Assessing Galileo ionosphere-free combinations: impacts of contrasting equatorial ionospheric conditions

Abstract

This study investigates the impact of Galileo ionosphere-free (IF) dual-frequency combinations on the performance of Precise Point Positioning (PPP). Four IF combinations were analyzed: E1–E5a, E1–E6, E1–E5b, and E1–E5 (AltBOC). The analysis was conducted using 60 days of GNSS data from seven Brazilian stations located between latitudes 15° S and 30° S, covering two distinct ionospheric conditions: the equinox month of March and the solstice month of June 2024. Positioning accuracy, convergence time, and observation residuals were evaluated during the first 3 h of each day (00:00–03:00 UTC), a period marked by strong ionospheric variability at low latitudes. Among the tested combinations, E1–E5a and E1–E5b achieved higher accuracy and robustness, maintaining centimeter-level positioning even under disturbed ionospheric conditions. The E1–E5 combination also performed well, exhibiting low noise in carrier-phase observations despite moderate dispersion in the code measurements. In contrast, the E1–E6 combination showed the weakest performance, particularly during March, with increased positioning errors and larger residual variability. These findings underscore the importance of frequency pair selection in multi-frequency PPP strategies and highlight the sensitivity of specific Galileo signal combinations to equatorial ionospheric effects.