A satellite-based augmentation system (SBAS) broadcasts differential global positioning system (GPS) corrections and integrity information to users which are computed from widely dispersed SBAS reference stations using geostationary earth orbit (GEO) satellites. The SBAS monitors and broadcasts the user differential range error (UDRE) to protect users from residual error which may persist after the removal of satellite clock and ephemeris errors. The UDRE is a single scalar confidence bound. Thus, it needs to be the largest projected value applicable for all users. To avoid over-conservatism in the calculation of the UDRE, Message Type 28 (MT28) is applied to specify the correction confidence as a function of the user location. A MT28 broadcasts the full information, which is a four-dimensional clock-ephemeris covariance matrix set from which the UDRE was derived. The covariance matrix is a function of the satellite location, reference station observational geometry, and reference station measurement confidence. Therefore, MT28 parameters need to be determined for reference stations and GEO satellite which are used for the system. In this paper, we determine the UDRE and MT28 parameter for Korean GEO satellites by assuming the use of Inmarsat-4 F1, which is available in the Korean region. The corresponding availability is analyzed in the Korean region. In addition, confidence bound of satellite clock and ephemeris error was compared for several GEO satellite locations.

Keywords: satellite-based augmentation system, user differential range error, MT28