Improved model for correcting the ionospheric impact on bending angle in radio occultation measurements

The standard approach to remove the effects of the ionosphere from neutral atmosphere GPS radio occultation measurements is to estimate a corrected bending angle from a combination of the L1 and L2 bending angles. This approach is known to result in systematic errors and an extension has been proposed to the standard ionospheric correction that is dependent on the squared L1 / L2 bending angle difference and a scaling term (kappa). The variation of kappa with height, time, season, location and solar activity (i.e. the F10.7 flux) has been investigated by applying a 1-D bending angle operator to electron density profiles provided by a monthly median ionospheric climatology model. As expected, the residual bending angle is well correlated (negatively) with the vertical total electron content (TEC). kappa is more strongly dependent on the solar zenith angle, indicating that the TEC-dependent component of the residual error is effectively modelled by the squared L1 / L2 bending angle difference term in the correction. The residual error from the ionospheric correction is likely to be a major contributor to the overall error budget of neutral atmosphere retrievals between 40 and 80 km. Over this height range kappa is approximately linear with height. A simple kappa model has also been developed. It is independent of ionospheric measurements, but incorporates geophysical dependencies (i.e. solar zenith angle, solar flux, altitude). The global mean error (i. e. bias) and the standard deviation of the residual errors are reduced from -1.3 x 10(-8) and 2.2 x 10(-8) for the uncorrected case to -2.2 x 10(-10) rad and 2.0 x 10(-9) rad, respectively, for the corrections using the kappa model. Although a fixed scalar kappa also reduces bias for the global average, the selected value of kappa (14 rad(-1)) is only appropriate for a small band of locations around the solar terminator. In the daytime, the scalar kappa is consistently too high and this results in an overcorrection of the bending angles and a positive bending angle bias. Similarly, in the nighttime, the scalar kappa is too low. However, in this case, the bending angles are already small and the impact of the choice of kappa is less pronounced.