Assessing the reliability of the GNSS Loss of Lock Indicator (LLI) index for PPP across different types of GNSS receivers

Abstract

Cycle slips are events in which a receiver experiences a disruption in its carrier phase lock to a Global Navigation Satellite System (GNSS) signal. Accurate detection of cycle slips is essential to prevent degradation of positioning accuracy when carrier-phase observations are used. In this study, the reliability of the Loss of Lock Indicator (LLI) index in RINEX observation files from six different receiver types was investigated and compared with the Phase Geometry-Free and Hatch–Melbourne–Wübbena (GF + HMW) cycle slip detection algorithm. Precise Point Positioning (PPP) in stand-alone mode was performed using GPS, GLONASS, Galileo, and BDS-3 observations, applying either the LLI index alone or the GF + HMW cycle slip detection algorithm. The resulting RMSEs and overlaps between the methods were analyzed based on receiver types. The results show that the most reliable LLI data comes from LEICA and TRIMBLE receivers, while SEPTPOLARX5-TR, SEPTPOLARX5, and JAVAD receivers exhibited relatively poor LLI performance. PPP results showed that the LLI, which outperforms GF + HMW on the ionospherically quiet days for some receivers and GNSS constellations, performs worse on ionospheric scintillation days.