A PN code optimal design based on unequal-length code modulation is proposed in this paper for better perfor-mance of code acquiring for weak signals .The principle is that by using un-equal-length code in spread-spectrum modulation the cross-correlations between different signals have been sliding ,thus suppressing multiple access interference effectively .Theoretical analysis and simulation show that this design has predictive power for compressing multi access interference .%强多址干扰下的弱信号捕获是高灵敏度GNSS接收机比较常见又难以解决的问题。本文提出一种非等长的伪码设计方法，通过使各路导航信号调制伪码码长互质，从时域上降低了伪码的互相关值，从频域上降低了目标信号和干扰信号的谱分离系数。理论分析和仿真表明，基于非等长的伪码优化方法，能够改善强多址干扰环境中弱信号的捕获性能。
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