A comparison of Magic and FORTE ionosphere measurements

To date, no formal validation of the new ionosphere nowcast system, United States–Total Electron Content (US-TEC), at the Space Environment Center in Boulder, Colorado, has been published. This paper therefore lays part of the validation groundwork by comparing solutions from Magic, the analysis version of US-TEC, with total electron content (TEC) data from the Fast Onboard Recording of Transient Events (FORTE) satellite. The Magic system uses ground-based GPS observations to reproduce a four-dimensional model of the electron density in the ionosphere. From this model, the TEC between any two points at any time can be obtained. The FORTE satellite, on the other hand, detects the arrival time versus frequency of a broadband signal from a transmitter at Los Alamos. The FORTE-observed group delay provides the TEC along the line of sight between the transmitter and the satellite. These FORTE line-of-sight observations can be compared with TEC values over the same lines of sight in the Magic model. A root-mean-square error (RMSE) calculation statistically compares 178 lines of sight. The RMSE indicates a statistical error of 2.87 total electron content units (1 TECU = 10¹⁶ el/m²) between FORTE and Magic, using the current operational GPS station list in US-TEC. How much FORTE and Magic individually contribute to this error remains indeterminable, although the errors are expected to be unique to either system and uncorrelated. Individual contributions of each method to the RMSE are estimated by eliminating observations most affected by raypath bending in FORTE and by varying the number of stations in Magic.