Seasonality of Nominal Ionospheric Gradient Using Time-Step Method Based on GNSS CORS Observations in Hong Kong

The Ground-Based Augmentation System (GBAS) is designed to augment the Global Positioning System (GPS) to provide precision approach and landing capabilities with essential accuracy and integrity. The variation of ionospheric delay between GBAS ground facility and the aircraft leads to residual error and need to be bounded. In order to do this, the GBAS ground facility broadcasts a parameter known as sigmavig to the aircraft, which represents the standard deviation of vertical ionospheric gradients that bounds the spatial gradients under nominal conditions. The sigmavig parameter is used to compute vertical protection levels for evaluating navigation integrity. As sigmavig varies with different region, the sigmavig for Hong Kong needs to be estimated to support GBAS implementation in this region. In this study, sigmavig for the year 2014 is estimated based on GNSS data obtained from Hong Kong Satellite Positioning Reference Station Network (SatRef). The results of sigmavig vary seasonally, with the maximum and minimum values occurring in spring and summer, respectively. To reflect this seasonality, quadratic polynomial expressions as functions of the day of the year were derived to upper-bound all sigmavig values.