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AIRCRAFT NAVIGATION Concept

Selasa, 16 Maret 2010 , Posted by saeful uyun at 20.57

AIRCRAFT NAVIGATION 353
AIRCRAFT NAVIGATION
Historically, pilots flew paths defined by VOR (VHF Omnidirectional
Radiorange) radials or by nondirectional beacon signals

using basic display of sensor data. Such paths are restricted
to be defined as a path directly to or from a
navigation station. Modern aircraft use computer-based
equipment, designated RNAV (Area Navigation) equipment,
to navigate without such restrictions. The desired path can
then be direct to any geographic location. The RNAV equipment
calculates the aircraft position and synthesizes a display
of data as if the navigation station were located at the
destination. However, much airspace is still made available
to the minimally equipped pilot by defining the paths in terms
of the basic navigation stations.
Aircraft navigation requires the definition of the intended
flight path, the aircraft position estimation function, and the
steering function. A commonly understood definition of the
intended flight path is necessary to allow an orderly flow of
traffic with proper separation. The position estimation function
and the steering function are necessary to keep the aircraft
on the intended flight path.
Navigation accuracy is a measure of the ability of the pilot
or equipment to maintain the true aircraft position near the
intended flight path. Generally, navigation accuracy focuses
mostly on crosstrack error, although in some cases the
alongtrack error can be significant. Figure 1 shows three components
of lateral navigation accuracy.
Standardized flight paths are provided by government
agencies to control and separate aircraft in the airspace. Path
definition error is the error in defining the intended path.
This error may include the effects of data resolution, magnetic
variation, location survey, and so on.
Position estimation error is the difference between the position
estimate and the true position of the aircraft. This component
is primarily dependent upon the quality of the navigation
sensors used to form the position estimate.
J. Webster (ed.), Wiley Encyclopedia of Electrical and Electronics Engineering. Copyright # 1999 John Wiley & Sons, Inc.
354 AIRCRAFT NAVIGATION
X NM
X NM
2X NM
2X NM
99.999% Integrity limit
95% Accuracy limit
Intended path
99.999% Integrity limit
95% Accuracy limit
Figure 2. RNP–X accuracy and integrity limits.
Flight technical error
Position estimation error
Estimated position
of aircraft
True position
of aircraft
Path definition
Error Path defined by Pilot
or RNAV equipment
Intended path
Figure 1. Aircraft navigation errors.
United States the airways below 18,000 ft are designated as
victor airways and have a prefix of V. Airways above 18,000 Flight technical error is the indicated lateral deviation of
ft are designated as jet airways with a prefix of J. In other the aircraft position with respect to the defined path. RNAV
parts of the world, airways are prefixed with a letter (A1, systems in larger aircraft have provisions to couple a steering
G21, etc.) that is the first letter of a color (amber, green, etc.) signal to a control system to automatically steer the aircraft
Those airways are divided at different altitudes, and the up- to the intended path. In less equipped aircraft, the RNAV sysper
airways are indicated with a prefix of U (UA1, for exam- tem simply provides a display indication of the crosstrack disple).
Airways have associated altitude restrictions to provide tance to the intended path, and the pilot manually provides
separation from terrain. In addition, published airways have the steering correction.
certain conditional restrictions. The restrictions can be on the
type of aircraft (only jet, for example) and on the direction of
RNP–RNAV STANDARDS travel, and they can have restrictions that are effective for
certain hours of the day.
In the interest of standardizing the performance characteristics
of airborne navigation systems and the airspace, the concept
of required navigation performance (RNP) for RNAV, denoted
as RNP–RNAV, is being developed. Reference 1
provides the current state of the concept. Because the separation
requirements for airspace depend on the proximity of obstacles,
density of traffic, and other factors, the RNP–RNAV
characteristic includes a measure, expressed in nautical miles
(NM), that is correlated to the accuracy and integrity requirements
for the airspace. To be more specific, the airspace or
route will be defined as RNP-X, where X is the associated
measure in nautical miles. This allows a consistent means of
designation for airspace from the en route environment to the
approach environment.
