Gravity Field Recovery from GRACE: Unique Aspects of the High Precision Inter-Satellite Data and Analysis Methods

The very high accuracy of the Doppler and range measurements between the two low-flying and co-orbiting spacecraft of the GRACE mission, which will be at the μm/sec and ≈10 μm levels respectively, requires that special procedures be applied in the processing of these data. Parts of the existing orbit determination and gravity field parameters retrieval methods and software must be modified in order to fully benefit from the capabilities of this mission. This is being done in the following areas: (i) numerical integration of the equations of motion (summed form, accuracy of the predictor-corrector loop, Encke's formulation): (ii) special inter-satellite dynamical parameterization for very short arcs; (iii) accurate solution of large least-squares problems (normal equations vs. orthogonal decomposition of observation equations); (iv) handling the observation equations with high accuracy. Theoretical concepts and first tests of some of the newly implemented algorithms are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

High Dynamic GPS Receiver Using Maximum Likelihood Estimationand Frequency Tracking

A new high dynamic global positioning system (GPS) receiver ispresented and its performance characterized by analysis,simulation, and demonstration. The demonstration receiver is abreadboard model capable of tracking a single simulated satellitesignal in pseudorange and range rate. Pseudorange and range rateestimates are made once every 20 ms, using a maximum likelihoodestimator, and are tracked by means of a third-order fadingmemory filter in a feedback configuration. The receiver trackspseudorange with rms errors of under 1 m when subjected tosimulated 50 g, 40 g/s circular trajectories. The tracking thresholdis approximately 28 dB·Hz, which provides 12 dB margin relativethe the minimum specified signal strength, assuming 3.5 dB systemnoise figure and 0 dB antenna gain.     

基于旋转加速度计原理的重力梯度测量技术研究与试验

旋转加速度计式重力梯度测量方法通过旋转调制的方式提取微弱的重力梯度信息。首先从测量原理出发,指出实现该方案的主要难点及对策,提炼出关键技术;其次,以多种方式开展引力梯度效应试验,验证了理论的正确性;最后,尝试了面向载体应用的动态适应性试验,达到地面低动态条件下的技术要求,为开展高动态条件下的重力梯度测量奠定基础。     

利用外场数据对产品工龄—可靠性特性的研究

本文采用了三参数相关系数优化法与图估法相结合的数据处理方法,利用外场数据,对某型航空发动机的工龄-可靠性特性进行了分析,得到相应的结论和对维修方式的建议。     

旋转加速度计式重力梯度仪动态测量适应性能试验与效果分析

基于旋转加速度计测量原理的重力梯度动态测量技术,仪器自身动态适应性能是该研究工作的重要内容。首先,从重力梯度动态测量误差机理出发,梳理出实现动态测量的关键技术;其次,以国内航空遥感作业常用的Y-12固定翼飞机为例,分析了典型测量作业工况;最后,开展了基于载体动态参数的地面模拟试验,初步达到了论证的技术要求,为开展真实动态条件下的重力梯度测量奠定基础。     

Decoding Navigation Data Messages from Weak GPS Signals

A new method for decoding navigation data bits from very weak Global Positioning System (GPS) signals is presented. Coherent signal integration is applied over a time period that includes multiple navigation data bits. A search is performed for a combination of bits that maximizes the signal energy during the coherent integration.     

GPS软件接收机的捕获与跟踪算法

研究了GPS软件接收机的捕获与跟踪算法。分析了时域串行滑动相关捕获算法和频域基于FFT的并行相关捕获算法,设计了适合GPS软件接收机的并行算法,实现了对空中可见卫星的捕获。针对GPS信号的特点,设计了基于DLL与PLL相结合方法的GPS跟踪算法。利用实测中频信号对上述捕获与跟踪算法进行了验证分析,测试结果表明,基于FFT的并行相关捕获算法能够有效增强软件接收机的捕获能力,采用DLL与PLL相结合的方法能够实现对GPS信号的有效跟踪。     

The Lunar Gravity Ranging System for the Gravity Recovery and Interior Laboratory (GRAIL) Mission

