The Jet Propulsion Laboratory (JPL) global assimilative ionospheric model (GAIM) combines first-principles physics with empirical measurements to estimate the global electron density structure of the ionosphere. As with other data assimilative models of the ionosphere, JPL GAIM faces numerous challenges, including: sparse data coverage in ocean regions, noisy measurements, and incomplete or empirically-specified physics drivers.

In this study, we utilize a unique data set for mid-November 2008 to evaluate and verify JPL GAIM in an equatorial regime. Provided by the Air Force Research Laboratory (AFRL), this data set includes slant TEC measurements from four GPS receivers near Kwajalein Atoll that have been calibrated against the ARPA Long-Range Tracking and Instrumentation Radar (ALTAIR) radar. We present results based on our latest JPL GAIM model which ingests ionospheric radio occultation soundings from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) and ground GPS observations from nearly 200 GNSS receivers. JPL GAIM is used to predict ionospheric delays along lines-of-sight to satellites tracked by the ALTAIR radar. We assess JPL GAIM performance by withholding AFRL TEC data from the assimilation and assess the GAIM prediction of TEC along the lines of sight of withheld data. This provides quantitative information on GAIM accuracy in low latitude regions where electron density is highly structured.