Global Availability and Reliability Assessment of the GPS and Galileo Global Navigation Satellite Systems

Abstract

The performance of a Global Navigation Satellite System can be quantified by availability, accuracy, and reliability. The deployment of Galileo by the European Union will provide double the number of navigation satellites currently available to users and offer improvements in accuracy and reliability. This paper briefly describes the design of the proposed Galileo system in comparison with the existing Global Positioning System (GPS). Availability and accuracy are then defined and statistical reliability theory is reviewed. Worldwide availability, accuracy, and reliability estimates for GPS, Galileo, and the combined GPS/Galileo system are obtained by software simulation. The simulation results are presented using Horizontal and Vertical Dilution of Precision as measures of availability and accuracy and maximum Horizontal Position Error due to one undetected blunder as a measure of reliability. The validity of these quantities as measures of accuracy and reliability is discussed and compared with other measures. The proposed 30 satellite, 3 orbital plane Galileo constellation shows a slight improvement over the existing GPS constellation when low-elevation masks are employed. At higher mask angles GPS slightly outperforms Galileo. The use of GPS and Galileo together is shown to provide both improved accuracy and reliability, especially in extreme masking environments.