弱信号下“北斗”接收机NH码的快速捕获

在全球卫星导航系统(GNSS)中，延长相关累积时间可以有效提高弱信号捕获灵敏度，但是“北斗”中圆地球轨道/倾斜地球同步轨道（MEO/IGSO）卫星信号的捕获性能很大程度上受到二次编码(NH)码的影响：调制NH码的信号在每一个码元周期(1 ms)内都有可能发生跳变，对相关累积时间造成一定影响，从而会导致系统捕获灵敏度的下降。为此，提出了一种针对NH码的码元遍历搜索算法，通过遍历累积时间内NH码的所有组合，消除NH码码元引起的误差。实验结果显示，所提算法可以在一定程度上提高卫星信号的捕获成功率，尤其可以至少提高2 dB弱信号的捕获灵敏度。同时，算法可以为信号跟踪提供更加准确的载波频率。

Fast acquisition of Neumann-Hoffman codes for BDS receivers in weak signal environment

Longer integrations can efficiently enhance the acquisition sensitivities of the Global Navigation Satellite System (GNSS). However, the performance of Beidou System(BDS) medium Earth orbit/inclined geostationary orbit(MEO/IGSO) satellite signal acquisition can be seriously affected by the secondary Neumann-Hoffman (NH ) codes. For the NH codes, bit-flipping may occur in each signal period (1 ms), which will affect the integration time and thus lower the acquisition sensitivities. To solve the problem, a symbol-based exhaustive searching algorithm is presented. The errors caused by the NH codes flipping are eliminated by traversal searching of all the possible coding combinations.Experiment results show that the presented algorithm can improve the acquisition successful rate 2 dB at least, especially for weak signals. Furthermore, the algorithm can provide more accurate carrier frequency.