HIGHER ORDER IONOSPHERIC ERRORS IN MODERNIZED GPS AND FUTURE GALILEO SYSTEMS

Modernized GPS and future Galileo systems will transmit a third frequency allowing to cancel out the first and second order ionospheric terms in the refractive index by double differencing carrier phases or code pseudoranges. However, differential bending of the signal and third order ionospheric term are not fully removed in this approach. This work estimates the magnitude of various higher order ionospheric errors using a large number of ionospheric vertical profiles reconstructed from CHAMP-GPS radio occultation measurements. Our investigation shows that triple-frequency residual range errors become significant at low elevation angles (< 15°) and at high level of total electron content. The triple-frequency residual range error reaches up to 1 and 4 cm at 5° elevation angle in carrier-phase and pseudo-range measurements, respectively. Different approximation formulas have been discussed to mitigate higher order ionospheric errors in operating satellite positioning systems. The Galileo system will allow four frequencies to be used for higher order ionospheric corrections. It has been found that ionospheric effects are successfully removed in quadruple-frequency measurements, i.e., no ionospheric correction is required in the range estimation. However, quadruple-frequency measurements for the ionospheric correction are not practically useful since the measurement noise exceeds the ionospheric correction