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.