Figure: The Flat Earth Society, from artsgr1e.

Eddie Sez:

For those pilots who want to avoid all things tech, or understanding all things tech, here is what you need to know about WGS-84 in a nutshell:

  • The United States Department of Defense first developed GPS for military uses and that eventually morphed into a worldwide civil system of navigation.

  • Various entities around the world started cataloging the positions of things on earth as a way of finding them and, of course, avoiding them. The standard most of us use is known as the World Geodetic System of 1984, or WGS-84.

  • If your aircraft and its database uses WGS-84 — and most do — then it is critically important that your navigation and approach charts are based on WGS-84 too.

  • Part of your mission planning to international destinations needs to ask this question for every procedure you fly: is this WGS-84 compliant?

  • Your aircraft manufacturer might require you to deselect GPS in non-WGS-84 areas while en route (Gulfstream does not in the G450) and for approach (Gulfstream does).

  • As the aviation world logs more and more GPS time around the non-WGS-84 world, we've come to the conclusion that GPS gives you better situational awareness everywhere in the world and that you probably ought to keep it connected en route, but not for approaches. Your manufacturer should have a position on this topic. The Honeywell position, as of April 2016, is given below.

What follows is the history and math that you don't need to know, but geeks like me find it interesting. What you do need to know follows under ICAO Requirement and Non-WGS84 Airspace.


History

Considering the Earth

Figure: Erastothenes' Size of the Earth, from Geodesy for the Layman, Figure 1.

Contrary to popular folklore, it has long been obvious that the earth is a sphere of some sort, and as early as 240 BC the chief librarian at the Great Library of Alexandria, Egypt, had come up with an idea just how big the sphere is. . .

[Geodesy for the Layman, Ch. 1] In Egypt, a Greek scholar and philosopher, Eratosthenes, set out to make more explicit measurements. He had observed that on the day of the summer solstice, the midday sun shone to the bottom of a well in the town of Syene (Aswan). Figure 1. At the same time, he observed the sun was not directly overhead at Alexandria; instead, it cast a shadow with the vertical equal to 1/50th of a circle (7° 12'). To these observations, Eratosthenes applied certain "known" facts (1) that on the day of the summer solstice, the midday sun was directly over the line of the summer Tropic Zone (Tropic of Cancer)-Syene was therefore concluded to be on this line; (2) the linear distance between Alexandria and Syene was 500 miles; (3) Alexandria and Syene lay on a direct north south line. From these observations and "known" facts, Eratosthenes concluded that, since the angular deviation of the sun from the vertical at Alexandria was also the angle of the subtended arc, the linear distance between Alexandria and Syene was 1/50 of the circumference of the earth or 50 x 500 = 25,000 miles. A currently accepted value for the earth’s circumference at the Equator is 24,901 miles, based upon the equatorial radius of the World Geodetic System.

Sphere "Flattening"

Figure: Oblate spheroid, from Haskel Library

The earth is basically round because gravity pulls with equal strength in all directions, tending to smooth variations towards a norm. But it isn't perfectly round, the centrifugal effects of its rotation tends to make it wider in the middle than it is tall. Technically, you would call the basic shape an oblate spheroid.

Sphere versus Geoid

Figure: Schematic diagram, from National Geodetic Survey.

It is helpful to think of the earth's shape as a "geoid," the shape it would most closely resemble figuring the effects of gravity.

[National Geodetic Survey] There have been many definitions of the "geoid" over 150 years or so. Here is the one currently adopted at NGS:

  • geoid: The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level

Even though we adopt a definition, that does not mean we are perfect in the realization of that definition. For example, altimetry is often used to define "mean sea level" in the oceans, but altimetry is not global (missing the near polar regions). As such, the fit between "global" mean sea level and the geoid is not entirely confirmable.

The earth doesn't conform to the geoid because the magnetic field isn't uniform and the earth's surface is filled with varying heights of land as well as a sea that does not maintain the same level throughout.

