| The Global Positioning System (GPS) is a satellite navigation system in which a user receives signals from at least four satellites and then can estimate the location of the receiver and the offset of the receiver clock from GPS reference time. The system is operated by the Department of Defense, but part of the system has been made available to the civil community. Currently, the accuracy and integrity of this system are being examined to determine the suitability of GPS for use in the National Airspace. The intrinsic accuracy which has been made available to the civil community has proven to be better than was originally planned and is quite satisfactory for civil aviation users.; The question which remains unanswered pertains to the integrity of the system; that is, can it be assured that an alarm will be raised if the user's horizontal error exceeds a specified protection level due to a satellite signal malfunction. The system has many built-in features which monitor the signal quality but to satisfy civil aviation specifications (pertaining mainly to safety), an independent verification of the system integrity is required. The current proposed techniques for performing the integrity check are either based on a civilian ground monitoring system and communication link or on algorithms performed inside the GPS receiver which perform a consistency check of redundant satellite signals. The latter approach is sometimes referred to as receiver autonomous integrity monitoring (RAIM).; This dissertation begins by presenting a review of the current RAIM techniques and then a new scheme is proposed which has some advantages over the previous approaches. It was found that even with the proposed 21-satellite configuration, which always yields six to nine satellites in view, it is possible that poor integrity geometries can exist where failures in certain satellites cannot be observed in the consistency check. It appears that this fundamental limitation will require the use of aiding information independent of the GPS signals if continuous GPS integrity is to be provided 100% of the time. |
