A study of ionospheric effects on IRNSS/NavIC positioning at equatorial latitudes

Abstract

The Indian Regional Navigation Satellite System (IRNSS/NavIC) is fully operational and broadcasting radio signals at L5 (1176.45 MHz) and S-band (2492.028 MHz). When these signals pass through the ionosphere, carrier phase delay, which depends on Total Electron Content (TEC), causes error in positioning. Corrections are applied in the augmented navigation system to minimize the position error due to TEC; however, 100% removal of error is practically impossible. Thus, a study has been carried out to understand the role of ionospheric determinants, such as TEC and scintillation S4, on the accuracy of augmented navigation over low latitude region using a dual-frequency NAVigation with Indian Constellation (NavIC) receiver installed at BITS-Pilani K.K. Birla Goa Campus (Geog. Lat. $15.39°$N, Geog. Long. $73.87°$E). Data collected using the Indian Space Research Organization (ISROs) NavIC receiver (make DataPattern) during the low solar period between March 2019 to December 2019 is utilized for this study. The important findings of the study are as follows: (a) Mean position error was in the range of 2–4 m. (b) Diurnal variation of the position error indicated a maximum at afternoon hours, coinciding with the time of maximum TEC over the EIA crest region. (c) Mean position error in afternoon hours indicated a linear relation with mean TEC, in a scatter-plot analysis. (d) Statistically, the position error during the scintillation nights was found to be similar to non-scintillation nights, indicating that the post-sunset equatorial density depletions during low solar do not significantly impact navigation accuracy. Absolute position error during the active phase of a moderate geomagnetic storm of 14 May 2019 was found to be significantly higher compared to a quiet period, which also depends on the mode of operation of the augmented navigation system. It was found that a hybrid NavIC (Dual) + GPS (SBAS) augmented navigation was more accurate than NavIC with dual- ionospheric corrections or NavIC with grid corrections, alone. Besides, the dynamic behavior of the ionosphere, i.e., diurnal, monthly, and seasonal variations of ionospheric TEC, have also been studied using the iono-delay values derived from the NavIC receiver.

Introduction

The NAVigation with Indian Constellation (NavIC) developed by the Indian Space Research Organization (ISRO) is a regional navigation system that operates at L5 (1176.45 MHz) and S-band (2492.028 MHz). NavIC aims to provide precise and accurate navigation and timing solutions within 1500 km around the Indian boundaries. However, various error sources limit the positioning accuracy of the IRNSS/NavIC. Among all errors, the ionospheric irregularities are considered the significant source of error, which degrades the receiver performance or even results in loss of lock. Due to the plasma density irregularities present in the Earth’s ionosphere, the trans-ionospheric signals from NavIC satellites often experience random fluctuations in the phase and amplitude of the received signal. These fluctuations are known as ionospheric scintillation caused due to small-scale irregularities. When sufficiently intense, it impairs the performance of the receiver by stressing the tracking stage, and results in loss of signal and cycle slip. Therefore, this phenomenon turns out to be a significant concern for a variety of navigation applications.

It is evident that the variability of the ionosphere is an immediate threat to all types of GNSS receivers. Mainly near the geomagnetic equator, VHF (244 MHz) to L-band signals, all are affected due to low-latitude ionospheric irregularities (Aarons, 1982, Basu et al., 1988, Rao et al., 2005, Joshi, 2016, Joshi et al., 2019a, Joshi et al., 2019b). During the adverse amplitude scintillation, the signal strength received at the receiver is less than the required tracking threshold, which might lead to abnormal functioning of acquisition and tracking of GNSS radio signals, and even result in complete receiver outage (Kintner et al., 2007).

Over the year, due to the rapid growth in the applications based on trans-ionosphere radio signals, the demand for more accurate and precise positioning and timing information has been increased drastically. However, with this increasing demand for precise navigation, it becomes an important consideration that the ionospheric delays models need to be updated. To an extent, it has been conventionally possible to reduce the ionospheric delay using dual and single -frequency receivers (Klobuchar, 1987, Datta-Barua et al., 2008, Rethika et al., 2013). However, during extreme conditions or ionospheric disturbances, these ionospheric models cannot accurately estimate the ionospheric delay errors (Gioia et al., 2015). Therefore, it becomes an essential task to provide updated information on ionospheric variability (in our case for NavIC system) and its effect on the position accuracy for the improvement of these systems. Several ionospheric models have been developed using GPS like (Coster et al., 1992, Bilitza, 2001).

Furthermore, it is well known that the physical and chemical characteristics of the ionosphere depend upon various factors such as geographical location, time of day, day of the year, solar and geomagnetic activity (Rees and Rees, 1989). Hence, investigation of such scientific behavior of the ionosphere over the low-latitude region will help in understanding, modeling, and forecasting the dynamics and unpredictable behavior of equatorial and low latitude ionosphere. Several researchers have investigated the variations in TEC during various period of solar cycle within the Indian region like (Bhattacharya et al., 2009, Kumar and Singh, 2009, Bagiya et al., 2009, Chauhan et al., 2011, Sharma et al., 2012, Joshi and Sripathi, 2016, Ratnam et al., 2017, Rao et al., 2019a, Rao et al., 2019b, Desai and Shah, 2019, Sridhar et al., 2020). The results of their analysis helped to characterize the TEC variations. Similarly, the characteristics and distribution of TEC during geomagnetic activities has also been studied by few researchers (Danilov and Lastovicka, 2001, Bhuyan et al., 2006, Mukherjee et al., 2010, Desai and Shah, 2019, Desai and Shah, 2020, Chakraborty et al., 2020). In addition, the amplitude and phase scintillation effects on GPS signal and its impact on positioning has been discussed by (Gwal et al., 2004, Dubey et al., 2005, Dubey et al., 2006). All these studies help to understand the morphological behavior of TEC, which ultimately improves the threat models for better navigation solutions.

This paper deals with a case study on the TEC and night-time plasma bubbles/scintillation, and their possible effect on the NavIC positioning at a low-latitude station (Goa). In this study, we report the scintillation characteristics observed on IRNSS/NavIC (L5 and S-band) signals recorded by the ISRO’s in-house IGS (IRNSS/GPS/SBAS) receiver installed at BITS-Pilani Goa Campus. Moreover, in this paper, we have investigated the diurnal and seasonal variations of ionosphere TEC during the low solar activity period of 2019. In Section 2., the data analysis and methodology have been presented. Section 3. present and discuss the results and morphology of amplitude scintillation, TEC variability, and response to a moderate geomagnetic storm on ionospheric delay, and NavIC positioning. Finally, the conclusion has been given in Section 4.