## 3 WAAS System Upgrade  
WAAS contains three segments: control segment, space segment, and user segment [3]. The WAAS control segment includes a geographically distributed set of GPS L1 (1575.42 MHz) and L2 (1227.6 MHz) dual-frequency receivers at precisely known reference locations. These receivers continuously monitor all of the GPS satellites, and are called wide area reference stations (WRSs). These WRSs send raw GPS measurements back to the wide area master stations (WMSs) where vector corrections are generated. These vector corrections consist of the satellite ephemeris and clock errors, and a grid of ionospheric delays. The data stream also includes confidence bounds for the corrections and \"Use/Do-Not-Use\" messages to provide integrity. These messages are then passed to the WAAS space segment through a Ground Uplink System (GUS). The current WAAS space segment contains two geostationary satellites (GEOs). These are the PRN-135 (Intelsat, 133\({}^{\circ}\) west) and PRN-138 (Telesat, 107.3\({}^{\circ}\) west). The GEOs broadcast the integrity messages and vector corrections on the same frequency as GPS L1 to user equipment (_i.e._, WAAS avionics sensor). These GEOs also act as additional ranging sources to enhance service availability. Figure 2 is the MAAST simulation result for the VPL contour of IOC (Initial Operational Capability) WAAS. The important parameters used in the simulation are: the twenty four standard GPS satellites constellation defined in the WAAS MOPS, one-degree user grid within the service volume, and thirty-second time steps over a twenty four hour simulation period. To be consistent and for ease of comparison in the results, these simulation parameters of MAAST in the remainder of this paper will remain the same. Figure 2 indicates that a user avionics sensor at each specific location had a VPL equal to or below the value indicated by the color bar 99.9% of the time. In this paper, we are interested in the following aviation navigation services: LPV (VAL (Vertical Alert Limit) = 50 m), APV II (VAL = 20 m), and CAT I (VAL = 12 m) [6]. As shown in Figure 2, most of CONUS has LPV  
Figure 1: The 99.9% LPV availability contours of the current WAAS. The left plot is the MAAST simulation result (a 24 hour average of LPV availability using the actual almanac data of August 15 th, 2009), and the right plot is the actual performance of the WAAS (a three month average of LPV availability, July 1 st to September 30 th, 2009). service of very high availability (equal to 99.9% or greater). However, there are some regions do not meet the LPV requirement (VPL \(>\) 50 m (LPV VAL)). WAAS has been upgraded to better meet the needs of civil aviation users. Since IOC, 13 new reference stations have been added to the WAAS network, with five of them added in Mexico, four in Canada, and another four added in Alaska. These new reference stations expand the WAAS service coverage. By adding new reference stations to Mexico and Canada, WAAS becomes an international system. The operating principle of ionospheric correction of the WAAS is to employ a set of reference stations to monitor the GPS signals so as to come up with corrections. Similar to the Nyquist sampling theorem, a key to the success of the approach is that the reference stations and ionospheric grid points (IGPs) must be dense enough to account for the variation of the ionosphere. Thus, the estimation of the ionospheric delay would be benefited from more reference stations. In addition, the IGP mask limits the WAAS precision approach and landing service region, because WAAS users must obtain the real-time ionospheric corrections in order to perform the vertical guidance [7]. The IGP mask around Alaska and Canada will be expanded to gain more samples of ionosphere observation. The WAAS ionosphere working group is working on the expansion of the IGP mask, and an example of the expanded IGP mask will be used in this paper for the WAAS service volume analysis.