A simultaneous study of ionospheric parameters derived from FORMOSAT-3/COSMIC, GRACE, and CHAMP missions over middle, low, and equatorial latitudes: Comparison with ionosonde data

Accurate ionospheric modeling efforts are partly restricted by lack of enough reliable ground-based data and the inability to validate the existing space-based data. In this article, we present a first time comprehensive reliability and validation check of ionospheric data derived using the GPS Radio Occultation (RO) Technique (from three separate missions: FORMOSAT-3/COSMIC, GRACE, and CHAMP) by comparing RO data with ionosonde data for low-latitude, equatorial, and midlatitude stations, simultaneously. This paper discusses two main objectives: (a) Determination of the appropriate spatial resolutions for effective RO and ionosonde data comparisons and (b) Estimating the accuracy of the ionospheric parameters derived from RO missions with respect to ionosonde data within the African sector. For the first time, ionospheric parameters retrieved from RO data have been compared (in details) to ionosonde data over the African sector, specifically for the South African midlatitude stations Grahamstown, GR13L (33.3 degrees S, 26.5 degrees E), and Madimbo, MU12K (22.4 degrees S, 30.9 degrees E). For the equatorial and low-latitude regions, data for Fortaleza FZA0M (3.8 degrees S, 38 degrees W), Brazil, and Ascension Islands AS00Q (7.9 degrees S, 14.4 degrees W) was analyzed. A simple but important method to determine the latitudinal and longitudinal range to be used in comparison with ionosonde data has been established. Based on statistical analysis, it is found that 4.5 degrees x 4.5 degrees 3 degrees x 3 degrees, and 4 degrees x 4 degrees are the approximate suitable spatial resolutions in both latitude and longitude spaces over an ionosonde station for effective comparisons for midlatitude, low-latitude, and equatorial regions, respectively. Appropriate spatial coverage for effective comparisons vary with region and therefore a constant assumption should not be applied on regional/global basis especially if the studies/investigations or modeling extends from middle to low/equatorial latitude zones. For the three latitude regions, COSMIC overestimates the maximum height of the F-2 layer (h(m)F(2)) and underestimates the maximum electron densities. These results provide a step-by-step basis and motivation for the usage of RO data in ionospheric modeling (especially for models which give average conditions) and characterisation over regions that do not have sufficient ground-based instruments/data.