Modernizing WAAS

The Wide Area Augmentation System (WAAS) became operational on July 10, 2003. Currently this system provides en-route through non-precision approach guidance. Further, the signals are capable of providing vertical guidance to bring an aircraft within 250 feet of the ground. WAAS offers significant benefit to both aviation and non-aviation users. However, the vertical guidance availability is vulnerable to disturbances in the ionosphere and to GPS satellite outages. Fortunately, solutions to these limitations are well underway in the form of GPS modernization and Galileo. GPS modernization will provide two civil frequencies that enable users to directly estimate and remove ionospheric effects. Galileo will provide many additional ranging sources that will make the user more robust to the loss of an individual one. Naturally, WAAS should also be modernized to take advantage of these new signals. We investigate different methods to incorporate the signals and model their improvement to the aviation user. The best performance comes to a fully modernized user who can incorporate both civil frequencies and both constellations (GPS and Galileo). However, there is still significant benefit to legacy users who can only receive single frequency GPS signals. By upgrading the WAAS Reference Station (WRS) receivers to measure Galileo signals we can double our sampling of the ionosphere. The increased sampling translates into smaller broadcast confidence values on the single frequency ionospheric corrections. These lower values lead to higher availability. Similarly, the L5 signal has better noise properties and can be acquired at a lower elevation angle. This leads less uncertainty in the ionospheric measurements and to smaller confidence values. Before the modernized GPS and the Galileo constellations are complete, we can take advantage of the first new satellites. Users who can recognize the signals can begin to take advantage of them if proper correction information is broadcast. Legacy users will begin to see confidences improve as the new signals are mixed in with the old. The airborne noise and multipath confidence factor will grow in importance. Currently, it is the smallest term in the protection level calculation. For a dual frequency user it may become the largest. Efforts are underway to better characterize this airborne environment. The level of service offered by the end state system will depend on the final curve used to bound the error.