## 1 Introduction  
The Global Positioning System (GPS) provides positioning, navigation and timing services to around 400 million users in sea, air, terrestrial, and space applications [1]. In order to improve the performance of GPS-based navigation system providing support for the very demanding requirements of aircraft approach operations, various augmentation systems are being developed [2-4]. One such system is the Wide Area Differential GPS (WADGPS) system. The WADGPS system utilizes a geographically distributed network of reference receivers at precisely known locations throughout the service region, and these reference receivers, which are called Reference Stations (RSs), continuously monitor all GPS satellites and their propagation environments in real time. These RSs send raw GPS measurements back to the WADGPS Master Station (MS) where the WADGPS messages are generated. The WADGPS system provides both differential corrections to improve accuracy and associated confidence bounds to ensure integrity. The corrections will improve the accuracy of the system and more importantly, the integrity will open the door for widespread aviation use. The WADGPS concept is summarized in Figure 1. The Satellite Based Augmentation System (SBAS) is an extension of the WADGPS concept and there are several SBASs which are developing around world to enhance aviation navigation performance. For instance, the MTSAT-based Satellite Augmentation System (MSAS) is deployed in Japan, the European Space Agency (ESA) is working on the European Geostationary Navigation Overlay Service (EGNOS), and the Wide Area Augmentation System (WAAS) is under development in the United States [4]. Among them, WAAS began operation on July 10, 2003, MSAS was commissioned for aviation use on September 27, 2007, and EGNOS open service started on October 1, 2009. However, in Taipei Flight Information Region (FIR), there is no SBAS service available for aviation users. Therefore, the objective of this paper is to implement a WADGPS system to investigate the augmented GPS performance for civil aviation users within the Taipei FIR.  
Figure 1: The Wide Area Differential GPS.  
Similar to this work, in the early 1990s, the Federal Aviation Administration (FAA) implemented the National Satellite Test Bed (NSTB) as a WAAS prototype system to ensure the success of the WAAS. In 2006, the authors co-developed one of the RSs of the Asia-Pacific Economic Cooperation (APEC) Global Navigation Satellite System (GNSS) test bed which is a WADGPS-like system to conduct the preliminary analysis of the SBAS performance within Asia-Pacific [5]. With the experience of developing the WADGPS RS, this paper further implements the WADGPS master station to assess the performance of various architectures for the WADGPS. The e-GPS observation stations operated by the Taiwan Ministry of Interior (MOI) are used as the WADGPS RSs. Our focus will be one particular aircraft approach procedure known as Lateral NAVigation (LNAV), and the Required Navigation Performance (RNP) for LNAV is summarized in Table 1[6, 7]. The metrics used to quantify the performance will be the positioning accuracy and the service quality at a pre-defined and fixed level of integrity.  
Accordingly, this paper is organized as follows: In Section 2, we define the metrics that are normally used to quantify the performance of GPS-based aircraft approach systems. In Section 3, we describe the details of the WADGPS architecture. Section 4 shows the main processes of the WADGPS master station. In Section 5, we first validate the implementation of the WADGPS using the U.S. NSTB data, and we then conduct several experiments to evaluate the LNAV performance of the WADGPS within Taipei FIR. Finally, Section 6 presents the summary and concluding remarks.