Accuracy of high frequency Maximum Usable Frequencies (MUF) prediction

The residuals between observed oblique sounder medium maximum observed frequencies (MOF) and predicted maximum usable frequencies (MUF) were used to assess the accuracy of high frequency MUF prediction. 4668 MOF's measured on 25 paths were compared against the predicted values from ITSA-1, HFMUFES 4 and MINIMUF-3.5. The data was screened into subsets to see the effect of particular paths, path length and orientation, season, month, latitude, sunspot number, diurnal trends, geographic region and sounder type. MINIMUF-3.5 was most accurate, having a bias of 0.08 MHz (0.6 percent) and a root mean square (rms) error of 3.71 MHz (3.6 percent). It was least accurate during the sunrise and sunset transition hours and for path lengths 5000 to 7000 km. Linear regression analysis identified the probable source of this error to be in the M factor calculation. Except for land paths the performance of HFMUFES 4 was disappointing. It was least accurate for paths over the ocean and with lengths between 4000 and 5000 km. Linear regression analysis identified the source of this error to be a bias in the foF2 numerical coefficient map over ocean areas. The 'k sec Phi'. factor accentuated this bias as the range increased. The version of ITSA-1 with ionospheric characteristics mapped in universal time appeared to be slightly more accurate than the one with its characteristics mapped in local time. All of the programs had difficulty predicting MUFs accurately at high latitude.