Detecting ionospheric gradients for GBAS using a null space monitor

In this paper, a carrier phase-based monitor is developed to instantaneously detect ionospheric gradients using measurements from multiple, spatially separated ground antennas. Two monitor approaches based on traditional detection and estimation techniques are initially explored, and it is shown that their performance is highly dependent on how receivers´ pairs (baselines) are selected for measurement differencing within a given spatial configuration. In order to eliminate this baseline selection dependency, a new monitor (termed the null space monitor) is proposed that combines measurements from a given set of spatially separated antennas through a null space transformation. In addition to the baseline selection independence, we show that the null space monitor has a larger gradient detection range than either of the two traditional monitors.