Prediction of the time accuracy and integrity of GPS timing

The application of Receiver Autonomous Integrity Monitoring (RAIM) algorithms internal to the GPS sensor have added an extra level of system reliability to timing applications that cannot tolerate system outages. This paper addresses the time integrity issue from GPS, introduces an algorithm that has been implemented in a low cost GPS core receiver, and provides analysis of the algorithm performance under unusual satellite conditions. The algorithm detects and removes from the solution those satellites that exceed a pre-determined time residual threshold. Scientific error simulations have been used to characterize the detection and isolation performance of the algorithm, and are reported on. The threshold selection is designed to support a detection and isolation probability of at least 99.99%, and a maximum false alarm rate no greater than 1 false alarm every 5.7 days. A multi-channel GPS satellite simulator was used to generate typical GPS satellite outages, and the performance of the algorithm was tested and results are reported. Accuracy of GPS with and without the T-RAIM algorithm is reported as well as the receiver's ability to protect the IPPS time reference output pulse from the influence of the failed satellite. One of the by-products of the T-RAIM algorithm is its ability to predict the time error performance of the receiver under the influence of SA with and without failed satellites. The paper reports on the algorithm's ability to predict the overall system time accuracy for possible exploitation by the end user.