Communication / Navigation

VHF/HF/SATCOM communications and the VOR/DME/ILS/GPS/INS navigation and surveillance systems.

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Main ideas

COMMUNICATIONS: VHF (118–137 MHz) is line-of-sight, so range is limited by altitude; HF uses ionospheric sky-wave for long (oceanic) range; SATCOM gives global voice/data; ACARS is a datalink for messages, and SELCAL alerts a specific aircraft so the crew needn't monitor continuously.
NAVIGATION aids: VOR gives BEARING to/from a station; DME gives slant-range DISTANCE; the ILS gives a precision approach (localiser for centreline + glideslope for descent); ADF points to an NDB.
AREA/GLOBAL navigation: GPS/GNSS gives position from satellites; INS/IRS gives position and attitude from inertial sensors with no ground aids; these feed the FMS.
SURVEILLANCE and safety: the transponder replies to radar (and feeds TCAS traffic alerts); weather radar paints precipitation; the radio altimeter measures height above terrain for low-level use; GPWS/EGPWS warns of terrain.
Signals rely on antennas and wave propagation; correct antenna, cable and bonding condition is a core B2 concern.
⚠ Exam trap: VHF is LINE-OF-SIGHT (short range, more with altitude) while HF uses SKY-WAVE for long range; VOR gives BEARING and DME gives DISTANCE (slant range) — don't swap them.

Solved examples

Why do oceanic flights beyond VHF range use HF radio, and what limits VHF range in the first place?
VHF signals travel essentially in straight lines (line-of-sight), so beyond the radio horizon — set by the aircraft's and station's height — they cannot be received; over an ocean there are no stations within range. HF signals refract off the ionosphere (sky-wave) and can travel thousands of miles, so HF (or SATCOM/CPDLC) is used for long-range oceanic communication.

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