The GPS (Global Positioning System) or Navstar (Navigation System using Timing And ranging) were the first generation satellites of the GPS navigation system. The first two satellites were also called NDS (Navigational Development Satellite).

The Block I satellites were positioned in the same orbital planes as the later Block II, but at an inclination angle of 63 degrees. Block I consisted of the original concept validation satellites and reflect various stages of system development. Lessons learned from the 11 satellites were incorporated into later generations.