A "gps_Intro" presentation comes with GIPSY software, in the "class_gd2p" directory. The presentation goes over the basics of GPS systems and the theory behind using GPS for positioning.
The three presentations from the GIPSY class are available from the GIPSY documentation web site. Please use your GIPSY Forum login/password information to access this area.
References on the basics of GPS theory and application
Blewitt, G. (2007), GPS and Space Based Geodetic Methods, chapter in Treatise on Geophysics, Vol. 3., pp. 351-390, Ed. Thomas Herring, Ed.-in-chief Gerald Schubert, Academic Press, Oxford, UK, ISBN: 0-444-51928-9. (PDF)
Dixon, T., The Global Positioning System, Review of Geophysics, 1991.
Segall, P. and J.L. Davis, GPS applications for geodynamics and earthquake studies, Annual Reviews of Earth and Planetary Science, 25, 301-36, 1997.
Strang, G. and K. Borre "Linear Algebra, Geodesy, and GPS", Wellesley-Cambridge Press, 1997. (available in print or also on Google books),