The main navigation requirements for RNP–RNAV equipment
are an accuracy requirement, an integrity requirement,
and a continuity of function requirement. For RNP-X airspace,
the accuracy requirement limits the crosstrack and
alongtrack error of aircraft position to less than X NM 95%
of the time. For RNP-X airspace, the integrity requirement
limits the undetected position error to less than 2 times X
99.999% of the time. The continuity of function requirement
limits the failure of the system to meet RNP–RNAV standards
to less than 0.01% of the time. Figure 2 illustrates
the accuracy and integrity limits for the RNP-X route.
AIRWAYS
Published airways provide defined paths for much of en route
airspace. Generally, airways are defined by great-circle segments
terminated by VOR stations. In remote areas, nondirectional
beacons (NDBs) are used in the airway structure.
Figure 3 shows an aeronautical chart of airways. In the Figure 3. Example of an airway chart.
AIRCRAFT NAVIGATION 355
For purposes of transitioning from one airway to another,
the intersections of airways are often defined by
named fixes. Navigation equipment can store the network
of airways and intersections for use by the pilot in defining
the path. This allows the pilot to enter the intended flight
path in terms of the airway identifiers. Airborne equipment
generally does not store directional or other conditional airway
restrictions.
For airways defined by VOR stations, the pilot is expected
to navigate using the VOR at the closest end of the segment
unless a changeover point (COP) is defined on the airway. The
defined changeover point may not be at the midpoint of the
airway segment to account for radio interference or other
unique characteristics of the situation.
Some airways are designated as RNAV airways and are
available only to aircraft operating with RNAV equipment.
Such airways do not have the restriction that a receivable
VOR or NDB be used to define the great-circle path. It is expected
that the RNAV equipment uses available navigation
stations or GPS to compute the aircraft position. Because conventional
non-RNAV airways are defined by VOR or NDB stations,
traffic becomes concentrated near those stations. RNAV
airways offer a significant advantage by allowing the airspace
planner the ability to spread the aircraft traffic over a greater
area without the installation and support of additional navigation
stations.
TERMINAL AREA PROCEDURES
To provide a fixed structure to the departure and arrival of
aircraft at an airport, published procedures are provided by
the authorities. Such procedures are known as standard instrument
departures (SIDs) and standard arrival routes
(STARs). Figure 4 is an example of an SID chart. Generally,
the instructions provided in SIDs and STARs are intended to
be flown by the pilot without the aid of RNAV equipment. In
order to incorporate the procedures into the RNAV equipment,
the instructions must be reduced to a set of instructions
that can be executed by the equipment. A subsequent section
describes this process in more detail.
Standard approach procedures are issued by the authori-
Figure 4. Example of an SID chart. ties to assist pilots in safe and standardized landing operations.
The generation of the approach procedures accounts for
obstacles, local traffic flow, and noise abatement. Historically,
the approach procedures are designed so that RNAV equip- tion algorithm will account for the quality differences and aument
is not required. That is, the pilot can execute the ap- tomatically use the data to generate a best estimate of posiproach
using basic sensors (VOR, DME, ADF) until landing tion. Complementary filters or Kalman filters are commonly
visually. For operations in reduced visibility situations, there used to smooth and blend the sensor data. The common senare
Category II and III instrument landing system (ILS) ap- sors used for position estimation are GPS, DME, LORAN,
proaches that require automatic landing equipment. In addi- VOR, and IRS. The data from each of the sensor types have
tion, there are RNAV and global positioning system (GPS) ap- unique characteristics of accuracy, integrity, and availability.
proaches that require RNAV equipment. Modern RNAV In addition, each of the sensor types requires unique support
equipment is capable of storing the defined approach path functions.
and assist the pilot in flying all approaches. Figure 5 is an
example of an approach chart. Sensor Accuracy
The accuracy characteristic of a sensor can be expressed as
the 95th percentile of normal performance. For any specific NAVIGATION SENSOR SYSTEMS
sensor, the wide variation in conditions in which it can be
used makes it difficult to generalize the accuracy with specific RNAV equipment receives information from one or more sensor
systems and forms an estimate of the aircraft position. If numbers. The following data represent the accuracy under
reasonable conditions. more than one sensor type is available, the position estima356
AIRCRAFT NAVIGATION
provide integrity is with redundant measurements. By comparison
of the redundant measurements, an error in one of
the measurements can be detected and in some cases removed
from consideration.
GPS has a function known as receiver autonomous integrity
monitoring (RAIM), which provides integrity. This function
can be used when sufficient signals of satellites are available.
