A penetration detection device earned by an earth penetrating bomb includes an inertial measurement unit and a penetrator processor and logic circuit contained in a shock isolated package which can withstand the transmitted environment encountered during earth penetration The inertial measurement unit derives information relative to the attitude of the bomb and the penetrator processor and logic circuit is responsive to the outputs of the inertial measurement unit as well as the outputs of external accelerometers to not only detect the transitions of adjoining layers but count the layers penetrated and also to determine the material of each layer This is compared with a stored layer characterization and if the stored and computed values are in correlation, a signal will be sent to start the arming initiation process A second layer charactepzation may also be stored for comparison if the first layer does not correlate
