Simulation Study Of Residual Ionospheric Errors In GNSS Radio Occultation

The global navigation satellite systems (GNSS) radio occultation (RO) techniquehas been widely used to observe the atmosphere for applications of numerical weatherprediction and global climate monitoring. The ionosphere is a major error source inGNSS RO measurements and an ionosphere-free linear combination ofdual-frequency bending angles derived from RO is commonly used to mainly removethe first-order ionospheric effect. However, the residual ionopheric error (RIE) is stillconsiderable, thus it needs to be further mitigated for high accuracy applications,especially in upper air where the RIE is more severe. To effectively mitigate the RIEeffect, characterization and quantification of the bending angle RIEs are important forobtaining benchmark-quality upper-air RO retrievals.A comparison study of the bending angle errors in high solar activity periods andlow solar activity periods was conducted by using the European centre for mediumrange weather forecasts (ECMWF) model and constellation observing system formeteorology, ionosphere, and climate (COSMIC) data. The results show that thebending angle RIE is a main error source in the upper stratosphere (US) and lowermesosphere (LM).End-to-end simulations and a detailed analysis of single-event RIEs have beenperformed to investigate the characteristics and magnitude of the bending angle RIEsin various ionospheric conditions. The results illustrate that the bending angle RIE issignificant in the mesosphere (MS) and US; its magnitude is dependant upon localtime, the intensity of solar activity and the direction of the RO plane; and ionosphericdisturbances can enlarge the bending angle RIEs by several to more than20times.This research has for the first time quantified daily-zonal-mean bending angleRIEs in the impact height layers of the lower stratosphere (LS), US, LM and uppermesosphere (UM) using end-to-end simulation. A global ensemble of one-day ROevents were simulated and divided into six geographic zones named global (GLO),north hemisphere high latitude (NHH), north hemisphere middle latitude (NHM),equatorial day time (EDT), south hemisphere middle latitude (SHM) and southhemisphere high latitude (SHH). In the simulation, the MSIS-90atmospheric modeland the3D NeUoG ionospheric model were used. The simulated bending angleprofiles were compared with the reference—the ones simulated using the neutralatmosphere only, for calculating the biases, standard deviations and uncertainties of the bending angle RIEs. The variations of the bending angle RIEs with solar activity,latitudinal region, and with and without the assumption of ionospheric sphericalsymmetry were assessed.The results show that the layer-average bending angle RIE biases in the US, LMand UM height layers and in all the six zones have an obvious negative tendency andthe magnitude of their absolute values increases with the rise of solar activity level.Comparison of the bending angle RIE biases of the six zones indicated that themaximum layer-average RIE biases, located in the EDT zone, in the UM, LM and USlayers are0.048μrad,0.041μrad and0.032μrad respectively. The minimumlayer-average RIE biases are in the SHH zone with the values close to zero. Theseresults suggest that the bending angle RIEs tend to have negative systematicbiases.This research has significance since these simulation results can be a referencefor the calibration of bending angles derived from real RO observations in future‘s ROdata processing.In addition, the mechanism of the bending angle RIEs was also investigatedusing the3D ray tracing technique and those RO events with exceptionally large RIEswere analyzed. The results indicate that the asymmetry of ionospheric electron densityalong inbound‘and outbound‘ray paths is the major cause of the exceptionally largebending angle RIEs.