Subcarrier modulated navigation signal processing in GNSS: a review

Abstract

Subcarrier modulated signals are widely used in the new generation of Global Navigation Satellite Systems (GNSS) to improve spectral compatibility and ranging accuracy. The unique structure of subcarrier modulated signals poses an ambiguity threat to acquisition and tracking, rendering traditional GNSS signal processing algorithms unsuitable. Designing unambiguous acquisition and tracking as well as observation extraction techniques for subcarrier modulated signals that can realize the high ranging accuracy potential while avoiding the ambiguity threat is a topic of common interest in both academia and industry. Numerous studies have addressed the ambiguity elimination problem of subcarrier modulated signals. However, to the best of our knowledge, no updated reviews on this topic reflecting recent advances in the field have been presented in the past few years. On the one hand, the unique composite subcarrier modulated signal structure in the B1 and B2 bands of the BeiDou Navigation Satellite System (BDS) and the E5 band of the Galileo satellite navigation system (Galileo) has been gradually emphasized by academics. On the other hand, unambiguous processing methods for subcarrier modulated signals based on multidimensional tracking loops have flourished, which provides a new way to solve the ambiguity problem. An overview of the most representative subcarrier modulated signal processing techniques, focusing on the achievements made in the last decade, is presented in this article, which can be a starting point for the researchers who start their study in this area. Furthermore, we aim to present a complete review on the recent breakthroughs that have taken place within this research area, providing links to the most interesting and successful advances in this research field. Remaining challenges with prospects are also given.