One major problem with GPS is if one satellite begins to transmit rubbish. This can result in incorrect positions being calculated and may take time to be detected and the satellite switched off or the constellation health message upgraded to broadcast this fact.
Which is precisely why IFR certificated aircraft receivers have their own integrity monitoring and warning ssytems... and why those States experienced in the use of GPS have developed and implemented technical, training, regulatory and operational procedures to ensure safety of flight......