Development of a Variable Sampling Rates GNSS/INS Tightly Coupled Navigation System Based on SDR

Abstract

Most Global Navigation Satellite System (GNSS) receivers typically work at a sampling rate of 1 Hz. However, variable sampling rates are required for optimal performance in different dynamic navigation applications. For example, high sampling rates are crucial for high-dynamic platforms such as Unmanned Aerial Vehicle (UAV) navigation. Higher sampling rates help decrease positioning interval time and improve travel distance measurements, especially when moving on a curvy route. On the contrary, lower sampling rates help save power consumption and computation. Currently, there is barely literature presenting the impact of sampling rates on positioning accuracy and what sampling rate is required for different vehicle dynamics, considering the positioning accuracy and computational load. In this thesis, a GNSS Software-Defined Receiver (SDR) is investigated and developed to implement variable sampling-rate capability at low cost. This approach provides flexibility, customisation, and scalability, as the GNSS SDR can be reconfigured or updated via software to support multiple GNSS signals, systems, or new techniques without requiring costly hardware modifications. While carrier phase measurements have been widely applied to conduct precise position determination in traditional positioning methods, the Time Differenced Carrier Phase (TDCP) measurements are increasingly used, particularly for precisely positioning in dynamic applications. TDCP is a measurement of the phase change over time, making it particularly effective for high-dynamic applications and positioning scenarios with low-cost GNSS receivers. The variable-rate phase observations from the GNSS SDR will be applied to form TDCP observations for precise positioning and attitude determination. In urban environments, GNSS navigation faces major challenges such as limited satellite visibility, multipath effects, Non-Line-of-Sight (NLOS) conditions, signal attenuation, and interference caused by buildings and other infrastructure. Due to the complementarity of GNSS and the Inertial Navigation Systems (INS), a tight GNSS/INS integration has the potential to mitigate the multipath effects at the baseband level. The RTK/INS tightly coupled system is introduced to aid the navigation and attitude determination of the TDCP-based system. Field tests and numerical results are presented to assess the developed SDR and the tightly coupled integrated systems. The test results show that TDCP can achieve high precision navigation. Besides, the research results on the impact of different sampling rates on the positioning performance can help facilitate the selection of an appropriate sampling rate for navigation systems.