Absolute Slant Ionosphere Gradient Monitor for GAST-D: Issues and Opportunities

Abstract

Strong ionospheric gradients above 300 mm/km remain a threat for single frequency GBAS users including those conducting CAT II and CAT III precision approaches under GAST-D protocols. This can be overcome using a network of single-frequency, double- differenced carrier-phase-based monitors as proposed in [1]. The detection performance of this monitor is very sensitive to the level of carrier phase residual error. Therefore, an architecture using 4 aligned receivers with optimal separation that allows the highest carrier phase residual error (8.65mm, one standard deviation) has been proposed in [2]. This level of carrier phase residual error provides 100% detectability of absolute slant ionosphere gradients in the range [-2000, -300] and [300, 2000] mm/km for a probability of missed detection and false alarm lower than 10􀀀4. Given this result, it is tempting to declare the ionosphere issue solved for GAST-D using the optimal distribution of receivers, provided the level of carrier phase standard deviation errors remains less than 8.65 mm. This would be the case if the gradient measured by the monitors located on the ground near the GBAS reference point is the effective gradient (ratio between the true iononsphere residual after GBAS correction and the distance to aircraft). In this paper, we investigate the case when the measured gradient and the effective gradient are not the same. In other words, the ionospheric gradient is spatially diffusive. This work was motivated by the fact that the optimal architecture proposed is not guaranteed to detect a gradient close to but below 300 mm/km. A small degree of diffusion or variation in the gradient would violate this requirement. While the threat model originally established for the Conterminous U.S. in [3] assumes a constant gradient, this assumption is made for analysis convenience and is not mandated as part of the threat model.