Aircraft navigation – a paradigm shift? Abstract

This paper examines the impact of satellite based navigation upon the aviation industry, identifies the benefits that have been forthcoming and the potential issues relating to the integrity of the system.

Traditional navigation techniques for aviation were developed using ground based navigation aids. There are various types of navigation facilities inter alia;

Non Directional Beacon (NDB) which is a radio transmitter at a known location which an aircraft can track to/from.

VHF Omnidirectional Range (VOR) which provide more accurate directional navigation information.

Distance Measuring Equipment (DME) that provides distance information to/from the facility.

Instrument Landing System (ILS) that provides lateral and vertical guidance to aircraft approaching to land.

These facilities had to be located in positions where they were accessible for maintenance and in the most optimum position for its navigation purpose.

Limitations of conventional navigation aids.

The basic principle of all of these navigation facilities is the fact that aircraft in general navigate towards and away from the navigation aid itself, “point to point”. This means that the location of the navigation aid must be in an optimized location. This optimized position is, in many cases, not achievable (due to being situated in high terrain, open seas, politically unacceptable areas, etc). Therefore the route structure must be aligned with the position of the navigation aid and not aligned in the ideal position for its purpose. This results in additional distances being flown by aircraft which has a number of disadvantages including economic, environmental and efficiency disbenefits.

In addition to the additional distance flown a number of other problem areas arise;

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  Utilisation of the same arrival/departure tracks. As aircraft need to fly to and from the same navigation facility aircraft which are travelling in opposite directions can in many cases be following the same track over the ground while climbing and descending. While the safety implications of this can be mitigated against by introducing air traffic control separation procedures these are sometimes inadequate as occurred at Delhi Airport in 1996 where a Saudi Arabian Airlines B747 (SVA763) collided mid air with a Kazakhstan Airlines (KZA1907).
  
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  High terrain. At airports located in high terrain with difficult accessibility arrival procedures, based upon conventional ground based navigation aids, may result in aircraft being unable to land at the airport safely during periods of low visibility. An example of this is Juneau Airport Alaska where Alaska Airlines Flight 1866 B727 crashed while attempting to land in 1971.
  
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  Environmental restrictions. Around airports environmental restrictions often require aircraft to follow complex tracks to avoid certain noise sensitive areas. Utilizing conventional ground based navigation aids these tracks are hard to define and to fly. An example of this is Manchester Airport UK where the adjacent village of Knutsford is a sensitive noise pollution area. To avoid the village aircraft are required to make complex manoeuvres soon after takeoff from Manchester Airport.
  
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  Lateral containment of tracks. With conventional ground based navigation aids the accuracy of the track to be flown is a factor of how close to the aid the aircraft is. The closer the aircraft is to the aid the more accurate the track keeping capability. As the aircraft gets further away from the aid the accuracy reduces. This requires that a maximum distance for the aid to be used must be published and that the route spacing requires to be established on the worse case scenario.
  
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  Infrastructure cost. At airports with low numbers of flights the cost of equipping with navigation aids to support aircraft arrivals during reduced visibility can be prohibitively high. This results in no, or limited, arrival capability during bad weather.
  
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  Limited aircraft position information over high seas. The traditional form of aircraft surveillance is by radar which is limited to the line of sight. Therefore radar coverage over the high seas is, in general, restricted to areas relatively close to a land mass where a radar can be located. As separation between aircraft must be increased when there is limited or no radar cover this results in restrictions being imposed upon traffic flows over the high seas where there is no radar cover.
  

GNSS is a satellite navigation system that can provide global coverage.

The first GNSS system in operation was the Global Positioning System (GPS).

GPS is a system of space based satellite navigation systems that provide location and time information anywhere on, or near, the earth is all weather conditions. It is operated and maintained by the United States Government and is freely accessible to anyone. Initially introduced for military purposes it was made available for civilian use during the mid 1990’s.

The utilization of this system of satellites revolutionized aviation navigation capabilities by introducing a flexible, accurate, low cost system that can be used globally.

