A Study on the Application of the Empirical Mode Decomposition to GPS Signal Acquisition Process

摘要

In recent years, researches on the weak GPS signal acquisition have attracted a lot of attentions to provide an accurate position in indoor environment. It has been studied that GPS signal acquisition process needs a longer integration time for conventional FFT acquisition method under the weak signal condition. However, the maximum data length for the coherent integration time method is limited by the navigation data bit transition effect. In fact, the GNSS weak signal may be "transient" in the receiver, with duration generally shorter than the observation interval; it therefore can be defined as non-stationary signal. Consequently, this paper presents a signal analysis technique for GPS signal based on the Empirical Mode Decomposition (EMD). The EMD process decomposed any complicated data into a finite and often small number of Intrinsic Mode Function (IMF) components and a residual trend. The general features of the IMF components are described along with the ability to combine IMF components and the residual to form low frequency or high frequency filters. This paper takes the characteristics of the EMD as a filter to reduce the noise prior to the signal acquisition process. As a result, the weak signal can be further acquired by focusing on the filtered GPS signals. In this paper, the analytical background of the EMD is introduced and its effectiveness is experimentally, evaluated using the GPS raw Intermediate Frequency (IF) signals which are collected by a software GPS receiver. The results demonstrate that EMD-based GPS signal acquisition method is adequate on capturing transient events such as the weak signal under the indoor environment.