Jeppesen-Radio-Navigation

INTRODUCTION

Satellite navigation was initially developed for military operations. The civil aviation world now
extensively uses the system. Available 24 hours a day, the system provides accuracy and
reliability never seen before in aviation systems.

Two systems of satellite navigation exist: GPS or Navstar Global Positioning Service developed
by the USA, and GLONASS, the Russian equivalent.

This chapter refers primarily to GPS. A discussion of GLONASS follows at the end of this text.

SYSTEM CAPABILITY

For civil aviation, GPS is capable of providing users with the following information:

¾ Position, in three dimensions
¾ Velocity determination
¾ Time

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FREQUENCY

Fundamental Frequency f0 10.23 MHz
Carrier Frequency L1 1575.42 MHz
Carrier Frequency 1227.60 MHz
L2

The use of the frequencies is discussed later in this chapter.

BASIC PRINCIPLE OF OPERATION

GPS uses a similar principle of operation to radar:

¾ The satellite transmits a signal.

¾ The receiver notes the time the satellite signal is received.

¾ Comparing the time of transmission to the time of receipt provides a distance from
the satellite.

Example: Assume that 2 people on the surface of the Earth are a set distance away from a
satellite (also on the Earth’s surface).

10 seconds

10 seconds

Suppose the satellite transmits a sound signal that both people receive 10 seconds after
transmission.

If the speed of sound is 340 m/s, each person is 3400 m from the satellite.

The distance measurement depends upon:

¾ The satellite transmitting at the correct time
¾ The speed of sound being exactly 340 m/s
¾ The clocks of the receivers being correct and synchronised to the satellite clock

The example above provides only one range measurement, indicating only that each person is
somewhere on a circular line of position at a range of 3400 m.

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If the pilot of an aircraft receives the same satellite signal 10 seconds after transmission, the
aircraft could be anywhere on a spherical surface with a radius of 3400 metres from the satellite.

10 10
seconds seconds

10
seconds

For a person on the ground, one satellite only gives one position line. Three satellites will give a
three position line fix. As soon as a fix is required in 3 dimensions, a fourth satellite is required.

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If the timepiece in the receiver is in error then the range from each satellite will have the same
range error. This would present a pilot with three possible positions. By biasing each of these
ranges equally, the pilot could deduce the accurate position and, at the same time, determine the
magnitude of the clock error (known as clock bias).

THE GPS SYSTEM

The GPS system consists of three segments:

¾ The Space Segment
¾ The Control Segment
¾ The User Segment

THE SPACE SEGMENT

The space segment consists of a group of satellites known as a constellation, which provides
the navigation signals. GPS consists of 24 satellites, 21 of which are operational, and 3
operational spares. The constellation is arranged in six orbital paths with four satellites in each
orbit. The orbits are inclined at 55° to the equator, and are separated from each other by 60° of
latitude as they cross the Equator.

A satellite is considered masked when it is less than 5° above the observer’s horizon. Masked
satellites are not used in the navigation solution.

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Orbit Height: Approximately 20 200 km
Orbit Time: Approximately 12 hours

There is a time difference of 3 minutes 57 seconds between two orbits of the satellite and one
rotation of the earth around its axis. This is due to the difference between the length of the
sidereal day and that of the solar day.

The system is designed so that an observer can always detect signals from a minimum of five
satellites.

The receiver system usually has a back-up capability for failures of any part of the system.

GPS TIMING

GPS timing is measured in seconds from a start date of 00:00:00 UTC, 06 January 1980. The
system then runs for 1024 weeks, after which the clock restarts at zero. Because the start date
was UTC the GPS has to run referenced to UTC. The time difference between the satellite clock
and UTC at present is approximately 13.5 seconds. The accurate time difference is transmitted in
the satellite broadcast.

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FREQUENCY AND CODING

GPS satellites transmit on two frequencies:
L1 1575.42 MHz
L1 transmits the Coarse/Acquisition (C/A) code, Precision (P) code, and system
data message.
The C/A code is repeated every millisecond on a frequency of 1.023 MHz.
The P code is repeated every 7 days on 10.23 MHz.
The navigation and system data message is transmitted at 50 Hz.

L2 1227.60 MHz
Transmits the P code only
This frequency determines ionospheric delay

L1 Carrier 1575.42 MHz L1
SIGNAL

C/A CODE 1.023 MHZ

NAV/SYSTEM DATA 50 HZ

P CODE 10.23 MHZ

L2 Carrier 1227.60 MHz L2
SIGNAL

New satellites will also have a capability for the inclusion of a new frequency (L5) upon which the
calculations for automatic correction of ionospheric time delays will be based, although as yet the
final frequency for L5 has yet to be decided. The frequency difference from L1 must be greater
than 200 MHz in order to optimise the ionospheric corrections. Sufficient separation must also be
allowed for other Radio Aids in the UHF band such as DME.

These signals use Pseudo Random Noise (PRN) to carry messages. PRN uses binary
mathematics generated in a predictable manner by the on-board clock. This is achieved by using
Binary Phase Shift Keying (BPSK) in which the phase of the carrier wave is reversed when the
PRN code changes.

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Since the on-board clock generates the PRN sequence, the start time of each sequence is
precisely known and can be carried with the sequence. The PRN sequences also contain two
codes known as:

¾ Coarse/Acquisition code C/A

¾ Precision code P

Remember: L1 carries both C/A and P
L2 carries only P

The P code is only available to authorised users. As a security measure, the military introduces a
security feature that changes the coding of the P code, making it the Y code. This does not affect
the C/A code which civilian aviation uses.

NAVIGATION MESSAGE
Superimposed on both C/A and P codes is a navigation message. (NAV-msg). This message
contains five discrete sub-frames containing:

¾ Clock data for the satellite being tracked
¾ Ephemeris (the satellite orbit) for the satellite being tracked
¾ Message – data on obtaining UTC and, for C/A users, ionospheric delay corrections
¾ Almanac data – information about all the satellites in the constellation.

1 TLM HOW SV Clock Correction Data

2 TLM HOW SV Ephemeris Data (1)

3 TLM HOW SV Ephemeris Data (2)

25 Pages of Subframes 4 & 5 take 12.5 minutes to download

4 TLM HOW Other Data

5 TLM HOW Almanac Data for all SVs

TLM Telemetry
HOW Handover word
SV Space Vehicle (satellite)

Each sub-frame takes 6 seconds to transmit, the total frame time taking 30 seconds. The 4th
subframe contains the ionospheric propagation data. Frame 5 transmits the current SV
constellation data.

Frames 4 and 5 change in every message; a series of 25 frames is required to download the
whole almanac. The almanac information takes 12.5 minutes to download, and is usually
downloaded every hour and is valid for 4 hours to several months.

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THE CONTROL SEGMENT

This provides the control and support system for GPSI and consists of:

Master Control Station (MCS) Colorado Springs
Monitoring Stations (MS) Ascension
Hawaii
Back up MCS Kwajalein
Diego Garcia
Onizuka

Onizuka Colorado
Kwajalein Hawaii Springs

Ascension

Diego
Garcia

The Master station tracks, monitors, and manages the satellite constellation. It also provides an
updating service for the Navigation Message.

The monitor stations are precisely surveyed and consist of very accurate receivers that receive
ranging data and navigation message from each GPS satellites in view.

The satellite internally computed position and clock time are checked at least once every 12
hours. This data is transmitted to the master control station. The MCS establishes the satellite’s
exact orbit and location, known as the satellite ephemeris, along with the actual and predicted
clock parameters. The MCS then transmits the updated ephemeris and clock data to each
satellite, to be included in its Navigation Message.

THE USER SEGMENT

Each GPS receiver decodes the space segment to determine position.

Multi-Channel Receiver
This receiver is preferred for civil transport aeroplanes. This system monitors all satellites
in view and selects the best four satellites in determining the position.

Sequential Receiver
This type of receiver scans the satellites in a sequential manner in order to determine the
pseudo range. As a result, fixing can be quite slow.

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Multiplex Receiver
The multiplex receiver is quicker than the sequential receiver. The system is still only
single or twin channel.

Advantage can be gained by using more than four channels in an all in view system. Using five
channels, it is possible to track all five satellites in view. If a satellite is temporarily obscured from
the aerial, there are still four satellites in view and providing the full position, velocity, and time
data.

