Which factor most strongly affects the range of VOR and DME signals?

VOR and DME use VHF/UHF signals that are largely line of sight. The altitude of the aircraft and the transmitting facility strongly affects the radio horizon and therefore service range. Groundspeed does not determine signal coverage, and ionospheric reflection is not the normal VHF/UHF propagation mechanism.\n\n**Key takeaway:** Higher antenna altitude generally increases line-of-sight coverage.\n\n**Glossary:** **Radio horizon**: geometric limit of direct radio propagation; **VHF/UHF**: very high and ultra-high frequency bands.