Ranging and Demodulation Performance of QZSS L1C Signal in Forest/Urban Environments

Abstract

The new GPS signal for civilian use, L1C, is a modernized signal designed to enhance various performance aspects for satellite positioning. L1C differs from the currently used L1C/A signal in numerous ways. The L1C codes are ten times longer than the L1 C/A codes. This increased length helps in reducing interference when multiple satellites are tracked at the same frequency. The L1C signal consists of a dataless pilot component and a data component with navigation data. Dataless signals enable more robust tracking under challenging conditions. Furthermore, the L1C signal provides a new CNAV-2 navigation message, which is the first navigation signal using modern advanced forward error correction (FEC) codes. Several studies have compared the performance of the L1C signal with that of the conventional L1C/A signal, but most of them are based on simulations. This is mainly because only the Japanese Quasi-Zenith Satellite System (QZSS) and one GPS Block III satellite launched in the year 2019 actually broadcast the L1C signal. Although few studies evaluated the ranging performance of the L1C signal, they were confined to open-sky environments. In this study, we evaluated the actual ranging accuracy of the L1C signal in forest and urban environments and the decoding performance of the CNAV-2 navigation message compared to the conventional L1C/A signal. This information is the most vital for GNSS users. The main contributions of this paper are as follows: (1) This is the first study to evaluate the actual reception performance of the L1C signal in urban and forest environments. (2) The error rate of CNAV-2 was compared with that of LNAV and CNAV in a real environment. The performance of the QZSS L1C signal was evaluated through static and kinematic experiments in actual forest and urban environments. We compared the performances of the L1C, L1C/A, and L5 signals in terms of the observed signal-to-noise ratio and signal availability in harsh environments, the ranging accuracy using multipath linear combinations, and the actual navigation message demodulation error rate. We employed the software GNSS receiver to evaluate the navigation message demodulation error rate. For this, we implemented the acquisition and tracking algorithms of L1C/A, L5, and L1C signals in the software GNSS receiver and extracted the raw navigation bits. Subsequently, we decoded each navigation message and evaluated the error rate of the navigation message. In the case of L1C signals, CNAV-2 is encoded by low-density parity-check (LDPC) and interleaving. Because the LDPC decoding performance is close to the Shannon limit, it can be expected to demodulate the most stable navigation messages in harsh environments. From the results of the evaluation experiments, it is confirmed that the L1C signal has a lower measurement noise and a higher multipath tolerance than those of the L1C/A signal. In addition, the L1C signal is approximately 2–3 dB-Hz stronger and more robust than the legacy L1C/A signal, and its availability is higher than that of the L1C/A signal in the forest and urban kinematic experiments. The CNAV-2 message in L1C was found to have a lower message error rate than that of the legacy LNAV owing to its data structure and advanced error correction codes.