Abstract:
This study presents unique perspectives of occurrence and strength of low latitude ionospheric scintillations on multiple signals of
Global Navigation Satellite System (GNSS) and its frequency dependence using continuous observation records of 780 nights. A robust
comparative analysis is performed using scintillation index, S4 and its variation during pre-midnight and post-midnight duration from a
GNSS receiver located at Waltair (17.7 N, 83.3 E), India, covering period from July 2014 to August 2016. The results, generally exhibit
the impact of declining phase of solar cycle 24 on occurrence and strength of scintillations, which, however, is evidently different over
different frequencies transmitted from different GNSS systems. A deeper quantitative analysis uniquely reveals that apart from the solar
cycle and seasonal effects, the number of visible satellites of a selected GNSS markedly affect the occurrence and also the strength. Processing
scheme of adopting 6 hourly time windows of pre-midnight and post-midnight brought a novel result that the strength and occurrence
of strong scintillations decrease with declining solar activity during pre-midnight hours but remarkably increase for moderate and
weak scintillations during post-midnight. The physical processes that dominate the post-midnight equatorial ionosphere are invoked to
explain such variations that are special during declining solar activity. Finally, inter-GNSS signal analysis in terms of the effect of strong,
moderate and weak scintillations is presented with due consideration of number of satellite passes affected and frequency dependence of
mean S4. The quantitative results of this study emphasize for the first time effect of low latitude scintillation on GNSS signals in Indian
zone under changing background solar and seasonal conditions.
