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Each location/fix defined in an airborne navigation
database is assigned latitude and longitude values in
reference to a geodetic datum that can be used by
avionics systems in navigation calculations.
THE WGS-84 REFERENCE DATUM
The idea of the earth as a sphere has existed in the scientific community since the early Greeks hypothesized
about the shape and size of the earth over 2,000 years
ago. This idea has become scientific fact, but it has been
modified over time into the current theory of the earth’s
shape. Since modern avionics rely on databases and
mathematical geodetic computations to determine the
distance and direction between points, those avionics
systems must have some common frame of reference
upon which to base those calculations. Unfortunately,
the actual topographic shape of the earth’s surface is far
too complex to be stored as a reference datum in the
memory of today’s FMS or GPS data cards. Also, the
mathematical calculations required to determine distance and direction using a reference datum of that
complexity would be prohibitive. A simplified model
of the earth’s surface solves both of these problems
for today’s RNAV systems.
In 1735, the French Academy of Sciences sent an
expedition to Peru and another to Lapland to measure
the length of a meridian degree at each location. The
expeditions determined conclusively that the earth is
not a perfect sphere, but a flattened sphere, or what
geologists call an ellipsoid of revolution. This means
that the earth is flattened at the poles and bulges
slightly at the equator. The most current measurements
show that the polar diameter of the earth is about 7,900
statute miles and the equatorial diameter is 7,926
statute miles. This discovery proved to be very important in the field of geodetic survey because it increased
the accuracy obtained when computing long distances
using an earth model of this shape. This model of the
earth is referred to as the Reference Ellipsoid, and
combined with other mathematical parameters, it is
used to define the reference for geodetic calculations
or what is referred to as the geodetic datum.
Historically, each country has developed its own geodetic reference frame. In fact, until 1998 there were
more than 160 different worldwide geodetic datums.
This complicated accurate navigation between locations of great distance, especially if several reference
datums are used along the route. In order to simplify
RNAV and facilitate the use of GPS in the NAS, a common reference frame has evolved.
The reference datum currently being used in North
America for airborne navigation databases is the North
American Datum of 1983 (NAD-83), which for all
practical navigation purposes is equivalent to the World
Geodetic System of 1984 (WGS-84). Since WGS-84 is
the geodetic datum that the constellation of GPS satellites are referenced to, it is the required datum for flight
by reference to a GPS navigation receiver certified in
accordance with FAA Technical Standard Order (TSO)
C129A, Airborne Supplemental Navigation Equipment
Using the Global Positioning System (GPS). The
World Geodetic Datum was created by the Department
of Defense in the 1960s as an earth-centered datum for
military purposes, and one iteration of the model was
adapted by the Department of Defense as a reference
for GPS satellite orbits in 1987. The International Civil
Database
Providers
• Collect the Data
• Format per ARINC 424
• Revise and Maintain Database
Avionics
Manufacturers
Decide on:
• Information to be Included
• How Information will be Processed
• User Interface
• Ensure Currency
• Execute Updates
• Responsible for Working Knowledge
of Avionics using Database
Pilots
(End Users)
A-5
Aviation Organization (ICAO) and the international
aviation community recognized the need for a common
reference frame and set WGS-84 as the worldwide geodetic standard. All countries were obligated to convert
to WGS-84 in January 1998. Many countries have complied with ICAO, but many still have not done so due to
the complexity of the transformation and their limited
survey resources.
ARINC 424
First published in 1975, the ARINC document,
Navigation System Data Base (ARINC 424), sets forth
the air transport industry’s recommended standards for
the preparation of airborne navigation system reference
data tapes. This document outlines the information to
be included in the database for each specific navigation entity (i.e. airports, navigation aides [NAVAIDs],
　
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