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WGS-84 and Data conversion

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WGS 84 Manual Doc 9674

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SESSION PLAN

History of the WGS-84

The basics of WGS-84

Data Quality Requirements

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WHAT IS A GEODETIC DATUM?

Cartesian datum

Set of shift parameters :

DX, DY, DZ

Set of rotation angles :

a, b, g

Scale factor : m

Ellipsoidal datum

Additionally the shape of the meridian

ellipse of Earth ellipsoid is added

Memo rule : Ellipsoidal datum =

Cartesian datum + Shape of the Earth

ellipsoid

ab

X,Y

Z

X

Y

Z

DX

Y

Z

WGS 84


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DEFINITIONS

Geodetic Reference System (GRS) : concept of ageocentric cartesian system (X, Y, Z)

Geodetic Reference Frame (datum) : practical

implementation of a GRS by means of surveys

Worldwide GRS : Origin : mass-center of the earth

Z-axis : mean rotation axis of the Earth

X-axis : Greenwich meridian plane, perpendicular to Z-axis

Y-axis : orthogonal

Local GRS : origin and axis are "arbitrary"


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HISTORY 1800 - 1945 : National reference frames

Aim : to provide a basis for charts and cartography

1945 - 1970 : Datum standardization

Aim : answer to WW2 military problems

Europe : European Datum (ED 50)

1970 - now : Wordwide geodetic frames

Aim : Common global reference thanks to space techniques

USA : WGS-72, WGS-84 (Transit and GPS system measurements)

Russia : SGS 85 (GLONASS system)

Europe : EUREF (european frame)


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MAIN REFERENCE FRAMES IN THE WORLD

Tokyo

Adindan

Indian

Pulkovo

Cap

North American

South American

European

Independent

Australian


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Easting (m)

Northing(m)

DATUM ISSUES IN AIR NAVIGATION (1)

Coordinates of DIEKIRCH

(Luxembourg) navaid in

dif ferent reference frames


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Radar

Datum 1

Radar

Datum 2

Horizontal Aircraft

Position

Datum 2Datum 1

Positional discrepancy 100m - 3000m


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In the early 1970's :

Reference frame problems encounteredduring the development of multi-radartracking systems (Belgium, Luxembourg,

Germany, Netherlands).

In the middle of the 1970's :

The use of DMEs located in differentcountries led to positional "jumps" ofexperimental paths.


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In the past Differences betweenreference frames could be accepted

Now The navigation accuracy improvementand the RNAV introduction lead to the needof a common reference frame

Use of the GNSS (based upon WGS-84) inair navigation


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THE AREA NAVIGATION (RNAV)

Constant increase in air traffic (doubling eachdecade)

Standard navigation and air traffic control cannot

manage the increase in air trafficThe need to increase infrastructure capacity can

be satisfied by

Lower distance between routes

Direct routings independent of navaids infrastructure


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Standard navigation : flying from / to a navaid

RNAV: allowing aircraft paths independent of navaids

location

RNAV CONCEPT (1)

VOR

NavAid 1

NavAid 2


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RNAV CONCEPT (2)

No need to fly from/to defined navaids RNAV concept relies on waypoint coordinates

Lower lateral distance between routes

The number of potential routes increases

more flexibility higher capacity of airspace

Perspective : use of GNSS for approach, landing and

ground movements


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RNAV REQUIREMENTS

Higher accuracy in air navigation

Accurate coordinates databases

Data is one of the key elements

The accuracy and integrity of the coordinates must beensured

New surveys theorically required

RNAV preliminary condition : WGS-84implementation


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March 1989 : the Council of the ICAO accepted arecommendation from its Special Comittee on

FANS for the adoption of the geodetic reference

WGS-84 as a standard for international air

navigation February 1994 : the ICAO Council adopted the

necessary amendments to Annexes 11 (ATS) and

15 (AIS)

