and possessing many of the functions provided by RNP systems, are not able to provide assurance of their performance. Recognising this, and to avoid operators incurring unnecessary expense, where the airspace requirement does not necessitate the use of an RNP system, many new as well as existing navigation requirements will continue to specify RNAV rather than RNP systems. It is therefore expected that RNAV and RNP operations will co-exist for many years. However, RNP systems provide improvements in the integrity of operation, permitting possibly closer route spacing, and can provide sufficient integrity to allow only the RNP systems to be used for navigation in a specific airspace. The use of RNP systems may therefore offer significant safety, operational and efficiency benefits. While RNAV and RNP applications will co-exist for a number of years, it is expected that there will be a gradual transition to RNP applications as the proportion of aircraft equipped with RNP systems increases and the cost of transition reduces. Functional requirements RNAV specifications include requirements for certain navigation functions. These functional requirements include: continuous indication of aircraft position relative to track to be displayed to the pilot flying on a navigation display situated in their primary field of view; display of distance and bearing to the active (To) waypoint; display of ground speed or time to the active (To) waypoint; navigation data storage function; and appropriate failure indication of the RNAV system including its sensors. Navigation error components and alerting Lateral navigation The inability to achieve the required lateral navigation accuracy may be due to navigation errors related to aircraft tracking and positioning. The three main errors are path definition error (PDE), flight technical error (FTE) and navigation system error (NSE). The distribution of these errors is assumed to be independent, zero-mean and Gaussian. Therefore, the distribution of total system error (TSE) is also Gaussian with a standard deviation equal to the root sum square (RSS) of the standard deviations of these three errors. PDE occurs when the path defined in the RNAV system does not correspond to the desired path, i.e. the path expected to be flown over the ground. Use of an RNAV system for navigation presupposes that a defined path representing the intended track is loaded into the navigation database. A consistent, repeatable path cannot be defined for a turn that allows for a fly-by turn at a waypoint (because nearness to waypoint and