Evaluation of QZSS Centimeter Level Augmentation System (CLAS): Open-Sky to Urban Environments and Geodetic to Low-Cost Receivers

Abstract

The Japanese Centimeter Level Augmentation Service (CLAS), which started operation from November 2018, provides centimeter level positioning accuracy within the service area of Japan. CLAS utilizes the Precise Point Positioning - Real Time Kinematic (PPPRTK) technique by broadcasting compact state space representation (SSR) messages including satellite orbit, clock, code bias, phase bias, tropospheric and ionospheric corrections. To date, SSR message quality and the resulting CLAS positioning performance have not been fully analyzed, especially for low-cost receivers and within urban environments. This research aims to evaluate the performance of CLAS from open sky to urban environments and from geodetic to low-cost receivers. Several issues with CLAS data quality are discussed in this research. Based on these issues, we propose an improved strategy of partial ambiguity resolution (PAR). To evaluate the performance of CLAS and the efficiency of the proposed PAR strategy, GNSS data in both open sky and urban environments was collected by Trimble NetR9 and u-blox F9 receivers. Our results show that the new PAR improves the ambiguity fix rate and time to first fix (TTFF). For the Trimble receiver, accuracy of less than 10 cm in RMS can be achieved in both horizontal and vertical directions. With a fix rate from 92.89% in a normal urban environment to over 99% in an open sky environment, the performance of CLAS-based PPP-RTK with PAR meets the performance standard of CLAS. However, the inexpensive u-blox receiver has issues with larger observation noise, inaccurate phase bias, and fewer satellite measurements, which lead to lower fix rates and more instances of false (incorrect) fixing. Hence, a more critical significance level in the ratio test used to validate ambiguity fixing is recommended for CLAS positioning using low-cost receivers.