Area Navigation (RNAV) allows aircraft to fly on any desired flight path rather than just between ground navigation aids. It works by using a navigation computer that inputs waypoints and estimates the aircraft's position to provide guidance. RNAV has historically developed from ground-based systems like VOR to satellite-based systems like GPS. The ICAO defines four RNAV specifications - RNAV 10, RNAV 5, RNAV 2, and RNAV 1 - which require different levels of navigational accuracy. RNAV differs from RNP in that RNP requires performance monitoring and alerting of the aircraft system.