Inversion of GPS meteorology data

The GPS meteorology (GPS/MET) experiment, led by the Universities Corporation for Atmospheric Research (UCAR), consists of a GPS receiver aboard a low earth orbit (LEO) satellite which was launched on 3 April 1995. During a radio occultation the LEO satellite rises or sets relative to one of the 24 GPS satellites at the Earth's horizon. Thereby the atmospheric layers are successively sounded by radio waves which propagate from the GPS satellite to the LEO satellite. From the observed phase path increases, which are due to refraction of the radio waves by the ionosphere and the neutral atmosphere, the atmospheric parameter refractivity, density, pressure and temperature are calculated with high accuracy and resolution (0.5±1.5 km). In the present study, practical aspects of the GPS/MET data analysis are discussed. The retrieval is based on the Abelian integral inversion of the atmospheric bending angle pro®le into the refractivity index pro®le. The problem of the upper boundary condition of the Abelian integral is described by examples. The statistical optimization approach which is applied to the data above 40 km and the use of topside bending angle pro®les from model atmospheres stabilize the inversion. The retrieved temperature pro®les are compared with corresponding pro®les which have already been calculated by scientists of UCAR and Jet Propulsion Laboratory (JPL), using Abelian integral inversion too. The comparison shows that in some cases large di€erences occur (5 K and more). This is probably due to di€erent treatment of the upper boundary condition, data runaways and noise. Several temperature pro®les with wavelike structures at tropospheric and stratospheric heights are shown. While the periodic structures at upper stratospheric heights could be caused by residual errors of the ionospheric correction method, the periodic temperature ̄uctuations at heights below 30 km are most likely caused by atmospheric waves (vertically propagating large-scale gravity waves and equatorial waves).