The low-frequency radio navigation system required, at a minimum, only a simple AM radio receiver on board the aircraft to accurately navigate the airways under instrument meteorological conditions, and even execute an instrument approach to low minimums.[note 8] On the downside, however, it had only four course directions per station, was sensitive to atmospheric and other types of interference and aberrations, and required pilots to listen for hours to an annoying monotonous beep, or a faint stream of Morse codes, often embedded in background "static". Its eventual replacement, the VHF band VOR navigation system, had many advantages. The VOR was virtually immune to interference, had 360 available course directions, had a visual "on course" display (with no listening needed), and was far easier to use.[18] Consequently, when the VOR system became available in the early 1950s its acceptance was rapid, and within a decade the low-frequency radio was mostly phased out.