Loss of lock on GNSS signals and its association with ionospheric irregularities observed over Indian low latitudes

Abstract

Loss of lock (LoL) may occur due to receiver dynamics, multipath, and interference apart from the ionospheric irregularities for a given receiver configuration. This study makes a robust analysis of the LoL using multi-frequency GNSS observations during 2014–2017 from a low latitude station at Waltair, India. With possibly the first such results from India, this study shows a strong seasonal, local time, and solar activity dependence of the LoL. While the LoL occurrence is found to increase with the severity of scintillation and maximize for strong scintillations, it is also found that the LoL may occur at any level of S4 due to other dominating factors. It is found that while the weak and moderate S4 events weigh the distribution, they correspond to fewer than 15% of the total LoL. A combination of 0.5 < S4 < 0.7 and 0.4 < $\sigma_{\varphi }$ < 0.6 is found to produce LoL for most of the seasons for all the signals. A limitation of the scintillation data is highlighted wherein spurious values of 60-s $\sigma_{\varphi }$ (i.e. $\sigma_{\varphi }$ > 1.5) are found to constitute more than 53% of total LoL events against the widely accepted $\sigma_{\varphi }$ < 1.5. The LoL events are found to increase over latitudes away from the dip equator for most of the signals, whereas they decrease with an increasing elevation angle of IPP, possibly due to multiple refractions over excess path lengths at lower elevation angles and a latitudinal gradient in the electron density. A combination is found composed of S4 > 0.6 and $\sigma_{\varphi }$ > 1.5 with lower elevation angles that produce more LoL events between 20:00 and 22:00 local times. New probabilistic forecast models for the occurrence of the LoL are finally given for the different seasons against S4, $\sigma_{\varphi }$ and local time that would be useful for different applications.