Improved Time-Step Method for Bounding Nominal Spatial and Temporal Ionospheric Gradients for Ground-Based Augmentation Systems in Hong Kong

Abstract

A ground-based augmentation system (GBAS) is a critical component in civil aviation that augments the Global Positioning System (GPS) in providing precision approach and landing capabilities with guaranteed accuracy and integrity.The GBAS ground facility broadcasts a parameter known as σ vig to the aircraft, which is used to compute vertical protection levels for evaluating navigation integrity. σ vig represents the standard deviation of the vertical ionospheric gradients, which bounds the spatial gradients under nominal conditions. Although the time-step method has been widely utilized to estimate ionospheric spatial gradients, this strategy suffers from temporal effects. In this paper, an improved time-step method is developed for separating temporal gradients from spatial gradients using observation data collected from the Hong Kong Satellite Positioning Reference Station Network. We investigated two parameters: σ tg , which bounds the standard deviation of temporal gradients, and σ vig . The results show that a constant value of 5.5 mm/km can serve as an upper bound for all σ tg values. However, the results of σ vig vary seasonally, with maximum and minimum values occurring at the equinoxes and summer, respectively. To reflect this seasonality, quadratic polynomial expressions, given as functions of the day of the year, were derived to provide an upper bound for all σ vig values.