Uncertainties in electron density profiles retrieved by COSMIC GPS radio occultations

P. ALEXANDER; A. DE LA TORRE; P. LLAMEDO; R. HIERRO; N. YEN

Congreso; 2nd International Conference on GPS Radio Occultation, Taipei (Taiwan); 2013

The close proximity of the COSMIC 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. We recall previous work on this issue, but most efforts have been devoted to the assessment of observational errors of this remote sensing technique in the neutral atmosphere. In trying to work with homogeneous sets, data are divided in ten groups according to day or night and latitude. In all cases the best precision values (about 1%) are found at the F peak height, which slightly degrade upwards. For all day groups it can be seen that electron density profiles above about 120 km height become quite homogeneous, which also coincides with a substantial improvement on precision. Night groups are rather diverse, in particular the precision becomes better than 10 % above different levels between 120 and 200 km height. From all the pairs studied, there were just 26 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 representativeness error in electron density profiles due to the spherical assumption above 120 km height. However, it is notably found that differences in precision between setting and rising GPS RO may be introducing significant biases above 120 km altitude. 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). Our calculations reproduce the results by other authors regarding the fact that low altitudes lack precision and that day profiles show in general a better performance than night ones.