The satellite receiver includes:

¾ Wide coverage aerials normally on the top of the aircraft fuselage
¾ Operating transmitters and receivers
¾ Quartz clocks

The satellite uses a combination of two caesium and two rubidium clocks to maintain atomic
frequency standards. This gives a clock accuracy of 10-13 x 3, about 0.003 seconds every 1000
years.

GPS OPERATING PRINCIPLES

Current satellite signals transmit on two frequencies. These are identified as L1 (1575.42 MHz)
and L2 (1227.60 MHz). Each RF signal is modulated by Binary Phase Shift Keying (BPSK).

This modulation provides Pseudo Random Noise (PRN) sequences that carry messages and
make up two codes. These two codes are known as:

Coarse/Acquisition code (C/A)
This code provides the Standard Position Service (SPS) and is available to all users.

Precision code (P)
A Precision Position Service (PPS) is provided. The availability of the P code is limited to
users authorised by the US Department of Defense.

The satellite generates and transmits the PRN code. The code is not truly random but follows a
strict mathematical process, so it is predictable, reproducible within the receiver, and referenced
to GPS time. It is important to remember that the signal reproduced with the receiver is not
transmitted.

The GPS user unit receives satellite signals at the aerial and feeds them to the RF amplifier and
the Phase Modulated receiver, where they are compared and matched with the appropriate PRN
codes held in memory. This determines the identity of the satellite and the time at which the
satellite transmitted the signal.

During the time that the receiver is identifying the PRN code it is also downloading the navigation
message, which is modulated onto the L1 carrier signal. This information provides the basis upon
which to refine the initial range into an accurate value to provide the required accuracy for which
GPS is certified.

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L-Band Antenna

PM Receiver
Data Processor

User Interface

Matching up the received signal and the reproduced signal determines a time of arrival. This
TOA is the measured value of elapsed time between transmission and reception of the satellite
signal. From this, it is possible to derive a range. This derived range takes no account of the fact
that the receiver clock may be in error and is referred to as a Pseudo Range (PR).

PSEUDO RANGE

A Pseudo Range measurement is equal to the TOA value determined by the receiver. It contains:

¾ Signal travel time errors
¾ The GPS receiver’s clock bias (error).

Pseudo Ranges (PR) feed from the receiver to the data processor.

Within the data processor (sometimes called the navigation processor), each PR is corrected for:

¾ Satellite clock errors (which are the difference between the satellite clock time and
the GPS system time)

¾ Atmospheric distortion of the radio signals
¾ Effects of relativity
¾ Receiver noise

The data for these corrections is determined from information that the processor collects from the
satellite’s navigation message. This message is superimposed on both C/A and P code signals.
The navigation message consists of 25 data frames. Each individual data frame contains 1500
bits of information, divided into five x 300 bit sub-frames. Each frame takes 30 seconds to
transmit so that the entire 25 data frames repeat every 12.5 minutes.

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The navigation message contains:

¾ GPS system time of transmission
¾ A hand-over signal for processors transferring from C/A to P code use
¾ Orbital position data for the satellite (its ephemeris)
¾ Clock data for the particular satellite being tracked
¾ Almanac data giving information about the operational status of all the satellites in

the constellation
¾ Coefficients for the calculation of UTC
¾ Coefficients for the determination of ionospheric propagation delay

The navigation message data is normally valid for up to four hours. It is monitored and maintained
as part of the master control station’s task.

Once the GPS receiver/processor has applied the corrections derived from the navigation
message, it can now set out to resolve its own clock bias.

4 Seconds 7 Seconds
(wrong time) (wrong time)

Actual No Possible Point of
Position Intersection Using Wrong
Time Measurements

9 Seconds
(wrong time)

In the example above, a TOA system has determined three ranges in which all variable errors
have been reduced to zero. If no clock bias existed, the three ranges would intersect at a single
point.

Clock bias affects all measured ranges equally. As a result, the actual position lies in the
geometric centre of the shaded area.

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Adjusting the measured TOA values by equal amounts until the three ranges intersect at a point,
determines the clock bias. This is what the receiver processor does in a mathematical solution
but, instead of solving for a two-dimensional fix, it solves for four unknown quantities, namely:

¾ The user’s x co-ordinate
¾ The user’s y co-ordinate
¾ The user’s z co-ordinate
¾ Time (t)

Z

Position X1 Y1 Z1

Prime
Meridian

Y

Equator

X

The same x, y, z co-ordinates are used in defining the positions of the satellites, referring to an
origin located at the centre of the earth as shown above. Using the origin, the processor then
converts the x, y, z co-ordinates into a fix referenced to the WGS – 84 ellipsoid. This conversion
allows for the shape of the ellipsoid and results in a position in terms of latitude, longitude, and
height.

The GPS receiver/processor uses the information in the navigation message to compute the
exact location of the satellite. The relative velocity of the satellite and the aeroplane does not
affect the fix accuracy in any way.

The time that it takes the GPS receiver to position the aircraft depends on the almanac currency
and the position and time in the receiver.

If these three parameters are correct, the fix is obtained within 30 seconds. If the almanac needs
updating, or the position and time are wrong, obtaining a fix occurs only after transmission of all
the subframes and updating the receiver. In this case, the time to fix is over 12.5 minutes.

VELOCITY MEASUREMENT

The processor generates a carrier signal at the same frequency as L1, which is compared to the
frequency received from the satellite L1 signal. A difference exists due to Doppler shift. This
relative motion between the receiver and the satellites is used to derive the aircraft velocity.

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The receiver can also derive an accurate UTC from the navigation message and display this to
the user. The GPS receiver solves for the three elements:

¾ Position
¾ Velocity
¾ Time

GPS RECEIVER

A schematic diagram of a typical GPS avionics receiver is shown below. The receiver is fed from
an aerial, generally mounted on the top of the fuselage.

The aerial is designed to provide uniform sensitivity for all signals from satellites above a
specified angle of elevation (normally 5°). It is shielded from low elevation signals that are prone
to multi-path signals, especially signals arriving from a low elevation satellite. Multi-path signals
may occur from:

¾ Reflections and refraction from the Earth
¾ Its environment

If received, multi-path signals would cause significant errors.

The RF amplifier sets the receiver’s noise level and rejects interference from other RF sources
that are not the satellite frequencies.

The Reference Oscillator provides the receiver’s time and frequency reference. The output from
the RO is fed to the Frequency Synthesiser, which converts the output to local oscillators.

The frequency synthesiser also provides the essential clock information to the Signal Processor.
The Signal Processor performs the following critical functions:

¾ Splits the signal into multiple channels to allow parallel processing of the separate
satellites in view of the aerial

¾ Generates the reference PRN codes
¾ Acquires the signals from the individual satellites
¾ Tracks the code and carrier of the individual satellites
¾ De-modulates and reads the navigation message
¾ Extracts the pseudo range rate and pseudo range from the satellite signals
¾ Maintains the relationship between the receiver and GPS time

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This information is fed to the Navigation (data) Processor, which performs some or all of the
following tasks:

¾ Selects satellites to be acquired and tracked by the signal processor, and computes
signal acquisition and tracking-aiding information for that function

¾ Collects navigation data messages and measurements from the signal processor and
maintains a database

¾ Computes the positions, velocities, and time corrections for each satellite and
corrects the measurements

¾ Uses external navigation data to assist with navigation processing

¾ Determines position, velocity, and time

When switching any GPS receiver on, or if the GPS signal has been interrupted, the signal
processor/ navigation processor must go through a search procedure in order to:

¾ Detect and identify the visible satellites
¾ Carry out the tracking and information gathering functions
¾ Determine the position, velocity, and time

These processes, if not assisted, occupy a noticeable period of time, known as Time to First Fix.
This can take up to 15 minutes on older systems. Pre-setting the receiver processor to the current
position can reduce the time delay.

SYSTEM LIMITATIONS

There are two levels of positioning service:

PPS, or Precision Positioning Service, is only available to authorized users at the time of
this writing.

SPS, or Standard Positioning Service, is carried on the C/A code, is available to all
aircraft. This is the one of concern to pilots.

NUMBER OF USERS

Since the pseudo ranges are determined from signals that are broadcast without specific
address, the number of users that can be served at any instant is virtually limitless.

COVERAGE

Of the 24 satellites in orbit, only 21 must be fully operational, with the remaining three considered
as reserve satellites that can be rapidly re-deployed to replace other failed units.