1st January 1998 : applicability date for WGS-84implementation

CONSEQUENCES


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1

SESSION PLAN





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THE SHAPES OF THE EARTH

The Earth as a geoid

The Earth as a sphere

The Earth as an ellipsoid


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THE EARTH AS AN ELLIPSOID

Global Ellipsoid

Local EllipsoidGeoid


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LOCAL COORDINATE SYSTEMS

Origin and axis orientation

arbitrary

Based upon national (local)

ellipsoids Adjustment for a given

country

Reference system for

horizontal coordinates

Local ellipsoidGeoid

Local area of

interest


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Origin O

Mass-center of the Earth

Z-axis Mean rotation axis of the

Earth

X-axis

Mean Greenwich plane,

perpendicular to Z-axis

P (X, Y, Z)

Greenwich

Meanrotation

axis

Mean equatorial plane

O

Mean Greenwich

meridian plane

GEOCENTRIC GEODETIC SYSTEM

Global ellipsoidGeoid


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Geographic (geodetic) latitude Angle (in the meridian plane)

between the equatorial plane

and the perpendicular to the

ellipsoid at the given point

Geographic (geodetic) longitude Angle (in the equatorial plane)

between the origin meridian and

the meridian plane of the point

Ellipsoidal height h

ELLIPSOIDAL GEOGRAPHIC COORDINATES

l h

Y

X

Z


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WGS-84 REFERENCE SYSTEM

1) Origin O

Mass-centre ofthe Earth

2) OZ axis

Conventional Earth

rotation axis3) OX axis

so as XOZ

is parallel withthe conventional

meridian plane

Z

X

YO4) OY-axisOXYZ = orthogonal


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WGS-84 ORIGIN AND ORIENTATION

Defined by the coordinates of five GPS stations

x Master Control Station

. Surveillance station

.

Kwajalein

.Hawaii

Colorado Springs .x

.

Ascencion.

Diego

Garcia


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WGS-84 : ASSOCIATED ELLIPSOID

Z

X

YO WGS84ellipsoid

a

b

a - baf = = 1/298.257223563

Semi major axis a = 6378137 m

Flattening f :


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THE EARTH AS A GEOID

Listing ( 1873 ) Equipotential surface of

the terrestrial gravity

matching the oceanssurface (under the relief) Mean Sea Level

Perpendicular tothe ellipsoid Perpendicular to

the geoid

Mean SeaLevel (geoid)

Geoid

Undulation

A complex reality : the geod


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A

B

PHYSICS CONCEPTS OF HEIGHT

Question : where doesthe water flow towards ?

Heights are equal on a

gravity equipotential

surface

This belongs to the

physics (vs mathematics)

area

" Point A is higher than point B"" Point B has the same height as C"

Waterfall

C


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HORIZONTAL AND VERTICAL COORDINATES

DGPS surveying techniques provide : WGS 84 horizontal coodinates : latitude f and longitude l

WGS 84 vertical coordinates : ellipsoidal height h (above the

ellipsoid)

The ellipsoidal height does not answer the question : " where

does the water flow towards ? "

Need to use physics to define the height (gravity potential)

Geodetic frames Horizontal reference : Ellipsoid

Vertical reference : Geoid (MSL)


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ELLIPSOIDAL HEIGHT VS ALTITUDE

Geoid (physics area),estimation by MSL

surveys

Ellipsoid

(mathematical area),

estimation by GPS

surveys

MSL can vary as 3M

Geoid UNDulation GUND = Difference between

geoid and el l ipso id

(

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ELLIPSOIDAL HEIGHT VS ALTITUDE

H

h Geod

ellipsod

Terrain

Geod

ellipsod

Terrain

h ellipsodal height

H MSL elevation

h - H : geoid undulation


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GEOID / WGS 84 (1)

- 100 m + 100 m+ 50 m0 m- 50 m


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GEOID / WGS 84 (2)


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THE VARIOUS VERTICAL REFERENCESEllipsoid, geoid, global and local models

Ellipsod

Actual geod (undetermined)

Earth surface

Local geoid model

(levelling network)

-

Global Geoid Model (EGM-96)

Tidal gauge


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VERTICAL REFERENCE ISSUES

Mean sea level (MSL) datum which gives therelationship of gravity-related height (elevation)

shall be used as vertical reference system

Earth Gravitational Model of 1996, EGM-96,

as the reference geoid model to be used in

civil aviation (Annex 15-Para 3.7.2.2).

