This information is from the GPS World website:

Almanac: GPS, Glonass, and satellite-based augmentation systems (SBAS) constellation data and information sources

How Does GPS Work?: a brief overview by GPS World "Innovation" columnist Richard Langley.

Acronyms
Glossary
Other Web Sites on GPS and Related Issues
On-line GPS Tutorials
Receivers vs. Tracking
Basic Timeline of GPS Development

Downloadable 34-page PDF of GPS History, Chronology, and Budgets