WGS 84 and
EUROCONTROL
In the ECAC area,
the implementation of WGS 84 was coordinated by EUROCONTROL as part of
EATCHIP (The European Air Traffic Control Harmonisation and Integration
Programme).
In March 1989 the
Council of the International Civil Aviation Organisation (ICAO) accepted
a recommendation from its Special Committee on Future Air Navigation
Systems (FANS/4) which stated:
"Recommendation
3.2/1 - Adoption of WGS 84
That ICAO adopts,
as a standard, the geodetic reference WGS 84 and develops appropriate
ICAO material, particularly in respect to Annexes 4 and 15, in order to
ensure a rapid and comprehensive implementation of the WGS 84
system."
Following its
acceptance of the FANS Recommendation, the ICAO Council requested that
the EUROCONTROL Agency act as the co-ordinating body for the
introduction of WGS 84 in the European region for the States of the
European Civil Aviation Conference (ECAC). The Agency’s Committee of
Management accepted this request and EUROCONTROL started preparing the
way for an implementation programme.
In February 1994
the ICAO Council adopted Amendment 35 to Annex 11 (Air Traffic Services)
and Amendment 28 to Annex 15 (Aeronautical Information Services) to the
Convention on International Civil Aviation which mandated the use of WGS
84 as the common geodetic reference system for civil aviation with an
applicability from 1 January 1998.
The applicability date for the
implementation of WGS 84 was in line with the ECAC Ministers’ decision
in relation to RNAV implementation in 1998, for which WGS 84
implementation was a pre-requisite.
In March 1997 the
ICAO Council adopted Amendment 29 to Annex 15 (Aeronautical Information
Services) to the convention on International Civil Aviation, which
mandated the use of the vertical component of WGS 84 with selective
applicability from 5 November 1998.
The programme
formed part of The European Air Traffic Control Harmonisation and
Integration Programme (EATCHIP). Currently, new issues are being
addressed under The European Air Traffic Control Management Programme (EATMP).
World Geodetic System 1984 (WGS 84)
WGS 84 is an earth fixed global reference frame,
including an earth model. It is defined by a set of primary and secondary parameters:
- the primary parameters define the shape of an earth
ellipsoid, its angular velocity, and the earth mass which is included in the ellipsoid
reference
- the secondary parameters define a detailed gravity
model of the earth.
These additional parameters are needed because
WGS 84 is used not only for defining coordinates in surveying, but, for example, also for
determining the orbits of GPS navigation satellites (Other
Related Websites).
Why the need for a common Reference System?
For historical reasons each country has its own geodetic network
and national geodetic reference frame.
Most of the national reference frames are not identical and are
not identical with the global WGS 84 reference frame. For practical reasons navigation
facilities, e.g. DME’s, are surveyed and coordinated with respect to the national
reference frame. The basic problem is to transform the national coordinates to WGS 84 and
express all coordinates in this global system.
EPIC
(European Programme for the Implementation of a Common geodetic reference frame)
The WGS 84 Implementation Group was set up in 1991,
meetings
being held biannually. Participation is drawn from EUROCONTROL, ICAO, the National
Administrations of ECAC States and several independent organisations involved in this
field. It acts as a forum for the exchange of information and experience on WGS 84
implementation
(document download available).
Relationship of WGS 84 to the ITRF
and ETRF reference frames
The WGS 84 reference
frame is consistent
with the International Terrestrial Reference Frame (ITRF). The differences
between WGS 84 and ITRF are in the centimeter range worldwide.
In practice,
co-ordinates are based on the most recent realisation and represented at
the time of computation as ITRF or ETRF using conversion formula through ETRS89. The ITRF or ETRF co-ordinates (followed by the year
reference) are
therefore a high accuracy, physical reference frame, where the
co-ordinates are fixed in time.
The values can be used and when deemed necessary, linked to later
realisations. ETRS89 is being established as the official geodetic datum
for continental Europe.
ETRS89
ETRS89 is the European geodetic
datum, introduced to unify national reference systems for surveying,
mapping, GIS and navigation in Europe. Its purpose is also to define the
precise reference system to monitor tectonic and geodynamic motions and to
integrate vertical datums in Europe.
ETRS89 is a geocentric reference
system fixed to the stable part of the European plate and identical to
ITRS89 at the epoch 1989.0, with geographical co-ordinates based on the
GRS80 ellipsoid. It is defined to 1cm accuracy and consistent with the
global International Terrestrial Reference System (ITRS). Co-ordinates and
velocities given in any ITRS year can be transformed into ETRS89 and vice
versa. In practice, ETRS89 co-ordinates are based on the most recent
realisation and referred to as the International Terrestrial Reference
Frame (ITRF) for a specified year. ETRS is accessible through the primary
realisations of the EUREF permanent GPS stations network and the validated
EUREF campaigns.
