Radio & navigation aids

How far a signal reaches depends on its frequency band:

• LF / MF: ground wave by day (steady range); at night skywave adds range but also causes night effect.
• HF: very long range by skywave off the ionosphere.
• VHF / UHF: line of sight only. Range grows with altitude and is blocked by terrain.

The aids

• VOR (VHF): line of sight, so higher is farther. Not bothered by night or weather.
• NDB, flown with the ADF: low/medium frequency, bends around terrain for range, but suffers night effect, terrain effect, and thunderstorm interference (the needle swings toward the storm).
• GNSS / GPS: position from satellites. Needs RAIM to check its own integrity, and can lose or degrade its signal.

Surveillance and distress

• Transponder: replies to radar with a code, and with Mode C also your pressure altitude.
• ADS-B Out: broadcasts your GPS position and identity automatically.
• ELT: sends its distress signal to the satellites on 406 MHz (121.5 MHz is only the local homing tone).

Note: VHF/UHF line of sight is in the AIM (COM 4.5, 4.7), but the HF-skywave and LF/MF night-range physics, and the ADF needle pointing toward a storm, are general radio theory (COM 4.6 lists electrical storms as an NDB error source without the direction).

Trap: expecting a VOR to reach as far down low as an NDB. VHF is line of sight, so near the ground terrain and the horizon cut it off.

Practice