A passive altimeter system comprising an angle between a point and a central boresight that is determined from distortion parameters of a lens in an infrared sensor in a countermeasure system on a mobile platform wherein the infrared sensor captures a first image for determining a distance between the platform and one of (i) a ground surface and (ii) a target, and the passive altimeter system further comprising a dimensional distance between two points in the first image that is determined from a secondary source external to the countermeasure system, and a processor to triangulate the distance between the platform and one of (i) the ground surface and (ii) the target based on the dimensional distance and the angle.