You may find the
Global Positioning System Standard Positioning Service Performance Analysis Report (PDF, 284 K)
and
GPS RISK ASSESSMENT STUDY (PDF, 2281 K)
interesting reference material on availability and accuracy.
Neither seems to specifically address the issue of receiver failure or software issues, which I would guess play a significant part in the overall availability and accuracy of GPS.
[This message has been edited by bookworm (edited 06 July 2001).]