GPS IIR-1

GPS IIR (Global Positioning System) or Navstar IIR (Navigation System using Timing And Ranging) are the third evolution stage of the second generation of the GPS satellites. The GPS IIR replenishment satellites were produced by General Electric Astrospace (later Martin Marietta and Lockheed Missiles & Space) under a contract issued in 1989, which included 21 satellites based on the commercial AS-4000 bus. These provided improved navigation accuracy and longer autonomous satellite operation than earlier model GPS satellites. The satellites were three-axis stabilized and operated at downlink frequencies of 1572.42 MHz and 1227.6 MHz (L-Band) and 2227.5 MHz (S-Band). Lockheed was responsible for launch and flight operations support of the GPS-IIR through 2006. Block IIR satellites provided dramatic improvements over previous blocks. They could determine their own position by performing inter-satellite ranging with other IIR vehicles. They also had reprogrammable satellite processors enabling problem fixes and upgrades in flight, increased satellite autonomy, and radiation hardness.