Improved model for correcting the ionospheric impact on bending angle in radio occultation measurements
Abstract. 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 ( κ ). The variation of κ 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). κ 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 κ is
approximately linear with height. A simple κ 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 × 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 a
small band of locations around the solar terminator. In the daytime, the
scalar 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, in
this case, the bending angles are already small and the impact of the choice
of κ is less pronounced.