A kinematic statistical method is proposed to determine the position for Chang'E-3(CE-3) lunar lander.This method uses both ranging and VLBI measurements to the lander for a continuous arc,combing with precise knowledge about the motion of the moon as provided by planetary ephemeris,to estimate the lander's position on the lunar surface with high accuracy.Accuracy analyses are carried out with simulation data using the software developed at Shanghai Astronomical Observatory in this study to show that measurement errors will dominate the position accuracy.Application of lunar digital elevation model(DEM) as constraints in the lander positioning is also analyzed.Simulations show that combing range/doppler and VLBI data,single epoch positioning accuracy is at several hundred meters level,but with ten minutes data accumulation positioning accuracy is able to be achieved with several meters.Analysis also shows that the information given by DEM can provide constraints in positioning,when DEM data reduce a 3-dimensional positioning problem to 2-dimensional.Considering the Sinus Iridum,CE-3 lander's planned landing area,has been observed with dedicated details during the CE-1 and CE-2 missions,and its regional DEM model accuracy may be higher than global models,which will certainly support CE-3's lander positioning.
HUANG YongHU XiaoGongLI PeiJiaCAO JianFengJIANG DongRongZHENG WeiMinFAN Min
空间飞行器需要实时的高精度轨道信息来完成对栽荷的指令操作和遥感数据的实时处理。除了星栽GPS技术，星载多普勒无线电定轨定位系统（DORIS，Doppler Orbitography and Radio—positioning Intergrated by Satellite）是仅有的有能力提供分米级精度的实时在轨轨道确定技术，它可通过测量星地相对多普勒频移，在星上完成实时定轨和预报，目前该技术已在国外多个卫星上实现，达到了较好的效果，而我国还没有建立这样实时自主定轨系统。为此，结合我国高分辨率空间对地观测系统的建设需求和我国航天器对实时自主定轨及其精度的要求，利用扩展卡尔曼滤波算法对多普勒测量进行了实时自主定轨仿真计算，分析了频率偏差估计与否、初轨误差、地面信标站地理分布以及观测精度等对实时自主定轨的滤波收敛时间和定轨精度的影响，为我国利用DORIS技术进行实时在轨轨道确定提供方案和软件原型。仿真计算表明，基于28个全球分布的地面站，对于高度为800km的卫星，在忽略其动力学模型误差的假设下，若初轨三维位置、速度误差分别为100m（或差至1km）、1m／S（1d），2h后滤波可以达到稳定收敛，收敛后的实时定轨误差可以达到0．1m（1d）。滤波估计参数除了6个卫星轨道状态参数，还估计了地面信标相对于卫星超稳定振荡器的频率偏差；
With strong support from European Space Agency (ESA),Shanghai Astronomical Observatory (SHAO) organized a tracking and orbit determination trails using Chinese VLBI Network (CVN) to track Mars Express,the first Mars probe launched by ESA. Using a high-resolution VLBI software correlator and Doppler measurement system developed in-house,two sets of tracking data,VLBI and Doppler,were acquired. The trials represent the first successful foray held in China to track a probe about 360 million kilometers away from the Earth. The tracking data are analyzed using a Mars satellite orbit determination software system developed at SHAO. The results show that the accuracy of 5 s integrated three-way-Doppler data is about 0.3 mm/s,or roughly the same accuracy as ESA's tracking data. Position discrepancies between the Doppler-based orbit solution of 8 h arc-length (about 1 orbital revolution) and ESA's reconstructed orbit are of the order of several hundred meters. In preparing for the Russia-China co-sponsored Mars exploration mission Phobos-Grunt-YingHuo,simulations were carried out to evaluate the achievable orbital accuracy levels and the contributions of VLBI and Doppler data respectively. Results show that Doppler data provide better orbit accuracy,so that for VLBI to be able to provide kilometer level orbit solutions,the accuracy of VLBI measurement needs to be improved by at least one order of magnitude.
CAO JianFengHUANG YongHU XiaoGongMA maoLiZHENG WeiMin
Constellations of regional satellite navigation systems are usually constituted of geostationary satellites (GEO) and inclined geostationary satellites (IGSO) for better service availability. Analysis of real data shows that the pseudorange measurements of these two types of satellites contain significant multipath errors and code noise, and the multipath for GEO is extremely serious, which is harmful to system services. In contrast, multipath error of carrier phase measurements is less than 3 cm, which is smaller than the multipath of pseudorange measurements by two orders of magnitude. Using a particular combination of pseudorange and dual-frequency carrier phase measurements, the pseudorange multipath errors are detected, and their time varying features are analyzed. A real-time multipath correction algorithm is proposed in this paper, which is called CNMC (Code Noise and Multipath Correction). The algorithm decreases the influence of the multipath error and therefore ensures the performance of the system. Data processing experiments show that the multipath error level may be reduced from 0.5 m to 0.15 m by using this algorithm, and 60% of GEO multipath errors and 42% of IGSO multipath errors are successfully corrected with CNMC. Positioning experiments are performed with a constellation of 3 GEO plus 3 IGSO satellites. For dual-frequency users the East-West position accuracy is improved from 1.31 m to 0.94 m by using the CNMC algorithm, the South-North position accuracy is improved from 2.62 m to 2.29 m, and the vertical position accuracy is improved from 4.25 m to 3.05 m. After correcting multipath errors, the three-dimensional position accuracy is improved from 5.16 m to 3.94 m.
