In the early 1970s, the Department of Defense, DOD, commissioned a study to define its future positioning needs. That study found nearly 120 different types of positioning systems in place, all limited by their special and localized requirements. They were of various kinds—some were terrestrial, some relied on electromagnetic signals, some were optical. There were various kinds of positioning systems in place. A plan was made to combine the systems—to integrate them. In general, the idea always was to take from the previous systems the aspects that worked the best and to leave behind those that left something to be desired. The study called for consolidation, and NAVSTAR GPS (NAVigation System with Timing And Ranging, Global Positioning System) was proposed. Specifications for the new system were developed to build on the strengths and avoid the weaknesses of its forerunners. What follows is a brief look at the earlier systems and their technological contributions toward the development of GPS.