Real-Time Specification of HF Propagation Support Based on GAIM Models of the 
Ionosphere 
 
Leo F. McNamara1, Craig R. Baker2 and William S. Borer2 
1.Institute for Scientific Research, Boston College, Chestnut Hill, 
Massachusetts, USA 
2.Air Force Research Laboratory, Research Vehicles Directorate, Hanscom Air 
Force Base, Massachusetts, USA 
 
Abstract: AFRL has been tasked with developing an HF propagation program that 
will take advantage of the world-wide USU-GAIM electron density profiles, which 
are updated every 15 minutes. In the initial planning stage, it was proposed 
that a current program such as VOACAP or ITU Rec-533 be modified, replacing the 
monthly median world maps of foF2 and M(3000)F2 by equivalent maps of GAIM 
values. However, the USU-GAIM profiles were found to be not amenable to this 
approach because of the presence of strong particle E (night E) profiles at high 
dip latitudes during the night, which preclude the determination of foF2 and 
M(3000)F2 in those regions. The lack of real-time observations of HF absorption 
and HF noise at the receiver also means that it is not possible to provide 
real-time specifications of the signal-to-noise ratio (SNR), the key parameter 
for HF communications. AFRL has therefore concentrated on determining the range 
of frequency support for a specified circuit, from 
 the lowest usable F-layer frequency (FLUF) to the highest (FMUF), together with 
the ELUF and EMUF. The FMUF is derived by first calculating the vertical 
incidence ionogram at the control point(s), and then transforming that ionogram 
to the oblique incidence (OI) ionogram for the given circuit length. The nose 
frequency in the OI ionogram is then the FMUF (or EMUF for a particle-E 
reflection). The amalgamated frequency range could be provided to an HF 
operator, or directly in terms of a task list to an Automatic Link Establishment 
(ALE) system.