World Geodetic System 1984 (WGS-84)
International Operations Appendices
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.
Figure: The Flat Earth Society, from artsgr1e.
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.
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]
- The Department of Defense, in the late 1950’s began to develop the needed world system to which geodetic datums could be referred and compatibility established between the coordinates of widely separated sites of interest. Efforts of the Army, Navy and Air Force were combined leading to the DoD World Geodetic System 1960 (WGS 60).
- In January 1966, a World Geodetic System Committee composed of representatives from the Army, Navy and Air Force, was charged with the responsibility of developing an improved WGS needed to satisfy mapping, charting and geodetic requirements. Additional surface gravity observations, results from the extension of triangulation and trilateration networks, and large amounts of Doppler and optical satellite data had become available since the development of WGS 60. Using the additional data and improved techniques, WGS 66 was produced which served DoD needs for about five years after its implementation in 1967.
- After an extensive effort extending over a period of approximately three years, the Department of Defense World Geodetic System 1972 was completed. Selected satellite, surface gravity and astrogeodetic data available through 1972 from both DoD and non-DoD sources were used in a Unified WGS Solution (a large scale least squares adjustment).
Adopting a Standard
[Honeywell Direct-To, pg. 11]
- ‘The World Geodetic System was developed by the U.S. Department of Defense in 1966 as a process for accurately surveying the earth and providing methods for assigning latitude/longitude/altitude coordinates for the purposes of navigation. With the deployment of the GPS constellation, the surveying process was updated in 1984 to incorporate GPS as the primary method of reference. Therefore, the ‘84’ is appended to the name to identify the year that the system was last updated.
- Throughout history, there have been numerous methods to survey the surface of the earth, but the WGS-84 system is the most accurate, having an overall fidelity of less than 1 meter. As an example, the WGS-84 system placed the actual Prime Meridian (0o line of longitude) approximately 100 meters east of where it traditionally lies in Greenwich, UK. This is a prime example of how different methodologies can yield different results.
- In 1989, ICAO officially adopted WGS-84 as the standard geodetic reference system for future navigation with respect to international civil aviation. With this policy, virtually all countries use the WGS-84 standard to publish waypoint coordinates for navigation (e.g., airports, runways, navaids, etc.). This data is compiled and disseminated throughout the world in formats such as navigation charts and is the data contained in most FMS Navigation Databases.
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 . 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 Doc 9613 ¶184.108.40.206] 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:
- all coordinate data must be referenced to the World Geodetic System — 1984 (WGS-84);
- all surveys must be based upon the International Terrestrial Reference Frame;
- all data must be traceable to their source;
- equipment used for surveys must be adequately calibrated;
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.
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. Note also that China is now WGS-84 compliant.
[Honeywell Direct-To, Dec 2011, pg. 11]
- Several countries such as China and Russia have not adopted the WGS-84 standard, and continue to use their own methods to survey their airports and navaids. Although the fidelity of their methods are not in question, the simple fact that using different methods exposes the possibility that position information may not match when compared to WGS-84 methods. In any event, non-WGS-84 countries publish their navigational information in similar fashion to WGS-84 countries. This information is also compiled and disseminated in formats such as navigation charts and FMS Navigation Databases.
- For the purposes of navigation, the FMS normally compares actual GPS location against the location of waypoints defined in the navigation database. Since these waypoints were derived using WGS-84 methods, the comparison between actual GPS position relative location among waypoints is consistent. This allows for accurate navigation since the aircraft’s computed position is using the same reference as how the waypoints were surveyed.
- In non-WGS-84 airspace, a GPS-derived position may not yield accurate results because the associated waypoints were not surveyed using WGS-84 methods. Therefore, pilots are instructed to deselect GPS and use DME/DME to determine position. This position is compared against the location of the waypoints defined in the navigation database. By using DME/DME, a consistent reference is being used between the actual position and the position relative to nearby waypoints because the navigation database uses the non WGS-84 data. One might notice a difference between the DME/DME position and a GPS position, but this is irrelevant due to the fact that the FMS is still comparing its position against the navigation database data supplied by the country developing the procedure or route.
- Most AFMs advise pilots to deselect GPS and use DME/DME as the primary navigation method when operating in non WGS-84 airspace. This is acceptable when navigating to the airport or when maneuvering to a conventional approach. It is important to note that aircraft with advanced navigation displays and EGPWS or RAAS must consider that much of the information displayed is based on WGS-84 data. For example, the terrain mapping data that is depicted on the INAV on EPIC platforms was developed using WGS-84 data. Therefore, it is possible that the depiction of terrain on the INAV will be inconsistent when compared to local charts and maps. The terrain data on the INAV is correct, but the relation between terrain and non WGS-84 information (airports, navaids, etc.) could be different. Other WGS-84 derived information, such as geopolitical boundaries could also show some inconsistencies.
Operational Considerations in Non WGS-84 Airspace
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 . . .
- It is Honeywell’s position that GPS updating provides a valuable safety benefit of enhanced situational awareness when operating in areas, routes, and on procedures that are not WGS-84 compliant. Accordingly, Honeywell plans to remove from its Pilot’s Guides the requirement to disable GPS in non-WGS-84 areas. The new guidance will continue to prohibit use of RNAV approaches and require the flight crew to monitor the underlying navaids during radio-based approaches to ensure GPS navigation is acceptable.
- Sample guidance is shown below: OPERATIONS IN NON-WGS-84 AIRSPACE
- Compliance with country-specific requirements relating to the use of GPS is mandatory.
- Check Airplane Flight Manual for any restriction and/or requirements for use of GPS in non-WGS-84 airspace.
- RNAV GNSS Approaches are prohibited in non-WGS-84 airspace. AFM requirements are controlling and supersede any conflicting guidance that may exist in this or other documentation.
- Radio-based (VOR, NDB, etc.) approaches are authorized using GPS updating provided the underlying navaid is tuned and monitored to ensure aircraft position accuracy relative to the published procedure. If at any time during the approach the GPS position does not match the raw data, the raw data shall be used for navigation. (Reference AC 90-108 for additional information.)
- If the underlying navaid is out of service or the onboard radio(s) is(are) inoperative, the use of RNAV to fly the procedure is not authorized.
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 WGS-84 to see the exact verbiage.
Portions of this page can be found in the book International Flight Operations Manual, Part VIII, Chapter 40.
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
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