Atmospheric sounding by GNSS radio occultation: An analysis of the negative refractivity bias using CHAMP observations

Validation studies of current GPS radio occultation experiments using meteorological analyses consistently report on a negative refractivity bias in the lower troposphere. It is shown that refractivity profiles obtained from Doppler-inverted bending angle profiles not only deviate significantly within zones of multipath propagation but also depend on the selected end point of the occultation signal in the Earth’s radio shadow. End-to-end simulations including the GPS receiver’s signal tracking process suggest that receiver-induced phase deviations contribute to this observed bias as well. We propose a heuristic retrieval algorithm based on the canonical transform and the sliding spectral technique that seems less susceptible to tracking phase errors than the canonical transform method. The approach is described using simulated profiles and validated on the basis of 4221 CHAMP occultations recorded between 14 May and 10 June 2001. Compared to the canonical transform results the heuristic method results in a significantly smaller tropospheric refractivity bias at low latitudes.