ACAC RNAV Procedures Workshop

1
World Geodetic System of 1984 (WGS-84)and Data
Conversion

• ACAC RNAV Procedures Workshop

2
CONTENTS

• History of the WGS-84 implementation
• Reference System considerations
• RNAV
• The ICAO decisions
• The basics of WGS-84
• The various coordinate systems
• The WGS-84 reference system and ellipsoïd
• Ellipsoïdal height VS altitude
• Data conversion and implementation of WGS-84
• The different methods of implementation
• Data conversion
• WGS-84 implementation through a survey campaign

3
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

Z
Z
WGS 84
D
Y
X
Y
X
4
DEFINITIONS

• Geodetic Reference System (GRS) concept of a
  geocentric 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"

5
HISTORY

• 1800 - 1945 National reference frames
• Aim to provide a basis for charts and
  cadastre
• 1945 - 1970 Datum standardization
• Aim answer to WW2 military problems
• Europe European Datum (ED 50)
• 1970 - now Wordwide geodetic frames
• Aim Common reference for the world thanks to
  space techniques
• USA WGS-72, WGS-84 (Transit and GPS
  system measurements)
• Russia SGS 85 (GLONASS system)
• Europe EUREF (european frame based on 20
  VLBI and 100 GPS stations)

6
THE MAIN REFERENCE FRAMESIN THE WORLD
7
DATUM ISSUES IN AIR NAVIGATION (1)
Coordinates of DIEKIRCH (Luxembourg) navaid in
different reference frames
Northing (m)
Easting (m)
8
LATITUDE DISCREPANCIES LOCAL DATUM vs WGS-84 (")
9
LONGITUDE DISCREPANCIES LOCAL DATUM vs WGS-84 (")
10
DATUM ISSUES IN AIR NAVIGATION (2)
Horizontal Aircraft Position
Radar Datum 1
Radar Datum 2
Datum 1
Datum 2
Positional discrepancy 100m - 3000m
11
DATUM ISSUES IN AIR NAVIGATION (3)

• In the early 1970's
• Reference frame problems encountered during
  the development of multi-radar tracking systems
  (Belgium, Luxembourg, Germany, Netherlands).
• In the middles of the 1970's
• The use of DMEs located in different countries
  led to positional "jumps" of experimental paths.

12
DATUM ISSUESIN AIR NAVIGATION (4)

• In the past Differences between reference frames
  could be accepted
• Now The navigation accuracy improvement
  and the RNAV introduction lead to
  the need of a common reference frame
• Use of the GNSS (based upon WGS-84)
  in air navigation

13
THE AREA NAVIGATION (RNAV)

• Constant increase in air traffic (doubling each
  decade)
• Standard navigation and air traffic control
  cannot manage the increase in air traffic
• The need to increase infrastructure capacity can
  be satisfied by
• Lower distance between routes
• Direct routings independent of navaids
  infrastructure

14
RNAV CONCEPT (1)

• Standard navigation flying from / to a navaid
• RNAV allowing aircraft paths independent of
  navaids location

VOR
NavAid 2
NavAid 1
15
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

16
RNAV REQUIREMENTS

• Higher accuracy in air navigation
• Accurate coordinates databases
• DATA ARE ONE OF THE KEY ELEMENTS
• The accuracy and integrity of the coordinates
  must be ensured
• New surveys theorically required
• RNAV preliminary condition WGS-84 implementation

17
CONSEQUENCES

• March 1989 the Council of the ICAO accepted a
  recommendation 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-84
  implementation

18
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
• Data publishing (text and charts)
• Annex 4 Aeronautical charts
• Annex 15 Aeronautical Information Services

19
ANNEX 11 - ATS

• Determination and report of geographic
  coordinates
• 2.18.1 Geographic coordinates must be
  determinated and reported according to the
  wordwide reference frame World Geodetic System of
  1984 (WGS-84).
• 2.18.2 The determination and report of
  geoographic coordinates must comply with appendix
  5 specifications.

20
ANNEX 11 - Appendix 5

• REQUIRED ACCURACIES
• a) FIR 1 NM
• b) P/R/D zones (out of CTA / CTR) 1 NM
• c) P/R/D zones (in CTA / CTR) 100 m
• d) Control areas, navaids, waypoints,
• holding points, SID and STAR 100 m
• e) Final approach / precision approach
• Points, essential points for instrument
• approach procedures 3 m

21
ANNEX 14 - Aerodromes

• Element Accuracy Resolution
• Obstacles (in approach
• and landing area) 3 m ddd.mm.ss.x
• Navaids (on aerodromes) 3 m ddd.mm.ss.x
• Navaids (enroute) 30 m ddd.mm.ss
• Runway thresholds 1 m ddd.mm.ss.xx
• ARP 30 m ddd.mm.ss

