LRNS Requirements RNAV
In order to fly in areas remote from ground based navaidnavaid —Navigation Aids (think oceanic transits) the aircraft must have two independant and serviceable long range navigation systems. Systems that fill this requirement are GPSGPS —Global Positioning System (USA), IRSIRS —Inertial Reference System and INSINS —Inertial Navigation System as they are either internal or accessible anywhere (something that VORVOR —VHF Omnidirectional Range, for example, is not.)
As you may know from instruments, internal systems like the IRSIRS —Inertial Reference System and INSINS —Inertial Navigation System tend to "drift" over time, so there is a standard time limitation on their use. However, if the position is known (say from seeing a remote island), you can extend that by manual updating.