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Effect of latitude and longitude on bounding sigma at low-latitude stations for GBAS applications

In modern satellite based navigation systems for aviation applications, Ground Based Augmentation System (GBAS) is set to become an important part of the systems for ensuring safe and reliable Precision Approach (PA) and landing of aircrafts. The performance of GBAS is limited by spatial decorrelation of ionospheric delays (spatial gradients). No significant work has been done on this aspect at low and equatorial latitude regions, especially in the context of LAAS. In our statistical analysis of spatial gradients, TEC data from Indian Space Based Augmentation System (SBAS), namely GAGAN stations is used. Probability Density Function (PDF) of ionospheric gradients is compared with one-sigma Gaussian curve, to compute the bounding sigma, which in turn will be used in the estimation of Protection Levels (PLs) by the aircraft. Variation of gradients with respect to two key parameters namely latitude and longitude have been analyzed. Our results show that, the bounding sigma is high at low latitudes and varies drastically as latitude changes. But for a given latitude, longitudinal variations do not have much effect on the bounding sigma. Results presented in this paper helps to design and develop an efficient GBAS for low-latitude regions.