Current Minimum Operational Standard calls for a minimum of five satellites to be above a mask
angle of 7.5°.

RELIABILITY/INTEGRITY

Monitoring of the satellite is carried out by:

¾ Internal systems within the satellite Radio Navigation
¾ The five monitor stations
¾ The master control station of the ground segment

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These ensure the detection of signal performance degradation at an early stage and give GPS a
similar reliability to most well-used navigation aids.

It is possible that the satellite health message, a component of the satellite’s navigation message,
may only be changed after a cycle of 25 frames transmission. So it is possible that a period of at
least 12.5 minutes may elapse before a receiver/processor corrects the source message.

ICAO does not accept this delay and GPS is not acceptable in precision approach landings.

RECEIVER AUTONOMOUS INTEGRITY MONITORING (RAIM)

RAIM is system integrity monitoring within the GPS unit. The receiver/processor evaluates the
information from a minimum of five visible satellites. The position is determined. From the
positions determined, the receiver looks to see if one of the satellites is providing an incorrect
range and if so, removes it from the position calculation. For RAIM activity there must be:

¾ Five satellites visible
¾ The receiver/processor must be capable of handling the extra data

The GPS receiver/processor can be integrated into on-board navigation systems, taking inputs
from other navigational sources. This is known as receiver augmentation. If the navigation
computer detects a sudden marked deviation of the GPS position, a satellite failure can be
suspected.

GPS INTEGRITY BROADCAST (GIB)

GIB is a ground-based satellite monitoring system that is sited and surveyed accurately. These
sites measure satellite range on a continuous basis and use the measured values to compute
and broadcast range errors, via satellite, to all users.

Modern equipment uses a combination of RAIM and GIB.

COVERAGE PROBLEMS

Transient holes in the coverage can interrupt or degrade GPS coverage. These holes can appear
in certain areas on a regular basis lasting from a few minutes to a few days. Normally this
weakens the GPS signal, causing a degradation of accuracy. In some cases, a total loss of the
GPS service can occur.

The cause of these problems can be the result of any of the following:

¾ Other transmissions on the same frequency
¾ Multiples (harmonics) of other transmissions
¾ Inter-modulation which causes an effect similar to above
¾ Suppressing or blocking interference where the GPS receiver is swamped by strong

RF signals and is de-sensitised
¾ Deliberate jamming

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ACCURACY AND ERROR SOURCES

ACCURACY FOR CIVIL USE

The expected accuracy of the SPS 95% of the time is:

¾ 35 metres horizontally
¾ 75 metres vertically

In practice, the accuracy achieved is much greater.

The reason for the discrepancy between the horizontal and vertical figures is due to geometry.
The surface of the sphere around each satellite is predominantly in the vertical plane when
perceived on or near the Earths’ surface. This gives rise to a stretching of the area of uncertainty
in the vertical plane.

The derived velocity is accurate to within ± 0.2 metres/second.
The time is accurate to 52 nanoseconds for GPS time and 340 nanoseconds for UTC.

The PPS accuracies are:

¾ 5 metres horizontally
¾ 27.7 metres vertically

USER EQUIVALENT RANGE ERRORS

User Equivalent Range Error (UERE) is the result of a number of sub elements as follows:

¾ Errors in the content of the satellite navigation message
¾ Predictability of the satellite’s orbit, which may be disturbed by perturbations resulting

from:
¾ Asymmetry of the earth’s gravitational field
¾ Lunar and/or solar gravity
¾ Atmospheric drag
¾ Electro-magnetic forces
¾ Solar wind
¾ Stability of the satellite’s clock

The satellite clock is monitored and its general error trend established and extrapolated to form
part of the navigation message. If the trend changes, this establishes an error source:

¾ Precision of the PRN tracking in the GPS receiver. The heart of this is a stable clock.
If the receiver clock drifts, it causes the tracking sequence to be in error.

¾ Errors in the processor’s ionospheric model. If the ionosphere/troposphere creates
different refractive indices from those used in the model, this causes errors.

Good satellite signal quality and good receiver/processor design minimise the UERE.

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DILUTION OF PRECISION (DOP)

Like all position-determining systems that use lines of position, the geometry of the intersecting
lines greatly affects the potential accuracy of any resultant fix. If the satellites in use, as viewed by
the GPS receiver aerial, are close together, the surfaces of position have a poor angle of
intersection and, consequently, there exists a significant loss of accuracy. This is referred to as a
high dilution of precision. A high DOP causes any UERE to have a greater effect.

Large angle gives
better solution

Small angle gives
less accurate
position

DOP can be referred to as:

GDOP: Geometric DOP as just described
VDOP: DOP in the vertical (altitude)
HDOP: DOP in the horizontal
PDOP: DOP of position (a combination of VDOP and HDOP)
TDOP: DOP in time

ERROR PREDICTIONS

Up to a point, both UERE and DOP error sources can be predicted for a specific receiver at a
specific time in a specific place. Not all error sources are totally predictable, such as ionospheric
refraction and solar wind. There is always the possibility of uncompensated errors.

DIFFERENTIAL GPS (DGPS)

GPS provides a world-wide navigation capability with a high level of accuracy. At present, the
system cannot be used for precision approaches unless using a differential system (ICAO
requires a 2 second warning of failure for a precision approach and 8 seconds for a non-precision
approach).

Differential GPS involves the cooperation of two receivers:

¾ An accurately surveyed stationary receiver
¾ A moving receiver that requires accurate position information

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DGPS PRINCIPLE OF OPERATION

GPS receivers use timing signals from at least four satellites to establish a position. Each of those
timing signals has some error or delay.

If each timing signal has an error, the position calculation is going to be a compounding of the
timing errors. The satellites are so far out in space that the short distances travelled on Earth
appear insignificant.

If two GPS receivers are close to each other, the signals that reach them will have travelled
through the same portion of the atmosphere. This means that the same errors have affected both.
By establishing these variable errors, they can be corrected. If a reference receiver is used to
measure these errors, it can be used as a referencing system.

This reference station receives the same GPS signals as the aircraft. By accurately knowing its
own position, it calculates the time required for a satellite signal to reach the earth. Comparing the
calculated to the actual time provides the error correction required for an accurate position.

The receiver transmits this error correction to the aircraft, which then corrects its own messages.
The reference receiver catalogues all visible satellites since it does not know which satellites the
aircraft might be using.

The aircraft receives the complete list of errors from the reference receiver and applies the
corrections for the particular satellites it is using. The use of DGPS significantly improves the
accuracy of GPS as illustrated:

SUMMARY OF GPS ERROR SOURCES

Typical Error in Metres

Standard GPS Differential GPS

Satellite Clocks 1.5 0

Ephemeris Errors 2.5 0

Ionosphere 5.0 0.4

Troposphere 0.5 0.2

Receiver Noise 0.3 0.3

Multipath Reception 0.6 0.6

Horizontal Typical Position Accuracy
Vertical 29 1.3
45 2.0

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PSEUDOLITE/DGPS

Using a pseudolite (a pseudo-satellite) makes it possible to reduce the VDOP. Trials show that a
reduction in VDOP to less than 1 m for a pseudolite-augmented DGPS is possible from 2.3 m for
DGPS.

The reference station is exactly the same as in DGPS. The pseudolite is set at a fixed location
and accurately surveyed. The pseudolite generates a GPS style signal, which is capable of
coding and serving as a ranging signal. The reference station monitors the transmission from the
satellites in view and the pseudolite. Monitoring these signals allows derivation of differential
corrections, which proceed to the pseudolite. The pseudolite broadcasts these corrections so that
user equipment can use the broadcast to:

¾ Determine the differential corrections
¾ Establish an additional ranging input

Pseudolite augmented DGPS provides a significant advance, especially in applications for
precision-approach landing. In addition to the enhanced accuracy, a pseudolite provides the
capability of overcoming multi-path effects. Additionally, their signals are unaffected by
ionospheric and tropospheric delays.

Because the range to the pseudolite changes rapidly, the receiver is subjected to a much greater
variation in signal strength. If the pseudolite is set up on the approach path, its transmitter power
may be set to provide a coverage of 20 nm. Approaching the pseudolite, the intensity of the
received signal increases inversely with the square of the range from the pseudolite. At 0.1 miles,
the signal is 40 000 times stronger, which could cause avionic saturation and swamp the satellite
signal. Modifying the broadcast signal structure resolves this problem.

