

We describe an approach for a medium accuracy position determination of a user terminal (UT) on the Earth surface, using one or two low Earth orbit (LEO) satellites. The positioning approach is intended to meet the requirements of a worldwide personal communications system using LEO satellites. The basic two requirements are: (1) immediate positioning, and (2) horizontal position accuracy of the order of 10 km. Those requirements stem from the need of the system to know the user's approximate location before it connects his call. The approach makes use of the two-way communication with the UT, which can receive, transmit, and make its own measurements. Delay and Doppler measurements are used in order to enable instantaneous positioning with one satellite, and in order to achieve unambiguous positioning with two satellites. A simplified Globalstar satellite constellation and the expected Globalstar delay and frequency measurement accuracy are used to demonstrate the concept and to evaluate its performances     

Existence and uniqueness of GPS solutions

The existence and uniqueness of positions computed from global positioning system (GPS) pseudorange measurements is studied. Contrary to the claims of S. Bancroft (1985) and L.O. Krause (1987), in the case of n=4 satellites a fix may not exist, and, if a fix exists, it is not guaranteed to be unique. In the case of n>or=5 satellites, a unique fix is assured, except in certain degenerate cases such as coplanar satellites. An alternate formulation of the direct n=4 pseudorange to three-space position solutions of Bancroft and Krause is presented, and simple tests for existence and uniqueness are derived.<>     

An alternative closed-form solution to the GPS pseudo-rangeequations

In the four satellite Global Positioning System (GPS) problem, the system of pseudo-range equations is shown to be equivalent to a system of two linear equations together with a range difference and pseudo-range equation. The formulation represents the user's position as the intersection of two planes and a hyperbola branch of revolution. The formulation is three-dimensional and includes almost all degenerate and special case geometries. It provides geometric insight into the characteristics of the solutions and resolves existence and uniqueness questions regarding solution of the pseudo-range equations     

三星定位原理研究

双星定位系统只需要两颗同步卫星便可以为用户提供定位服务,但却存在着用户位置易暴露和用户数量易饱和这两大缺陷。建议采用三颗同步导航卫星像GPS卫星那样向用户播发导航电文,用户接收机根据这些导航电文信息以及气压高度表信息,像GPS接收机那样解算用户位置,从而克服了双星定位的两大缺陷。仿真表明本定位原理行之有效。     

三维空间中目标机动加速度的估值问题

建立了描述目标在三维空间中进行切向与法向机动的非线性状态模型。目标切向与法向机动加速度的幅值表示为修正的瑞利-马尔可夫随机过程;法向加速度的方向角则假定在2π区间内具有均匀的概率密度。在仅有含噪声位置观察数据的情况下,发展了一种推广的卡尔曼滤波和自适应算法,并由此获得一种机动目标切向与法向加速度估值的直接方法。提供了某些计算结果以证实方法的有效性。     

Autonomous orbit determination using epoch-differenced gravity gradients and starlight refraction

Autonomous orbit determination via integration of epoch-differenced gravity gradients and starlight refraction is proposed in this paper for low-Earth-orbiting satellites operating in GPS-denied environments.Starlight refraction compensates for the significant along-track position error that occurs from only using gravity gradients and benefits from integration in terms of improved accuracy in radial and cross-track position estimates.The between-epoch differencing of gravity gradients is employed to eliminate slowly varying measurement biases and noise near the orbit revolution frequency.The refraction angle measurements are directly used and its Jacobian matrix derived from an implicit observation equation.An information fusion filter based on a sequential extended Kalman filter is developed for the orbit determination.Truth-model simulations are used to test the performance of the algorithm,and the effects of differencing intervals and orbital heights are analyzed.A semi-simulation study using actual gravity gradient data from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) combined with simulated starlight refraction measurements is further conducted,and a three-dimensional position accuracy of better than 100 m is achieved.     

