Performance Analysis on Multi-Constellation SBAS of the Modified L1-only SBAS Message

All the current SBAS systems broadcast the augmentation message based on the L1-only SBAS MOPS (Minimum Operational Performance Standards), which is described in the RTCA DO-229 as an international standard. The L1 SBAS message was originally designed to support multi-GNSS constellations, but all the SBAS systems except for the SDCM of Russia provide GPS-only augmentation message. There are several reasons (i.e. a low data-rate) why most SBAS systems cannot support the multi-constellation GNSS. In order to overcome the limitations of the existing L1-only SBAS MOPS and to improve the performance of the current operating SBAS systems, studies on the L1/L5 DFMC (Dual-Frequency, Multi-Constellation) SBAS are actively ongoing. FAA (Federal Aviation Administration) in United States and the ESA (European Space Agency) have held the SBAS IWG (Interoperability Working Group) and have continued to discuss the development of the L1/L5 DFMC SBAS. However, even if the development of the L1/L5 DFMC SBAS is completed, many users with a L1 single frequency and multi-constellation GNSS receiver, which occupies most of the GNSS receiver market share, cannot use the L1/L5 DFMC SBAS. Therefore, it is necessary to study on a method to improve the performance of the L1-only SBAS. We propose to remove the fast corrections and to assign additional time slots for providing the long-term corrections of GLONASS and other constellations. As we had already proven at the ION ITM 2017, the PRC (Pseudo-Range Correction) in MT (Message Type) 2 to 5 needs not to be broadcast every 6 seconds, and it can be included to the satellite clock correction term in MT 25 after being converted into the unit of second. If the fast correction is not broadcast, half of the time slots are available and enough to send the long-term corrections for other constellations than GPS. And, we generated the long-term corrections for GLONASS satellites using GLONASS satellite precise ephemeris from the IAC (Information and Analysis Center). GLONASS long-term corrections were packed in MT 25 and these additional messages were scheduled in the empty time slots obtained by removing the fast correction. In this paper, we suggested to remove the fast correction and to modify the broadcasting timing and very few parameters of the current L1 SBAS message. We also showed that even this minor modification is working with the live SBAS message and effective to reduce the size of the protection level by supporting the multi-constellation GNSS, which will also contribute to improve the performance of integrity, availability and continuity.