PSEUDOLITE SIGNAL
DIFFERENTIAL CORRECTION→

Aerial screening also poses a problem. The pseudolite is on the ground and the satellite

navigation system receiver aerial is on top of the aeroplane. The fuselage can screen the aerial

and prevent reception of the pseudolite signal. Using separate aerials solves this problem, with

the satellite aerial on top of the fuselage and the pseudolite aerial below. This is not the ideal

solution. Normally, the pseudolite position is offset from the approach path and, if possible, on an

elevated site to overcome fuselage screening.

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SATELLITE BASED AUGMENTATION SYSTEMS (SBAS)

The pseudolite is capable of providing high accuracy in small areas. The accuracy deteriorates
rapidly away from the pseudolite until 100 km. From this distance, there is little gain from its use.
SBAS is being implemented to allow high accuracy at greater ranges.

Three systems, all of which work on the same principle, are under development. These systems
are:

¾ Wide Area Augmentation System (WAAS), developed in the US
¾ European Geostationary Navigation Overlay System (EGNOS), developed in Europe
¾ Metsat Satellite Based Augmentation System (MSBAS), developed in Japan

All operate on the same basic principle and aim to provide accuracy sufficient to enable Category
1 precision approaches.

SBAS uses:

¾ A ground segment
¾ A space segment
¾ A user segment just like GPS

The ground segment consists of a number of precisely surveyed:

¾ Wide Area Reference Stations (WARS)
¾ Wide Area Master Station (WAMS)
¾ A Ground Earth Station (GES)

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GROUND EARTH GPS
STATION (GES) SATELLITE

Reference Station (WARS) Geostationary
Satellite

The WARS and WAMS stations are connected by communication data links. The network of
reference stations (WARS) track the satellites and data link information to the WAMS. The
WAMS:

¾ Collates all the data

¾ Determines the differential corrections for each tracked satellite

¾ Organises the data

¾ Formats a data broadcast.

This is sent to the ground earth station (GES) and uplinked to geostationary satellite.

In the American WAAS and the European EGNOS systems, the geostationary satellites used are
from the INMARSAT 3 series of marine communications satellites. Up to 4 of these will be
equipped with navigation packages. The Japanese system envisages the use of their
meteorological satellites MTSAT 1 and MTSAT 2.

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The geostationary satellite receives the data and transmits it to all users as a broadcast. The
broadcast uses the GPS L1 frequency, modulated with a C/A code of the same category as the
GPS C/A codes, and uses the same time basis. The signal also includes a ranging message so
that the geostationary satellite can also serve as an extra positioning satellite.

The message, broadcast by the geostationary satellite consists of:

¾ An integrity message indicating the status of all GPS satellites in a use/don’t use
format

¾ Wide Area DGPS error corrections
¾ An ionospheric delay model
¾ Ephemeris and clock data for the geostationary satellite

At the aircraft, the broadcast from the geostationary satellite is decoded. Using a wide-area
ionospheric delay almanac, the aircraft receiver determines the ionospheric delay for its position
and applies the necessary corrections. No tropospheric delay correction data is included, so any
correction calculation must come from the stored standard model.

RAIM IN THE WIDE AREA AUGMENTATION SYSTEM

The Required Navigation Performance, as a sole means of navigation, makes it necessary to
extend the role of Receiver Autonomous Integrity Monitoring. In a single navigation solution,
RAIM has to detect, isolate, and exclude the failed source.

The RAIM specification for RNP must include:

¾ Alarm Limits
¾ Alarm Response Time
¾ Limitations On Nuisance Alarms
¾ High detection probability
¾

GLONASS

GLONASS is the Russian-developed counterpart to GPS.

BASIC CONCEPTS OF THE GLONASS SYSTEM

The Russian Global Navigation Satellite System (GLONASS) is based on a constellation of active
satellites similar to GPS Navstar.

The planned space segment is:
¾ A constellation of 24 satellites
¾ Arranged with 8 satellites in 3 orbital planes
¾ Satellites are identified by a slot number, which defines the orbital plane (1-8, 9-
16,17-24) and their locations within the plane
¾ The three orbital planes are separated by 120° of longitude at the equator and the
satellites within the same orbit plane by 45° of arc.
¾ GLONASS orbits are roughly circular with an inclination of about 64.8°
¾ The orbit is at 19 000 km with a period of 11 hrs 15 m 44 s.

The ground control segment of GLONASS is entirely located in Russia.

18-22 Radio Navigation

Global Navigation Satellite System (GNSS) Chapter 18

The co-ordinate system definition of the GLONASS satellite orbits is in accordance with the PZ-
90 system, formerly the Soviet Geodetic System 1985l1990.

The time scale is defined as Russian UTC.

One difference between GLONASS and GPS is that the GLONASS time system includes leap
seconds. Satellites transmit simultaneously on two frequency bands, allowing the aircraft receiver
to correct for ionospheric delays. Each satellite is allocated a particular frequency within the band,
determined by the channel number of the satellite. Aircraft receivers identify each satellite by its
frequency channel.

Superimposed onto the carrier frequency, the GLONASS satellites modulate the navigation
message. Two modulations can be used for ranging purposes, the Coarse Acquisition code, with
a modulation length of 586.7 metres and the Precision code, of 58.67 metres. The satellites also
transmit:

¾ Orbital information
¾ An almanac of the entire constellation
¾ Correction parameters to the time scale

The ground control centre predicts the orbital values for a 24-hour period. The satellite transmits
a new set of orbital data every 30 minutes. The almanac is updated once per day.

INTEGRATED NAVIGATION SYSTEMS

The integration of the Navstar GPS and the GLONASS systems is a method of providing a truly
Global Navigation Satellite System (GNSS). The system could provide for an essential high level
of redundancy if the Required Navigation Performance (RNP) levels are to be achieved.

Receivers are available that are designed to decode both GPS and GLONASS signals.
Europe has announced that, in association with Russia, the GLONASS system will see further
development. This would considerably enhance the reliability of the system and could bring
GNSS close to meeting the needs of RNP.

GNSS tends to be used:

¾ As an integral part of a multi-sensor navigation system.
¾ As a secondary and supplemental system in which the GNSS and its augmentations

may be used, but another approved system must be available and useable at all
times.
The integration of GNSS and an inertial reference system (IRS) is one means of achieving RNP
today. This provides the levels of accuracy, integrity, availability, and continuity viewed as the
essential elements of RNP.

The advantage of this hybrid system is that the IRS continuously determines vector velocities to a
very high degree of accuracy. Combining these with the positional accuracy derived from GNSS,
results in navigation performance of a very high order.

Radio Navigation 18-23

Chapter 18 Global Navigation Satellite System (GNSS)

GNSS APPLICATIONS

GNSS with suitable augmentation will provide for:

¾ Accurate enroute navigation including area navigation capabilities
¾ Terminal area routing
¾ Precision approaches

In general, the navigation computer of the FMC processes the information and feeds it to the
AFCS and to a display such as an EFIS or something similar.

The Automatic Dependent Surveillance Broadcast (ADS-B) is also using GNSS. The system
digitises the position information derived from the GNSS and broadcasts it as part of a data
stream, which includes:

¾ Aircraft identification/flight number
¾ Aircraft type
¾ Altitude
¾ Speed
¾ Heading
¾ Flight condition (climbing, turning levels, etc.)

This data renews several times a second.

If the data stream is linked to a communications satellite ATC will be given a continuous
stream of real time data. Ideal for oceanic sectors, the same data stream transmitted
through the transponder mode S could also be used by ATC for both short range ATC
and, possibly, TCAS warnings.

18-24 Radio Navigation

INTRODUCTION

In the early days of commercial air transport expansion, a system of air routes was developed to
provide a safe means of control and separation of aircraft. These routes, which became known as
airways, were defined by radio navigation facilities sited at strategic distances apart and at
significant navigational points such as:

¾ Airway intersections
¾ Turning points
¾ FIR boundaries

Following an airway provides an aircraft with a system of navigational checkpoints, and, by being
enclosed (by ATC) in a clear box of airspace, separation from other known aircraft in the vicinity.
For many years, the airway system has provided an adequate means of routing aircraft, in spite
of the fact that navigation from departure aerodrome to destination is not normally the most direct
route.