Further useful
information and background to EUREF and ETRS89 can be found at the
following site:
www.euref-iag.net.
EUROCONTROL Standard
‘Surveying of Navigation Facilities’
This document was developed as one of the principal elements of
the work programme. It sets out the minimum requirements for the surveying of the
geographical position of radio navigation aids and points whose coordinates contribute to
air navigation (navigation fixes) brought about by the implementation of WGS 84
(document download available).
Quality Assurance
The objective of the WGS 84 Implementation Programme is to
produce coordinate data referenced to a common datum in which a high degree of confidence
can be placed on the accuracy and integrity of the data.
The method used to acquire all the positional data to the
required standard has to address the problems of the size of the task. All the coordinates
produced should be traceable back to their origin along an unbroken trail. As each point
is unique making quality control checks on a sample would not be suitable. It would be
impractical to independently check every single point considering the large number of
points and the geographical spread. However, it is possible to check the method by which
the data is acquired. This can be achieved by quality assurance.
To assist States that have no existing Quality System in place,
or resources to develop one, the WGS 84 support office has produced a QA package,
comprising:
1. Quality Plan
- An
example outlining the formal structure of a State Quality Plan, which describes how
quality assurance can be applied to the survey programme (document download
available).
2. Project Plan
Template - set of checklists based on the minimum requirements described in the
EUROCONTROL Standard ‘Surveying of Navigation Facilities’. It is intended that
these checklists be used as an aid to auditing surveys by assessing that the reports
received meet the requirements as specified in the Standard (document download
available).
3. Preliminary Assessment Matrix
- Contains a sequence of tasks identified as necessary for setting up a quality system,
assigning resources and planning individual surveys.
Other Documentation
WGS 84 Implementation Manual: A manual compiled
from the WGS 84 training course which provides guidance in the provision of geographical
coordinates referenced to the WGS 84 datum
(document download available).
Managing the Change to WGS 84: A collection of
the papers and copies of the associated presentations made at the IATA sponsored seminar
held in Geneva (September 1995) to brief the airlines on the implementation of WGS 84 and
its implications for users.
State Progress
Resources
Each State's task had to be viewed as different
due to the size of the resurvey programme, resources available and any
local requirements.
Allocation of resources was dependent on the
internal administrative structure:
- Surveys being the responsibility of a
different State authority
- Responsibility spread over independent
authorities (e.g. each aerodrome)
In some cases the WGS 84
programme was conducted in parallel with other State survey requirements,
particularly in those cases where the ETRF 89 connection had recently been
established.
Geodetic Connection
It was agreed that the high accuracy geodetic
reference ETRF be used as the realisation of WGS 84 for civil aviation in
Europe (refer 4.1.3 in the EUROCONTROL Standard 'Surveying of Navigation
Facilities'). The geodetic datum associated with ETRF or ITRF reference
frames is ETRS89.
There was an on going programme
for the establishment of a network of first order geodetic points
conducted by the German Institute (IFAG). For some States this provided a
suitable opportunity to co-ordinate their survey programmes with the
establishment of their first order network.
Data Acquisition
73% of the States have had to
resurvey their coordinates. This is because there was insufficient detail
for traceability to the origin of the data or that the cost / time of the
process to recover such information would be comparable to that of a new
survey.
Those States that had detailed
records which enabled them to establish that existing data was of
sufficient quality to meet the accuracy requirements, have been able to
implement by using mathematical transformation methods.
The added benefit in resurveying
is that height data could also be recorded to meet future requirements.
Publication
In order to meet the target date
of 1 January 1998 the proposed timetable included a survey completion date
of mid 1997 to allow for the pre publication tasks such as validation
checks and calculation of associated computed points to be completed.
As the European region involved
areas with a substantial volume of navigation coordinate data, it was
recognised that the simultaneous publication of all the data would present
its own problems. The WGS 84 support office conducted some initiatives on
informing the industry of the expected implications concerning the
publication of WGS 84 data.
These initiatives included
presentations at an IATA seminar (Geneva September 1995, ref Managing The
Change) and a meeting with representatives from the major data producers
in the ECAC region, the Significant Data Volumes group, in which some form
of phased publication was encouraged in order to avoid problems associated
with a ‘big bang’.
States were asked to report their
intended publication schedules in order that EUROCONTROL could monitor the
relative volume of data expected to be published. Those States completing
their programmes early were encouraged to publish and make their data
available, where national regulations permitted, to avoid all data being
published with an effective date of 1 January 1998.
