The Block I SVs, built by Rockwell International as developmental prototypes, were launched between 1978 and 1985 from VAFB, CA. These SVs supported most of the system testing. The Air Force launched the first Block I research and development satellite in February 1978, and as of February 1991, the GPS network consisted of six Block I R&D satellites. These satellites were used for the development and testing of Navstar receivers and user equipment. Out of a total of 11 Block 1 SVs launched on the Atlas-Centaur, one was lost as a result of a launch failure, three have reached end-of-life due to wear-out of their atomic clocks, and two suffered failures of their three-axis attitude control system. The oldest Block I SV (PRN #6) operated reliably for more than double its five-year design life. The total number of GPS satellites was originally planned to be 21 active plus 3 spares. This number was reduced to 18 active and 3 spares as a cost saving measure in the early 1980's,(1) but the number of active satellites was returned to 21 in 1987,(2) with this number planned for implementation by 1993.(3)
1. Clarke, Christopher, "...and a Star to Steer By," Defense Electronics, June 1989, page 57-64.
2. Klass, Philip, "Defense Dept. Will Seek Funds to Expand Navstar Constellation," Aviation Week & Space Technology, 5 October 1987, page 30-31.
3. "Magnavox Prepares for GPS Buildup," Military Space, 25 September 1989, page 3-5.