Weighted RAIM for Precision Approach

The use of differential GPS is becoming increasingly popular for real-time navigation systems. As these systems migrate to safety-of-life applications (e.g. precision approach and landing), their integrity becomes more important than their accuracy. One method for increasing both accuracy and integrity is the use of weighting in the navigation solution. This method uses a priori information to weight certain satellites (e.g. those at higher elevation) over other satellites. The accuracy increases because we better use the information available. The integrity increases because satellites that are more likely to introduce error contribute less to the solution. A weighted position solution by itself does not provide sufficient integrity to support precision approach. However, this method can be combined with a weighted form of Receiver Autonomous Integrity Monitoring (RAIM) to increase the level of integrity. RAIM uses redundant measurements to check the consistency of an overdetermined solution. This check is crucial because only a user can detect certain error types (e.g. severe airframe multipath or local interference). A differential reference station can detect many types of errors. However, it is only at the user where all the information is combined. The use of RAIM (or some form of integrity at the user) must be combined together with integrity checking at the reference station to provide the overall safety of the system. Weighted RAIM is investigated for application to Category I precision approach as supported by a Wide Area Augmentation System (WAAS). This paper details how to implement weighted RAIM and how to use geometry selection to guarantee a certain level of protection. Also, we provide information on the availability of these geometries. The results are based upon analysis, Monte Carlo simulation and actual data collected from Stanford University’s wide-area differential GPS network.