Evaluation of Orbit, Clock and Ionospheric Corrections from Five Currently Available SBAS L1 Services: Methodology and Analysis  
[PERSON]  
1 School of Geosciences, China University of Petroleum, Qingdao 266580, China; [EMAIL_ADDRESS] (Z.N. ); [EMAIL_ADDRESS] (Z.W. )1 School of Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada; [EMAIL_ADDRESS] (F.L. ); [EMAIL_ADDRESS] (Y.G. )1 School of Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada; liu19@ucalgary. broadcast the corrections and integrity messages by geostationary satellites over its service area [1]. Although the SBAS system is originally developed for civil aviation use, it could also be used for other applications, such as precision farming, on-road vehicle fleet management, positioning of oil exploration platforms at sea, and so on. Several countries and regions have developed their own SBAS systems. Currently, there are 4 operational SBAS systems, including the US Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS), the Indian GPS Aided GEO Augmented Navigation (GAGAN), and the Japanese Multi-functional Satellite Augmentation System (MSAS) ([[http://mpex.igs.org/IGS_MGEX_Status_SBAS.php](http://mpex.igs.org/IGS_MGEX_Status_SBAS.php)]([http://mpex.igs.org/IGS_MGEX_Status_SBAS.php](http://mpex.igs.org/IGS_MGEX_Status_SBAS.php))). Russia has also deployed its SBAS system, named as the System for Differential Corrections and Monitoring (SDCM). Currently, three SDCM satellites are broadcasting augmentation messages for GPS and GLObal NAvigation Satellite System (GLONASS) over Russia ([[http://www.sdcm.ru/index_eng.html](http://www.sdcm.ru/index_eng.html)]([http://www.sdcm.ru/index_eng.html](http://www.sdcm.ru/index_eng.html))). In addition, some countries, such as Australia and Nigeria, are testing their SBAS systems ([[http://mpex.igs.org/IGS_MGEX_Status_SBAS.php](http://mpex.igs.org/IGS_MGEX_Status_SBAS.php)]([http://mpex.igs.org/IGS_MGEX_Status_SBAS.php](http://mpex.igs.org/IGS_MGEX_Status_SBAS.php))). China is also planning to broadcast SBAS messages via BeiDou-3 geostationary satellites [2]. The SBAS systems have been developed in accordance with the common Radio Technical Commission for Aeronautics (RTCA) aviation standard, MOPS 229D (Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment version D). The SBAS corrections include satellite orbit correction, satellite clock correction, and ionospheric correction. The satellite orbit and clock corrections consist of long-term and fast correction data. The long-term corrections are intended to correct satellite orbit errors and the slow-varying part of satellite clock errors, while the fast corrections are designed to compensate for rapidly changing part of GPS clock errors. The ionospheric corrections are provided in the form of gridded vertical ionospheric delay. The users can calculate the vertical delay at the ionospheric pierce point (IPP) of the observed satellite through interpolation using vertical delays at nearby predefined ionospheric grid points (IGPs). It should be noted that there is no fixed update rate for specific SBAS correction types. Generally, fast corrections are updated at a high rate, while long-term satellite corrections and ionospheric corrections are updated at a relatively low rate [1]. In recent years, real-time high-precision Global Navigation Satellite System (GNSS) positioning has been attracting more and more attention in the GNSS community. The SBAS correction messages are transmitted in real time through SBAS satellites. Unlike the International GNSS Service (IGS) real-time service (RTS) [3; 4], which requires users to access the internet, the SBAS message transmission does not require extra data links. Therefore, SBAS correction is a good choice or alternate to conduct real-time high-precision GNSS positioning.