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Typically, they formulate the aircraft’s current position
using a combination of conventional distance measuring
equipment (DME) signals, inertial navigation systems
(INS), GPS receivers, or other RNAV devices. Like
stand-alone navigation avionics, they rely heavily on airborne navigation databases to provide the information
needed to perform their numerous functions.
DATABASE CAPABILITIES
The capabilities of airborne navigation databases
depend largely on the way they are implemented by the
avionics manufacturers. They can provide data about a
large variety of locations, routes, and airspace segments
for use by many different types of RNAV equipment.
Databases can provide pilots with information regarding airports, air traffic control frequencies, runways,
special use airspace, and much more. Without airborne
navigation databases, RNAV would be extremely limited.
PRODUCTION AND DISTRIBUTION
In order to understand the capabilities and limitations
of airborne navigation databases, pilots should have a
basic understanding of the way databases are compiled
and revised by the database provider and processed by
the avionics manufacturer.
THE ROLE OF THE DATABASE PROVIDER
Compiling and maintaining a worldwide airborne navigation database is a large and complex job. Within the
United States (U.S.), the Federal Aviation Administration
(FAA) sources give the database providers information,
in many different formats, which must be analyzed,
edited, and processed before it can be coded into the
database. In some cases, data from outside the U.S.
must be translated into English so it may be analyzed
and entered into the database. Once the data is coded
following the specifications of ARINC 424 (see
ARINC 424 later in this appendix), it must be continually updated and maintained.
Once the FAA notifies the database provider that a
change is necessary, the update process begins.
1
The
change is incorporated into a 28-day airborne database
revision cycle based on its assigned priority. If the
information does not reach the coding phase prior to its
cutoff date (the date that new aeronautical information
can no longer be included in the next update), it is held
out of revision until the next cycle. The cutoff date for
aeronautical databases is typically 21 days prior to the
effective date of the revision.
2
The integrity of the data is ensured through a process
called cyclic redundancy check (CRC). A CRC is an
error detection algorithm capable of detecting small
bit-level changes in a block of data. The CRC algorithm
treats a data block as a single (large) binary value. The
data block is divided by a fixed binary number (called a
“generator polynomial”) whose form and magnitude is
determined based on the level of integrity desired. The
remainder of the division is the CRC value for the data
block. This value is stored and transmitted with the corresponding data block. The integrity of the data is
checked by reapplying the CRC algorithm prior to distribution, and later by the avionics equipment onboard
the aircraft.
RELATIONSHIP BETWEEN EFB AND
FMS DATABASES
The advent of the Electronic Flight Bag (EFB) discussed in Chapter 6 illustrates how the complexity of
avionics databases is rapidly accelerating. The respective FMS and EFB databases remain independent of
each other even though they may share some of the
same data from the database provider’s master navigation database. For example, FMS and GPS databases
both enable the retrieval of data for the onboard aircraft
navigation system.
Additional data types that are not in the FMS database
are extracted for the EFB database, allowing replacement of traditional printed instrument charts for the
1
The majority of the volume of official flight navigation data in the U.S. disseminated to database providers is primarily supplied by FAA
sources. It is supplemented by airport managers, state civil aviation authorities, Department of Defense (DOD) organizations such as the
National Geospatial-Intelligence Agency (NGA), branches of the military service, etc. Outside the U.S., the majority of official data is provided by each country’s civil aviation authority, the equivalent of the FAA, and disseminated as an aeronautical information publication
(AIP).
2
The database provider extract occurs at the 21-day point. The edited extract is sent to the avionics manufacturer or prepared with the
avionics-packing program. Data not coded by the 21-day point will not be contained in the database extract for the effective cycle. In order
for the data to be in the database at this 21-day extract, the actual cutoff is more like 28 days before the effective date.
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Instrument Procedures Handbook (IPH)仪表程序手册上(166)
