To improve reliability of autonomous unmanned helicopter, GPS-INS hybrid navigation systems using Kalman Filter are utilized. Barometer can be integrated into the hybrid navigation system, but the problem is that there is no sufficient model which can describe the relation between the altitude and the air pressure in ground effect. The purpose of this paper is to develop a measurement model of a barometer on an unmanned helicopter which can be used in the hybrid navigation system for autonomous unmanned helicopters. In particular, we propose that the model equation of atmospheric pressure under ground effect can be applied to estimate the terrain clearance of the helicopter. Results of flight experiments show that the proposed method is effective when the helicopter is close to the surface. © 2011 IEEE.