THE IONOSPHERE AND GALILEO - NEW CHALLENGES AND OPPORTUNITIES

Abstract

Galileo is a civil space based navigation system that will provide positioning and timing services with different levels of accuracy, integrity and availability. In combination with tropospheric delay and multipath, wave propagation effects in the ionosphere reduce the performance of Galileo. Since the used L-band signals may suffer ionospheric range errors up to about 100 meters, a proper modelling and mitigation of propagation effects is required. Whereas multi-frequency users take benefit of the dispersive nature of the ionosphere, single frequency users will apply an ionospheric model to estimate the range error for correction. Combining the phase measurements at two different frequencies, ionospheric range errors may be first-order corrected. For achieving full gain of the high precision GALILEO service, higher order propagation effects have to be taken into account. Due to the availability of more than two frequencies, new options of mitigating higher order effects exist. As learned from GPS, small scale irregularities of the electron density in the ionosphere may cause severe degradation of L-band signal strength causing occasionally loss of lock. Consequently, the signal availability is reduced. Localized severe spatial gradients and rapid changes of ionization may violate ionospheric error bounds in space based augmentation systems such as the European Geostationary Navigation Overlay Service (EGNOS) or local Ground Based Augmentation Systems (GBAS). Safety of Life (SoL) applications require reliable detection of ionospheric threats, otherwise loss of system integrity results. To reliably detect ionospheric distortions as required by the system is a challenging task for ionospheric scientists in monitoring, modelling and predicting the ionospheric behaviour. Fortunately, GALILEO itself offers new opportunities for monitoring and studying ionospheric effects in an unprecedented matter. In combination with dual frequency signals transmitted by other Global Navigation Satellite Systems (GNSS) such as GPS and GLONASS, the number of available data increases enormously both for ground and space based sounding techniques. Besides the high precision of GALILEO signals also the availability of a third frequency offers new possibilities for improved estimation of the Total Electron Content (TEC) of the ionosphere. The high potential for ionospheric sounding by GNSS techniques is demonstrated.