Expansion on above:
The point of radio, including radio navigation is to transmit information. That information is typically transmitted over a carrier wave via modulation. AM radio modulates the amplitude (or strength/EMF potential) of the carrier while FM (or phase modulated) systems vary the frequency (or phase) of a sub-carrier or carrier.
While both radio systems are technically prone to interference, Phase-modulated Doppler VORs are universally more stable in all but a relatively small number of problematic installations, where terrain, man-made structures, or even bodies of water or atmospheric phenomena create inaccuracies that go well beyond the set standards, rendering the system unreliable for navigation in select areas of coverage.
Last edited by vapilot2004; 3rd Jul 2016 at 03:54.