GPS data is used for ADAHRS performance monitoring, so at least one source of GPS data is required. This requirement can be met by installing a GPS 20A, or by connecting a GPS antenna to at least one GDU GPS receiver. A GTX Series Transponder with built-in GPS, or a GNX 375 connected to the system will also satisfy this requirement. In a system with multiple GDU displays, additional GPS antennas may be connected to the other displays for redundancy, if desired.