Assessment of global TEC mapping using a three-dimensional electron density model

Dual-frequency transmissions from the Global Positioning System satellites can be used to measure and map ionospheric total electron content (TEC) on global scales, Using data exclusively from ground-based GPS networks, globa l ionosphere mapping has been successfully applied using either two- or thr ee-dimensional techniques. Two-dimensional TEC maps retrieve a horizontally -varying distribution of total electron content, assuming a fixed vertical electron density profile. In three-dimensional mapping, both the horizontal and vertical distribution density are adjusted to fit the data. We describ e a three-dimensional TEC mapping algorithm that uses three independent con stant-density slabs stacked vertically to model the electron density, and c ompare with a more conventional two-dimensional approach using a single sla b. One apparent benefit of the new method is reduction in a level error of the TEC maps, which decreased by 1.7 TECU using the three-dimensional retri eval on simulated data (1 TEC Unit corresponds to 10(16) electrons/m(2)). A nother benefit of the multilayer approach is improved slant TEC modeling, U sing actual data from an equatorial site at Cocos Islands (96.8E, 12.2 S), three slab modeling improved estimates of slant TEC by a factor of 2 for el evation angles between 10 and 20 degrees (9 versus 4.4 TECU, root-mean-squa re). However, the global structure of the vertical TEC retrievals we analyz ed did not improve using three-dimensional modeling. This may be due to a c ritical approximation shared by both techniques that TEC persists unchanged at a given local time. This assumption is required to produce global maps from observations acquired from widely scattered ground receivers, Further improving the retrieval of global TEC structure with ground-based data prob ably requires improved dynamical models of TEC behavior. New data available from GPS receivers in low Earth orbit is also promising. (C) 2000 Elsevier Science Ltd. All rights reserved.