Estimation of overbound on ionospheric spatial decorrelation over low‐latitude region for ground‐based augmentation systems

Abstract

In ground-based augmentation systems (GBAS), such as local area augmentation system (LAAS) of USA, variation of ionospheric delay between LAAS ground facility and the user leads to residual ionospheric error. Hence, the users need to compute the bounds on residual ionospheric error with the help of a parameter, σvertical_ionospheric_gradient (σvig). As σvig is a function of geographic region, the estimated value of σvig for mid-latitude regions (4 mm/km) is not appropriate for low-latitude regions where ionosphere is highly varying and is characterised by large spatial gradients. In this study, gradients are computed with processed total electron content data from 16 stations of global positioning system aided geo augmented navigation network and using the time-step method. The distribution of gradients is compared with one-sigma Gaussian distribution to obtain ‘inflation factor’. σvig overbound is computed for several days data comprising of quiet, moderate and storm conditions of ionosphere using the corresponding inflation factors. It is found that overbound values of 4.5 and 14.4 mm/km are required to bind the gradients under quiet and active conditions. These findings are useful for designing a safe and reliable GBAS for low-latitude stations.