Development and Evaluation of a Multi-constellation Multi-frequency GBAS Prototype for Air Navigation

GBAS (Ground Based Augmentation System) is an augmentation system designed to ensure the maintenance of essential quality parameters (integrity, continuity, accuracy, and availability) related to GNSS (Global Navigation Satellite System) positioning in aviation. It uses the DGNSS (Differential GNSS) positioning method, where ground reference stations generate pseudorange corrections transmitted to aircraft, improving positional accuracy to support approach procedures. According to the International Civil Aviation Organization (ICAO) (2007), GBAS is authorized to use the GPS and GLONASS L1 frequencies, while the L2 frequency is not permitted, as it falls outside the aviation-protected spectrum. In the future, the L5 frequency and additional constellations, such as Galileo and BeiDou, are expected to be incorporated, making GBAS more robust, especially in mitigating ionospheric effects. The integration of new GNSS signals and constellations in aviation depends on ongoing research and investigation to support upcoming changes and implementations. This work aimed to develop a GBAS Prototype, called Experimental UNESP GBAS, which is capable of operating with multi-constellation and dual-frequency GNSS, enabling its use in air navigation. For this study, GNSS data collected from five reference stations of the INCT GNSS NavAer network, with four stations representing the ground system and one station simulating an aircraft, all located in the low latitude region. The selected day was DOY 227/2024 (August 14, 2024), during which no ionospheric scintillation was observed, and 281/2024 (October 7, 2024), having an intense ionospheric condition. The Experimental UNESP GBAS was implemented in Matlab using code and carrier phase observations, collected at a 0.5 second interval, with a 10° elevation mask for the L1/E1 and L5/E5a frequencies of GPS and Galileo systems, respectively. Using the GNSS data in each ionospheric condition, the performance of each GBAS subsystem (Ground and Aircraft) was evaluated, focusing on the L1/E1 frequencies (GPS and Galileo) and IonoFree combination (IF), with 100 seconds for smoothing constant. In the experiment, the ground subsystem produced results aligned with the literature and the characteristics of the data used, for each method used. Results for air systems showed that both methods achieved consistent performance in weak scintillation scenario, with 100% operational for SF and 99.98% for IF in the horizontal component, and 97% operational epochs for the vertical component and only minimal occurrence of protection level (PL) exceeding the alert limit (AL), for the Iono-Free. Under intense scintillation, the IF combination outperformed SF, achieving 99% operational epochs versus 97% for SF, and demonstrating lower unusable epochs. However, the IF method detected slightly more cases of position error exceeding PL, due to dual-frequency noise and multipath effects. Overall, the IF combination proved more effective in mitigating ionospheric effects, especially during intense scintillation. Future work could include enhanced integrity monitoring using statistical tests to further improve performance and address undetected effects.