Abstract: An error detector apparatus (42) producing a signal used in the computation of a missile guidance signal in an aircraft-based missile guidance and tracking system. The error detector apparatus (42) incorporates means (146), (148), (150), (152), and (154) for producing a first envelope signal from a tracking signal emitted by a missile. The envelope signal is then converted, along with a reference signal, to a digital signal by an analog to digital converter (158). A microcontroller (168) calculates an error correction signal from the digital signals. A digital to analog converter (160) converts the digital error correction signal to an analog error correction signal and outputs the analog error correction signal to the system for use in computing a missile guidance signal.