This is usually the case when the GPS receiver is
receiving signals from five or more satellites. The status of
RAIM is provided to the RNAV equipment and is important
in approach operations using the GPS sensor.
For RNAV systems that use VOR and DME signals, if
there are not redundant signals available, the position solution
is vulnerable to the effects of radio signal multipath and
to the navigation database integrity. The DME signal
multipath problem occurs in situations where the local terrain
supports the reflection of the radio signal to or from the
DME station. The navigation database integrity is difficult to
ensure, especially for DMEs that are associated with military
TACANs. Military TACANs are sometimes moved, and the
information does not get included in the navigation database
in a timely fashion.
NAVIGATION COORDINATE REFERENCE
The WGS-84 ellipsoid has become the standard for aeronautical
navigation. This reference can be viewed as a surface of
revolution defined by a specified ellipse rotated about the
earth polar axis. The semimajor axis of the ellipse lies in the
equatorial plane and has a length of 6378137.000 m. The
semiminor axis is coincident with the earth polar axis and
has a length of 6356752.314 m. Paths between two fixes on
the WGS-84 spheroid are defined as the minimum distance
path along the surface, known at the geodesic path between
the two points. In general, the geodesic path does not lie on a
plane but has a geometric characteristic of torsion. However,
for reasonable distances, there is no significant error by approximating
the path as a portion of a great circle of the appropriate
radius.
Most of the fixes defined in the world were specified in a
reference system other than WGS-84. An effort is under way
to mathematically convert the data from the original survey Figure 5. Example of an approach chart.
coordinate system to that of the WGS-84 coordinate system.
At the same time, when possible, the survey of the location is
GPS has an accuracy of better than 0.056 NM in approach being improved.
conditions, with some degradation allowed at higher speeds.
DME range is accurate to about 0.1 NM with some degra-
COURSE OF THE GREAT CIRCLE PATH dation for longer ranges. The accuracy of a position estimate
based on two or more DME ranges will be dependent upon
The basic path for airways is a direct path between two fixes, the geometry of the DME stations relative to the aircraft.
which may be a VOR station, an NDB station, or simply a LORAN accuracy is about 0.25 NM when receiving a good
geographical location. In terminal area procedures the most ground wave signal.
VOR bearing is generally accurate to within 2 . When used common path is defined by an inbound course to a fix. The
as a position sensor, the position estimate accuracy is depen- RNAV equipment approximates such paths as segments of a
dent upon the range to the VOR station. great circle. Considering the case of a path defined as a radial
IRS accuracy is dependent upon the time since alignment of a VOR, the actual true course depends upon the alignment
and the accuracy of the entry of the position at alignment. An of the VOR transmitter antenna with respect to true north.
accuracy of better than 2 NM/h since alignment is normal. The angular difference between the zero degree radial of the
VOR and true north is called the VOR declination. When the
Sensor Integrity VOR station is installed, the 0 VOR radial is aligned with
the magnetic north so the VOR declination is the same as the Integrity is the ability of the system to warn the pilot of significant
errors in a timely manner. The most common way to magnetic variation at the station at the time of installation.
AIRCRAFT NAVIGATION 357
Magnetic variation is the difference between the direction this difference is necessary to provide a display of course that
is consistent with the magnetic heading of the aircraft as it of north as indicated by a magnetic compass and true north
progresses along the path. defined by the reference ellipsoid. As such, it is subject to the
local anomalies of the magnetic field of the earth. The magnetic
field of the earth varies in a systematic manner over the ARINC-424 NAVIGATION DATABASE
surface of the earth. It is much too complex to be defined as
a simple bar magnet. The magnetic field is also slowly chang- The navigation database installed in the RNAV system stores
ing with time in a manner that has some random characteris- information about airways, SIDs, STARs, approaches, navigatics.
Every 5 years a model, both spatial and temporal, is de- tional aids, and so on. Such information changes continually
fined by international agreement using worldwide data. A as navigational aids are removed or installed, airports are imdrift
of magnetic variation of 1 every 10 years is not uncom- proved, and so on. To ensure that the pilot has current data,
mon on the earth. The model is defined in terms of spherical new data become effective every 4 weeks by international convention.