Performance of phased-array antennas with mechanical errors

The performance of planar phase-array antennas with mechanical errors is investigated. Errors in array element positions as a result of structural distortions are considered as deterministic and predictable. Detailed calculations for two assumed modes of distortion reveal that their effects on antenna performance are the loss of peak response in the scan direction and the broadening of the mainlobe, while the far-out sidelobe structure remains relatively intact. For large antennas, performance improvement can be expected by suitable phase compensation. Performance of antennas with random errors in their element positions must be treated statistically. Expressions of average directivity and sidelobe level corresponding to arbitrary error magnitudes in element position, amplitude and phase of excitation as well as finite rate of failure of element modules were derived and verified by direct numerical calculations from the antenna directivity patterns. For a planar phased-array antenna typical for space-based radars, the standard deviation of element position errors must not exceed 1% of the operating wavelength in order to maintain a -10 dBi sidelobe level     

空间失效慢旋卫星视觉特征跟踪与位姿测量

空间碎片或者失效卫星往往绕其惯性主轴做自旋运动,虽然处于一种稳定状态,但是对其运动状态测量的难度相比三轴稳定目标而言又增加了许多,主要体现在复杂光场环境下和目标观测面周期性进出视场引起的特征点丢失、尺度变化和旋转变化,以及长时间连续观测引起的累积误差增大、位姿解算不收敛等难题。通过构建优化目标特征数据库,将传统测量方式中前后帧的匹配转变为当前帧与特征数据库的匹配和特征点的优化,即使中间过程帧出现了偏差仍然可以较好地跟踪后续图像帧的正确位置。在李代数空间下建立位姿向量微分扰动方程,获得测量值与估计值的残差目标函数,基于贝叶斯法则最大后验概率和李群与李代数的对指变换法则,求取位姿向量最优解。解决了李群空间下由于位姿变换矩阵不可加性而无法优化的问题,提高了连续测量过程中系统的测量精度。实验结果表明,无优化测量方法无法保证整个旋转周期的有效测量;通过增加位姿优化过程,连续稳定测量时间明显延长,针对以12（°）/s自旋运动的目标,测量稳定段误差在2°以内,旋转角速度测量误差为0.12（°）/s。     

三星时差无源跟踪算法研究

基于卫星平台的无源定位系统可以通过卫星的绕地运动和轨道覆盖,实现对全球地面辐射源的被动侦察,具有侦察隐蔽性强、侦测范嗣广和不受地理位置限制的优点。把一种新的修正协方差扩展卡尔曼滤波（MVEKF）方法引入三星对运动目标的时差无源定位跟踪中,克服了EKF受初始状态和测量误差影响大的缺点,也不用像MGEKF一样需要观测方程的修正函数,仿真表明该滤波方法相对来说收敛速度更快,跟踪性能更好。     

Sensitivity analysis of dual-satellite geolocation

An analytic solution is presented for the linearized estimation problem arising from the dual-satellite geolocation of the source of a narrowband signal using time- and frequency-difference-of-arrival (TDOA and FDOA, respectively) observations and an altitude constraint. This solution is used to analyze the covariance matrix of the resultant source location estimate, along with the sensitivity of its mean to errors in the presumed source altitude and velocity, and the measured or predicted positions and velocities of the satellites. Following the practice of differential Global Positioning System, the technique of differential calibration is used to reduce or eliminate various sources of solution bias which would otherwise require tight, and consequently expensive, individual calibration     

超低轨航天器气动特性快速预测的试验粒子Monte Carlo方法

超低轨(LEO)卫星气动特性的快速准确计算是对其进行轨道预测和控制的关键输入条件。基于真空技术领域中计算管道分子流率的试验粒子Monte Carlo(TPMC)方法,结合自由分子流理论,发展了一套快速准确预测低轨卫星气动特性的TPMC方法,给出了其模拟步骤及主要关键技术点,并采用该方法模拟了带电池翼超低轨卫星的气动特性和航天器典型构件之间的多次反射效应。结果表明:TPMC方法在计算超低轨航天器气动力、力矩时具有较高的可靠性和对工程复杂外形的适用性;该方法能够准确模拟自由分子流理论无法求解的多次反射问题,给出正确的气动力系数;该方法的计算速度比直接模拟Monte Carlo(DSMC)快3~4个量级,存储量要求也比后者低1~2个量级,是超低轨航天器气动特性快速准确预测的一个理想方法。     

离心压气机S1流面流场的计算

本文对计算轴流压气机 S1流面的流函数矩阵解法的计算机程序作了修改,使之能适合于计算离心压气机的 S1流面。本方法是引入人工密度,积分运动方程求速度场,而后求密度场,进行迭代求解,计算速度非常迅速。对离心压气机的几个算例表明:只要不发生明显分离,计算结果与实验值是符合的。      

