Future GBAS Processing - Do we need an ionosphere-free mode?

The Ground Based Augmentation System (GBAS) is a landing system for aircraft. It consists of carefully sited reference receivers at an airport, generating corrections for the navigation signals from Global Navigation Satellite Systems (GNSS). Along with the corrections, integrity parameters are generated and transmitted to arriving aircraft that allow the aviation users to bound their residual position errors after applying the corrections. Currently, corrections are generated for the GPS constellation and the L1 frequency. However, with the ongoing buildup of the European Galileo, the Chinese BeiDou and the modernized Russian Glonass the number of available GNSS constellations is increasing. This provides the opportunity to design systems more robust against disturbances, such as ionospheric scintillation effects, through a larger number of available ranging sources. Furthermore, all Galileo and the latest generation of GPS satellites feature signals in the L5 band that can be used by aviation. Therefore, it is possible to apply dual frequency techniques for mitigation of the ionospheric gradient threat. This paper discusses the advantages and disadvantages of using an ionosphere free combination of the signals for positioning and if such a mode for a future generation of GBAS should be developed