1. A passive altimeter, comprising: an imaging device; a groundspeed system; and an image processing device operatively connected to receive an indication of groundspeed from the groundspeed system and to receive at least a first image from the imaging device taken at a first position and a second image from the imaging device taken at a second position spaced apart from the first position, wherein the image processing device is configured to compare the first image and the second image to determine an altitude above ground using the indication of groundspeed from the groundspeed system.