Because the aircraft may not be available for data- harmonic coefficients. Data from this model are used by senbase
update at the day the new data become effective, most sors and RNAV systems to calculate the magnetic variation
RNAV systems have provisions to allow the new data to be at any location on the earth. In particular, inertial navigation
loaded several days before it is to become effective. In effect, systems are references to true north and produce magnetithe
RNAV system stores two databases, and the day of flight cally referenced data by including the local magnetic variais
used to determine the database that is effective for the tion as computed from a magnetic variation model.
flight. Because the magnetic variation of the earth is slowly
An international standard for the interchange of naviga- changing, a VOR whose 0 radial is initially aligned with the
tional database information is encompassed in the ARINC magnetic north will lose this quality after a period of time.
specification 424 entitled Navigation System Data Base. This This discrepancy between the VOR declination and the local
specification provides for standardized records of 132 ASCII magnetic variation is one reason for ambiguity in course
characters. Record formats are provided to store a wide set of values.
navigational information. As one progresses from along the great circle path, the de-
RNAV systems have packing programs that process sired track changes due to the convergence of the longitude
ARINC-424 records into packed data that are loaded into lines and due to the magnetic variation. Figure 6 shows the
the airborne equipment. The packing programs select only effect of position on true and magnetic courses. The true
those records that are applicable to the RNAV system and are course at the fix, CT, is different from the true course, C T , at
in the desired geographic area. It must also be ensured that the aircraft because the longitude lines are not parallel. The
the selected subset of data is consistent; that is, all references difference in the magnetic courses is the result of the differto
other records are satisfied in the subset. Finally, the se- ence in the true courses together with the difference in maglected
data are packed in the format required for the particu- netic variation at the two locations. lar RNAV system.
For the pilot, an important piece of information is the mag- The reduction of terminal area procedures to a set of innetic
course to be flown to stay on the great circle path. With structions that can be automatically flown by the RNAV
no wind, when on track, the current magnetic heading of the equipment is particularly complex. A considerable fraction of
aircraft should agree with the displayed magnetic course. To the ARINC-424 specification is devoted to this issue. A set
achieve this goal, the RNAV equipment first computes the of leg types have been established to encode terminal area
true course of the desired path and then adjusts it for local procedures. Each leg type has a path definition and a termimagnetic
variation. nation definition. The intended flight path is encoded as a
On the aeronautical charts, the magnetic course of the sequence of legs. The RNAV equipment will automatically fly
path is defined as the termination point of the path. When the procedure by processing the sequence of leg types. As each
the aircraft is some distance from the termination point, both leg becomes active, the path definition of that leg will form
the true course and the magnetic variation are different. This the current flight path intent, and the termination definition
causes the FMS to display a magnetic course at the aircraft will provide information when the successor leg is to become
that is different than that of the chart. As explained above, active.
Table 1 lists the 23 leg types defined by the ARINC-424
specification. Note that generally the first letter of the leg
type can be associated with the intended path and the second
letter can be associated with the termination of the path.
Leg types CA, CD, CI, and CR are provided to handle instructions
such as ‘‘fly 310 track until . . .,’’ whereas leg
types VA, VD, VI, VM, and VR will handle similar instructions
such as ‘‘fly 310 heading until . . ..’’ These leg types
have no specified geographic path but will cause the aircraft
to be steered to the proper track or heading from the current
position of the aircraft whenever the leg becomes active. The
other leg types are referenced to some geographic location.
Limitation of ARINC-424 Coding
True
north
Magnetic
north
C′M
C′T
CM
CT
True
north Magnetic
north
Using the ARINC-424 leg types, most terminal area procedures
can be encoded in such a way that the RNAV equip- Figure 6. True and magnetic courses vary with position.
358 AIRCRAFT NAVIGATION
Missed approach
path
Approach
transitions
Final approach path
Figure 8. Data structure for approaches.
appear on the procedure chart. Because the chart is written
with the pilot in mind, the chart may include logical instructions
that cannot be coded with the 24 leg types of ARINC-
424 specification. An instruction such as ‘‘fly to an altitude or
DME distance, whichever occurs first’’ cannot be encoded as
a sequence of ARINC-424 legs. Current charts exhibit a wide
variety of logical instructions involving altitude, DME distance,
aircraft category, landing direction, and the like. Many
of these instructions cannot be directly encoded as a sequence
of ARINC-424 legs.
