1.  Field of the Invention
This invention relates generally to fuzes for explosive devices and more particularly to determining a detonation time related to when an explosive device impacts with a target. 
2.  Description of Related Art
Explosive projectiles must be capable of being handled safely under considerable stress and environmental conditions.  In addition, explosive projectiles must be capable of detonating at the proper time.  Depending on the application, this proper time may be before impact, at a specific point during flight, during impact, or at some time delay after impact.  As used herein the terms “warhead,” “explosive device,” and “explosive projectile” are generally used to refer to a variety of projectile type explosives, such as, for example, artillery shells, rockets, bombs, and other weapon warheads.  In addition, these explosive projectiles may be launched from a variety of platforms, such as, for example, fixed wing aircraft, rotary wing aircraft (e.g., helicopters), ground vehicles, and stationary ground locations.  To determine the proper detonation time, these explosive projectiles frequently employ fuzes. 
A fuze subsystem activates the explosive projectile for detonation in the vicinity of the target.  In addition, the fuze maintains the explosive projectile in a safe condition during logistical and operational phases prior to launch and during the first phase of the launch until the explosive projectile has reached a safe distance from the point of launch.  In summary, major functions that a fuze performs are; keeping the weapon safe, arming the weapon when it is a safe distance from the point of launch, detecting the target, and initiating detonation of the warhead at some definable point after target detection. 
The first two functions of keeping the weapon safe and arming the weapon are conventionally referred to as Safing and Arming (S&A).  Safing and Arming devices isolate a detonator from the warhead booster charge until the explosive projectile has been launched and a safe distance from the launch vehicle is achieved.  At that point, the S&A device removes a physical barrier from, or moves the detonator in line with, the explosive train, which effectively arms the warhead so it can initiate detonation at the appropriate time. 
Some S&A devices function by measuring elapsed time from launch, others determine distance traveled from the launch point by sensing acceleration experienced by the weapon.  Still other devices sense air speed or projectile rotation.  For maximum safety and reliability of a fuze, the sensed forces or events must be unique to the explosive projectile when deployed and launched, not during ground handling or pre-launch operations.  Most fuzes must determine two independent physical parameters before determining that a launch has occurred and a safe separation distance has been reached. 
The last two functions conventionally performed by a fuze of detecting the target and initiating detonation may depend on target type, explosive projectile type, and tactical operational decisions.  Target detection may occur using a simple timer, determining a predetermined time after launch, using sensors to detect proximity to a target, or using sensors to detect impact with a target.  Conventionally, impact fuzes, as opposed to proximity fuzes, are designed to detect the target by sensing some type of impact or contact with a target. 
In an impact fuze, the final fuze function of initiating detonation of the warhead may occur as temporally close to impact as possible or may be delayed for a certain period of time allowing the warhead to penetrate the target prior to detonation.  Conventionally, delayed detonation has been performed by defining a fixed delay after impact to initiate detonation.  However, generally there may be an optimum penetration depth at which the warhead should detonate.  A fixed delay may cause the warhead to detonate significantly earlier than or later than this optimum penetration depth is reached.  In addition, the impact event may be the only parameter available for determining the fixed delay.  When impact is the only event parameter available, the impact velocity is conventionally unknown. 
If the impact velocity were known, a penetration delay proportional to the impact velocity could by incorporated to optimize the penetration delay and, as a result, detonate the warhead at a depth closer to the optimum penetration depth.  There is a need for an apparatus and method for generating an impact velocity estimate and for determining a more optimum delay time in which to detonate an explosive projectile after impact with a target.