Since the introduction of the GPS constellation (which currently consists of 32 satellites) the GLONASS constellation (currently consisting of 29 satellites) is in existing global operational use. Further global constellations will be introduced into operational use in the near future such as China’s COMPASS and the European GALILEO. In addition a number of regional constellations will operate e.g. India’s IRNSS.

The use of such satellite navigation systems by aircraft is known as Area Navigation (RNAV). In further stages, under the auspices of the International Civil Aviation Organisation (ICAO), the concept of Required Navigation Performance (RNP) was developed. This introduced the requirement for on-board performance, monitoring and alerting of the RNAV system to allow the aircrew to detect if the navigation system is not achieving the performance required. During 2008 this was further refined with the launch of Performance Based Navigation (PBN). This concept specifies the aircraft RNAV system performance requirements be defined in terms of accuracy, integrity, availability, continuity and functionality required for any proposed aircraft operation. It does not prescribe the system that should be utilized to achieve these performance requirements. PBN is currently limited to operations with linear/lateral performance requirements (2D).

PBN has introduced the ability to design aircraft routes without reference the constraint of needing to fly “point to point” from one ground based navigation aid to another. It makes it possible to design routes which may be used in conjunction with the aircraft Flight Management System to fly complex tracks over the ground in a consistent and accurate manner. The main advantages of PBN are;

1. 
   Reduction of the need to maintain sensor-specific routes and the associated cost of installation and maintenance.
   
2. 
   Avoids the need for development of sensor-specific operations with each new evolution of navigation systems.
   
3. 
   Allows for more efficient use of airspace (route placement, fuel efficiency, noise abatement, etc)
   
4. 
   Clarifies the way in which all the diverse RNAV systems are used
   
5. 
   Facilitates the operational approval process for aircraft .
   

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  Utilisation of the same arrival/departure tracks. As navigation does not need to be “point to point” routes can be designed so that separation of traffic flows is achieved.
  
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  High terrain. Airports which could not previously be accessed easily can have a route designed to avoid high terrain (for example for an aircraft to fly down a valley).
  
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  Environmental restrictions. Routes can be designed to fly around noise sensitive areas.
  
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  Lateral containment of tracks. As the aircraft position is established on a constant update basis it is not dependent upon the distance from the navigation aid. Therefore the containment value is constant. This allows for routes to be placed closer together in busy airspace areas.
  
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  Infrastructure cost. PBN procedures can be established at relatively low cost therefore making it cost effective to introduce at low density airports.
  
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  Limited aircraft position information over high seas. As the aircraft navigation system knows its accurate position anywhere on the globe this information can be used to supplement radar information or when no radar data is available.
  

Atlanda airport USA, $35 million saving for aircraft operators in one year.

Brisbane airport Australia, reduction of 11.4 miles for each arriving flight (143 kg of fuel).

Reduction globally of 13 million tonnes of Carbon Dioxide emissions per year (source IATA).

Aircraft navigation systems are safety-critical. If the system fails and there is not suitable back up, this could result in a serious or fatal accident. Therefore a number of back up systems have, or are in the process of being, introduced e.g.

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  Receiver autonomous integrity monitoring (RAIM) is a technology developed to assess the integrity of the GPS in a GPS receiver system and can predict areas in which the GPS signal may be compromised.
  
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  Wide Area Augmentation System (WAAS) is a satellite based aid developed by the USA to augment GPS with the goal of improving its accuracy, integrity, and availability.
  
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  European Geostationary Navigation Overlay Service (EGNOS) is a satellite based augmentation system developed in Europe.
  
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  Ground-Based Augmentation System (GBAS) is a safety-critical system that augments GPS and provides enhanced levels of service.
  
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  Multiple Mode Receivers. Capable of operating utilizing more than one of the satellite systems information.
  

As previously stated, PBN only supports two dimension (2D) navigation at present. For the future this will progress to three dimension (3D) which includes a vertical component and eventually four dimension (4D) which included a time component. As more reliance is place upon GNSS it will be increasingly important to ensure the coherent integration of navigation, communication and air traffic control surveillance enables to provide integrity of the overall system.