Mapping the Earth

[Geodesy for the Layman, Ch. 8]

Adopting a Standard

[Honeywell Direct-To, pg. 11]


Technical Definition

Figure: WGS84 Coordinate System Definition, from NIMA, Figure 2.1

[NIMA, ¶2.1] The WGS 84 Coordinate System is a Conventional Terrestrial Reference System (CTRS). The definition of this coordinate system follows the criteria outlined in the International Earth Rotation Service (IERS) Technical Note 21 [1]. These criteria are repeated below:

  • It is geocentric, the center of mass being defined for the whole Earth including oceans and atmosphere
  • Its scale is that of the local Earth frame, in the meaning of a relativistic theory of gravitation
  • Its orientation was initially given by the Bureau International de l’Heure (BIH) orientation of 1984.0
  • Its time evolution in orientation will create no residual global rotation with regards to the crust

The WGS 84 Coordinate System is a right-handed, Earth-fixed orthogonal coordinate system and is graphically depicted in [the figure].

  • Origin = Earth’s center of mass
  • Z-Axis = The direction of the IERS Reference Pole (IRP). This direction corresponds to the direction of the BIH Conventional Terrestrial Pole (CTP) (epoch 1984.0) with an uncertainty of 0.005
  • X-Axis = Intersection of the IERS Reference Meridian (IRM) and the plane passing through the origin and normal to the Z-axis. The IRM is coincident with the BIH Zero Meridian (epoch 1984.0) with an uncertainty of 0.005
  • Y-Axis = Completes a right-handed, Earth-Centered Earth-Fixed (ECEF) orthogonal coordinate system

The WGS 84 Coordinate System origin also serves as the geometric center of the WGS 84 Ellipsoid and the Z-axis serves as the rotational axis of this ellipsoid of revolution.

The World Geodetic System 84 is a standard used by most of the world to define exactly where a set of coordinates are on the earth. The issues on using one standard versus another are more than just determining where something is left, right, forward, and aft. Another issue is that the world isn't a perfect sphere, or geoid, and defining where something is can also vary in height above the center of the earth.


ICAO Requirement

[ICAO Doc 9613 ¶1.2.5.2] The navigation data published in the State AIP for the routes and supporting navigation aids must meet the requirements of Annex 15 — Aeronautical Information Services. All routes must be based upon WGS-84 coordinates.

[ICAO Doc 9613 ¶3.4] Navigation data may originate from survey observations, from equipment specifications/settings or from the airspace and procedure design process. Whatever the source, the generation and the subsequent processing of the data must take account of the following:

  1. all coordinate data must be referenced to the World Geodetic System — 1984 (WGS-84);
  2. all surveys must be based upon the International Terrestrial Reference Frame;
  3. all data must be traceable to their source;
  4. equipment used for surveys must be adequately calibrated;

WGS-84 Compliance

The Jeppesen State pages list WGS-84 compliance but are not always up-to-date. Jeppesen offers a better list at: http://www.jeppesen.com/company/publications/wgs-84.jsp.


Non-WGS-84 Airspace

The Honeywell position has evolved over the years. I am showing the 2011 position followed by the 2016 position just to illustrate that there is a bit of controversy on the topic. Gulfstream is on board with the 2016 position and the G450 manual has reflected that for quite some time now.

[Honeywell Direct-To, Dec 2011, pg. 11]

There is no doubt you are better off flying an ILS in Russia or China than hoping your GPS coordinates are the same as theirs. But what about en route? Honeywell used to say that you should deselect GPS and rely on DME/DME. But the DME/DME network is spotty in China and Russia, and when at altitude wouldn't the WGS-84 coordinates in your FMS be close enough? So things change . . .

[Honeywell Direct-To, Apr 2016]


What About the G450?

The G450 Aircraft Flight Manual used to require the GPS be deselected in non-WGS-84 countries but now only says to do that while on approach. See G450 Normal Procedures & Techniques / WGS-84 to see the exact verbiage.


Book Notes

Portions of this page can be found in the book International Flight Operations, Part VIII, Chapter 40.


References

http://artsgr1e.deviantart.com/, Deviantart.com

Geodesy for the Layman, Defense Mapping Agency, Building 56 U.S. Naval Observatory DMA TR 80-003, Washington DC 20305, 16 March 1984

Honeywell Direct-To, FMS Quarterly Update and Newsletter, December 2011

Honeywell Direct-To, FMS Quarterly Update and Newsletter, April 2016

http://www.ngs.noaa.gov/GEOID/geoid_def.htm, National Geodetic Survey

ICAO Doc 9613 - Performance Based Navigation (PBN) Manual, International Civil Aviation Organization, 2008

World Geodetic System 1984, Department of Defense, National Imagery and Mapping Agency (NIMA), NSN 7643-01-402-0347, NIMA TR8350.2, Third Edition, Amendment 1, 3 Janaury 2000