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.
Background
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;
 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).
 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.
 Environmental restrictions.
Around airports environmental
restrictions often require aircraft to
follow complex tracks to avoid
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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.
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.
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.
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.
Global Navigation Satellite System
(GNSS)
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.
GNSS as used by aviation
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).
Advantages of Performance Based
Navigation (PBN)
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 .
These advantages can then be compared
against the existing limitations of
conventional navigation aids to be able to
measure the benefit gained.
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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.
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).
Environmental restrictions.
Routes can be designed to fly
around noise sensitive areas.
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.
Infrastructure cost. PBN
procedures can be established at
relatively low cost therefore making
it cost effective to introduce at low
density airports.
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.
Some examples of quantified benefits
are;
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).
Integrity of safety-critical navigation
systems.
Aircraft navigation systems are safetycritical. 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.
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.
European Geostationary
Navigation Overlay Service
(EGNOS) is a satellite based
augmentation system developed in
Europe.
Ground-Based Augmentation
System (GBAS) is a safety-critical
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system that augments GPS and
provides enhanced levels of service.
Multiple Mode Receivers. Capable
of operating utilizing more than one
of the satellite systems information.
The future
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.
Conclusion
The introduction of satellite based RNAV
operations and the subsequent
development of RNP and PBN have
significantly impacted the aviation industry.
It has improved safety, efficiency capacity
and reduced the impact upon the
environment.