The presence of code Doppler and navigation bit sign transitions means that the acquisition of global positioning system (GPS) signals is difficult in weak signal environments where the output signal-to-noise ratio (SNR) is significantly reduced. Post-correlation techniques are typically utilised to solve these problems. Despite the advantages of these techniques, the post-correlation techniques suffer from problems caused by the code Doppler and the navigation bit sign transitions. We present an improved semi-bit differential acquisition method which can improve the code Doppler and the bit sign transition issues in the post-correlation techniques. In order to overcome the phenomenon of navigation bit sign transitions, the proposed method utilises the properties of the navigation bit. Since each navigation bit takes as long as 20 ms, there would be 10 ms correlations duration integration time between the received signal and the local coarse/acquisition (C/A) code in which the navigation bit sign transitions will not occur. Consequently, this problem can be cancelled by performing 10 ms correlations in even and odd units separately. Compensation of the code Doppler is also accomplished by shifting the code phase of the correlation results. To validate the performance of our suggested method, simulations are performed based on three data sets. The results show that the quantity of required input SNR to detect at least four satellites in the proposed method is − 48·3 dB, compared with − 20 dB and − 9 dB, respectively, in traditional differential and non-coherent methods.

中文翻译：

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弱信号环境下导航比特符号转换和码多普勒补偿的改进半比特差分采集方法

代码多普勒和导航位符号转换的存在意味着在输出信噪比 (SNR) 显着降低的弱信号环境中难以获取全球定位系统 (GPS) 信号。后相关技术通常用于解决这些问题。尽管这些技术具有优势，但后相关技术仍存在由代码多普勒和导航位符号转换引起的问题。我们提出了一种改进的半比特差分采集方法，可以改善后相关技术中的码多普勒和比特符号转换问题。为了克服导航位符号转换的现象，所提出的方法利用了导航位的特性。由于每个导航位需要长达 20 毫秒，在接收信号和本地粗略/采集 (C/A) 码之间将有 10 ms 的相关持续时间积分时间，其中导航位符号转换不会发生。因此，可以通过分别以偶数和奇数单位执行 10 ms 相关来消除此问题。代码多普勒的补偿也是通过移动相关结果的代码相位来完成的。为了验证我们建议的方法的性能，基于三个数据集进行了模拟。结果表明，在所提出的方法中检测至少四颗卫星所需的输入 SNR 数量为 - 48·3 dB，而在传统差分和非相干方法中分别为 - 20 dB 和 - 9 dB。