1. A computer-implemented method for analyzing ionospheric slant total electron content (TEC) using global navigation satellite systems (GNSS)-based estimation comprising: estimating slant TEC for a given set of raypath geometries by fitting historical GNSS data to a specified delay model; evaluating an accuracy of the specified delay model by calculating delay estimate residuals and plotting a behavior of the delay estimate residuals; computing an ionospheric threat model based on the specified delay model; and computing ionospheric grid delays (IGDs) and grid ionospheric vertical errors (GIVEs) based on the ionospheric threat model.