This article explains how aviation navigation has evolved from traditional ground-based aids to modern satellite-based systems such as GPS and Performance-Based Navigation (PBN).
Focus: Aviation navigation systems and evolution
Level: Introductory
Topics: GPS, PBN, RNAV, RNP, ground-based navigation
In a previous post, we explored some of the ground-based devices used by the aviation community for navigation, specifically how they are utilised to conduct approaches into aerodromes (see post). However, this type of navigation is slowly becoming dated; much like me!
With the advent of new technologies such as GPS, the future of aviation is moving away from static, ground-based systems towards a more flexible approach to navigation. This allows aircraft to follow routes that are no longer constrained by the location of ground-based navigation aids.
This concept is known as “Performance-Based Navigation”, so called because it defines navigation in terms of the required performance of the aircraft’s systems, rather than relying solely on fixed infrastructure.
The way this system is used is not too different from satellite navigation in cars, albeit with the additional regulatory overlay associated with movement in three dimensions and the responsibility of carrying millions of passengers each year. Taking this analogy further, it is like cars no longer needing to follow fixed roads between cities, but instead being able to travel directly to their destination using satellite-based and other navigation technologies. This has obvious benefits in terms of traffic management in the sky, fuel burn and flexibility of routes to avoid hazards.
Why do we need common standards?
Like the rules of the road, navigation in the air requires adherence to common standards in terms of “accuracy, integrity, continuity and functionality”. This helps prevent accidents and reduces chaos by ensuring that everyone follows the same standards. The operations being conducted will determine the navigation specification being applied. For instance, an approach into an aerodrome will require a higher standard than enroute operations.
More on Performance Based Navigation…
Navigation specifications under the “Performance-Based Navigation” framework are based on two main systems:
- RNAV; and
- RNP.
Put simply, a navigation specification that includes a requirement for on-board performance monitoring and alerting is based on the RNP system (International Civil Aviation Organization, 2023). This means the aircraft can monitor its own navigation performance and alert the pilot if it falls below a defined standard. By contrast, while still requiring accurate navigation, RNAV does not include this specific on-board performance monitoring and alerting requirement (International Civil Aviation Organization, 2023).
The Specification in More Detail…
The two main categories can then be subdivided as outlined in the illustration below.
(International Civil Aviation Organization, 2023)
The specifications are defined according to accuracy, integrity, continuity, and functionality. For instance, an RNP 0.3 specification requires the aircraft to maintain lateral navigation (side-to-side navigation) within 0.3 nautical miles for at least 95% of the flight time, along with on-board performance monitoring and alerting.
By contrast, an RNAV 10 specification requires the aircraft to maintain a lateral navigation accuracy of within 10 nautical miles for at least 95% of the flight time, but without the same requirement for on-board performance monitoring and alerting.
Functionality in these cases, refers to the specific navigation features required to support a procedure or airspace concept. Without going into lots of detail, these can include things such as waypoint sequencing, the ability to fly defined path terminators and curved path capability.
Going back to our car example, we might think of a Tesla using auto lane control. On a narrow, winding country road, the system would need to maintain a much higher level of precision to keep the car safely within the lane. By contrast, on a wide, straight motorway, there is more margin for error, and the required level of precision is lower.
Aviation works in a similar way. In complex phases of flight, such as an approach into an aerodrome, the required navigation performance is much higher. In simpler phases, such as en-route flight, the acceptable margin of error is greater.
As alluded to in the illustration, the International Civil Aviation Organization’s (ICAO) PBN (Doc 9613), identifies the navigation specifications with their intended applications.
What does this mean in terms of Approaches?
In a previous post, I briefly explained the concept of ILS and NDB approaches, which use ground-based radio aids to guide an aircraft into an aerodrome. These approaches allow the crew to land in reduced visibility or cloud, under what are known as Instrument Flight Rules (IFR).
The concept of Performance-Based Navigation (PBN) has encouraged the use of a different type of approach, commonly referred to as an RNAV (GNSS) (“RNP”) approach, which relies on satellite navigation. This means that, in principle, such approaches can be conducted at aerodromes without ground-based navigation equipment, provided the aircraft and procedure meet the relevant specification requirements.
For those with some aviation experience, these approaches can be flown using different types of minima, depending on the level of capability available. These are summarised below:
| Approach | Functionality/Minima | Guidance |
| Non-precision approach | LNAV (MDA/MDH) | Lateral guidance only based on satellite positioning |
| APV Baro-VNAV approach | LNAV/VNAV (DA/DH) | Lateral guidance via GNSS and vertical guidance via barometric systems |
| APV SBAS approach | LPV (DA/DH) | Lateral and vertical guidance via GNSS with SBAS augmentation |
(International Civil Aviation Organization, 2023)
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