Abstract

A fundamental observable that will be useful for future long-term climate studies is the GPS radio occultation (RO) bending angle. In this study we attempt to better understand and quantify random and systematic bending angle differences in collocated occultation pairs from the Constellation Obse... Show moreA fundamental observable that will be useful for future long-term climate studies is the GPS radio occultation (RO) bending angle. In this study we attempt to better understand and quantify random and systematic bending angle differences in collocated occultation pairs from the Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) / Formosa Satellite 3 (FORMOSAT-3) mission. An important feature of the COSMIC six-satellite constellation is that immediately after launch, the six satellites were clustered together in one orbit. During the first months after the launch the separation between the satellites was about 1-2 s in time (about 10 km along the orbit) and gradually increased with time. This small separation allowed for pairs of closely collocated occultations from one GPS satellite with almost parallel occultation planes. Thus, this COSMIC cluster mode gives a unique opportunity to evaluate RO bending angle errors by examining the differences of the retrieved profiles from the collocated occultation pairs. An initial analysis of 4,700 pairs of collocated COSMIC (satellite #3 - satellite #4) occultations from Apr - Dec 2006 shows an RMS difference of bending angles of about 0.3 % at an impact height of 20 km and near zero mean differences from 5 to 40 km impact heights (for tangent point separations less than 10km). This analysis also shows mean bending angle differences of about 1 % at 50 km impact height and about 0.5 % at impact heights less than 5 km. An investigation into the causes of these mean differences will be presented. Additional results will be presented that consider variations of RO bending angle precision due to impact height, latitude, local time, tangent point separation distance, azimuth angle of occultation plane, GPS satellite oscillator type and near real-time versus post-processed analyses. Although the analysis of collocated occultations only provides estimates of precision and not accuracy, results from this study provide RO bending angle error characteristics that will be useful for the assimilation of these data by numerical weather models and also by climate models in the future. Show less