Alerting a user of an integrity error in a global positioning system (GPS) signal transmitted from a transmitter in a global positioning system (GPS) satellite, by detecting an error in the global positioning system (GPS) signal, alerting the user of the error by shifting a phase of the global positioning system (GPS) signal from an initial value by a predetermined phase-shift amount, and returning, in the case that the detected error is corrected, the phase of the global positioning system (GPS) signal to the initial value. Preferably, the GPS signal is phase-shifted by the predetermined phase-shift amount using an alert timing pattern which indicates the level of detected error.