The global assimilative ionospheric model (GAIM) has been developed since 1999 under the Multidisciplinary University Research Initiatives (MURI) program sponsored by the U.S. Department of Defense. The development has been conducted at the University of Southern California and the Jet Propulsion Laboratory.

GAIM is a global, fully three-dimensional, and time-dependent ionospheric model. It numerically solves for ion and electron volume densities through the hydrodynamic equations for individual ions. The model is based on the first-principle ionospheric physics and incorporates state-of-the-art optimization techniques that provide a powerful capability of assimilating various types of ionospheric measurements. GAIM's data assimilation capability enhances the modeling accuracy significantly and helps to specify the Earth's ionosphere realistically.

The optimization techniques incorporated into GAIM include the Kalman Filter and 4-dimensional variational (4DVAR) approaches. Assimilating ionospheric data, the former technique esitimates the state covariances and updates the state with the measurements mapped to the model through the observation matrix or operator. The latter technique applies an ajoint method to compute the gradient of the cost function with respect to the model parameters, which are related to the model drivers, minimizes the cost function with a qausi-Newton method, and estimate the model drivers that satisfies the minimization requirement.

The data types being currently practiced with GAIM include line-of-sight TEC measurements made from ground-based GPS receiver networks and space-borne GPS receivers, satellite UV limb scans, and ionosonde. Intensive validation has also been conducted using various independent data sources, including vertical TEC measured using satellite ocean altimeter radar (such as those aboard TOPEX and Jason-1 missions), ionosonde, incoherent scatter radar.