In certain geographical areas (e.g. the North Atlantic, the northern portion of Canada) it is a requirement that aircraft carry navigational equipment which permits an enhanced degree of accuracy, especially as regards tracking. To this end, equipment redundancy (two independent long range navigation systems) may be legislated to provide for the failure case.
The prescribed minimum navigation performance specifications (MNPS) and the procedures governing their application are published in the Regional Supplementary Procedures (ICAO Doc 7030) as well as in national Aeronautical Information Publications (AIPs).
Doc 7030: Regional Supplementary Procedures;
Doc 9613: Manual on Required Navigation Performance.