Estimation of Satellite-Based Augmentation System Grid Size at Low Latitudes in the Indian Zone

Abstract

The Total Electron Content (TEC) measured from a station situated near the northern crest of the equatorial anomaly is compared with that obtained from models such as the Parameterized Ionospheric Model (PIM 1.6) and International Reference Ionosphere (IRI-95) for well over one solar cycle (1977–1990). The limitations of conversion from vertical to slant TEC and vice versa as required for GPS ionospheric corrections in the equatorial region are discussed. It is found that the correspondence among the vertical TEC at the ionospheric pierce point, geometrically (sec χ) converted slant TEC, and slant TEC along a GPS signal propagation path becomes poor for elevation angles of less than 80 deg. Based on this finding, an optimum grid size for reliable operation of the Satellite-Based Augmentation System (SBAS) in the Indian subcontinent (GAGAN, or GPS and Geo Augmented Navigation) is estimated. The suggested grid size is much smaller than the standard 5 × 5 deg.