Navigation PPLintermediate
Why does VDF range depend on aircraft altitude, and what equipment does the aircraft need to use the service?
Answer
VHF signals propagate approximately line of sight, so greater aircraft altitude generally increases the radio horizon and potential range. The VDF bearing is determined at the ground station; the aircraft needs a functioning VHF radio to transmit, rather than a dedicated VDF receiver.
Explanation
VDF uses a ground facility to determine the direction of an aircraft's VHF transmission. Because VHF propagation is approximately line-of-sight, a higher aircraft has a greater radio horizon and may be detectable at a longer range. The aircraft needs a VHF radio to communicate with the station; the bearing-finding equipment is on the ground.\n\n**Key takeaway:** VDF is ground-based direction finding, and altitude affects its line-of-sight range.\n\n**Glossary:** **VHF**: very high frequency; **radio horizon**: maximum line-of-sight radio range set by geometry.
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