Estimates of the precision of GPS radio occultation bending angles in the neutral atmosphere from the COSMIC/ FORMOSAT-3 Mission

The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) / Formosa Satellite 3 (FORMOSAT-3) is a six-satellite radio occultation (RO) mission that was successfully launched in mid-April, 2006. During the first months after the launch the separation between each satellite pair 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 estimate the precision of RO by examining the differences of the retrieved parameters from the collocated occultation pairs. An analysis of ~4,700 pairs of collocated COSMIC occultations from Apr – Dec 2006 shows an RMS difference of bending angles of about 0.3% at an impact height of 20 km (for tangent point separations less than 10km). The atmospheric structure function (observational differences versus tangent point separation distance) from this analysis of collocated occultations will also be presented that considers differences of RO bending angles due to impact height and latitude. In addition, estimates of precision of the atmospheric boundary layer depth and tropopause height and temperature will be presented. Although the analysis of collocated occultations only provides estimates of precision and not accuracy, results from this study provide RO error characteristics that will be useful for the assimilation of these data by numerical weather models and also potentially by climate models in the future.