Session B4: GNSS Integrity and Augmentation

Approach to Utilizing SFMC SBAS Message for Dual-frequency Users to Enable Universally Applicable SBAS
Jina Lee, Yongrae Jo, Donguk Kim, Byungwoon Park, Sejong University
Location: Beacon B

Currently operational L1 Legacy SBAS (Satellite-Based Augmentation System) provides L1 GPS users with precise satellite orbit and clock correction, as well as vertical ionospheric delay correction, delivering meter-level accuracy. With the development of global navigation satellite systems (GNSS) like Galileo and BeiDou, and the provision of navigation signals in various frequency bands, research on DFMC (Dual-Frequency Multi-Constellation) SBAS, which considers L1/L5 dual frequency and multi-constellation GNSS, has been actively promoted, and standardization has recently been completed. Following the Legacy SBAS for GPS L1 users, the use of DFMC SBAS is expected to lead to more receivers adopting SBAS capabilities in the future. However, when using DFMC SBAS, single-frequency users cannot eliminate ionospheric delay errors, and the L1 Legacy SBAS service does not support multi-constellation GNSS systems. Therefore, each of these two augmentation services is targeted at different user groups, so additional communication bandwidth must be imposed, such as using separate channels like different satellites. Furthermore, L1 multi-constellation receivers may fall into service shadow groups and be excluded from both services.
In our previous research, a study was conducted on SFMC (Single-Frequency Multi-Constellation) SBAS, which provides multi-constellation correction within the previously set bandwidth of 250bps by redesigning the L1 SBAS. The proposed SFMC SBAS was not designed to consider dual-frequency DFMC users since it uses the L1 single frequency. In this study, we proposed a method of utilizing the proposed SFMC SBAS in a single and dual frequency environment through a new correction parameter, DCB (Differential code bias), so that DFMC SBAS users can also use SBAS correction.
The reference clock for SFMC SBAS is aligned with the L1 C/A code, while the reference clock for DFMC SBAS is aligned with the Ionospheric-free linear combination (IFLC) of L1 C/A and L5. Since the navigation message's reference clock is based on the IFLC of L1P(Y) and L2P(Y), the satellite clock correction provided by SBAS utilizes TGD (Time Group Delay) and DCB within the satellite clock term of the navigation message. The difference between the satellite clock correction values provided by DFMC SBAS and those provided by SFMC SBAS becomes the new correction. The new correction parameter, DCB, is provided by CNAV, enabling dual-frequency users to utilize DFMC with the SFMC SBAS service.
To verify the applicability of the dual-frequency user of the SFMC SBAS Message proposed in this study, a static field test based on actual measurement data was performed. The precise satellite orbit and clock used the actual measured data SP3 provided by IGS, the ionosphere delay error was IONEX, and the new correction was DCB provided by IGS. Through this experiment, it was confirmed that the accuracy of L1 GPS, SFMC, and DFMC improved using the SFMC single correction.