Philosophical Society of Washington

The Global Positioning System

Thomas McCaskill
Naval Research Laboratory (Retired)

2218 th Meeting Abstract
Friday, March 2, 2007 at 8:15 PM


The Global Positioning System is a U.S. Department of Defense space-based system consisting of a constellation of Earth-orbiting “Navstar” satellites that provides worldwide navigation and timing information to U.S. and NATO military users, and to millions of civilian users, including the 911 emergency services used to locate cell phones. A GPS user can determine their position and time instantaneously and also, velocity, nearly-instantaneously. Instantaneous positioning with GPS is accomplished by making four, or more, simultaneous passive ranging measurements, using the received signals broadcast by atomic clocks that are synchronized, and syntonized, to a common GPS time, and is functionally related to a four-dimensional space-time defined by three position coordinates and one time coordinate.

Each Navstar satellite continuously broadcasts precise timing signals and a navigation message that is used to calculate the position and clock offset for each satellite tracked by a user's GPS receiver. The orbital elements and the clock offset of each satellite are determined by the GPS Master Control Station using tracking data collected by a network of five tracking stations. The satellites are located in nominally circular orbits of 12-hour period with an inclination of 55 degrees. The principal effects of special and general relativity on the orbiting atomic clocks are constant with a circular orbit and the signals broadcast by each Navstar satellite incorporates a constant frequency offset so that the rate of GPS time is the same as that of Universal Time.

Thomas McCaskill

About the Author:

Thomas McCaskill received an Associate of Arts Degree from Wingate Junior College in 1958, a Bachelor Science in Engineering Mathematics from NC State University (honors) in 1960, and then began work as a Mathematician with the Space Applications Branch of the U.S. Naval Research Laboratory. In 1964 he was selected to participate in the NRL Edison Memorial Training Program and completed all required courses for a Ph.D. in Mathematics at the University of Maryland. In 1977 he was reclassified as a Research Physicist. As an NRL scientist, he made key contributions to the design of GPS, including the first solution of the instantaneous positioning problem, determination of the optimal inclination for the GPS constellation, and verification of the effects of relativity on an orbiting cesium atomic clock. McCaskill received five NRL Research Publication Awards, one Patent Award and retired from NRL in 2002 after 42 years of service. He is a Senior Member of the Institute of Electrical and Electronic Engineers (IEEE) and member of the Institute of Navigation (ION).

In 2003 he was selected by the Institute of Navigation to serve as a Congressional Fellow. In 2005 he became a candidate for U.S. Senator in the Maryland primary election with the Campaign Motto:

GPS Saves Lives

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