Abstract: Navigation system errors, which result from a bias or error in a transmitter's data or timing, are detected and isolated. The transmitters may be ground based or they may be in a satellite. The least square error residual is calculated using the information received from the transmitter and the last estimate of the aircraft's position. Using Baye's Rule, the residual is used to calculate the a posteriori conditional probability of each transmitter being in error. The a posteriori conditional probabilities, and an estimate of each transmitter's error or bias are used to estimate the system loss associated with the elimination of a particular transmitter's information from the calculation of the aircraft's position. The faulty transmitter is identified by determining which transmitter's elimination will result in minimal system loss. In some instances, the system loss is minimized by eliminating no transmitters.