The Lunar Gravity Ranging System (LGRS) flying on NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission measures fluctuations in the separation between the two GRAIL orbiters with sensitivity below 0.6 microns/Hz^1/2. GRAIL adapts the mission design and instrumentation from the Gravity Recovery and Climate Experiment (GRACE) to a make a precise gravitational map of Earth’s Moon. Phase measurements of Ka-band carrier signals transmitted between spacecraft with line-of-sight separations between 50 km to 225 km provide the primary observable. Measurements of time offsets between the orbiters, frequency calibrations, and precise orbit determination provided by the Global Positioning System on GRACE are replaced by an S-band time-transfer cross link and Deep Space Network Doppler tracking of an X-band radioscience beacon and the spacecraft telecommunications link. Lack of an atmosphere at the Moon allows use of a single-frequency link and elimination of the accelerometer compared to the GRACE instrumentation. This paper describes the implementation, testing and performance of the instrument complement flown on the two GRAIL orbiters.     

Earth Gravity Field from Space — from Sensors to Earth Sciences: Closing Remarks

Space Science Reviews -     

基于组件网络方法的微加速度计建模与仿真

 提出多端口组件网络方法对MEMS 进行建模与仿真, 建立了惯导MEMS 典型功能结构部件的参数化多端口组件模型, 联结组件模型形成网络构建了蟹腿式微加速度计的系统模型。用硬件描述语言作为编码工具, Saber 作为平台对该微加速度计模型进行了仿真分析。研究表明, 采用多端口组件网络可以实现M EMS 整体行为的快速仿真, 并能分析局部尺寸参数对系统性能的影响。与有限元分析相比较, 微加速度计系统模型仿真得到的检测方向的固有频率误差小于1.0%; 仿真结果表明梁的宽度变化对系统灵敏度有较大影响, 故需要尽量减小梁宽的加工误差。借助该模型还研究了系统对正弦及脉冲输入信号的响应特性。     

外弹道测量数据采集与处理技术

数据采集与处理技术是数据处理的关键。主要介绍外测事后数据处理过程中测量数据采集、异常数据识别、数据中奇异项剔除、数据丢点、重点检测方法、数据取齐、加密、压缩与采样时间校正、数据平滑滤波等关键技术,其对于研究数据处理新方法和提高数据处理精度,具有一定的参考价值。     

极弱信号环境下GPS位同步和载波跟踪技术

为解决环路未锁定、无法找出正确的导航数据位跳变点时极弱全球定位系统(GPS)信号跟踪问题,通过分析信号跟踪模型和相关器1 ms采样数据特性,将最优路径动态规划算法应用于GPS位同步,并提出基于平方根无迹卡尔曼滤波(SRUKF)和位同步结合的载波联合跟踪算法。建立了适合位同步的SRUKF载波环参数估计模型,针对位同步的实现条件和无相位频率先验信息的情况,将位同步模块提前,研究设计了环路未锁定时串行和并行两种弱信号跟踪方案。位同步跟踪完成后环路采用单独的SRUKF工作方式。实验结果证实该文提出的算法具有较高的数据位边缘检测概率(EDR)和参数估计性能,而无须关心环路是否处于锁定状态,能够对载噪比低至22 dB/Hz的弱信号实现跟踪。     

一种兼容GPS和BD系统的跟踪环路相干积分方法

在城市、峡谷等复杂环境下,卫星导航信号衰减严重,接收机导航定位性能变差,甚至不能工作,给国民生产生活带来诸多不便.提高接收机弱信号的跟踪能力,研究高性能的卫星导航接收机具有重要的意义和价值.在研究分析GPS、北斗D1导航电文的调制方式和相干积分方法的基础上,构建了一种改进直方图的位同步方法,该方法借鉴直方图的思想,通过设定比特跳变的统计门限上限和多组门限下限找到比特跳变边沿.在此基础上设计实现了一种长时间相干积分方法,该方法兼容GPS电文和北斗D1导航电文,可以有效地消除导航电文跳变或者NH码跳变带来的比特翻转.最后,在FPGA平台验证了该方法可以有效提高信号信噪比,以及接收机在弱信号环境下的跟踪能力.     

