Future Dual Frequency Multi Constellation GBAS

Abstract

The Ground Based Augmentation System (GBAS) is a landing system for aircraft. It provides differential corrections for navigation systems from Global Navigation Satellite Systems (GNSS). Currently available GBAS installations only provide corrections for the US GPS constellations for signals in the L1 frequency band. Sharp gradients in the ionospheric plasma are a serious concern for differential systems like GBAS. Different ionospheric delays experienced at the GBAS ground station and by a GBAS user approaching the airport have the potential to result in unacceptably large ranging and thus position errors. In current systems highly conservative assumptions (worst ever observed gradient always present) and/or a significant number of sensitive monitors in the ground station and the airborne system are used to ensure safe operation under any conditions. While these concepts are working in mid-latitude regions with benign ionospheric characteristics, areas in equatorial and polar regions see frequent disturbances that are significant enough to limit the availability of GBAS to an operationally unacceptable level. With the introduction of Galileo, potentially also a modernized Glonass, the Chinese BeiDou and other constellations, such as the Japanese QZSS, a large number of additional GNSS satellites are becoming available for navigation. With all these navigation satellites available, the vulnerability of a GNSS service to ionospheric disturbances can be reduced dramatically. Furthermore, all operational Galileo satellites and all GPS satellites since the latest generation (Block IIF) are providing additional ranging signals in the L5 frequency band. This allows using a second frequency for calculating an ionosphere free position solution or effective monitoring for ionospheric gradients. Using dual frequency methods, however, comes at the cost of a significant increase in residual errors due to noise and multipath since the errors of the measurements on both frequencies are combined. Furthermore, both frequencies have to be tracked simultaneously at all times which may be challenging during the daily period of scintillations after sunset in equatorial regions. In this work we therefore investigate the performance of different future processing modes and discuss the implications of the respective modes and their performance under nominal and disturbed ionospheric conditions.