This paper gives details of the failure rates and the failure modes experienced with SPS GPS receivers certificated for aviation use to the TSO-C129 specification. These failure rates are significant compared to those from other modern avionics and the rates are comparable to that currently experienced from un-scheduled failures in the GPS signals-in-space. To achieve certification, Flight Management System (FMS) design should consider the way in which the satellite navigation receivers fail. The paper also gives details concerning the 2-D errors that have been experienced with these receivers, and indicates further work that needs to be done with C 129a receivers.