PERFORMANCE SPECIFICATION PEL PATH The direction of progression of successive pels along a line in an image. PEL SPACING The distance between any two successive pels along a scan line of an image. It is the inverse of such often-used terms as pel density, transmission density, and resolution. PENETRATION The minimum depth a groove or flange weld extends from its face into a joint, exclusive of reinforcement. PENETRATION AIDS Components of the re-entry system uti- lized to penetrate and confuse enemy defenses, thus enhancing the mission probability of delivering a re-entry vehicle to its intended destination. PENETRATION BALLISTICS The part of terminal ballistics that treats the motion of a projectile, such as a guided missile war- head, as it forces it way into targets of solid or semisolid substances. PENETRATION VELOCITY The minimum velocity at which a particular projectile, such as a guided missile warhead, is expected to consistently perforate plates of given thickness and physical properties at a specific angle of obliquity. PERCENT DEFECTIVE The percent defective of any given quantity of units of product is one hundred times the number of defective units of product contained therein divided by the total number of units of product. PERCENT DEFECTIVE ALLOWABLE (PDA) The percent defective allowable of a production lot of parts or materials is the maximum allowable percentage of parts or material specimens that fail to pass one or more tests before the entire production lot is con- sidered to be unacceptable. PERCUSSION WELDING (PEW) A resistance welding process wherein coalescence is produced simultaneously over the entire abutting surfaces by the heat obtained from an arc produced by a rapid discharge of electrical energy with pressure percussively applied during or immediately following the electrical discharge. PERFORMANCE SPECIFICATION A performance specifica- tion is a specification that states requirements in terms of the desired results with criteria for verifying compliance, but without stating the methods for achieving the required results. A perfor- mance specification defines the functional requirements for the item, the environment in which it must operate, and interface and interchangeability characteristics. 193