Geostationary Satellite Navigation Systems

The concept of position determination using geostationary satellites as an alternative to the global positioning system (GPS) is studied. The advantage of a geostationary system is that only three, or at most four, satellites are required to cover the continental United States. A total of twelve satellites are sufficient for global coverage (excluding polar regions), or eight if only longitude and latitude, but not altitude, are measured. The system involves the determination of the range to either four geostationary satellites or, if the altitude is not measured, three geostationary satellites. The accuracy of the proposed systems are evaluated to obtain the rms error associated with position determination, and the concept for the implementation of measurements required by the systems is presented. The accuracy of the systems are adequate for civilian use in the continental United States; however, there is a degradation in accuracy as the location of the user approaches the equator.     

基于非合作低轨卫星的测向交叉定位技术

根据非合作低轨卫星的特点，可以被动测量多颗卫星信号的来向，通过测向交叉的方式进行定位。但是通过星历解算出的卫星位置位于地心地固坐标系，用户测量的方位角和俯仰角基于站心坐标系。针对非合作低轨卫星测向交叉定位时目标用户角度信息与卫星位置基于不同坐标系的问题，提出了一种迭代最小二乘定位算法，通过迭代的方式不断收敛定位结果，能够在目标用户角度信息与卫星位置基于不同坐标系的情况下，解决非合作低轨卫星的测向交叉定位问题。仿真结果表明，基于迭代最小二乘定位算法能够实现非合作低轨卫星仅利用角度定位，并分析了测角精度、卫星轨道高度、参与定位卫星数与定位误差之间的关系。针对迭代的计算方法，分析了迭代过程中不同收敛条件下迭代次数与定位误差之间的关系。在保证定位精度的情况下，将迭代收敛范围设置为8~30 km，可以降低2~3次迭代次数。     

Solution and performance analysis of geolocation by TDOA

One method of geolocation is based on measuring the time difference of arrivals (TDOAs) of a signal received by three or four geostationary satellites. The received signals are cross-correlated to determine the TDOAs and a set of nonlinear equations are solved to produce the location estimate. An exact solution for the transmitter position is derived for the three or four receiver cases. Extension of the solution method to more receivers is straightforward. An analysis of the performance of the system is given, together with expressions for predicting the localization mean-square errors (MSEs) and bias, and the Cramer-Rao bound. Both precision in TDOA measurements and the relative geometry between receivers and transmitter affect the localization accuracy. The geometric factors act as multipliers to the TDOA variance in the bias and MSE formulae. A study of the dependency of the geometric factors on transmitter position and satellite spacings are provided, as well as simulation results     

低马赫数流动中的预处理Euler方程的收敛特性

对预处理后Euler方程在低马赫数流动数值模拟中的收敛特性进行了细致研究。首先采用量级分析法研究了Weiss-Smith预处理方法对低速情形下Euler方程收敛性的影响;然后针对低速情形下预处理后的Euler方程组中各方程的收敛性,提出了一个改进的预处理矩阵。数值模拟结果表明：Weiss-Smith预处理方法可以很好地改善Euler方程在低速流动时的收敛特性,并且改进后的预处理矩阵在连续方程的收敛特性方面效果明显。     

Kalman Filter Design for Target Tracking

The problem of solving the matrix Riccati differential equation in the design of Kalman filters for the target tracking problem is considered. An algebraic transformation method is used to reduce the order of the Riccati differential equation and to obtain explicit expressions for the filter gains (in terms of the interceptor /target separation range) which results in a substantial reduction of the computer burden involved in estimating the target states. The applicability of the transform technique is demonstrated for the receiver thermal noise and the target glint noise cases.     

全球导航卫星系统的测高研究

以美国全球定位系统为代表的全球导航卫星系统具有广泛的应用领域，特点是能够在全球范围和近地空间，全天候地提供飞行器的位置、速度和时间信息。     

直升机旋翼的非定常可压缩升力面气动力

本文得出了三元可压缩非定常连续尾迹面条件下直升机等速前飞时旋翼压力分布的解答,使压力偶极子沿尾迹面运动而不沿桨叶的真实轨迹移动以满足尾迹条件,叠加移动压力偶极子获得翼面作复杂运动时三元非定常可压缩的强奇性积分方程,得出了处理强奇性的方法使之成为带强峰值的连续积分方程,并得出了求解该方程的数值解法。在小型计算机上获得的简单算例证明了方程与解法的正确性。