22
WGS-84 AERONAUTICAL COORDINATES

• Aerodrome Reference Point
• Runway thresholds
• Runway ends
• Navigation control points
• ILS
• MLS

• VOR/DME
• DVOR/DME
• TACAN
• VORTAC
• VOR
• NDB

23
CONTENTS

• History of the WGS-84 implementation
• Reference System considerations
• RNAV
• The ICAO decisions
• The basics of WGS-84
• The various coordinate systems
• The WGS-84 reference system and ellipsoïd
• Ellipsoïdal height VS altitude
• Data conversion and implementation of WGS-84
• The different methods of implementation
• Data conversion
• WGS-84 implementation through a survey campaign

24
THE SHAPES OF THE EARTH
25
THE EARTH AS AN ELLIPSOID
Geoid
26
LOCAL COORDINATE SYSTEMS

• Origin and axis orientation arbitrary
• Based upon national (local) ellipsoids
• Adjustment for a given country
• Reference system for horizontal coordinates

Northing
Easting
27
GEOCENTRIC GEODETIC SYSTEM

• Origin O
• Mass-center of the Earth
• Z-axis
• Mean rotation axis of the Earth
• X-axis
• Mean Greenwich plane, perpendicular to Z-axis

Global ellipsoid
Geoid
Mean rotation axis
Mean Greenwich meridian plane
P (X, Y, Z)
Greenwich
O
Mean equatorial plane
28
ELLIPSOIDAL GEOGRAPHICCOORDINATES

• Geographic (geodetic) latitude f
• Angle (in the meridian plane) between the
  equatorial plane and the perpendicular to the
  ellipsoid at the given point
• Geographic (geodetic) longitude l
• Angle (in the equatorial plane) between the
  origin meridian and the meridian plane of the
  point
• Ellipsoidal height h

Z
h
j
Y
l
X
29
WGS-84 REFERENCE SYSTEM
2) OZ axis Conventional Earth rotation axis
Z
3) OX axis so as XOZ is parallel with the
conventional meridian plane
4) OY-axis OXYZ orthogonal
O
Y
1) Origin O Mass-centre of the Earth
X
30
WGS-84 ORIGIN AND ORIENTATION

• Defined by the coordinates of five GPS stations

31
WGS-84 associated ellipsoid
Semi major axis a 6378137 m Flattening f
32
THE EARTH AS A GEOID

• Listing ( 1873 )
• Equipotential surface of the terrestrial gravity
  matching the oceans surface (under the relief)
• Mean Sea Level

Geoid Undulation
Mean Sea Level (geoid)
Geoid
Ellipsoid
Ocean
Perpendicular to the ellipsoid
Perpendicular to the geoid
33
PHYSICS CONCEPTS OF HEIGHT
A

• Question where does the water flow towards ?
• Heights are equal on a gravity equipotential
  surface
• This belongs to the physics (vs mathematics) area

Waterfall
B
C
"Point A is higher than point B" "Point B has the
same height as C"
34
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)

35
ELLIPSOIDAL HEIGHT VS ALTITUDE

• Geoid (physics area), estimation by MSL surveys
• Ellipsoid (mathematical area), estimation by GPS
  surveys
• MSL can vary 1 ... 3 m

H
MSL
Geoid
h
Ocean
N
Ellipsoid
Geoid UNDulation GUND Difference between
geoid and ellipsoid
(lt100 m) Altitude H
(orthometric height) Geoid elevation
H h - N
36
GEOID / WGS 84 (1)
37
GEOID / WGS 84 (2)
38
VERTICAL REFERENCE ISSUES

• Ellipsoidal heights are never reported on charts
• MSL differences up to 3 meters
• No current agreement regarding a Worlwide
  Unified Vertical Geodetic Reference, but
  adoption of Earth Gravitational Model of 1996,
  a.k.a. EGM-96, is currently under study by the
  ICAO

39
CONTENTS

• History of the WGS-84 implementation
• Reference System considerations
• RNAV
• The ICAO decisions
• The basics of WGS-84
• The various coordinate systems
• The WGS-84 reference system and ellipsoïd
• Ellipsoïdal height VS altitude
• Data conversion and implementation of WGS-84
• The different methods of implementation
• Data conversion
• WGS-84 implementation through a survey campaign

40
STATEMENT OF PROBLEM
Zi
Z
Airport

• Xi, Yi, Zi Local Reference Frame
• X, Y, Z Global Reference Frame

Yi
Y
Xi
X

• Given Airport coordinates in a local
  (national) reference frame
• Find Airport coordinates in a global (common)
  reference frame (WGS-84)

41
POTENTIAL METHODS

• 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

42
DATA CONVERSIONPrinciple

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

Mathematical Rule f(local datum)
P (fWGS 84, lWGS 84, hWGS 84 aWGS 84, fWGS 84)
43
DATA CONVERSIONMethods

