Second-order ionospheric effects on ionospheric electron density estimation from GPS Radio Occultation

The GPS ionospheric delay was considered as an error source, and now it can be determined as the useful ionospheric parameter, e.g., total electron content (TEC) and electron density, particularly from high spatial resolution GPS Radio Occultation (RO). However, second-order ionospheric delay was normally ignored in ionospheric electron density estimation from GPS RO. In this paper, the second-order ionospheric effect on ionospheric electron density estimation is investigated from COSMIC GPS Radio Occultation. Firstly, we use the ionospheric Nequick2 model and IGRF11 geomagnetic field model to calculate the second-order ionospheric delay and then analyze the characteristics of this error. Results show that the second-ionospheric delay has large effect with up to 800 electron/cm
^-3
, which should be corrected in high-precision ionospheric electron density estimation from GPS radio occultation. The second-order ionospheric error affects the electron density at 250km-300km obviously. In addition, the second-order ionospheric effects are investigated at different RO azimuths, which show the maximum negative value at around 0° and the maximum positive value at around ±150°. Some possible errors and mechanism are further discussed.