Inversion error analysis and validation of GPS/M

Authors: Steiner A K Kirchengast G Ladreiter H P

Publish Date: 1999/01/31

Volume: 17, Issue: 1, Pages: 122-138

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Abstract

Abstract The global positioning system meteorology GPS/MET experiment was the first practical demonstration of global navigation satellite system GNSSbased active limb sounding employing the radio occultation technique This method measures as principal observable and with millimetric accuracy the excess phase path relative to propagation in vacuum of GNSStransmitted radio waves caused by refraction during passage through the Earths neutral atmosphere and ionosphere in limb geometry It shows great potential utility for weather and climate system studies in providing an unique combination of global coverage high vertical resolution and accuracy longterm stability and allweather capability We first describe our GPS/MET data processing scheme from excess phases via bending angles to the neutral atmospheric parameters refractivity density pressure and temperature Special emphasis is given to ionospheric correction methodology and the inversion of bending angles to refractivities where we introduce a matrix inversion technique instead of the usual integral inversion The matrix technique is shown to lead to identical results as integral inversion but is more directly extendable to inversion by optimal estimation The quality of GPS/METderived profiles is analyzed with an error estimation analysis employing a Monte Carlo technique We consider statistical errors together with systematic errors due to upperboundary initialization of the retrieval by a priori bending angles Perfect initialization and properly smoothed statistical errors allow for better than 1 K temperature retrieval accuracy up to the stratopause No initialization and statistical errors yield better than 1 K accuracy up to 30 km but less than 3 K accuracy above 40 km Given imperfect initialization biases 2 K propagate down to below 30 km height in unfavorable realistic cases Furthermore results of a statistical validation of GPS/MET profiles through comparison with atmospheric analyses of the European Centre for Mediumrange Weather Forecasts ECMWF are presented The comparisons indicate the high utility of the occultation data in that very good agreement of upper troposphere/lower stratosphere temperature better than 15 K rms 05 K bias is found for a region Europe+USA where the ECMWF analyses are known to be good but poorer agreement for a region Southern Pacific where the analyses are known to be degraded Key words Atmospheric composition and structure pressure density and temperature Meteorology and atmospheric dynamics instruments and techniques  Radio science remote sensing

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