http://www.gpstk.org/bin/view/Documentation/WebHome The goal of the GPSTk project is to provide an open source library and suite of applications to the satellite navigation community--to free researchers to focus on research, not lower level coding.
http://sopac.ucsd.edu/ SOPAC provides GPS-related data from its GARNER archive via ftp and http . The GARNER archive contains over 5 terabytes of data, dating back to 1990, with a 15 terabyte data capacity.