## 7 Conclusions  
This paper investigated the vertical guidance performances of different phases of GPS/WAAS user avionics sensor for the next 15-20 years. First, this paper showed the performance of the IOC WAAS user avionics sensor. This paper then showed that upgraded WAAS could provide LPV (50 m VAL) service to the GPS/WAAS user avionics sensors in all of CONUS and in most of Alaska with 38 reference station, three GEOs, and an expanded IGP mask. Second, with the second civil signal (L5) WAAS could provide APV II (20 m VAL) service to the GPS/WAAS user avionics sensors in most of CONUS, Alaska, and Canada. Because the ionosphere is currently the largest error source on GPS, the second civil frequency provided a significant improvement on the GPS/WAAS user avionics sensor performance. An L1-L5 dual-frequency user avionics sensor could estimate the ionospheric delay directly and then subtract this estimation from the pseudorange observations. This direct use of the dual-frequency signals will be more accurate and offer higher availability. Because the new civil signals will have stronger power than current signal, this paper therefore lowered the floor of the residual user receiver noise and multipath error to evaluate if the GPS/WAAS user avionics sensor could meet 12 m VAL (CAT I). Unfortunately the L1-L5 dual-frequency GPS/WAAS user avionics sensor with the enhanced signal model could not meet the CAT I requirement (VPL \(>\) 12 m VAL).  
This paper then analyzed the VDOP improvement from the new satellite constellation composed of the combination of GPS and Galileo. It is shown that the dual-mode (GPS \(+\) Galileo) and dual-frequency WAAS user avionics sensor could provide CAT I service to users in most of CONUS, Alaska, and Canada. Importantly, the VPL values are less than 10 m VAL (a more stringent landing requirement) in most of CONUS and Canada, which is a significant improvement. Finally, this paper also presented the analysis results for the similar aviation navigation performance enhancement to Japanese MSAS and European's EGNOS. The results are equally encouraging.  
This paper treated Galileo satellites the same as GPS satellites, but in different orbits, the MAAST simulation results for the dual-mode (GPS \(+\) Galileo) and dual-frequency WAAS user avionics sensor  
Figure 13: The 99.9% VPL contours of the EGNOS user avionics sensor. The left figure is for the current EGNOS user avionics sensor, and the right figure is for an L1-L5 dual-frequency EGNOS avionics sensor with GPS and Galileo.  
might be optimistic. It does not consider and model the time reference difference between GPS and Galileo signals (group delays) and the coordinate difference between these two systems. These differences might have some impact at performance.  
MAAST was intended as an efficient and effective tool for algorithm development. It is strictly deterministic, and does not model asset failures in a probabilistic manner. Despite these limitations, the results of this paper show that the performance of WAAS can be dramatically improved with the upgraded system which features new additional civil signal (L5), and a new additional satellites constellation (Galileo).