Observing Large Ionospheric Spatial Decorrelation for Ground-Based Augmentation System in the Brazilian Region
Abstract:Ground-Based Augmentation Systems (GBAS) support aircraft precision approach and landing by broadcasting differential Global Positioning System (GPS) corrections and integrity information to aviation users. Under anomalous ionospheric condition, unacceptably large residual errors can occur due to anomalously large ionospheric spatial decorrelation, and this can pose integrity threats to GBAS users. Thus, the development of an ionospheric anomaly threat model is required to simulate worst-case ionospheric errors and develop mitigation strategies. Ionosphere in low latitudes is known to be much more intense than that in mid latitudes due to active geomagnetic effect, and investigation of low latitude ionospheric anomalies must take precedence before operation of GBAS.
In this paper, ionospheric spatial decorrelation is investigated for GBAS operation in the Brazilian region. Dual-frequency observation data are collected from Brazilian GPS reference stations. This analysis is performed using data sets collected on scintillating days, less-scintillating days, and storm days from 2012 to 2014. Precise ionospheric spatial gradient on the L1 signal is automatically estimated from dual-frequency observation data using simple truth method and station pair method. In the Brazilian region, however, intense ionospheric scintillations cause a large numbers of cycle slips in carrier-phase data. The simple truth process removes a considerably large number of those data through short-arc and outlier removals, and thus potential ionospheric gradients may not be detected. This motivates a data recovery process which skips short-arc and outlier removals if there appears a large ionospheric spatial gradient in the removed data. We also use a series of methods to validate anomalous ionospheric spatial gradients using manual validation with L1 single frequency measurement, station-wide check, satellite-wide check, and time-step check. In particular, the time-step check validates localized ionospheric anomalies in a scale of several tens of kilometers. This method is useful when the anomalies are not validated by station-wide and satellite-wide checks due to the sparse distribution of Brazilian GPS reference stations. Using the above methods, we observe and validate large ionospheric spatial gradients.