• Three different methods
• Helmert formulae
• Molodensky formulae
• Regression method
• The method choice depends on the initial
  coordinate system (cartesian or geographic), and
  on the overall shape of the covered area

44
DATA CONVERSIONExample Helmert formulae

• (f, l, h)Local (X, Y, Z)Local
• Helmert formulae
• (f, l, h)WGS 84 (X, Y, Z)WGS 84

X
X
X
X

-
m
e
e
D
é
ù
é
ù
é
ù
é
ù
é
ù
Z
Y
ê
ú
ê
ú
ê
ú
ê
ú
ê
ú
Y
Y
Y
Y


-


e
m
e
D
Z
X
ê
ú
ê
ú
ê
ú
ê
ú
ê
ú
Z
Z
Z
Z

-
e
e
m
D
ë
û
ë
û
ë
û
ë
û
û
Y
X
WGS
Local
Local
84
Origin translation
Rotation and Scale factor
45
DATA CONVERSIONConstraints

• The quality of the initial coordinates must
  be checked before applying data conversion
• Do the coordinates correspond to the correct
  aeronautical point?
• Is the initial reference system precisely known?
• Is the accuracy of initial coordinates compliant
  with the requirements?

46
DATA CONVERSIONSome issues

• Some important shifts may exist for the
  parameters to use between differents souces
  (National Geographic Institute, NIMA, etc.)
• The initial reference system accuracy may be
  unknown
• The initial coordinates may not be compliant with
  the ICAO requirements for final data quality

47
DATA CONVERSIONIllustration of the issues
70
60
50
40
Error in Final coordinates
30
20
10
m
0
0
0,5
10
0,4
20
0,3
30
0,2
40
Error in rotation parameters
Error in translation parameters
0,1
50
0
m
"
48
DATA CONVERSIONConclusions

• Even if data conversion is the quickest and
  easiest
• method to implement WGS-84, one should remind
  that
• The results depend on the initial coordinates
  quality accuracy of the initial survey,
  integrity of the data, ...
• The application of data conversion requires
  knowledge of the reference system parameters,
  which are not always accurately known
  (differences between sources, sometimes no
  assigned accuracy)
• Any initial error will affect the data conversion
  results (final data)

49
IMPLEMENTATION THROUGHA SURVEY CAMPAIGN
50
WGS-84 IMPLEMENTATIONTHROUGH SURVEYS

• Aim determination and report of geographic
  coordinates in the WGS-84 reference system,
  concerning
• Key elements of aerodromes
• Navaids
• Application field
• Aerodromes (international, IFR, others)
• Navaids
• Work surveys, materialization, et calculations
  of surveyed points (list established during an
  initial inventory phase)
• Processing the results data verification and
  validation, data recording, publication

51
SURVEY OF AERODROMES

• Initial implementation of a local geodetic
  network on each surveyed aerodrome used for all
  the relevant surveys of concerned air navigation
  points
• Network constituted of at least 4 stations with
  spacing, coverage and intervisibility constraints
• Very precise DGPS static mode survey of the
  stations (relative to known points, like IGS
  stations)
• Detail points survey (DGPS fast static and
  kinetic mode, or conventional techniques)
  relative to the local network

52
AERODROME NETWORK
Other points determination relative to R1
(centimetric accuracy survey)
R3
R4
R2
R5
R1
Basic point relative to IGS
R6
53
AERODROME NETWORK (2)

• It makes it possible to use faster DGPS surveying
  modes (fast static, kinetic) or conventional
  techniques for relative surveys
• Updates will be easier to implement (basis for
  further surveys)
• The network of local networks constitutes a
  national geodetic frame

54
NAVAIDS

• Navaids to be surveyed VOR, ILS, MLS, DME, NDB,
  L, TACAN
• Surveys relative to the concerned (or the
  nearest) aerodrome network
• The surveys are especially used for procedure
  design and navaid in-flight inspection

55
WGS-84 IMPLEMENTATIONProcessing the results

• Quality assurance
• Reports concerning general geodetic information,
  aerodromes (one per aerodrome), and navaids
  surveys
• Results verification and validation
• Data integrity assurance (via CRC wrapping) for
  critical uses
• Data publishing
• Data processing for publishing compliant with
  ICAO SARPS

56
CONCLUSION

• A common geodetic reference system is required in
  the RNAV context
• The RNAV procedures rely on the quality of the
  waypoint coordinates, as expressed in WGS-84
• WGS-84 implementation through data conversion is
  feasible but supposes a very accurate knowledge
  of the previously used reference system, and high
  quality initial data
• WGS-84 implementation through new surveys is one
  of the keys for high quality aeronautical data

57
Thank you for your attention
Some questions?