Ionospheric delay estimation in the European Global Navigation Overlay Service

The European Geo-stationary Navigation Overlay Service (EGNOS) was conceived mainly to improve the integrity and accuracy of the GPS and GLONASS satellite navigation systems. This paper describes the basic principles behind the dual-frequency and all single-frequency ionospheric delay estimation techniques. The main objective of the paper is to present a brief summary of new results illustrating the potential performance of the single-frequency techniques, by providing comparisons with equivalent dual-frequency results. These results were obtained by processing data recorded on a number of days at different locations in Europe. Line-of-sight (i.e. slant-range) delay estimates were computed using both the dual-frequency and a well-known single-frequency method. In order to make sensible comparisons, measures had to be taken to overcome the problems arising from the unknown L1-L2 satellite and receiver group delay biases. One method for estimating the L1-L2 biases which was found to be most useful was based on the SCORE (self calibration of pseudo-range errors) method, developed by Bishop et al. [1995]. The basic principles of this technique are discussed