In a modern battle, when the sky is filled with friendly and enemy  aircraft, and a variety of advanced weapons are ready to fire from both ground and airborne platforms, positive identification of friend and foe is critically important.  For example fratricide due to identification errors has been a major problem in all 20th century wars.
	Current IFF systems use an interrogation/response method which employs cryptographically encoded spread spectrum signals.  The interrogation signal received by a friend is supposed to result in the "correct" code being automatically sent back via a transponder on the friendly platform.  The "correct" code must change frequently to prevent a foe from recording and transmitting that code ("repeat jamming"), thereby appearing as a friend.  The code is changed at the end of what is called the code validity interval (CVI).
	The better the clock accuracy, the shorter can be the CVI, the more resistant the system can be to repeat jamming, and the longer can be the autonomy period for users who cannot resynchronize their clocks during a mission.
	J. R. Vig, "Military Applications of High Accuracy Frequency Standards and Clocks," IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 40, pp. 522-527, 1993.