Validation of the Utah State University Global Assimilation of Ionospheric Measurements (GAIM) model predictions of the maximum usable frequency for a 3000 km circuit

[ 1] The accuracy of the Utah State University Global Assimilation of Ionospheric Measurements (GAIM) monthly median values of the maximum usable frequency for an HF communications circuit has been determined for 3000 km circuits centered on Australian ionosonde locations for March and April 2004. The accuracy is discussed in terms of f(o)F(2), M( 3000) F-2, and the product MUF(3000) F-2 = f(o)F(2) * M(3000) F-2. Ground truth is provided by hourly values of f(o)F(2) and M( 3000) F-2 that have been hand-scaled from ionograms. In general, the accuracy of the GAIM predictions of f(o)F(2) exceeds that for M(3000) F-2. Given that M(3000) F-2 is a function of the height and shape of the F-2 subpeak, it follows that these parameters of the GAIM F-2 peak need to be improved. The GAIM errors in f(o)F(2) are quite large at Learmonth during the day, even though Learmonth is a digital ionospheric sounding systems (DISS) station that provides real-time data to GAIM. We interpret these errors in terms of limitations of the current GAIM model. Comparisons of the GAIM errors in the monthly median MUF( 3000) F2 with those given by the propagation prediction program Advanced Stand Alone Prediction System (ASAPS) show that the GAIM errors are generally somewhat greater than the ASAPS errors, but we consider GAIM's performance to be creditable given that most of the data available for assimilation is GPS total electron content observations. Overall, the daytime GAIM errors are 3 MHz or 10%, while the nighttime errors are 3 MHz or 16%. On average, the GAIM errors are about 3/2 the ASAPS errors during the day and twice the ASAPS errors at night.