Performance Analysis of NavIC Software Receiver for Single Frequency Ionospheric Delay Corrections

Abstract

IRNSS is a regional navigation satellite system, indigenously developed by Indian Space Research Organization (ISRO) of India. It aims at providing better than 20m positional accuracy in the targeted coverage area. At present there are seven active satellites operated from the desired orbits with an operational name as NavIC (Navigation with Indian Constellation). NavIC offers Standard Positioning Service (SPS) for civilian applications on both L5 (1176.45MHz) and S (2492.028MHz) bands. India, being at low latitude region, the received NavIC signals at the antenna gets adversely effected by travelling through the ionospheric layer and may observe substantial error in pseudoranges. Since the difference between these two frequencies is large, that is frequency of S signal is more than the frequency of L5 signal by 1315.578MHz, L5 signal undergoes more code phase delay than S signal. There are still challenges in design and development of antenna and low-cost RF-frontend for simultaneous reception of NavIC L5 and S band signals. So, the main objective of this work is limited to investigate the capability of NavIC software receiver for single frequency ionospheric corrections and compare both signals. For a single frequency NavIC receiver, ionosphere corrections can be employed using a broadcast coefficient based model and/or grid-based model. The major advantage of NavIC system is that the ionospheric corrections are broadcasted as part of the navigation messages. In this paper, authors try to analyze the positional performance with and without ionospheric corrections from both the models for single frequency L5 and S bands, by considering the differences in both the signals. The performance analysis is carried out using NavIC software receiver (NavICSR), developed at the Research and Training Unit for Navigational Electronics (NERTU), Osmania University, India. The algorithm is implemented as given in signal in space IRNSS-ICD document, published by ISRO in August 2017 for standard positioning service. Two different data sets are used in this preliminary study. The main aim of this work is to make NavICSR capable of mitigating the single frequency ionosphere effects and analyze the performance. Performance analysis is carried out with the implemented models for single frequency NavIC L5 and S band signals for the same acquisition and tracking parameter configurations by considering the differences in both the signals. Similarly the performance of coefficient based and grid based models with the single frequency NavIC L5 receiver are presented. The results shows that the performance of single frequency NavIC S receiver outperforms single frequency NavIC L5 receiver for the same receiver configuration. The grid based corrections approximate the ionosphere delays, caused by the ionospheric layer slightly better than the broadcast coefficient based model in the observed data sets. This initial work paves the foundation for the developed NavICSR to use in atmospheric related research in future. It may be possible to investigate the performance of single frequency L5 and S bands in various signal degraded environments, where dual frequency measurements are noisy. It may be also useful for developing robust NavIC receiver by adaptively changing the tracking parameters based on the predicted ionospheric scintillations.