Availability and Reliability Advantages of GPS/Galileo Integration

The performance of a Global Navigation Satellite System (GNSS) 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. 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 Dilution of Precision (HDOP) as a measure of availability and accuracy and maximum Horizontal Position Error (HPE) due to one undetected blunder as a measure of reliability. 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 the two systems together is shown to provide major improvements in accuracy and reliability. A local simulation comparing the availability of GPS and Galileo in an urban environment is also presented. The use of GPS and Galileo together is shown to provide the necessary satellite availability to obtain a navigation solution in extreme masking environments