Galileo Performance Assessment for CAT III GBAS

Landing aircrafts under low visibility conditions has always been a challenge even with conventional navigation systems like ILS (Instrument Landing System). CAT III, with GBAS (Ground Based Augmentation Systems) and GPS only can't be reached without relaxing the alarm limits and continuity requirements of air navigation. The objective of this work is to analyze the impact of Galileo in the performances of GBAS and to compare the integrity levels obtained with the requirements of CAT III under severe ionospheric gradients. Galileo is providing some promising features like the possibility offered to the aviation community to acquire 3 frequencies: L1, E5a and E5b in the ARNS band (Aeronautical Radio Navigation Service). This will augment the robustness of the complete system and will provide efficient smoothing techniques when considering also the phase observations. Another feature concerns the possibility to have a different constellation with another geometry characteristic; this would improve the Geometry Dilution Of Precision (low GDOP) and the number of visible satellite at any epoch can reach 15 or 16 when coupled with GPS. The BOC (Binary Offset Carrier) signal structure in L1 provides a high multipath rejection capacity that provides promising results at both GBAS ground subsystem and airborne receiver level. The probability of satellite outages coupled with the number of available satellites (27 + 3 spares) will dramatically improve the availability of the combined GPS and Galileo system. Different smoothing algorithms developed at Stanford University (GPS Lab) are applied: The ionosphere free and the divergence free smoothing to mitigate or even to cancel the ionosphere gradient. The availability of the vertical protection level with respect to CAT III vertical alarm limit is analysed. As a major result, the ionosphere free combination for a dual constellation dual frequency using an all in view satellites can fulfil CAT III with respect to the protection levels and it provides a robust solution with a very low risk of misleading information.