ARINC-424 Procedure Database Structures
SIDs can be defined in such a manner that a single identifier
implies a single path. In other cases, a single identifier can
be used to describe the departure paths from more than one
runway. In such cases, the departure path specification must
include the runway together with the SID identifier. In addi-
Table 1. ARINC-424 Leg Types
Leg Type Path and Termination Description
AF Fly a constant DME arc path to the fix
CA Fly the specified course to the altitude
CD Fly the specified course to a distance from a DME
CI Fly the specified course to intercept the following leg
CR Fly the specified course until crossing a specified VOR
radial
CF Fly the specified course into the fix
DF Fly directly to the fix
FA Fly the specified course from the fix to an altitude
FC Fly the specified course from the fix for a specified distance
FD Fly the specified course from the fix to a distance from
a DME
FM Fly the specified course from the fix until manually terminated
HA Fly the holding pattern until terminated at an altitude
HF Fly the holding pattern course reversal, terminated
after entry maneuver
HM Fly the holding pattern until terminated manually
IF An initial fix (no path defined)
PI Fly a procedure turn course reversal
TF Fly the great circle path defined by two fixes
RF Fly the arc defined by a constant radius to a fix
VA Fly the specified heading to an altitude
VD Fly the specified heading to a distance from a DME
VI Fly the specified heading to intercept of following leg
VM Fly the specified heading until terminated manually
VR Fly the specified heading until crossing a VOR radial
tion, the single identifier can further be used to describe the
path to several en route terminations. The multiple optional
paths are known as transitions. In the most general case, a ment can generally fly the procedure in a fashion that is simirunway
transition path is linked to a common route and then lar to the pilot navigation. However, there are significant
linked to an en route transition path. The complete path is limitations to this concept.
therefore specified by the SID identifier, the runway identi- First, the concept assumes that the RNAV equipment has
fier, and the termination of the en route transition. To allow sufficient sensor data to accomplish the proper steering and
the encoding of the complete set of options, the ARINC-424 leg terminations. Lower-end RNAV systems designed for
specification incorporates a database structure similar to that smaller aircraft often do not have sensors providing heading
shown in Fig. 7. or barometric altitude. Without a heading sensor, the system
STARs can have the same structure as SIDs, with the first cannot fly the heading legs properly. Substituting track legs
leg of the star beginning at the first fix on the en route transi- for heading legs is not always satisfactory. In the same way,
tion. With the complete SID or STAR encoded in the naviga- legs that are terminated by an altitude (CA, FA, VA, and HA)
tion database, the RNAV system allows the pilot to select the require that the RNAV system have access to barometric altiproper
runway and en route transition and links a single path tude data. The use of geometric altitude determined by GPS
from the selection. data will introduce several errors. The geometric altitude ig-
SIDs and STARs in the United States commonly use the nores the nonstandard state of the pressure gradient of the
branched structure. Outside the United States, this is gener- atmosphere. The geometric altitude ignores the undulations
ally not the case. That is, a single identifier is used to define of the mean sea level. Finally, the GPS sensor is accurate
the complete path from the runway to the final en route ter- in the vertical axis to about 150 m, which is less accurate
mination with no optional segments. than altimeters.
The general structure for approaches is a set of en route A second limitation to the concept of using the ARINC-424
transitions followed by a common route. The common route leg types has to do with the diversity of instructions that may
includes both the final approach path and a single missed approach
path. Virtually all approaches throughout the world
have this structure (Fig. 8).
BIBLIOGRAPHY
1. RTCA Inc., Minimum Aviation System Performance Standards for
Runway
transitions
En route
transitions
Common route
Required Navigation Performance RNP-RNAV, DO-236, current
ed., Washington, DC: RTCA Inc. Figure 7. Data structure for SIDs and STARs.
AIR POLLUTION CONTROL 359
Reading List
ARINC, Inc., Navigation Data Base Specification 424, current ed.,
Annapolis, MD: Aeronautical Radio.
M. Kayton and W. R. Fried, Avionics Navigation Systems, 2nd ed.,
New York: Wiley, 1997.
GERALD E. BENDIXEN
Rockwell Collins, Inc.
AIR DEFENSE. See ELECTRONIC WARFARE.
AIRPLANE MAINTENANCE. See AIRCRAFT MAINTENANCE.

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