In recent years however, a number of factors have led to a review of this situation. These include
such elements as increasing congestion on the airways system, resulting in flow control and
subsequent, frequently extensive, flight delays, and the need to conserve costs, which demands
following the shortest route from departure to destination. The development of improved and
enhanced navigation and communication systems permit an aeroplane’s position to be
determined accurately and transmitted speedily to the responsible ATC unit.

These enhanced ATC systems make it possible to provide aircraft with safe separation from other
air traffic without the need to confine them to narrow corridors of airspace.

Stave Faire Secon
VOR VOR Waypoint 2

Hanley Waypoint 1

Radio Navigation 19-1

Chapter 19 Area Navigation Systems

To respond to these problems and enhanced capabilities, a system known as Area Navigation
(RNAV) is being introduced. This is a system of navigation that is not dependant upon routing
between points coincident with the position of a radio facility. Instead, it is capable of providing
navigational guidance along other non–airway routes marked by waypoints.

A waypoint is a predetermined geographic position defined in terms of latitude and longitude.
Where appropriate, it may also be defined as a radial and range from VOR/DME beacons, which
is known as rho/theta (ρ/θ). Waypoints may also be defined by ranges from two DME beacons,
which is known as rho/rho (ρ/ρ).

Subject to an aeroplane being properly equipped, area navigation is available as follows:

Fixed Published RNAV Routes
These are usable in a flight plan only if the aeroplane has an approved RNAV capability.

Contingency RNAV Routes
These are published routes useable by suitably equipped aeroplanes during specified
times.

Random RNAV Routes
These are unpublished routes and may be flight planned within designated areas

AREA NAVIGATION CONCEPTS

JAR OPS 1 requires that an aeroplane’s navigation equipment should include “an Area
Navigation System when area navigation is required for the route being flown”. There are three
types of area navigation systems.

Self-contained systems obtain their navigation data from onboard sensors and are therefore
independent of external sources. Typical self-contained navigational systems use the outputs
from inertial reference systems (IRS) or inertial navigation systems (INS). The difference between
INS and IRS is covered later, but both provide navigation solutions without reference to external
systems. They update the position of the aircraft by sensing its accelerations and integrating them
with respect to time, establishing distance and direction of movement from the start position. An
integral navigation computer carries out all related navigation calculations. Although they are
extremely accurate from the initial position, the accuracy degrades with time.

Externally referenced systems derive information from external sources in order to provide
navigational guidance.

Hybrid systems use information from a combination of self-contained and externally referenced
navigation systems. Such systems are initially extremely accurate and maintain good accuracy

19-2 Radio Navigation

Area Navigation Systems Chapter 19

over time because the long-term accuracy of external radio navigation aids such as VOR and
DME counter the degradation of the self-contained system.

Many commercial operators have been quick to realise the benefits of such a system and are not
only specifying a suitable system for new aeroplanes but are, at considerable cost, actively
retrofitting their older fleets with similar equipment. Most equipment installations on commercial
aeroplanes form an integral part of a comprehensive avionics package and are capable of
providing area navigation even when out of range of ground-based navigation facilities. Such
systems are normally of the hybrid type.

Most modern general aviation aeroplanes have a basic RNAV system (so called BRNAV) as the
standard unit. These are generally based on the rho/theta or rho/rho system using inputs from
VOR/DME. GNSS, the Global Navigation Satellite System, based on satellite navigation, is
increasingly utilised as the prime source for the required navigation information.

ACCURACY OF RNAV EQUIPMENT

There are two types of RNAV:

Basic RNAV
The lateral track-keeping accuracy of basic RNAV is ± 5 nm for 95% of the flight time.

Precision RNAV
The lateral track-keeping accuracy of precision RNAV is ± 1 nm for 95% of the flight time.

The track keeping accuracy is dependent on the navigation system error and Flight Technical
Error. For obstacle clearance the Flight Technical Error is:

Departure ± 0.5 nm (at the DER a value of ± 0.1 nm is assumed)

Initial and Intermediate Climb ± 1 nm

Enroute 2 nm

BASIC RNAV

VOR/DME based area navigation is a navigation and guidance system that uses basic signal
inputs to compute track and distance to a waypoint. VOR bearings and DME slant ranging
provide the required information. More sophisticated systems may also provide barometric
altitude input. A block diagram of a simple navigation system appears below. The simple system,
most commonly installed in general aviation aircraft, usually consists of a computer that defines
each waypoint as a radial and range from a VOR/DME. Such waypoints are often called phantom
or ghost stations.

The computer’s memory is able to store a limited number of successive waypoints, normally a
maximum of nine, which enables the pilot to enter the planned route before departure. A more
sophisticated system utilises a navigational database stored either within the navigation computer
or in an external storage unit. The navigational database contains all the necessary information
regarding routes between airports, VOR/DME stations, and waypoints. It is obviously important
that this database is kept up to date. It should be updated every 28 days.

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Chapter 19 Area Navigation Systems

VOR / DME Computer Display
GNSS Pilot's CDU

Air Data Memory Auto Pilot

The Control Display Unit (CDU) is used to enter information into the computer and to display
navigation information.

In a basic system, the navigation computer resolves the navigation problem. The computer
receives a radial from the VOR receiver, DME distance from the DME interrogator, and altitude
from the encoding altimeter. It uses these parameters to establish the aeroplane’s current
position.

It compares this to the position of the next waypoint and generates an error signal, which
provides steering signals to a Course Deviation Indicator (CDI), Horizontal Situation Indicator
(HSI) or other suitable display. It also derives a distance to go reading.

In some aircraft installations, the computer may also send track correction signals (lateral steering
commands) to the autopilot roll channel. Since the use of Area Navigation Systems permits
waypoints to be accurately defined, determined and flown, this removes the need to follow the
facility-determined structure of the airways and permits direct routing and more effective use of
the available airspace. The aircraft equipment consists of the normal VOR/DME receivers, a
navigation (course line) computer, and a simple interface display.

19-4 Radio Navigation

Area Navigation Systems Chapter 19

USE OF BASIC RNAV

When operating the course line computer, the pilot selects a VOR/DME station that is within the
line of sight range of the desired waypoint. The radial and distance from the station to the desired
waypoint is then manually entered. This can be repeated for a number of waypoints (if the
equipment permits).

Once the waypoint information is stored, the pilot can select the sector (waypoint from and to)
and the course deviation indicator acts as if a VOR radial has been selected. Note that it is
possible to select sectors that do not connect successive waypoints. This allows waypoints to be
bypassed.

Begin navigating in the same manner as when tracking a VOR station. Distance to go appears in
the normal display.

With area navigation, the amount of CDI needle deflection does not vary with distance to the
waypoint as it would when tracking inbound to a VOR. It always represents a constant distance
off track for a given deflection.

On a 5 dot CDI, one dot deflection equals one mile of deviation, regardless of the distance to the
waypoint.

VOR/DME based RNAV has several applications:

¾ The ability to file a RNAV route flight plan
¾ The opportunity of tracking a direct route (subject to ATC restrictions and

requirements)
¾ The ability to navigate directly to an intersection bypassing waypoints.
¾ The ability to set up a holding pattern when the ATC instructs the pilot to “hold at

present position”
¾ The ability to locate and approach an airfield that is not equipped with navigation aids

Radio Navigation 19-5

Chapter 19 Area Navigation Systems

The plate shown above is the Oklahoma City RNAV approach procedure, which gives all
waypoints both in terms of radial/distance from the Oklahoma City VOR/DME, and
latitude/longitude. The chart is for information purposes only.

When using the RNAV approach mode, the system sensitivity is multiplied by 4. This means that
each dot on the CDI represents a linear displacement of 1/4 of a nautical mile.

19-6 Radio Navigation

Area Navigation Systems Chapter 19

RNAV LIMITATIONS

The VOR/DME based BRNAV suffers the dependent limitations of VOR/DME equipment. If this
signal is lost and the Nav warning flags appear on the CDI or HSI, the pilot must use an
alternative method of navigation.

A pilot may fly the area navigation route, using guidance signals to the waypoint, only as long as
the aircraft is within the operational range of the appropriate VOR/DME station. The practical
range limit is approximately 200 nm from the associated VOR/DME at the highest altitudes
normally used for civil aviation. Remember that operational altitude makes the range limit for a
light aeroplane normally considerably lower.
The accuracy of the position information and any derived steering signals are affected by the
same sources of error as the VOR/DME in use. Some systems use DME/DME navigation, with
frequency-scanning DME interrogators. Such systems provide more accurate navigation. All other
area navigation systems that depend on a single type of source, whether self-contained or facility-
dependent are affected by the errors of that source.

