1. A method of mapping refraction angles in a planet's atmosphere as a function of altitude above the planet, comprising the steps of: providing a database of known star locations; providing a field imager onboard a platform above the planet, said field imager having a field-of-view; positioning said field imager to observe a region that includes a portion of the planet's atmospheric limb as viewed from the platform; capturing a star field image for said region using said field imager wherein said star field image includes images of stars appearing in said field-of-view; generating, using said field imager and said database, a modeled star field image sized to match said field-of-view wherein said modeled star field image defines absolute locations of stars corresponding to stars represented by said images of stars appearing in said field-of-view; and comparing said modeled star field image to said star field image wherein differences between said absolute locations and locations of said images of stars are indicative of a limb refraction angle field for said field-of-view.