An Effective Algorithm for Pull-in and Tracking Processing of Weak GNSS Signal

Abstract

Usually, a GPS receiver uses the digital phase-locked loop (DPLL) or digital frequency-locked loop (DFLL) as the carrier tracking loop. The success rate of pull-in process may be low under the condition of very low signal-to-noise rate (SNR), which will affect the time to first fix (TTFF) even if the receiver can successfully acquire the weak signal. An effective algorithm based on FFT for pull-in and tracking processing of weak GNSS signal is presented, which overcomes the influence of the navigation bit transition. The algorithm realizes a lot non-coherent integration through FFT processing and every input point of FFT is the accumulation of coherent integration of 20 ms. The outputs of FFT show the frequency offset, according to which the local carrier frequency is adjusted. The experimental results show that this method can effectively improve the success rate of the pull-in process and reduce the time-consuming of the pull-in process when the GPS signal power level is as low as −155 dBm. After pull-in process, the method also can keep steady tracking under the same condition.