Ellipsoidal heights are never reported on charts


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THE SOLUTION : GEOID MODELS

Standardization of the use of a global geoid model EGM-96

replacing national / local geoid models

A path towards harmonization : maintaining local models, and

grid publication for conversion with EGM-96

EGM-96A local

model

RAF-98


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Q

WGS-84 IMPLEMENTATION

AIM : implementation of the WGS-84 for air navigation

Inventory of concerned coordinates

Data conversion / New surveys

Mathematical rule to transform coordinates from one

reference frame to another reference frame

OR, FOR HIGHER ACCURACY

Survey of the concerned points relative to accurately known

WGS-84 stations


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WGS-84 IMPLEMENTATION-TRANSFORMATION

Xi, Yi, Zi Local Reference

Frame

X, Y, Z Global Reference

Frame Given : Airport coordinates in a local (national)

reference frame

Find : Airport coordinates in a global (common)

reference frame (WGS-84)

Xi

Yi

ZiZ

Y

X

Airport


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DATA CONVERSION

Princip le

P (WGS 84, lWGS 84, hWGS 84; aWGS 84, fWGS 84)

P (fLocal, lLocal, hLocal; aLocal, fLocal)

Mathematical

Rule

(local datum)


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WGS-84 IMPLEMENTATION - SURVEYS

Aim : determination and report of geographiccoordinates in the WGS-84 reference system,

concerning :

Key elements of aerodromes

Navaids

Application field :

Aerodromes (international, IFR, others)

Navaids


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IMPLEMENTATION THROUGH

A SURVEY CAMPAIGNTheodolite

Total

station

Either via conventional techniques ...

... or via satellite techniques...


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SESSION PLAN





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AERONAUTICAL DATA

Evaluated aeronautical data

positional data, lengths, distances, bearings

Reference aeronautical data

identifiers, frequencies, facilities


Correct

or

incorrect

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POSITIONAL DATA TYPES Surveyed Point:

Determined by survey conducted in accordance with

applicable criteria

Caculated point:

Calculated by mathmatical manipulation from a surveyed point

Declared Point:

Not dependent upon nor formally related to any other point.


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ICAO ANNEXES RELATED TO

WGS-84 IMPLEMENTATION

Determination and report of geographic coordinates in the

WGS-84 geodetic reference system

Annex 11: Air Traffic Services

Annex 14: Aerodromes

Promulgation of Data

Annex 4: Aeronautical charts

Annex 15: Aeronautical Information Services


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DETERMINATION AND REPORTING

Annex-11 (2.19.5) Geographical coordinates indicating

latitude and longitude shall be determined and reported to the

aeronautical information services authority in terms of the

World Geodetic System 1984 (WGS-84)

Annex-14 (1.3.1) World Geodetic System

1984 (WGS-84)

shall be used as the horizontal (geodetic) reference system.

Annex-15 (3.7.1) World Geodetic System 1984 (WGS-84)

shall be used as the horizontal (geodetic) reference system

for international air navigation.


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ACCURACY, RESOLUTION & INTEGRITY

Accuracy A deg ree o f confo rmance between the

estimated or measured value and the tru e value.

Resolut ion A number o f un i t s or d ig i t s to wh ich a

measured or calcu lated value is exp ress ed and used.

Integ ri tyA deg ree o f assurance that an aeronauti cal data

and its value has no t been los t nor altered s ince the data

orig inat ion or authorized amendment.


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ACCURACY & INTEGRITY REQUIREMENTS

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ACCURACY & INTEGRITY REQUIREMENTS

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CONCLUSION

A common geodetic reference system is required in the

aviation context.

Implementation of WGS-84 as horizontal reference

system has resolved associated problems. WGS-84 implementation is necessary for all states to

enhance safety in the new aviation perspective.

Data quality requirements are also to be met.


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Thank you for

your attention

Any question?


wgs-84

Documents

basic ipd

ipd cell hqcaawhat

ipd cell hqcaahistory

ipd cell hqcaadatum

basics of wgs

geodetic datum

mellipsoidal datum

datum standardizationaim