Single frequency ionospheric error correction using coefficients generated from regional ionospheric data for IRNSS

Indian Regional Navigation Satellite System (IRNSS) is the satellite based navigation system being developed by India for locating user’s position in 3-dimensional space and time over IRNSS service area. The users estimate their position by using a receiver which receives the navigation signal from the IRNSS satellites which is operating on L5 (1176.45 MHz) and S (2492.028 MHz) frequencies. There are three types of IRNSS users: dual frequency (L5 and S); single frequency (L5); and single frequency (S). The signal from the satellites before reaching the user receiver passes through the ionospheric layer of the atmosphere and suffers a delay. The delay in the signal introduces error in the position computed by the user. The dual frequency users of IRNSS correct the ionospheric error by taking advantage of the dispersive nature of ionosphere. On the other hand, single frequency user requisite an algorithm for computing the ionospheric delay along his line-of-sight. In IRNSS, the ionospheric error correction for single frequency (L5 or S) user is provided through a set of eight ionospheric coefficients. These coefficients are computed on ground by using previous 24-hour data and broadcasted as secondary navigation parameters (messages) to the user. These corrections are available for any user in the primary service area (extending up to 1500 km from Indian boundary) of IRNSS with a validity period of one day. The algorithm designed and developed for the estimation of these coefficients is explained in this paper which uses the global ionospheric model based on cosine curve approximation of diurnal variation of ionospheric delay. The estimation technique has been tested initially with simulated data and then with measurement driven model data for days of different ionospheric activity. The results are promising when compared with GPS transmitted Klobuchar coefficients over IRNSS primary service area. This algorithm is a part of the IRNSS navigation software which is operational at IRNSS Navigation Center (INC), Bangalore.