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. The standard approach to remove theeffects of the ionosphere from neutral atmosphere GPS radio occultationmeasurements is to estimate a corrected bending angle from a combination ofthe L1 and L2 bending angles. This approach is known to result in systematicerrors and an extension has been proposed to the standard ionosphericcorrection that is dependent on the squared L1  ∕  L2 bending angle differenceand a scaling term ( κ ). The variation of κ with height, time,season, location and solar activity (i.e. the F10.7 flux) has beeninvestigated by applying a 1-D bending angle operator to electron densityprofiles provided by a monthly median ionospheric climatology model. Asexpected, the residual bending angle is well correlated (negatively) with thevertical total electron content (TEC). κ is more strongly dependent on the solar zenith angle,indicating that the TEC-dependent component of the residual error iseffectively modelled by the squared L1  ∕  L2 bending angle difference term inthe correction. The residual error from the ionospheric correction is likelyto be a major contributor to the overall error budget of neutral atmosphereretrievals between 40 and 80 km. Over this height range κ isapproximately linear with height. A simple κ model has also beendeveloped. It is independent of ionospheric measurements, but incorporatesgeophysical dependencies (i.e. solar zenith angle, solar flux, altitude). Theglobal mean error (i.e. bias) and the standard deviation of the residualerrors are reduced from - 1.3 × 10 - 8 and 2.2 × 10 - 8 for the uncorrected case to - 2.2 × 10 - 10  rad and 2.0 × 10 - 9  rad, respectively, for the corrections using the κ model.Although a fixed scalar κ also reduces bias for the global average,the selected value of κ (14 rad −1) is only appropriate for asmall band of locations around the solar terminator. In the daytime, thescalar κ 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 κ is too low. However, inthis case, the bending angles are already small and the impact of the choiceof κ is less pronounced.

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