A system for independently monitoring received instrument landing system signals onboard an aircraft, comprising: an ILS receiver, comprising a localizer receiver adapted to receive and demodulate ILS signals; an altimeter configured to sense aircraft altitude and supply altitude data representative of the sensed aircraft altitude; a global positioning system receiver configured to determine aircraft position and supply an aircraft position data representative thereof; and a processor in operable communication with the localizer receiver, the altimeter, and the GPS receiver, the processor configured, in response to the altitude data, to determine when the aircraft is in pre-approach phase and when the aircraft is in an approach phase, characterized in that: the processor is further configured to: sample, during the pre-approach phase, the ILS signals demodulated by the localizer receiver to thereby generate pre-approach phase ILS data; determine a statistical approach course of the aircraft wherein the statistical approach course is an approach course that is generated based on the pre-approach phase ILS data and the aircraft position data; sample, during the approach phase, the ILS signals demodulated by the localizer receiver to thereby generate approach phase ILS data; compare the approach phase ILS data to the statistical approach course to determine a course deviation; and selectively generate an alert signal when the course deviation exceeds a predetermined magnitude. an ILS receiver, comprising a localizer receiver adapted to receive and demodulate ILS signals; an altimeter configured to sense aircraft altitude and supply altitude data representative of the sensed aircraft altitude; a global positioning system receiver configured to determine aircraft position and supply an aircraft position data representative thereof; and a processor in operable communication with the localizer receiver, the altimeter, and the GPS receiver, the processor configured, in response to the altitude data, to determine when the aircraft is in pre-approach phase and when the aircraft is in an approach phase, the processor is further configured to: sample, during the pre-approach phase, the ILS signals demodulated by the localizer receiver to thereby generate pre-approach phase ILS data; determine a statistical approach course of the aircraft wherein the statistical approach course is an approach course that is generated based on the pre-approach phase ILS data and the aircraft position data; sample, during the approach phase, the ILS signals demodulated by the localizer receiver to thereby generate approach phase ILS data; compare the approach phase ILS data to the statistical approach course to determine a course deviation; and selectively generate an alert signal when the course deviation exceeds a predetermined magnitude. sample, during the pre-approach phase, the ILS signals demodulated by the localizer receiver to thereby generate pre-approach phase ILS data; determine a statistical approach course of the aircraft wherein the statistical approach course is an approach course that is generated based on the pre-approach phase ILS data and the aircraft position data; sample, during the approach phase, the ILS signals demodulated by the localizer receiver to thereby generate approach phase ILS data; compare the approach phase ILS data to the statistical approach course to determine a course deviation; and selectively generate an alert signal when the course deviation exceeds a predetermined magnitude.