Statistical relation of scintillation index S4 with ionospheric irregularity index ROTI over Indian equatorial region

Abstract

Ionospheric scintillation can cause severe degradation in the GNSS services, particularly at the polar and equatorial regions, by deteriorating the positioning performance in terms of accuracy. The continuity of the GNSS service is also affected as severe scintillation may lead to the loss of lock of the receiver. In this work, the occurrence probability of amplitude scintillation index S4 has been statistically related to the ionospheric irregularity index of ROTI. Simultaneously measured S4 and TEC data from the GAGAN TEC network stations, installed across India, were used for the purpose. TEC is used for the derivation of ROTI. The distribution of the S4 observed over different finite ranges in ROTI was separately matched with standard parametric models. This was done for both quiet and disturbed geomagnetic conditions. Log-normal distribution has been found to be the best match for most of the cases. The parameters for the matched log-normal distributions were found to vary systematically with the mean ROTI values in the range. Therefore, these distribution parameters were modelled in the terms of the corresponding ROTI values. The derived model was successfully validated with independent data and the conformity was established using statistical methods. This work may find usefulness in estimating the probable scintillation strength S4 once the ionospheric irregularity is known in terms of ROTI. Moreover, given an observed value of ROTI, the probability of losing receiver lock can be determined, provided the corresponding threshold S4 that the receiver can withstand is known.