Abstract: For low accuracy and heavy computation in acquisition module of BeiDou-2 navigation satellite system software receiver, a high accuracy and fast acquisition algorithm based on three-step approximation is proposed. Based on variable-step three-step approximation for narrowing search range of Doppler frequency shift gradually, it quickly acquires accurate initial code phase and Doppler frequency offset. Frequency circular shift is used to reduce computation burden of acquisition module by replacing FFT operations with one multiplication in the first step. Experimental results show that accurate Pseudo-Random Noise（PRN） code offset and carrier frequency error within 100 Hz can be acquired. Compared to the computation amount of traditional parallel code phase algorithm, it is reduced by average 70.56% in the optimal circumstances.

Key words: BeiDou-2, satellite navigation, three-step approximation, acquisition, frequency circular shift, IF signal source

摘要： 针对BeiDou-2（BD2）卫星导航软件接收机捕获模块中捕获精度低和运算量大的问题，提出了一种基于三步逼近的高精度快速捕获算法。算法采用变步长的三步逼近方法逐步缩小多普勒频移搜索范围，快速得到了精确的初始码相位和多普勒频偏；利用频率圆周移位方法，在第一步搜索以一次乘法代替FFT操作，较大地减少了捕获模块的运算负担。实验结果证明，该算法捕获的伪码偏移精确、载频偏移误差在100 Hz以内，且最优情况下的运算量比传统的并行码相位捕获算法平均减少了70.56%，运算效率高。

关键词: 北斗二代, 卫星导航, 三步逼近, 捕获, 频率圆周移位, 中频信号源