I'm busy revising for a license conversion... Out of curiosity, does anyone know of a radio navigational aid that used the frequencies we call HF (2-25 MHz)?
Current airborne radionavigation systems use MF (NDB), VHF (VOR/LOC, marker beacons), UHF (G/S), and L-band microwave (GPS/GNSS, DME) frequencies. (MLS operates in the C-band, and airborne radar in the X-band, but those are niche applications.)
Historically, the only system that used HF was the UK Gee system. LORAN-A had options for the use of HF frequencies, but these were never used, as MF operation just below the 2 MHz mark gave much more consistent results in both reception range and performance, and LORAN-C migrated into the LF band at 180kHz, partially to avoid the difficulties of being co-located with the amateur radio 160m band. Decca also used LF frequencies, and the Omega system operated at VLF to provide (imprecise) global coverage.
Furthermore, the FCC frequency table shows no allocations for the Radionavigation service anywhere from 2 to 30MHz, in any ITU region.