## 1 Introduction  
Presently, the only fully operational Global Navigation Satellite System (GNSS) is the Global Positioning System (GPS), which was developed, implemented, and is operated by the US Department of Defense (DoD) to provide position, velocity, and time information to users worldwide [1,2]. GPS currently has two operational signals, L1 at a center frequency of 1575.42 MHz and L2 at a center frequency of 1227.6 MHz. The L1 signal is modulated by both a 10.23 MHz clock rate precision P(Y) code and by a 1.023 MHz clock rate C/A code. On most GPS satellites, the L2 signal is modulated by only the P(Y) code. The P(Y) code is for authorized users and the C/A code is for civil users [2], therefore, the current civil users can only access the L1 C/A. A stand-alone GPS user can typically estimate location with an accuracy of 10 meters [1]. The GNSS is undergoing substantive changes that will enhance its capabilities in all applications. This future GNSS includes three key elements:  
\(\bullet\) _GNSS Augmentation Systems,_ which monitor the GNSS satellites and provides error bounds to safety-critical users in real time. The first of these GNSS Augmentation Systems is the Wide Area Augmentation System (WAAS) [3] in the United States, which achieved Initial Operational Capability (IOC) on July 10, 2003. WAAS currently monitors the GPS constellation to provide differential corrections and a certified level of integrity. The corrections will improve the accuracy of the system, and more importantly, the integrity will open the door for widespread aviation use. There are many countries working on similar systems. The European Space Agency (ESA) is working on the European Geostationary Navigation Overlay Service (EGNOS) which became operational on October 01, 2009, and Japan deploys the MTSAT-based Satellite Augmentation System (MSAS), which was commissioned on September 27, 2007. Taiwan Civil Aeronautics Administration (CAA) plans to replace the current ground based assisting navigation equipment with the augmented GNSS within the next few years. \(\bullet\) _GPS modernization_ will add two new civil signals to the GPS positioning and timing services. A second civil signal (L2C) will be added at the second GPS frequency, L2, at 1227.6 MHz. A third civil signal, L5, will be added at a lower frequency 1176.45 MHz [4]. Both L1 and L5 are in Aeronautical Radio Navigation Services (ARNS) bands, and are for safety-of-life services. On the other hand, L2 is not in an ARNS band and is for non-safety critical applications. The use of these additional frequencies is expected to enhance the performance of GPS. These three civil GPS signals will improve the performance of the GPS receiver, and will enable a new algorithm to estimate and mitigate the ionospheric delay, which currently is the largest obstacle for the GPS to become the primary navigation aid in civil aviation. \(\bullet\) _New GNSS Constellations_ are under development by many countries worldwide, for instance, Galileo is the Europe's contribution to a global navigation satellite infrastructure [5]. Using both infrastructures (GPS and Galileo) in a coordinated fashion offers real advantages in terms of availability and in terms of security, should one of the two systems becomes unavailable. Similarly, China is working on the Compass system and Russia is updating the GLONASS. This paper will use Galileo as an example to evaluate the performance improvement from the additional GNSS constellation. While the Federal Aviation Administration (FAA) WAAS works well under nominal conditions, it is susceptible to the ionospheric disturbance and to the satellite outages. Because of the addition of the new frequencies (L2 and L5) and the new constellation (Galileo), WAAS should be modernized to take advantage of the modernized GPS and Galileo. Therefore, the objectives of this paper are to assess the following two questions.