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WGS 84 IMPLEMENTATION
BACKGROUND

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.

Local/global reference system

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.

Reference frame graphic

Geodetic Datum Transformation

A geodetic datum transformation is a mathematical rule used to transform surveyed coordinates given in Reference Frame 1 into coordinates given in Reference Frame 2. The mathematical rule is a function of the set of necessary datum transformation parameters. Hence the accuracy of any datum transformation result is not only dependent on the accuracy of the original data but also on the accuracy of the determination of the transformation parameters used.

images/mathrule.gif (12496 bytes)

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.

WGS 84 web site and  Data Quality Tool Set, commissioned by EUROCONTROL,
produced and supported by Mileridge Limited
(Any comments about the WGS 84 web site to webmaster@dqts.net please)