Abstract: The Fault Detection and Exclusion (FDE) system for use in navigational systems that rely upon multiple ranging signals, such as GPS satellites, uses an FDE algorithm to detect, as soon as possible, whether a fault exists in a signal associated with one or more of the GPS satellites. The system makes this determination by comparing a computed residual error with a first fault detection threshold. After determining that the computed residual error has exceeded the first, relatively low, fault detection threshold, the system transmits a signal to one or more external systems indicating that one or more signals associated with the various GPS satellites may be faulty. The system then monitors horizontal estimated position error (HUL) until this value has met or exceeded a relatively high fault isolation threshold value. The system then attempts to isolate and exclude the faulty satellite from the current navigational solution.