TITLE:  GPS, Relativity, and Time

ABSTRACT:

The Global Positioning System is an example of an engineering system in
which the principles of the general theory of relativity are not merely a
matter of scientific interest or scientific speculation but have become 
issues of engineering necessity.  GPS has provided both a theoretical model 
for relativistic time transfer and a laboratory for the validation of the 
accuracy and consistency of relativistic algorithms. The historical
evolution of relativity will be described, beginning with the theories of
gravitation and electromagnetism and continuing with the fundamental ideas
underlying the special and general theories of relativity. 

The major relativistic effects important to GPS at the nanosecond level 
of precision will be outlined, including time dilation, the gravitational 
redshift, and the Sagnac effect.  Refinements to the existing corrections 
that would be desirable to model the effects on clock rates due to 
stationkeeping maneuvers and new corrections necessary for cross-link 
ranging and interoperability across systems will be identified.  Additional 
corrections at the subnanosecond level that must be considered in future 
generations of GPS will also be described.  The presentation will conclude 
with a discussion of the implications of corresponding relativistic effects 
for high precision time transfer involving other types of satellite systems, 
such as Low Earth Orbit satellites, the International Space Station, 
Highly Elliptical Orbit satellites, and geostationary satellites.