A hybrid system, in which the data from a number of sources is electronically compared and the
best information used, tends to provide a higher and more consistent degree of accuracy.

Radio Navigation 19-7

Chapter 19 Area Navigation Systems

19-8 Radio Navigation

INTRODUCTION

There is considerable pressure to make more effective use of the airspace in which pilots fly. As a
result, the industry is increasingly adopting the Area Navigation philosophy, and the modern
transport aeroplane is extremely well equipped to comply with the needs of an Area Navigation
System.

In simple terms, the fundamentals of navigation have not changed since ancient times. Pilots
must know where they are (current position), and where they are going (the destination). If they
cannot see their destination, they must be able to compute the direction and distance to that point
and must continuously monitor their progress to ensure that they are following the correct path.
The objective of navigation remains the same, but today the applicable tools to create a solution
are powerful beyond the dreams of navigators of even 50 years ago. The heart of this power is
the modern Flight Management System (FMS).

This chapter is an introduction to the FMS.

THE ROLE OF FMS

The FMS is an integrated automatic flight management system that provides, through precision
control of engine power and flight path, optimum economy of flight. At the same time, the FMS
reduces flight deck workload, which can considerably enhance safety. The diagram that follows
illustrates the many tasks the FMS can perform.

In such an integrated system, the FMS is interfaced with the Power Management Control System
(PMCS) and the Automatic Flight Control System (AFCS) so that it manages both control of
power and flight path (vertically and horizontally) against a pre-planned flight path.

The FMS consists of two units:

The Command Display Unit (CDU) is the crew’s interface with the system.

The Flight Management Computer (FMC) handles all the complex calculations and
memory items required.

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Chapter 20 Introduction to the Flight Management System (FMS)

The FMC has a data storage capacity, similar to the hard disk on a PC, which is in two broad
sections. One section is dedicated to aircraft performance data for take-off, climb, cruise,
descent, holding, go-around, and abnormal flight (e.g. engine-out) situations. This data has a
comparatively long life.

The second section, known as the Navigation database, stores all the data relevant to the
airline’s route structure. This includes:

Navigation Facilities: Position, frequency, identification, type

Waypoints: Latitude, longitude, type (enroute, etc.)

Airports & Runways: Designations, elevations, locations, etc.

Terminal Procedures: SIDs, STARs, holds, etc.

Approach & Go-around Procedures

Routes: Airway identifier, magnetic course

Company Routes: Details of normal routings.

This data requires frequent updates, and must be renewed every 28 days.

Using these two packages, along with the variable inputs (such as current position, wind velocity
from the aeroplane’s navigation computer, and air traffic clearance), it is possible to generate, or
modify, a flight plan to meet the current needs.

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Introduction to the Flight Management System (FMS) Chapter 20

Since the FMC has all the data required, the activities associated with following a precision RNAV
(PRNAV) route in three dimensions can be easily accommodated. All it requires is telling the
system the route to follow, the preferred flight profile, and the ATC clearance, accomplished
through the CDU. The aeroplane’s navigation sensors feed information to the FMC and, from
those sensors, it becomes possible to derive the best position information and find a solution to
the questions of how far to fly and in what direction.

The navigation sensors normally consist of a combination of inputs from facilities selected by the
FMC. A hybrid combination provides the necessary Required Navigation Performance to comply
with the needs of a precision RNAV (PRNAV).

These sensors may include some or all of the following:

¾ VOR/DME
¾ ILS
¾ IRS
¾ LORAN
¾ GNSS

At this time, none of these sensors alone can dependably provide the reliability, integrity, and
accuracy necessary.

¾ VOR/DME units are limited in range ability and the position accuracy deteriorates
with range from the facility.

¾ IRS suffers from cumulative position errors.
¾ LORAN does not provide reliable coverage nor is it worldwide.
¾ GNSS still requires augmentation.

The FMC evaluates the data from the available sources to derive the best position. In addition,
using inputs of altitude, airspeed, temperature, and Mach number from the air data computer,
along with engine parameters and fuel data, complete control of the flight profile can be
exercised. This ensures flight optimisation in terms of fuel efficiency.

USE OF FMS

The FMS is now such a critical item of equipment that two are normally installed. Two units allows
flexibility in usage:

Master/Slave Operation is the normal condition where one FMC is controlling and the
other is monitoring. All data entered into the controlling FMC is shared with the other
FMC. The computers talk to each other and, as well as sharing data, they compare each
other’s outputs. Each FMC retains control of its associated AFCS, auto-throttle, and
selection of radio navigation aids.

Independent use occurs wherein the FMS units operate independently. This allows the
pilots to operate with one unit displaying performance pages while the other displays
navigation data. Alternatively, one may be used to revise or review activities without
disrupting either the active flight plan or the commands of the other CDU.

Single use occurs when only one FMS is operational.

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Chapter 20 Introduction to the Flight Management System (FMS)

Back-up occurs when the FMC is suffering from some failure but a limited FMS function
still exists.

Within those parameters, the crew must decide how much control to give to the FMS. The options
are normally Managed Guidance, in which the FMS performs the task of maintaining the pre-
planned route, speed, and altitude profiles, or to control some parameter, such as a heading or
speed hold, through the use of the flight control panel.

A typical CDU appears below. Through this unit, the flight crew can:

¾ Construct a detailed flight plan
¾ Select data pages to view
¾ Respond to FMC requests for data entry
¾ Change displayed data

FMS design varies slightly from manufacturer to manufacturer but all have similar capabilities and
systems.

At the top of the CDU is the data screen. This is a flat CRT, normally providing up to 14 lines of
characters, each line providing space for up to 24 characters. Small sized characters are either
default or predicted values. The crew can change these if the data originates from the computer.
Large size characters show data entered by the crew.
The bottom line of the data screen provides for three activities.

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Introduction to the Flight Management System (FMS) Chapter 20

The left-hand side is a scratch pad that shows data entered by the pilot. The FMS uses the next
ten characters to pass messages to the crew, while the last two characters display up or down
arrows to show the direction of any necessary scroll (movement) up or down the screen. At the
end of each line on the CRT is a line key.

When the FMC requires data, a question mark (?) appears at the relevant line. For example, the
FMC needs to know the start position of the aeroplane in terms of gate number, so a question
mark appears. The pilot can now type the gate number in, using the alpha-numeric keypad. The
details typed appear on the scratch pad and, once verified, can be transferred to the correct line
by pressing the adjacent line key.

If an arrow appears against a line, it normally indicates an optional activity for the flight crew. This
could be either functional (e.g. aligning the IRS, deselecting a GNSS satellite) or display (e.g.
selecting another page of data). To choose the option, simply press the line key.

Function keys are also located below the CRT.

OUTPUT INFORMATION

Information derived from the FMS may be linked directly to the AFCS so that the aeroplane may
be flown through or by the FMS.

Information will normally be presented on the aeroplane’s EFIS system, with a map shown on the
nav section showing planned route, active sector, waypoints, etc. Details of attitude, speed,
vertical speed, etc., will appear on the PFD as normal.

The Instruments section of this manual provides full details of EFIS.

Information from the FMS may also be fed to the data transmission system for use in the ADS – B
system, as covered in the next chapter.

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Chapter 20 Introduction to the Flight Management System (FMS)

20-6 Radio Navigation

INTRODUCTION

This chapter looks at trends in the Communication, Navigation, and Surveillance/Air Traffic
Management (CNS/ATM, previously referred to as FANS) and provides an overview of each.

COMMUNICATIONS

Current activity in Europe, as elsewhere in the world, shows a much greater sensitivity to the
economic impact that ATM procedures have on aircraft operations. Much consideration is being
given to creating new procedures (or modification of existing procedures) that will lead to more
efficient fleet-wide operations. These services span oceanic, enroute, and terminal regimes. Most
of the changes are dependent upon effective Data Link facilities rather than increased complexity
of the FMS.

AIR TRAFFIC MANAGEMENT AND FANS 1

The integration of future Air Traffic Management ground systems with the Communication-
Navigation-Surveillance functions in aircraft avionics will enhance the ability to provide concise
scheduling, optimised time control, and well-defined, reduced air traffic separation. Aircraft
equipped with ATM-compatible avionics will likely benefit by receiving clearances as filed for the
scheduled landing time combined with minimising delays caused by weather and other aircraft.

