Research On Tracking Techniques Of High Sensitivity GNSS

As an important application of civil navigation, high sensitivity Global Navigation Satellite System (GNSS) receivers have been widely used recently. In high sensitivity environments, the received signal power will be attenuated heavily due to shadow, multi-path and radio frequency interference (RFI). All of these factors lead signal acquisition and stably tracking to more challenges. This thesis aims to explore more robust algorithm of carrier and code tracking in high sensitivity environments.The thesis proposed an algorithm for weak signal carrier tracking based on dynamic compensated fast Fourier transform (FFT), and analyzed some methods of eliminating multi-path propagation. The thesis is organized as follows:(1). Based on the model of GNSS receivers, the thesis discussed the properties of auto-correlation and cross-correlation of traditional L1 C/A signals, and described the difficulties in signal tracking in weak signal environments. Furthermore, the thesis analyzed various sources of error for GNSS receivers. Especially, a method was introduced to obtain the frequency error sequence of crystal oscillators from simulation.(2). According to the mathematical model of the basic phase locked loop (PLL), this thesis presented those sources of error of frequency locked loop (FLL) and PLL. By comparing their performances, it was concluded that the carrier tracking threshold was decided by FLL, and both of FLL and PLL were not suitable to the high sensitivity application. Then, considering the relationship between maximum likelihood estimation (MLE) and frequency domain discrete Fourier transform (DFT), a weak signal tracking algorithm was proposed based on dynamic compensated FFT. Moreover, performance simulation results were shown under different dynamic conditions.(3). Concerning code tracking in weak signal environments, the thesis introduced and compared two delay locked loop (DLL) discriminators based on dot product (DP) and early-minus-late power (EMLP) respectively in terms of discriminator gain and linear range. Besides, the error sources of code tracking including thermal noise, dynamic stress error and multi-path were represented. In terms of eliminating the tracking error caused by multi-path, this thesis analyzed the performance of two methods based on narrow correlator and early-late slope (ELS).(4).A scheme of the high sensitivity GNSS receiver was proposed. Also here described procedure of interrelated signal processing. According to the plan of GNSS modernization, which has been paid more and more attention to by researchers, the thesis introduced the characteristics of L2C, L5 and binary offset carrier (BOC), and showed their advantages to high sensitivity application at last.