Ionospheric Gradient Monitoring for Dual Frequency Multi-Constellation GBAS

The International Civil Aviation Organization (ICAO) Ground Based Augmentation System (GBAS) Working Group (GWG) is working on development of standards for Dual-Frequency Multi-Constellation (DFMC) GBAS. In the initial stage of the development, an architecture trade study is being conducted in order to select the overall architecture and to develop the concept of operation (ConOps) for DFMC GBAS. A previous paper discussed an alternative architecture candidate proposal for consideration for the future DFMC GBAS based on uplinking raw measurements in addition to the differential corrections that are already uplinked to support legacy GBAS service types. That paper discussed various advantages and disadvantages of the proposed architecture known as GAST X. This paper extends the work done in that paper to provide greater clarity regarding proposed ionospheric gradient monitoring techniques. This paper describes three strategies for Ionospheric Gradient monitoring and compares the relative performance of those strategies. Two of the strategies are based on the GAST X architecture and the third is based on the architecture proposed by Eurocontrol developed under the SESAR program. The paper defines the test statistics and processing for each strategy. The paper then expands on expected noise characteristics and errors sources relevant to observation of the iono gradient monitor with each strategy. Methods for bounding residual ionospheric errors and for detection and isolation of ranging sources with large iono gradients are discussed. Finally, the results of an availability study comparing the performance of the three different strategies is presented. The results presented in the paper show clearly the advantages of the GAST X architecture for ionospheric anomaly monitoring. Also, the paper highlights areas where validation by analysis of measurements in representative environments is needed.