To insure safe approach and landing operations of GAST D GBAS, this study develops a three-step worst-case parameter search and integrity risk evaluation method to identify the worst-case sets of ionospheric threat parameters and evaluate the worstpossible integrity risk. Parameters of the ionospheric gradient impact simulation are first classified into two groups, “Worst-Case (WC)” and “average,” based on the underlying “specific risk” integrity requirements. “WC” parameters are those for which a clear basis for averaging could not be established because the probabilistic distribution of these parameters cannot be established with confidence due to the lack of sufficient independent observation data. These “WC” parameters must use the worst-case value of that parameter in calculating integrity risk, meaning the value of the parameter that maximizes the integrity risk. Parameters chosen to be “average” are those which are known to be essentially random from the point of view of a GBAS precision approach impacted by an ionospheric front. The parameters treated as “average” can apply values from a sampled distribution (usually from a Uniform distribution) between the minimum and maximum values of that parameter as the basis for calculating integrity risk. The first step of this method performs a randomized search over all of the parameters with a very large total number of samples to identify one or more regions of the ionospheric threat space where the worst-case result (maximum integrity risk) is likely to lie. In the second step, the search for the worst-case set of threat parameters is more refined so that a single worst-case set of parameters can be selected for each region. Once each identified region has a single set of worst-case threat-model parameters, separate “Step 3” simulations can be conducted for each region to estimate integrity risk values, and these final results are what is compared to the SARPs anomalous ionosphere integrity requirements. This paper also evaluates the maximum integrity risk probabilities based on this three-step method using the SARPs ionospheric anomaly threat model and geometric parameters. The resulting integrity risk values for all regions are well below 10-9 , showing that the GAST D SARPs integrity requirement is met based on this combined worst-case/average definition of the anomalous ionospheric threat.