Assessment of precision in ionospheric electron density profiles retrieved by GPS radio occultations

ALEXANDER, P.; A. DE LA TORRE,; R. HIERRO; P. LLAMEDO

ADVANCES IN SPACE RESEARCH

ELSEVIER SCI LTD

Lugar: Amsterdam; Año: 2014

0273-1177

The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) is a six satellite radio occultation mission that was launched in April 2006. The close proximity of these satellites during some months after launch provide a unique opportunity to evaluate the precision of Global Positioning System (GPS) radio occultation (RO) retrievals of ionospheric electron density from nearly collocated and simultaneous observations. RO data from 30 consecutive days during July and August 2006 are divided in ten groups according to daytime or nighttime and latitude. In all cases the best precision values (about 1%) are found at the F peak height, which slightly degrade upwards. For all daytime groups it can be seen that electron density profiles above about 120 km height exhibit a substantial improvement on precision. Nighttime groups are rather diverse, in particular the precision becomes better than 10% above different levels between 120 and 200 km height. In our overall results we have seen that up to 100-200 km (depending on each group), the uncertainty associated to the precision is on the order of the measured electron density values. Even worse, the retrieved values tend sometimes to be negative. Then, although we cannot in general straightforwardly rely on electron density values at these altitudes, the shape of the profiles could be indicative of some ionospheric features (e.g. waves and sporadic E layers). Above 200 km the profiles of precision against height are qualitatively quite independent from daytime or latitude. From all the nearly collocated pairs studied, there were just 49 that exhibited a difference between line of sight angles of both RO at the F peak height larger than 10º. After analyzing them we find no clear indications of a significant representativeness error in electron density profiles due to the spherical assumption above 120 km height. Differences in precision between setting and rising GPS RO may be attributed to the modification of processing algorithms for rising cases during the initial period of the COSMIC mission.