CNS-ATM is currently in the early stages of system definition. The first step in the airborne side of
the equation (CNS) is the FANS 1 concept. Aircraft equipped with FANS 1 are already receiving
benefits on specific routes. The avionics functions required for FANS 1 are:

¾ Controller/Pilot Data Link Communication (CPDLC, also called TWDL or, offering
easier comprehension of what is meant, ATC Comm Data Link)

¾ Required Time of Arrival (RTA)
¾ GNSS input for time and position
¾ Required Navigation Performance (RNP)
¾ Automatic Dependent Surveillance (ADS)

These capabilities result in a fundamental change in airspace management today.

The automatic transmission of ground-to-air and air-to-ground clearance messages, flight
planning data, along with knowledge of the aircraft's intent, its scheduled arrival time and a
negotiated RTA at the metering fix provide the ground ATM systems with an unprecedented
amount of detailed information for each aircraft. This creates a better-managed airspace that
makes the intervention of the manager an exception rather than the rule. It will not be controlled
and will not require the intervention of the air traffic controller on a routine basis.

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Chapter 21 An Overview of CNS/ATM

Operators can realise the benefits associated with reduced aircraft separation as well as the
ability to fly preferred (or direct) routings, provided their aircraft are properly equipped to meet the
requisite RNAV level.

CONTROLLER/PILOT DATA LINK COMMUNICATION (CPDLC)

The definition and design of Data Link systems for communications between aircraft and Air
Traffic Control (ATC) agencies already exists in a very advanced state. To this end,
manufacturers have worked extensively with regulatory authorities, airlines, and service providers
to develop the Minimum Operational Performance Standards (MOPS) for airborne ATC
implementations. Data Link communications systems are being designed to provide more
efficient aircraft communications for ATC and Flight lnformation Services (FIS). Although these
systems essentially replace normal voice radio communications, a voice radio backup is essential
for ATC communications at this time.

Flight plan data, including aircraft position and intent (in the form of future waypoints), arrival
times, selected procedures, aircraft trajectory, destination airport, and alternates, will be
transferred to the ground systems for traffic management. The data sent to the ground ATM
system will aid in the process of predictions of where each aircraft will be at a given time. Conflict
management becomes simpler for the ground equipment, based on the use of actual flight plan
data, compared with making predictions on the ground. The ground system may decide that a re-
clearance (e.g. new flight plan) is necessary for one of the aircraft in a predicted conflict situation.
Through the utilisation of the RTA function on board the aircraft, a profile negotiation capability is
possible based on the available RTA and the current flight plan.

Voice Comm.
Data Links

FMS ATM
Air Ops

The diagram above shows how Data Link messages are exchanged between aircraft and the
ground ATM systems. Some operators may elect to use another link to their Airline Operations
Centre (AOC), flight operations, or flight planning services. This additional link may be used for
monitoring or intervention of flight planning route data, management of alternate airports, and
weather data. All links are two-way and require no pilot intervention, except to monitor messages
and to confirm and implement flight plan modifications. The monitoring may be accomplished via
the MCDU.

21-2 Radio Navigation

An Overview of CNS/ATM Chapter 21

AERONAUTICAL TELECOMMUNICATIONS NETWORK (ATN)

The Aeronautical Telecommunications Network (ATN) provides the means for world-wide data
communication between ground and airborne host computers within the aeronautical community.
It is a data communications network designed to allow interoperability between aircraft, airlines,
and air traffic control facilities and authorities.

ATN utilises satellite, VHF, Mode-S, and other specific sub-networks to transmit data from one
end system to another, linked by a common working protocol. When fully completed, the ATN will
allow any ATN host computer to communicate with any other ATN host computer without having
a direct physical link between them.

International agreements for the ATN are currently being documented in the International Civil
Aviation Organisation (ICAO), with regard to Secondary Surveillance Radar improvements,
Collision Avoidance Systems Panel, and Standards and Recommended Practices (SARPs).
These will establish communications protocols, and will ensure international interoperability
services and operational requirements are met. The ATN protocols are designed using existing
international standards as a basis.

The ATN will pave the way for major improvement in FAA/CAA Air Traffic Management services.
The use of the bit-orientated, transparent ATN protocols allows any type of application data to be
sent (including messages, graphics, and video). The aeronautical telecommunications industry is
currently defining Air Traffic Services (ATS) applications to operate over the ATN. These
applications provide a means for ATM facilities to communicate with the avionics and the flight
crew on Data Link equipped aircraft.

NAVIGATION

EUROCONTROL BRNAV AND PRNAV

The Eurocontrol region recognises two forms (or levels) of RNAV namely basic RNAV (BRNAV)
and Precision RNAV (PRNAV). BRNAV became mandatory in 1998 and PRNAV should be
mandatory by 2005.

The original BRNAV requirement of 99.999 percent availability has been removed. The existing
VOR route structure and an operating onboard VOR receiver are now expected to provide this
availability requirement.

The use of RNP 1 is intended to meet PRNAV requirements.

REQUIRED NAVIGATION PERFORMANCE (RNP)

ICAO created the RNP concept defining a 95 percent confidence area of position. In the RNP
concept, the equipment carried no longer defines the ability to fly in a particular airspace but
rather the equipment’s ability to meet defined accuracy, integrity, and continuity of service
requirements.

TERMINAL AREA INITIATIVES

For some time there has been a sustained effort to get more benefits from the existing FMS
equipment on board EFIS equipped aircraft. This effort has resulted in the definition of FMS
procedures for the terminal area airspace (often referred to as slant E procedures). A
considerable effort has been made on the use of Baro VNAV to provide additional final approach
benefits.

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Chapter 21 An Overview of CNS/ATM

Actual benefits received will typically be negotiated with the local authorities and may vary
depending on the ability to highlight situational awareness, coupled with a flight control system
and an auto throttle.

SURVEILLANCE

AUTOMATIC DEPENDENT SURVEILLANCE (ADS)

Automatic Dependent Surveillance (ADS) allows ground facilities to track aircraft current and
predicted status with minimal crew interaction. Crew interaction consists of selecting ON, OFF, or
EMERGENCY modes. All other operations are transparent to the crew. The general operation is
for the ground application to request data, and the aircraft application to supply the requested
data at the required interval.

AUTOMATIC DEPENDENT SURVEILLANCE – BROADCAST (ADS-B)

Automatic Dependent Surveillance - Broadcast, or ADS-B, is a system that transmits information
about an aircraft's state and intent at a predefined interval for use by both ground-based air traffic
control and other aircraft. This data is used for situational awareness and conflict resolution. The
expectation is for this information to be transmitted using the Mode S transponder Data Link. All
the information to be transmitted by ADS-B resides in the FMS function.

MODE S DATA LINK

This manual has already examined the principle of operation of the aeroplane’s transponder and
looked at the introduction of Mode S transponders.

Units are now available with a capacity to transfer such data as:

¾ Traffic information service
¾ Graphical Weather information
¾ Differential GPS uplinks
¾ Windshear alerts
¾ TCAS traffic resolution

The next development will be to provide the capacity to communicate simultaneously with
multiple ground stations while maintaining the capacity to conduct the TCAS function.

Development will, no doubt, continue in these important subjects and, as a professional pilot, it
will be your duty to remain up-to-date.

