GPS Block II
In the aftermath of the Challenger accident, the Air Force decided to remove the Navstars from the Shuttle manifest, and to launch them on an improved version of the proven Delta launch vehicle, known as the Delta II. A total of 20 of these rockets were ordered. Block II SVs were built by Rockwell International.
Significant Block II SV enhancements to the signal-in-space (SIS) interface were:
- Radiation hardened electronics to prevent random memory upset events to improve SIS reliability and survivability.
- Capacity to store 180 days worth of 50 Hz navigation message data, compared to only 3.5 days worth of storage in the Block I SVs, to guarantee SIS availability.
- Full selective availability (SA) and anti-spoof (A-S) capabilities to provide for SIS security.
- Automatic detection of certain error conditions and switching to non-standard PRN Code transmission or default navigation message data (alternating ones and zeros) to protect users from tracking a faulty SV and maximize SIS integrity.
With the launch of five new Navstar satellites in 1989, the Air Force began to implement the full complement of 21 active and 3 spare satellites will be required for the system can provide nearly continuous global coverage.(1) The first of 28 Block II SVs, SV PRN number 14, was launched on 14 February 1989 from Cape Canaveral AFS, FL using a Delta II MLV and was set "healthy" in its broadcast 50 Hz navigation message for global use on 15 April 1989. Although the first two satellites launched had been nicknamed Elvis (Presley) and Janis (Joplin), this practice ceased with the third and subsequent launches.(2) Rockwell International was also the prime contractor for Block IIA. The design life of the operational satellites is seven and one-half years.(3)
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