辅助北斗的弱信号导航理论与方法

Assisted-GPS has been researched for many years, but hybrid positioning among Cell-ID, Wi-Fi and GPS is the main technology in practice because of the lack of infrastructure support, such as reference stations and communication system. BeiDou Navigation Satellite System (BDS) has the capacity of regional service since 2012. Nowadays some companies can provide Assisted-BDS (A-BDS) services but there are no terminal supports. Research on the algorithms of A-BDS has important theoretical significance and practical value..This project is developed by software receiver which considers the hybrid constellation, multi-frequency signals, and short message functions of BDS. An assisted positioning server would be constructed based on local Continuous Operational Reference System (CORS) in Space Datum and Location Services Laboratory, to compute the assisted data produced by the CORS and IGS online positioning services. Besides, the data communication model would be designed combined by web data stream and BDS short message. For example, when the terminal has no mobile wireless communication support, the assisted information would be sent to users by BDS short message. .At mobile terminal side, satellite signal acquisition is very difficult because BDS signal has modulated NH code. The navigation data cannot be rightly decoded if the signal is weak and bit error ratio is too high, so the part or total pseudo-range would be simulated. When the quantity of visible satellite is less than four in sheltered environment, an integrated navigation algorithm of pseudo code phase and pseudo-range would be adopted. .The main goal is to resolve the navigation with short time to first fix in weak signal environment such as city canyons, indoor, etc. The achievements would provide ubiquitous location service to applications.

A-GPS研究由来已久，但缺乏基准站和通信等基础设施的支持，实用中往往以Cell-ID、Wi-Fi和GPS的混合定位为主。北斗卫星导航系统已经具备区域服务能力，也有企业开始提供支持北斗的辅助定位服务，但却没有导航终端支持，研究辅助北斗的导航定位方法具有重要的理论意义和现实价值。本课题将结合北斗卫星导航系统的混合星座、多频信号、短消息通信等特点，以软件接收机为用户终端平台，展开弱信号条件下的辅助北斗导航定位理论研究。研究基于CORS和IGS定位产品的辅助数据生成算法，设计Web数据流和北斗短信息相结合的辅助数据传输模型；分析北斗卫星信号特征，设计顾及NH码的弱信号捕获方法，研究无偏的卫星信号发射时刻恢复算法，实现北斗粗时段导航算法，解决城市峡谷、建筑物内等弱信号环境下的应急定位问题，降低首次定位时间，为位置服务应用提供泛在的位置支持。

在城市环境下，尤其是在城市峡谷、林荫树下、半遮蔽室内，由于北斗卫星导航信号受到遮挡而显得特别微弱，这对信号接收提出了更高要求，甚至有时因为可观测卫星数量不足而无法定位。为了提高北斗定位终端的可用性，本课题针对城市环境里卫星信号微弱场景下的定位问题，结合北斗系统的混合星座、多频信号、短消息通信等特点，以软件接收机为用户终端平台，开展了弱信号条件下的辅助北斗系统的导航定位算法研究。研究内容包括：（1）研究了加速信号捕获的辅助数据生成算法，基于CORS网络实时数据流，根据用户概略位置，实时动态生成捕获辅助数据（时间、码延迟和多普勒频移等），并以Web数据流或北斗短信息的模式发送给用户，其中Web数据流主要采用改造RTCM的方式，将生成的捕获辅助数据和定位辅助数据（导航电文）嵌入到RTCM数据中；北斗短消息通常只能发布捕获辅助数据，定位辅助数据需要进行编码和压缩，以减小数据量。（2）分析了北斗卫星信号特征，考虑到北斗D1导航电文调制有NH码，不利于在弱信号环境下通过相干积分提高信号的信噪比，设计了半比特补零和全比特补零算法，实现了-155dBm的弱信号捕获。并使用圆周移位的伪码补零算法，将复杂的载波剥离运算用计算量很小的频谱序列搬移操作来替代，有效地提高了运算量和捕获耗时。研究了北斗粗时段导航算法，提出了一种无偏的卫星信号发射时刻恢复方法，在仅有一颗完整的卫星伪距测量支持下，完成了辅助北斗定位，提高了北斗系统在城市峡谷、建筑物内等弱信号环境的定位能力。