21-4 Radio Navigation

Reference: JAR 25 (Advisory Joint Material AMJ 25-11Electronic Display Systems)

GLOSSARY OF TERMS

Abbreviation Decode Abbreviation Decode
AC FAA
Advisory Circular (Published Federal Aviation
ACJ By The FAA) FAR Administration
ADF Advisory Circular Joint HSI Federal Aviation Regulations
ADI Automatic Direction Finder ILS Horizontal Situation Indicator
AFCS Automatic Direction Indicator INS Instrument Landing System
Automatic Flight Control JAA Inertial Navigation System
AFM System JAR Joint Aviation Authorities
AIR Aeroplane Flight Manual JTSO Joint Aviation Requirements
Aerospace Information Joint Technical Standing
AMJ Report (SAE) MEL Order
ARP Advisory Material Joint PFD Minimum Equipment List
Aerospace Recommended RNAV Primary Flight Display
AS Practice (SAE) ROM Area Navigation
CDI Aerospace Standard (SAE) RTCA Read Only Memory
CRT Course Deviation Indicator Radio Technical Commission
DOT Cathode Ray Tube RTO For Aeronautics
Department Of SAE Rejected Take-off
EADI Transportation Society Of Automotive
Electronic Attitude Director STC Engineers
ED Indicator Supplemental Type
EFIS EUROCAE Document TSO Certificate
Electronic Flight Instrument VOR Technical Standing Order
EHSI System VHF Omni-range Station
Electronic Horizontal
EUROCAE Situation Indicator
The European Organisation
For Civil Aviation Equipment

INTRODUCTION

The contents of the reference provide guidance related to pilot displays and specifications for
CRTs in the cockpit of public transport aircraft. This digest is intended as a reference document to
supplement the FMS and EFIS notes found in the Instruments and General Navigation sections
of the notes. This document has relevance to the exams and does provide the source for
numerous questions in both examinations. The questions in the exams will refer to JAR 25 and to
the Boeing 737-400.

Radio Navigation 22-1

Chapter 22 Electronic Display Systems

GENERAL

The document covers general information on the following General Certification Considerations:

¾ Display functions
• Colour symbology
• Coding
• Clutter
• Dimensions
• Attention getting requirements

¾ Display visual characteristics
• Failure modes
• Instrument display and formatting
• Specified integrated display and mode considerations including:
¾ Maps
¾ Propulsion parameters
¾ Warning and advisory checklist procedures
¾ Status displays

Initial electronic displays tended to follow the electromechanical display formats of the older style
aircraft. Expect significant improvements as electronic displays evolve. The JAA allows for
certification environments that will give flexibility yet still take into account flight safety.

GENERAL CERTIFICATION CONSIDERATIONS

DISPLAY FUNCTION CRITICALITY

New designs of electronic displays allow designers to integrate systems that are simpler to
operate. The automation of navigation, thrust, aeroplane control, and related display systems has
accompanied this integration. While the above statement is true regarding normal operations,
does it apply during the failure of a system?

Certainly, failure state evaluation and determination may become more complex.

LOSS OF DISPLAY

Criticality of flight and navigation data displayed is evaluated in accordance with JAR 25.
Normally, pilot evaluation is used in determining the criticality of electronic displays. During test
flying the test pilot will check:

¾ The detectability of failure conditions
¾ The required subsequent actions
¾ That the actions taken are within a line pilot’s capabilities

Note: For the remainder of this document the word “improbable” means “remote”.

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Electronic Display Systems Chapter 22

The following are listed as critical functions, and the loss of information to both pilots must be
improbable. Displaying hazardously misleading information must be extremely improbable.

¾ Attitude
¾ Airspeed
¾ Barometric Altitude

As with the critical functions, loss of information to both pilots of the following essential functions
must be improbable:

¾ Vertical Speed
¾ Slip/Skid Indications
¾ Heading
¾ Navigation

The display of erroneous information to both pilots must also be improbable.

Rate of turn information is a non-essential function.

NAVIGATION INFORMATION

Where there is a relationship between navigation information and communicated navigation
information, a non-restorable loss must be extremely improbable.

For navigation displays, hazardously misleading information on either pilot’s display must be
improbable. The term hazardously misleading has to be agreed for certification purposes, and
depends on the type of installation and flight phase. Generally, displaying both raw navigation
and multi-sensor data ensures that the displays to both pilots are correct.

PROPULSION SYSTEM PARAMETER DISPLAYS

The failure of a system in one engine must not adversely affect the accuracy of any parameter for
the remaining engines. No single fault shall result in the permanent loss of display of more than
one propulsion unit.

CREW ALERTING DISPLAY

The alerting display should be compatible with the safety objectives associated with the system
function for which it provides an alert.

FLIGHT CREW PROCEDURES

The display of hazardously misleading flight crew procedures must be improbable.

INFORMATION DISPLAY

The Basic T is generally retained in the glass cockpit.

Radio Navigation 22-3

Chapter 22 Electronic Display Systems

INFORMATION DISPLAY COLOURS

Display features should be colour coded as follows:

Display Colour

Warnings Red
Flight Envelope and System Limits Red
Cautions, Abnormal Sources Amber/Yellow
Earth Tan/Brown
Engaged Modes Green
Sky Cyan/Blue
ILS Deviation Bar Magenta
Flight Director Bar Magenta/Green

Specified display features should be allocated colours from one of the following colour sets:

Colour Set 1 Colour Set 2

Fixed Reference Symbols White Yellow*
Current Data, Values White Green
Armed Modes White Cyan
Selected Data, Values Green Cyan
Selected Heading Magenta** Cyan
Active Route/Flight Plan Magenta White

* The extensive use of the colour yellow for other than caution/abnormal
information is discouraged.

** In colour set 1, the intention is for magenta to be associated with those analogue
parameters that constitute fly to or keep centred type information.

Precipitation and turbulence areas should be coded as follows:

Precipitation Colour

0 – 1 mm/hr Black
1 – 4 mm/hr Green
4 – 12 mm/hr Amber/Yellow
12 – 50 mm/hr Red
Above 50 mm/hr Magenta
Turbulence White or Magenta

22-4 Radio Navigation

INTRODUCTION

The EFIS provides displays for most of the aircraft navigation systems. The system provides:

¾ Colour displays of pitch and roll
¾ Navigational maps
¾ Weather
¾ Radio altitude and decision height
¾ Autopilot
¾ ADF/VOR bearings
¾ ILS data
¾ Stall warning information

SYSTEM ARCHITECTURE

The system comprises the following components (See Diagram 1 — EFIS System Architecture):

¾ Electronic Horizontal Situation Indicators (EHSI) — located directly in front of the
captain and first officer

¾ Electronic Attitude Director Indicators (EADI) — located directly in front of the
captain and first officer

¾ EFIS Symbol Generators (SG)
¾ EFIS Control Panels
¾ EFI Transfer Switch

Both the captain and the first officer have EFIS display units. Left and right SGs provide video
signals to drive the respective display units. An EFI transfer switch determines whether:

¾ The left and right SGs drive the captain’s and first officer’s EADIs and EHSIs
respectively

¾ One SG drives all four displays

Radio Navigation 23-1

Chapter 23 Boeing 737 Electronic Flight Instrument System (EFIS)

EFIS SYMBOL GENERATOR

Two symbol generators receive inputs from various aircraft systems. The SGs respond to these
inputs and then generate the proper visual displays for the respective EADI and EHSI.

The EFIS control panel provides the system control.

Diagram 1 – EFIS System Architecture

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Boeing 737 Electronic Flight Instrument System (EFIS) Chapter 23

Navigation Systems Inputs Symbol Generator 1 Symbol Generator 2
VOR 1 and 2
VHF/NAV DME VOR 1 and 2 DME 1 and 2
Flight Management DME 1 and 2 LOC 1 and 2
Computer LOC 1 and 2 ADF 1 and 2
Air Data Computer ADF 1 and 2 SWC 2
Auto Throttle SWC 1 ADC 2
Inertial Reference System ADC 1 LRRA 2
Digital Flight Control System LRRA 1 G/S 1 and 2
Stall Warning Computer G/S 1 and 2 FCC A and B
Automatic Direction Finder FCC A and B IRS L and R
Weather Radar IRS L and R MCP
Radio Altimeter MCP A/T
Ground Proximity Warning A/T
System

EFIS CONTROL PANEL

The EFIS control panel controls:

¾ Display modes
¾ Ranges on the EADI and EHSI
¾ Selection of Decision Height
¾ Weather Radar on/off control

Radio Navigation 23-3

Chapter 23 Boeing 737 Electronic Flight Instrument System (EFIS)

Diagram 2 – EFIS Control Panel

EADI CONTROLS

The left-hand side of the panel, shown in Diagram 2, controls the EADI.

BRT: Controls the brightness of the EADI display.

DH REF: The LCD displays the selected decision height (DH). The DH also appears on
the EADI in the bottom right corner.

Underneath the DH REF display is the Decision Height Set Knob. This
control has a range of -20 to +999 ft. The decision height defaults to 200 ft
when power is applied. Turning the knob changes the DH.

RST: This manually resets the DH alert on the EADI. The radio altimeter display
changes from yellow to white.

23-4 Radio Navigation