Research on the effects of user antenna´s attitude on the availability of RAIM

Receiver autonomous integrity monitoring (RAIM) is one of the most important integrity enhancing methods in navigation system, which has been equipped in many GPS receivers of civil aircrafts. RAIM detects GNSS (Global Navigation Satellite System) fault through redundant measurements. The performance of navigation system will be dramatically affected by the availability of RAIM. A classical RAIM algorithm is to compare the value of Horizontal Protection Level (HPL) with Horizontal Alert Limit (HAL). If the HPL is exceeds HAL, RAIM is unavailable. The computation of HPL is based on the geometry of visible satellites. The effects of GPS receiving antenna´s attitude on the availability of RAIM are investigated. We simulate the different attitudes of receiving antennas which fixed on the aircrafts by the use of receivers on the land. The performances of RAIM are compared in different receiving attitudes by computing the HPLs. The simulation results demonstrate that HPL increases with the increase of pitch angle, and the availability of RAIM is threatened by the increase of HPL.