A GPS-Based ADS for determining attitude and heading of a moving body comprises a GPS receiver, an inertial sensor unit, a blockage area calculator, a sensor anomaly detector and a monitor. The GPS-Based ADS provides information useful for making a judgment on appropriateness of installation sites and mounting conditions of a GPS antenna and inertial sensors based on blockage area information concerning each satellite signal blockage area obtained by the blockage area calculator and sensor signal anomaly information concerning each inertial sensor anomaly obtained by the sensor anomaly detector.