PROJECTILE RANGING WITH DIGITAL MAP

A terrain-referenced navigation system (600) for an aircraft comprises: a stored digital terrain map (606); a position calculation unit (608) arranged to calculate aircraft position relative to the stored digital terrain map (606) to determine a terrain-referenced aircraft position; a fall line calculation unit (612) arranged to calculate a fall line for a projectile starting from the terrain-referenced aircraft position as a launch point; and an impact point calculation unit (614) arranged to directly compare the fall line with the digital terrain map, by incrementally comparing a height of the projectile along the fall line with a height of the terrain according to the stored digital terrain map (606) in order to find an expected impact point on the terrain.