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GPS APPLICATIONS AND REQUIREMENTS 19 2 GPS Applications and Requirements INTRODUCTION GPS specifications were originally developed by the DOD (Department of Defense) in the late 1960s with the primary objective of satisfying military navigation requirements. A secondary objective was to provide a separate, less accurate signal for both military and civilian use. This signal, described in Appendix C, and known as the Standard Positioning Service (SPS), was intentionally degraded in accuracy (100 meters, 2 drms) to avoid its exploitation by potentially unfriendly users. As the GPS satellite constellation expanded and was eventually completed in 1993, the use of the freely available SPS signal for civil applications also continuously expanded. GPS is now used for positioning, navigation, and timing applications in a number of civil and commercial activities related to aviation; maritime commerce and recreation; land transportation; mapping, surveying, and geodesy; scientific research; timing and telecommunications; and spacecraft. Each of these broadly defined civilian user categories, along with military applications, is discussed in this chapter. Many of the innovative civilian applications that this chapter will address were not foreseen by the original designers and developers of GPS and cannot be accomplished without augmenting and/or enhancing the stand- alone capabilities of the system as currently configured. As a result, differential correction methods and user equipment integrated with other positioning technologies, as described in Appendix C, have been utilized to meet the requirements of many of these applications. Within this context, there have been no requirements imposed on the basic GPS by civilian users to date beyond the assurance that the basic SPS signal-in-space will remain freely available at its currently defined accuracy level.1 Users have taken this signal and adapted it to their applications. The basic GPS has therefore become a ''dual-use" system, which is still designed to meet the requirements of only a single user, the Department of Defense.2 1 This official U.S. government policy is currently reiterated every 2 years in the Federal Radionavigation Plan. 2 The term "dual-use" usually refers to use by both the military and civilians.