Performance-based navigation (PBN), described in ICAO (International Civil Aviation Organization) Doc 9613, defines airspace by the navigation performance required in it, flown by area navigation (RNAV) and required navigation performance (RNP) systems. The break with everything before it is one sentence: unlike conventional navigation, PBN is not sensor-specific. A conventional route was a VOR (VHF omnidirectional radio range) route or an NDB (non-directional radio beacon) route; a PBN route only demands performance, and whichever sensors deliver it may fly it.
The measures of performance
What is that performance? The RNAV and RNP system performance requirements are defined by accuracy, integrity and continuity, with a fourth measure defined alongside them:
accuracy — the conformance of the true position and the required position: how close the aircraft actually is to where it should be;
integrity — a measure of the trust that can be placed in the correctness of the information supplied by the total system, including the ability of the system to provide timely and valid alerts to the user: not just being right, but warning honestly when it may be wrong;
continuity — the capability of the system to perform its function without unscheduled interruptions during the intended operation;
availability — the percentage of time (annually) during which the system is available for use.
Why, and who must know
The point of demanding performance is room: RNAV and RNP systems are necessary to optimise the utilisation of the available airspace: aircraft that navigate provably well can be packed closer, on routes drawn where traffic wants them rather than where beacons happen to stand. The demand comes with a duty of awareness: flight crew and air traffic controllers must be aware of the on-board RNAV or RNP system capabilities, in order to determine whether the performance of the system is appropriate for the specific airspace requirements: the letters on the flight plan are a promise, and both sides of the frequency must know whether the aircraft can keep it.
Raw and computed data
One distinction, the difference between raw data and computed data, underpins the whole module:
raw data — what a sensor itself measures: a VOR radial, a DME (distance-measuring equipment) distance, readings straight from the aid, displayed as received;
computed data — what the navigation computer makes of them: a position, a track, a deviation from the intended path, calculated by combining and filtering the raw inputs.
Conventional navigation flew raw data; an RNAV or RNP system flies computed data, and everything it displays stands or falls with the computation behind it.
The three components
PBN has three components:
the NAVAID infrastructure — the navigational aids, ground- and space-based, that the sensors use;
the navigation specification — the performance and functionality the airspace requires, the RNAV X and RNP X labels of the next lesson;
the navigation application — the use of the two above in routes, procedures and airspace: what actually gets flown.
Scope: lines and angles
PBN's guidance comes in two lateral shapes. In the oceanic/remote, en-route and terminal phases of flight, PBN is limited to operations with linear lateral performance requirements and time constraints: a corridor of constant width either side of the path, the same tolerance at every point along the leg. In the approach phases of flight, PBN accommodates both linear and angular laterally guided operations. The difference is the shape of the protected lane: linear guidance keeps a fixed cross-track distance, while angular guidance works like the ILS (instrument landing system) of module 4: the allowed deviation is an angle from a point ahead, so the lane narrows continuously as the runway approaches, demanding ever finer flying exactly where precision matters most.