光电工程, 2009, 36 (4): 113, 网络出版: 2009-10-09
GPS 接收机伪码捕获自适应判决门限估计算法
Adaptive Threshold Estimation Algorithm for PN Code Acquisition in GPS Receivers
伪码捕获 自适应门限 平均捕获时间 恒虚警概率 GPS GPS PN code acquisition adaptive threshold average acquisition time constant false alarm rate
摘要
GPS 接收机输入信号的功率随时间而变化,如果采用固定门限判决是否完成捕获会出现检测概率降低或虚警概率提高的问题。鉴于此,提出了采用自适应捕获门限判决策略完成信号捕获。采用最大似然估计原理,根据判决统计量的样本值得到了噪声方差的最大似然估计,进而求得恒虚警率下的自适应捕获门限。同时,信号的平均捕获时间是GPS 接收机性能的重要指标之一,研究了载波频率与伪码相位均为不确定值时的平均捕获时间的计算方法,推导了平均捕获时间与信噪比及门限值之间的约束关系。分析结果表明,自适应门限捕获方法能减小平均捕获时间,实现信号的快速捕获。
Abstract
Input signal power of GPS receivers changes with time. If fixed threshold is used to determine completion of the acquisition stage, two problems will arise, namely, decreasing in detection probability or increasing in false alarm probability. Therefore, it is proposed to apply a decision method for adaptive acquisition threshold in signal acquisition. Specifically, maximum likelihood estimation method is used on the sample data for decision statistic to obtain the maximum likelihood estimation value for noise variance. Afterwards, adaptive acquisition threshold can be obtained based on the Constant False Alarm Rate (CFAR). As the average acquisition time is one of the most important indexes of GPS receivers, calculation methods for the average acquisition time are analyzed for the case when both carrier frequency and PN code phase are uncertain. Furthermore, constraint relationship among acquisition time, Signal-to-noise Ratio (SNR) and threshold are derived. Analysis results show that adaptive acquisition threshold method can reduce acquisition time effectively, thus accomplish faster acquisition.
刘娣, 薄煜明, 邹卫军. GPS 接收机伪码捕获自适应判决门限估计算法[J]. 光电工程, 2009, 36(4): 113. LIU Di, BO Yu-ming, ZOU Wei-jun. Adaptive Threshold Estimation Algorithm for PN Code Acquisition in GPS Receivers[J]. Opto-Electronic Engineering, 2009, 36(4): 113.