An assessment of ionospheric delay correction at L5 and S1 frequencies for NavIC Satellite System

The NavIC (Navigation with Indian Constellation) satellite system is launched by India to achieve Point Precise Positioning (PPP) system for various navigational and geodetic applications in the Indian region at S1 (2492.028 MHz) and L5 (1176.45 MHz) frequencies. However, the positional accuracy is greatly affected by the ionospheric delay introduced in NavIC measurements. The estimation of the ionospheric delay requires accurate knowledge of the line-of-site ionosphere electron content i.e. Slant Total Electron Content (STEC) present between the satellite and receiver. As the STEC is a function of solar radiation, the assessment of diurnal variability of ionospheric delay correction is necessary to achieve precise positioning and modeling of delay correction for single-frequency users. In this work, the STEC has been estimated using NavIC dual-frequency code and carrier phase measurements at S1 and L5 frequencies. The estimated STEC is also corrupted by differential instrumental biases (DIBs) of satellite and receiver. The DIBs are determined by implementing a five-stage Kalman filter and are removed to arrive at true STEC. An analysis of the first-order ionospheric corrections at S1 and L5 frequencies have been done based on data collected over June 2017 from NavIC satellites. To reduce the data volume and processing complexity, the determination of a monthly mean value has been proposed for the delay correction analysis. The delay correction at S1 frequency has been observed significantly less as compared to L5. Thus this work proposes S1 as an operating frequency in a single-frequency NavIC receiver.