Methodology and consistency of slant and vertical assessments for ionospheric electron content models

Abstract

A summary of the main concepts on Global Ionospheric Map(s) (hereinafter GIM(s)) of Vertical Total Electron Content (VTEC), with special emphasis on their assessment, is presented in this paper. It is based on the experience accumulated during almost two decades of collaborative work in the context of the International Global Navigation Satellite Systems (GNSS) Service (IGS) Ionosphere Working Group. A representative comparison of the two main assessments of ionospheric electron content models (VTEC-altimeter and difference of Slant TEC, based on independent Global Positioning System data GPS, dSTEC-GPS) is performed. It is based on 26 GPS receivers worldwide distributed and mostly placed on islands, from the last quarter of 2010 to the end of 2016. The consistency between dSTEC-GPS and VTEC-altimeter assessments for one of the most accurate IGS GIMs (the tomographic-kriging GIM 'UQRG' computed by UPC) is shown. Typical error RMS values of 2 TECU for VTEC-altimeter and 0.5 TECU for dSTEC-GPS assessments are found. And, as expected by following a simple random model, there is a significant correlation between both RMS and specially relative errors, mainly evident when large enough number of observations per pass is considered. The authors expect that this manuscript will be useful for new analysis contributor centers and in general for the scientific and technical community interested in simple and truly external ways of validating electron content models of the ionosphere.

Plasmaspheric Electron Content Inferred from Residuals between GNSS-Derived and TOPEX/JASON Vertical TEC Data

“…This is the opposite phenomenon to the PEC diurnal variation pattern but is expected, because IEC has a constant diurnal variation whereby values are larger during the day. Furthermore, the dispersion between the plasmasphere and ionosphere is significantly smaller, and PEC may exceed IEC around sunrise at low latitudes, which benefits the higher nighttime PEC contribution [9,36]. In addition, the PEC characteristics is different from the ionosphere, and the PEC growth magnitude is small with increasing solar activity though the IEC greatly increases, suggesting that the coupling processes linking these two regions seem not to be linear.…”

“…Similar to the process suggested by Imel et al [32], T/J vTEC used in our analysis was averaged over a 21-s smoothing window along track to reduce the inherent noise effects of the altimeters. Applying this technique continuously suggests that the precision of T/J vTEC estimates is accepted to be 1 TECu [32,35,36]. In addition, considering the resolution of the GIM in this study, the smoothed T/J vTEC was sampled for 15 s after the 21-s running window.…”

Consistency of seven different GNSS global ionospheric mapping techniques during one solar cycle

“…The direct VTEC measurements, obtained by dual-frequency altimeters like TOPEX, JASON and JASON2, among others (V a in TECUs), provide a GNSS-independent reference data very useful to assess the performance of GIMs in some of the most challenging scenarios: over the oceans, typically far from the permanent GNSS receivers contributing to the GIM estimation (see for example [19], and in more detail [22]). It is computed from the vertical phase ionospheric delay provided in Ku-band frequency (I Ku , for f Ku = 13.575GHz).…”

“…In this sense, to compare the performance of the GIMs the standard deviation of the difference between the VTEC of the GIMs and the VTEC obtained from the dual-frequency altimeters is preferred and will be discussed below, since it will minimize this difference [22].…”

“…Indeed, estimation techniques have been developed based, among others, on VTEC expansion in terms of spherical harmonics or electron content described in few layers or voxels in some cases. Assessments based on independent measurements of VTEC and STEC variation have provided a fair ranking and optimal weights for combination (see introduction and references in [22]). Moreover additional recent works are focused on GIM assessment from vessels, [27], at mid and low latitude GNSS receivers, [1], and looking the apparent stability of associated Differential Code Biases but not assuring an independent assessment, [32].…”