an RNP system include: Capability to follow a desired ground track with reliability, repeatability, and predictability, including curved paths; and Where vertical profiles are included for vertical guidance, use of vertical angles or specific altitude constraints to define a desired vertical path. RNP APCH supports all leg types and path terminators used in standard RNAV, including TF and RF. RNP AR procedures support only two leg types: TF leg: Track to Fix: a geodesic path between two fixes. RF leg: Radius to Fix. This is a curved path supported by positive course guidance. An RF leg is defined by a radius, arc length, and a fix. Not all RNP capable FMS systems support RF legs. Use of RF legs is allowed prior to the Final Approach Fix. For RNP AR APCH operation see Special Aircraft and Aircrew Authorization Required approach section below. The performance-monitoring and alerting capabilities may be provided in different forms depending on the system installation, architecture, and configurations, including: display and indication of both the required and the estimated navigation system performance; monitoring of the system performance and alerting the crew when RNP requirements are not met; and cross track deviation displays scaled to RNP, in conjunction with separate monitoring and alerting for navigation integrity. An RNP system utilises its navigation sensors, system architecture, and modes of operation to satisfy the RNP navigation specification requirements. It must perform the integrity and reasonableness checks of the sensors and data, and it may provide a means to deselect specific types of navigation aids to prevent reversion to an inadequate sensor. RNP requirements may limit the modes of operation of the aircraft, e.g. for low RNP, where flight technical error (FTE) is a significant factor, and manual flight by the crew may not be allowed. Dual system/sensor installations may also be required depending on the intended operation or need. An RNAV system capable of achieving the performance requirements of an RNP specification is referred to as an RNP system. Because specific performance requirements are defined for each navigation specification, an aircraft approved for a RNP specification is not automatically approved for all RNAV specifications. Similarly, an aircraft approved for an RNP or RNAV specification having stringent accuracy requirements is not automatically approved for a navigation specification having a less-stringent accuracy requirement. Designation For oceanic, remote, enroute and terminal operations, an RNP specification is designated as RNP X, e.g. RNP 4.