Interoperation and Integration of Satellite Based Augmentation Systems

In recent years there has been widespread growth in the independent development of Satellite Based Augmentation Systems (SBASs). The Federal Aviation Administration’s Wide Area Augmentation System (WAAS) will be the first of multiple systems to become operational in the near future. Of current interest is interoperability of these physically separate, independent SBASs. In particular, designers are investigating the type, amount, and methodology of information shared between systems. There are interface issues related to a user passing from one SBAS to another. Among these is the degradation of the ephemeris and clock integrity bounds for users operating outside of the SBAS network of reference stations. While some degradation in performance and integrity limits can be expected, it is shown that the ability to verify the quantity of error can deteriorate very rapidly. Both geometric and dynamic ephemeris estimates will suffer this increase in uncertainty due to dramatic decreases in GPS satellite observability. The effect of adding periphery stations to reduce this degradation is demonstrated through use of the National Satellite Test Bed (NSTB) network. Distributed systems have the ability to provide complementary and cooperative data while retaining specific levels of independence. This extends as well to worldwide interoperability of continuous navigation services and in the standardization of international aviation navigation aids. A risk/benefit analysis of SBAS interoperability is presented. The analysis presents strategies of information integration that optimizes overall user integrity for the typical SBAS architecture by monitoring data sent from systems external to the SBAS. The performance of this integrity monitoring is characterized. The conceptual distributed design is supported by results from the NSTB network. INTRODUCTION To improve the accuracy, availability and integrity of GPS the FAA is currently developing the Wide-Area Augmentation System (WAAS) which is an example of a Space Based Augmentation System (SBAS) as depicted in Figure 1. This will be accomplished by utilizing measurements from a network of GPS wide-area reference stations located throughout the coverage region. These measurements will be gathered by a wide-area master station through a communications network to compute corrections to GPS errors that are common at each reference station (or a subset of reference stations). Among these errors are ionospheric delay, satellite clock and broadcast satellite ephemeris (position) uncertainties. These corrections will be transmitted through an uplink center to one or more geosynchronous spacecraft which will broadcast the messages to users at the GPS L1 frequency using Pseudo-Random Noise (PRN) codes not in use by and orthogonal to GPS codes. Navigation Satellites Correction Satellite(s) Users Correction Uplink(s) Reference Stations Communications Network Master Station(s) Figure 1. SBAS Architecture. The users operating within a SBAS can be in one of four modes: 1) Precision approach (three-dimensional guidance in close proximity to an airport); 2) Non-precision approach (navigation in close proximity to an airport); 3) Terminal (navigation at an airport); and 4) En-Route (navigation between departure and arrival airports). Each of these modes has prescribed integrity and availability limits that are specified by the SBAS within its primary design area or service volume. Different groups around the world are currently implementing SBASs. Currently these are the European Geostationary Navigation Overlay System (EGNOS) [1], the Japanese MTSAT Satellite Augmentation System (MSAS) [2] and the Wide Area Augmentation System (WAAS) [3] in the United States. Stanford University is part of the National Satellite Test Bed (NSTB) [4]. The NSTB is a network of GPS reference receivers located throughout the United States with additional sites in Canada and Europe. The NSTB is being used as a research and development system to test concepts and algorithms for WAAS. We collect data through the NSTB network and process the measurements for corrections for both passive reference stations (not used for corrections) as well as being able to transmit this data to independent users. Processing can either be applied to users in real-time or stored for post-processing. This paper will study the benefits of integrated space based augmentation systems of the GPS constellation. First an overview of each of the major SBASs under development today will be given. A risk/benefit analysis of the different types of SBAS interoperation will be presented. The paper will then proceed to show the correction availability of users in the individual systems versus the combined cases. Since the NSTB has wide geographic coverage it is possible to study the effects of distributed SBAS architectures by breaking the NSTB into parts and treating each part as a separate system. The last part of the paper will break the NSTB into two major groups of stations to compare the results for both UDRE and border-case user accuracy for both the distributed and combined systems.