Augmented Empirical Models of Plasmaspheric Density and Electric Field Using IMAGE and CLUSTER Data

Empirical models for the plasma densities in the inner magnetosphere, including plasmasphere and polar magnetosphere, have been in the past derived from in situ measurements. Such empirical models, however, are still in their initial phase compared to magnetospheric magnetic field models. Recent studies using data from CRRES, Polar, and Image have significantly improved empirical models for inner-magnetospheric plasma and mass densities. Comprehensive electric field models in the magnetosphere have been developed using radar and in situ observations at low altitude orbits. To use these models at high altitudes one needs to rely strongly on the assumption of equipotential magnetic field lines. Direct measurements of the electric field by the Cluster mission have been used to derive an equatorial electric field model in which reliance on the equipotential assumption is less. In this paper we review the recent progress in developing empirical models of plasma densities and electric fields in the inner magnetosphere with emphasis on the achievements from the Image and Cluster missions. Recent results from other satellites are also discussed when they are relevant.     

Gravity Recovery and Interior Laboratory (GRAIL): Mapping the Lunar Interior from Crust to Core

The Gravity Recovery and Interior Laboratory (GRAIL) is a spacecraft-to-spacecraft tracking mission that was developed to map the structure of the lunar interior by producing a detailed map of the gravity field. The resulting model of the interior will be used to address outstanding questions regarding the Moon’s thermal evolution, and will be applicable more generally to the evolution of all terrestrial planets. Each GRAIL orbiter contains a Lunar Gravity Ranging System instrument that conducts dual-one-way ranging measurements to measure precisely the relative motion between them, which in turn are used to develop the lunar gravity field map. Each orbiter also carries an Education/Public Outreach payload, Moon Knowledge Acquired by Middle-School Students (MoonKAM), in which middle school students target images of the Moon for subsequent classroom analysis. Subsequent to a successful launch on September 10, 2011, the twin GRAIL orbiters embarked on independent trajectories on a 3.5-month-long cruise to the Moon via the EL-1 Lagrange point. The spacecraft were inserted into polar orbits on December 31, 2011 and January 1, 2012. After a succession of 19 maneuvers the two orbiters settled into precision formation to begin science operations in March 1, 2012 with an average altitude of 55 km. The Primary Mission, which consisted of three 27.3-day mapping cycles, was successfully completed in June 2012. The extended mission will permit a second three-month mapping phase at an average altitude of 23 km. This paper provides an overview of the mission: science objectives and measurements, spacecraft and instruments, mission development and design, and data flow and data products.     

Field test results prove GPS performance and utility

Statistical and operational results of extensive government field-testing of the Rockwell-Collins Global Positioning System (GPS) units are summarized. The equipment has exhibited better than 16-m spherical-error probable-position accuracy in over 6300 hours of testing conducted during the past two years. One-channel, two-channel, and five-channel receivers were subjected to thorough evaluation. Their respective signal-processing and data-processing architectures are described. Data highlighting dynamic position accuracy, static position accuracy, acquisition times, and field reliability are presented. GPS equipment integration and operation with nine different host vehicles is described. The results of mission scenarios, such as area navigation, rendezvous, and weapon delivery, are presented     

Earth Gravity Field from Space – From Sensors to Earth Sciences: Closing Remarks

Space Science Reviews -     

中继卫星系统动力学分析

应用单摆模型来等效液体晃动，并利用拉格朗日方程建立了中继卫星的动力学方程。在确定天线跟踪规律的基础上，从工程实际的角度分析了天线和星体之间的耦合作用。考虑天线不同的跟踪模式，将卫星的本体视为刚性体，在星体无控情况下进行系统的动力学响应分析。通过数值仿真确定天线在不同跟踪模式下对星体姿态的影响，并通过有无柔性振动和液体晃动结果的分析，得出天线运动特别是高速回扫、附件柔性振动都会对星体姿态产生比较大的影响，而液体晃动对星体姿态影响不大的结论。