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visual inconsistencies between chart and GPS displays
as well as confusion with approach clearances and
other ATC instructions for pilots unfamiliar with specific manufacturer’s naming conventions.
The manufacturer determines the capabilities and limitations of an RNAV system based on the decisions that
it makes regarding that system’s processing of the airborne navigation database.
USERS ROLE
Like paper charts, airborne navigation databases are
subject to revision. Pilots using the databases are ultimately responsible for ensuring that the database they
are operating with is current. This includes checking
“NOTAM-type information” concerning errors that may
be supplied by the avionics manufacturer or the database
supplier. The database user is responsible for learning
how the specific navigation equipment handles the navigation database. The manufacturer’s documentation is
Figure A-2. Naming Conventions of Three Different Systems for the VOR 34 Approach.
A-4
the pilot’s best source of information regarding the capabilities and limitations of a specific database.
[Figure A-3]
Figure A-3. Database Roles.
COMPOSITION OF AIRBORNE
NAVIGATION DATABASES
The concept of global position is an important concept
of RNAV. Whereas short-range navigation deals primarily with azimuth and distance on a relatively small, flat
surface, long-range point-to-point navigation must
have a method of defining positions on the face of a
large and imperfect sphere (or more specifically a
mathematical reference surface called a geodetic
datum). The latitude-longitude system is currently used
to define these positions.
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
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Instrument Procedures Handbook (IPH)仪表程序手册下(163)
