深空探测高精度VLBI测定轨和火星大气掩星观测研究

Very long baseline interferometry (VLBI) has played an important role in determining orbit in Chang'E series satellites. For example, VLBI much improved the accuracy of orbit determination on Chang'E-3 approach to the moon and circling the moon. We also performed the same-beam VLBI observations between the rover and the lander of Chang'E-3, monitored the rover motion with sensitivity of 5 cm, and measured the relative position between the rover and the lander with an accuracy of 20 cm. China will also launch Mars spacecraft, which consists of a rover and an orbiter, in 2018. The distance between Mars spacecraft and Earth is more than 200 million km, which increases hundreds of times compared with 380000 km of the moon, VLBI delay measurement accuracy must be greatly increased. On the other hand, using same-beam VLBI observations between the Mars orbiter and rover, the influence of the earth's atmosphere and ionosphere can be thoroughly canceled, and high-accuracy differential delay data can be obtained. These data not only can significantly improve the accuracy of orbit determination, but also can be used for innovative occultation observation researches in the Martian atmosphere and ionosphere. This project will focus on the researches of improving VLBI time delay measurement accuracy up to 0.4 ns and solving key techniques, such as detailed error analysis and improvement methods, weak signal processing technology using Doppler stopping method, technology of real-time decoding phase delay, atmospheric delay correction technique based on GPS and water vapor radiometer, plasma delay correction technology, etc. In addition，we will carry out advance research of Mars/Venus atmospheric and ionospheric occultation observations and orbit determination of the Mars orbiter and rover using the same-beam VLBI technique.

VLBI不仅对嫦娥三号在奔月环月段测定轨做出了重要贡献，刘庆会等还利用同波束VLBI把月球车的相对定位精度提高至20cm，并以5cm的灵敏度监视月球车动作。我国将发射由轨道器和火星车组成的火星探测器。捕获时，地火距离2亿km，与探月相比，星地距离增加数百倍，VLBI时延精度需大幅提高。另一方面，利用同波束VLBI技术观测火星轨道器和火星车，通过差分去除地球大气电离层影响，得到更高精度的差分时延数据,即可大幅提高定轨定位精度，又可创新性地开展火星大气电离层掩星观测研究。本项目将针对我国深空探测迫切需求，研究把VLBI时延精度提高至0.4ns的方法并攻克关键技术，如误差详细分析和改进措施研究、基于多普勒旋转的弱信号处理技术、相时延实时解算技术、基于GPS/水汽辐射计的大气时延改正技术、等离子体时延改正技术等。同时开展同波束VLBI技术在火星金星大气电离层掩星观测和环流特性的研究.

完成了实现深空探测单探测器VLBI时延测量精度提高的技术途径和关键技术研究，完成了基于同波束VLBI技术的大气电离层掩星观测的预先研究。发表论文16篇，培养博士生3名，硕士生4名。项目负责人刘庆会获得8项科技奖励。. 开展了深空探测器VLBI时延测量精度提高的技术研究。详细分析了深空探测VLBI测量精度提高的因素，并给出了解决办法。研究了大型射电望远镜对深空探测器VLBI时延测量精度提高的贡献。利用上海65米代替25米望远镜，ΔDOR型VLBI时延定轨后的残差由嫦娥二号时的1.77ns rms降至嫦娥三号时的0.67ns rms，最好时0.3ns。分析了月球热辐射对嫦娥三号信噪比的影响，得出环月期间的嫦娥三号信噪比周期性变化的主要原因是由进入到天线波束的月球热辐射能量随时间变化引起的。分析并明确了嫦娥三号群时延的数百皮秒幅度抖动以及差分相时延数皮秒的抖动皆是由于着陆器表面的反射波干扰造成的。研究了一种利用窄带宽的两个频点相关相位解算相时延的新方法。.开展了基于同波束VLBI 技术的大气电离层掩星观测的预先研究。研究了VLBI差分相关相位和角距离的关系模型，为大气电离层掩星观测的研究打下了良好基础。基于日本探月工程SELENE的两个小卫星4个测站长达1年的同波束VLBI观测数据，首次得到了差分相关相位和角距离的关系模型。同时，利用4测站6基线的相关相位及两卫星间差分相关相位，首次反演出各个测站的大气电波相位抖动及两卫星间差分相位抖动的阿伦方差、构造函数、以及差分相位和角距离的关系模型。差分相关相位和各台站的阿伦方差符合Kolmogorov乱流理论。利用同波束VLBI技术把嫦娥三号巡视器相对定位精度提高至1米，并有望把嫦娥五号轨道器和上升器的定轨精度提高至10米、把火星环绕器定轨精度和火星车定位精度提高至100米量级等。