1. A method for providing rapid intervisibility determination, comprising: receiving an active flight plan via a computing system of an aircraft; creating a terrain pre-processing area based on the active flight plan; determining geodetic coordinates of the terrain pre-processing area; querying and retrieving terrain elevation data from an on-board database based on said geodetic coordinates of the terrain pre-processing area; processing the terrain elevation data into a data structure; assigning the terrain pre-processing area and the data structure as an active planning area; dividing the active planning area into multiple, uniformly-sized zones in geodetic coordinates; processing the terrain elevation data of the active planning area and storing said terrain elevation data in a two-dimensional (2-D) grid coordinate system via an Ellipsoidal Transverse Mercator (ETM) projection; and computing a Line-Of-Sight (LOS) vector between a first location and a second location in Earth-Centered, Earth-Fixed (ECEF) coordinates, the first location being a start location for the vector, the second location being an end location for the vector.