一种基于GPS软件接收机的优化捕获算法

在GPS软件接收机系统中，卫星的捕获操作需要进行大量的相关运算，消耗大量的运算时间和硬件资源.虽然软件接收机采用快速傅里叶变换(FFT)算法来代替相关运算，可以一定程度上减少处理时间，但是对于嵌入式系统来说，由于耗时难以令人满意，限制了其在嵌入式平台上的应用.提出一种自适应搜索算法,利用循环相关操作的原理，在多普勒频移的搜索过程中，采用多级步进，通过不同长度搜索的步长，在保证搜索正确性的前提下，能够减少捕获操作中相关运算的数量；同时，为了减少不必要的硬件消耗和运算消耗，将多级步进预先设置并储存到数据表，从中得到足够精确的多普勒搜索步长，因此利用自适应搜索算法可以减少部分不必要的快速傅里叶变换的数量，提高捕获操作的时间效率.实验证明此方法能够一定程度上提升捕获操作的效率，从而减少捕获所需时间.

An Optimized Acquisition Algorithm in GPS Software Receiver

In GPS software receiver, there are massive correlating operations which cost huge time and hardware resources in the acquisition. The GPS software receiver uses the FFT algorithm to decrease process time obviously. However, this method is used limitedly on the embedded devices due to the limited hardware resources and time-consuming. Thus, this paper proposes an adaptive search algorithm. This algorithm makes a partition of different levels of steps and uses different steps to improve the efficiency of the acquisition. This method implements multi-level steps search in finding the 2-D spectrum peak of circular cross-correlations. In the search process, it preferres to big steps to reduce search operations as far as possible. Because the search process of every satellite uses multi-level steps other than the steps of conventional partition which is lowest level in the multi-level steps, it can remove FFT operations obviously. And to avoid unnecessary computation, there are some tables to restore the various levels of steps. This method reduces redundancy FFT operations in the acquisition and improves the efficiency of the acquisition. Experimental results show this method has improved the efficiency of the acquisition and decreased the process time significantly.