A system is provided that determines the length of an optical fiber unwound from or remaining on at least one storage reel. The fiber is used as a medium for transmitting information between a weapon and an underwater vehicle, from which the weapon was fired. The determination system measures the backscattered light power in the fiber, analyzes this power signal in order to detect a backscattered light power discontinuity that corresponds to the location of the fiber where the fiber leaves the reel, localizes this location along the fiber, and determines from this localization and from the length of fiber initially wound on the reel, the length of fiber unwound from or remaining on the reel.