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The Space Shuttle program began flying atmospheric flight navigation units in 1993, in support of Shuttle avionics upgrades. In the early 1990s, it was anticipated that proven in-production navigation units would greatly reduce integration, certification and maintenance costs. However, technical issues arising from ground and flight tests resulted in a slip in the Shuttle GPS certification date. A number of lessons were learned concerning the adaptation of atmospheric flight navigation units for use in low-Earth orbit. They are applicable to any use of a navigation unit in an application significantly different from the one for which it was originally designed. Flight experience has shown that atmospheric flight navigation units are not adequate to support anticipated space applications of GPS, such as autonomous operation, rendezvous, formation flying and replacement of ground tracking systems