Final answer:
The 'X' in RNAV X or RNP X refers to the required lateral navigation performance accuracy in nautical miles for an aircraft. RNAV is for area navigation and RNP includes onboard monitoring and alerting systems to ensure that the aircraft adheres to the required navigation performance.
Step-by-step explanation:
When you come across RNAV X or RNP X in aviation, the 'X' refers to the lateral accuracy in nautical miles that is required for the particular operation. RNAV stands for Area Navigation, which allows an aircraft to fly on any desired flight path within the coverage of ground- or space-based navigation aids, within the limits of the capability of the self-contained systems, or a combination of these. RNP, which stands for Required Navigation Performance, is a type of RNAV system with onboard performance monitoring and alerting.
RNP systems are named with the required lateral navigation performance accuracy in nautical miles. For example, RNP 1 requires the aircraft to stay within 1 nautical mile of the intended course 95% of the flight time. These navigation specifications are crucial for ensuring the safe and efficient movement of air traffic, especially in congested or obstacle-rich environments.