Tracking Tolerances

All navigation systems (earth based and satellite based) have a tolerance associated with them. The different types of system tolerances are specified by the International Civil Aviation Authority (ICAO) in the document Procedures for Air Navigation Services and Operations (Pans-Ops).

Adding the tolerances of the ground based and on-board equipment with the slight in-flight deviations results in a total protection area surrounding the flight path. For example, an aircraft tracking from a Non-directional Beacon (NDB) starts with an overhead tolerance of 4.6 Km which increases away from the beacon at a splay of exactly 10.3 degrees. This means that an aircraft flying a specific track could be anywhere within a 5.7-kilometer radius after flying just 10 miles.

Such NDB beacons are old technology and have been superceded by more accurate systems. However even very new systems still have a position tolerance that must be taken into account when designing a track to the runway clear of obstacles.