Space-based navigation - Past, present, and future

This paper traces the history of mankind's experience in the use of electromagnetic energy radiating from objects in space to navigate vehicles on or near the earth. It also provides a glimpse of probable future applications and promising improvements to today's most advanced radionavigation methodologies. The discussion begins with the earliest known examples of celestial navigation, then moves on to the 18th century, when observations of the heavenly bodies were combined with the impressive timekeeping capabilities of the marine chronometer. The accurate measurement of time was critically important because it allowed early navigators to determine longitude as well as latitude. The basic principles of radionavigation are then reviewed in general terms, and more specific discussions of systems that employ space-based transmitters follow. Most of the emphasis is placed on the Navy's Transit navigation satellite and the more modern Navstar Global Positioning System (GPS). The concluding sections of the paper discuss the future of radionavigation, including promising upgrades to the GPS that could enhance its performance and make it more valuable to a broad and varied class of users.