Optimizing WAAS Accuracy/Stability For a Single Frequency Receiver

The Wide Area Augmentation System (WAAS) provides accuracy (95%) to better than 1m in horizontal and 2m in vertical, which is quite an improvement compared to the stand-alone GPS accuracy and well within the WAAS specification. However, WAAS was designed and optimized for integrity rather than accuracy. Integrity requirements caused the selected carrier-smoothing time to be short and the weighting matrix for least-squares solutions to use overbounding variances. These choices are suboptimal for accuracy/stability. Even though the WAAS accuracy/stability is not optimized, airborne users accept this trade-off because integrity is critically related to safety. But, if one wants to use the WAAS for ground navigation, the same integrity requirements may not be necessary. Therefore, one can improve the WAAS accuracy/stability by relaxing the strict integrity specifications. This paper investigates how to use the existing WAAS broadcast information to better optimize for accuracy and stability. First, we propose a new weighting matrix which appropriately represents the most likely signal conditions instead of using overbounding variances. Second, a method for dynamically adjusting the carrier smoothing time is investigated, which determines a right smoothing time based on the measurements of the rate of change of ionosphere. In addition to these methods, the Range Rate Correction (RRC) is removed, which is no longer necessary now that Selective Availability (SA) is permanently turned off. It has been observed that the RRC introduces noise with a 12 second period and15~30 cm amplitudes in position domain. Overall, we expect that these methods improve the WAAS accuracy/stability and open a new way of using the WAAS for accuracyoriented applications.