Impact of Issue of Data Update Periods on Satellite-Based Augmentation Systems

The aim of the paper is to analyse the distribution of update periods of Issue of Data (IOD) parameters in Galileo F/NAV and Navstar Global Positioning System (GPS) LNAV messages. For this purpose, a new method aiming at the reconstruction of IOD update periods reported by a network of monitoring stations is proposed. Over one year of Galileo F/NAV and over three years of GPS LNAV data is processed and consolidated using the proposed method. A reconstruction rate of 99.66% is achieved for the Galileo F/NAV messages. To extract the information on the consecutive update periods of IOD parameters, two metrics are proposed. It was observed that Galileo F/NAV messages can experience fast single and fast double updates. Double updates happen at worst over two subsequent Galileo F/NAV sub-frames. The obtained results suggest a possible impact on Satellite-Based Augmentation System (SBAS). In order to assess the impact on a drafted Dual Frequency Multi Constellation (DFMC) SBAS Standards And Recommended Practices (SARPs), a Galileo F/NAV IOD selector, whose task is to select a specific IOD for augmentation in SBAS, is proposed and implemented. The design of the Galileo F/NAV IOD selector takes into account the observed IOD update periods as well as relevant requirement defined in the drafted DFMC SBAS Minimum Operational Performance Standards (MOPS). The selector aims at maximising the minimum time that the SBAS user has to decode the navigation message identified by the IOD before it is selected for augmentation. In addition, the maximum time that the SBAS would be using IOD, which is no longer transmitted by the Satellite Vehicle (Sv), is also over bounded by the design. According to the analysed data, it is identified that an optimal time to trigger the selection of new IOD of Galileo F/NAV message for augmentation is 350s when maximising the minimum decoding time. The trigger time guarantees that the SBAS user would always have at least 200s available to decode the Galileo F/NAV message.