GBAS ground monitoring requirements from an airworthiness perspective

Abstract

The ground-based augmentation system (GBAS) provides corrections for satellite navigation signals together with integrity parameters to aircraft and enables precision approach guidance. It will eventually replace the currently used instrument landing system. GBAS Approach Service Type C stations supporting CAT-I operations have been fully developed and certified, and first stations are operational. For the service type D, which is intended to support CAT-III operations including automatic approaches and landings, requirements have been drafted and are currently undergoing validation. One remaining issue is the requirement for monitoring of ionospheric anomalies in the ground subsystem. Large gradients in the concentration of free electrons in the ionosphere can lead to significant positioning errors when navigation is based on differential methods. We give a review of the derivation of currently proposed performance requirements for such a monitor. Next, we show that the required level of safety from an airworthiness perspective can be achieved even with relaxed monitoring requirements compared to the currently drafted standards. These relaxations result from satellite geometry assessments on the ground and actual approach characteristics toward a runway. We show that with this method, it is sufficient to monitor for gradients in the range of about 450-550 mm/km while current standards require detection already from 300 mm/km. A remote monitoring receiver near the touchdown point can monitor the post-correction differential range error and use it as test statistic for GBAS performance monitoring and protection against ionospheric disturbances.