Abstract: Herein described is an electronic control system for guiding a search unit of a nondestructive material tester over sloped or curved surfaces and shapes. More particularly described is the index system used on an indexing-scanning (inspection) device. An oscillator provides a preset number of pulses through an angle rate generator and direction logic to a binary coded decimal pulse counter. The counter keeps track of the input pulses provided thereto (each pulse represents an 0.1.degree. increment) therefore, its contents are indicative of the angle accumulated therein. A pair of read only memories provides the sine function and cosine function of the accumulated angle as a binary coded decimal trigonometric representation of the input angular degree. The signals therefrom control respective Y and Z rate generators whose input clock rate is indicative of the velocity along the curve of the pivot point of the search unit.