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.
In 1974 Rockwell International was awarded a contract to build 8 Block I satellites. In 1978 the contract was extended to build an additional 3 Block I satellites.
The stellites were 3-axis stabilized, nadir pointing using reaction wheels. Dual solar arrays supplied over 400 Watts (EOL) and charged NiCd batteries. S-Band (SGLS) communications were used for control and telemetry. A UHF channel provided cross-links between spacecraft. A hydrazine propulsion system was used for orbital correction. The payload included two L-Band navigation signals at 1575.42 MHz (L1) and 1227.60 MHz (L2)
