A Ground Based Augmentation System (GBAS) comprising: a plurality of reference receivers having known positions; at least one processing module communicatively coupled to the plurality of reference receivers; at least one aircraft communication device communicatively coupled to the at least one processing module; wherein the at least one processing module is configured to check a Global Navigation Satellite System (GNSS) satellite measurement for each of a plurality of Global Navigation Satellite System (GNSS) satellites to determine the proximity of each Global Navigation Satellite System (GNSS) satellite measurement's Ionosphere Pierce Point to a plurality of Ionosphere Grid Points (IGPs) derived from at least one Space-Based Augmentation System (SBAS) geostationary satellite; wherein the at least one processing module is further configured to determine that the Global Navigation Satellite System (GNSS) satellite measurement for each of the plurality of Global Navigation Satellite System (GNSS) satellites is safe for mitigation using at least one overbounded Vertical Ionosphere Gradient standard deviation sigma-vig (σ vig) when the Ionosphere Grid Points (IGPs) possess acceptable Grid Ionosphere Vertical Error (GIVE) values; wherein the at least one processing module is further configured to determine whether a number of Global Navigation Satellite System (GNSS) satellite measurements determined safe for mitigation using the at least one overbounded Vertical Ionosphere Gradient standard deviation sigma-vig (σ vig) are able to produce a Vertical Protection Limit that meets a Vertical Alert Limit required for a precision approach; and wherein the at least one processing module is configured to cause the at least one aircraft communication device to communicate the at least one overbounded Vertical Ionosphere Gradient standard deviation sigma-vig (σ vig) along with differential corrections and an indication of which Global Navigation Satellite System (GNSS) satellite measurements that are safe for mitigation using the at least one overbounded Vertical Ionosphere Gradient standard deviation sigma-vig (σ vig) are able to produce the Vertical Protection Limit that meets the Vertical Alert Limit required for a precision approach to an aircraft GNSS receiver when the number of Global Navigation Satellite System (GNSS) satellite measurements determined safe for mitigation using the at least one overbounded Vertical Ionosphere Gradient standard deviation sigma-vig (σ vig) are able to produce a Vertical Protection Limit that meets a Vertical Alert Limit required for a precision approach. a plurality of reference receivers having known positions; at least one processing module communicatively coupled to the plurality of reference receivers; at least one aircraft communication device communicatively coupled to the at least one processing module; wherein the at least one processing module is configured to check a Global Navigation Satellite System (GNSS) satellite measurement for each of a plurality of Global Navigation Satellite System (GNSS) satellites to determine the proximity of each Global Navigation Satellite System (GNSS) satellite measurement's Ionosphere Pierce Point to a plurality of Ionosphere Grid Points (IGPs) derived from at least one Space-Based Augmentation System (SBAS) geostationary satellite; wherein the at least one processing module is further configured to determine that the Global Navigation Satellite System (GNSS) satellite measurement for each of the plurality of Global Navigation Satellite System (GNSS) satellites is safe for mitigation using at least one overbounded Vertical Ionosphere Gradient standard deviation sigma-vig (σ vig) when the Ionosphere Grid Points (IGPs) possess acceptable Grid Ionosphere Vertical Error (GIVE) values; wherein the at least one processing module is further configured to determine whether a number of Global Navigation Satellite System (GNSS) satellite measurements determined safe for mitigation using the at least one overbounded Vertical Ionosphere Gradient standard deviation sigma-vig (σ vig) are able to produce a Vertical Protection Limit that meets a Vertical Alert Limit required for a precision approach; and wherein the at least one processing module is configured to cause the at least one aircraft communication device to communicate the at least one overbounded Vertical Ionosphere Gradient standard deviation sigma-vig (σ vig) along with differential corrections and an indication of which Global Navigation Satellite System (GNSS) satellite measurements that are safe for mitigation using the at least one overbounded Vertical Ionosphere Gradient standard deviation sigma-vig (σ vig) are able to produce the Vertical Protection Limit that meets the Vertical Alert Limit required for a precision approach to an aircraft GNSS receiver when the number of Global Navigation Satellite System (GNSS) satellite measurements determined safe for mitigation using the at least one overbounded Vertical Ionosphere Gradient standard deviation sigma-vig (σ vig) are able to produce a Vertical Protection Limit that meets a Vertical Alert Limit required for a precision approach.