AE8751 - UnitIV.pdf

AE8751 - AVIONICS
Dr. K. Kannan, M.E., M.E., Ph.D.,
Professor & Head,
Department of Mechatronics Engineering
UNIT IV
Introduction to Navigation Systems(9)

OBJECTIVES
• To introduce the basic of avionics and its need
for civil and military aircrafts
• To impart knowledge about the avionic
architecture and various avionics data buses
• To gain more knowledge on various avionics
subsystems

UNIT IV
INTRODUCTION TO NAVIGATION SYSTEMS
• Radio navigation
– ADF, DME, VOR, LORAN, DECCA, OMEGA,
ILS, MLS
• Inertial Navigation Systems (INS)
– Inertial sensors, INS block diagram
• Satellite navigation systems
– GPS.
CO4 : To realize the concepts of navigation systems

Navigation
The process of going from one place to other place
is called NAVIGATION.
For visual navigation, one of the most important
objects for navigation is a landmark, called
CHECKPOINT.
Navigation ability is needed in every flight, since
we go to other place and we need to know how to
get there and come back again.
Remember, if you can’t navigate properly, it could
lead to something called lost position.

Navigation is the process monitoring and controlling the
movement of an aircraft from a place to another place.
Pilots are navigators, since their duty is to monitor and
control the movement of Aircraft. As a pilot, to be able
to control it correctly, they need to AVIATE. Then, they
need to be able to tell where we are going. So the
pilot’s role is to NAVIGATE.
But , a pilot didn’t work alone. Along way, there must be a
contact with another aerodrome, i.e. Approach, Tower,
and Radar. So, there is need to COMMUNICATE.
Navigation

COURSE – It is the intended horizontal
direction of travel.
HEADING - It is the horizontal direction in
which an aircraft is pointed.
TRACK - Actual horizontal direction made by
the aircraft over the earth.
BEARING - Horizontal direction of one
terrestrial point from another.
Directions in Navigation

Radio navigation is the application of radio frequencies
to determine a position of an air craft on the Earth.
Inertial navigation system (INS) is a navigation device
that uses a computer, accelerometers and gyroscopes to
continuously calculate by dead reckoning the position,
the orientation, and the velocity of an aircraft without
the need for external references.
Satellite Navigation is based on a global network of
satellites that transmit radio signals from an aircraft
medium to earth orbit.
Navigation

Radio Navigation
Radio navigation is the application of radio frequencies
to determine a position of an air craft on the Earth.
The basic principles are measurements from / to electric beacons,
especially
- Angular directions, e.g. by bearing, radio phases or
interferometry
- Distances, e.g. ranging by measurement of time of
flight between one transmitter and multiple receivers or vice
versa,
-Distance differences by measurement of times of arrival of
signals from one transmitter to multiple receivers or vice versa
Combinations of these measurement principles also are
important—e.g., many radars measure range and azimuth of a
target.

• Automatic Direction Finder
• Distance Measuring Equipment
• VHF Omni Range
• LOng-Range Aid to Navigation
• Decca
• Instrument Landing System
• Microwave Landing System
Radio Navigation

Automatic Direction Finder
The Automatic Direction Finder (ADF) is a very important and
integral part of radio navigation. The ADF provides the pilot
with an indication of the direction of radio signals received
from selected stations operating in the low and medium
frequency range of 90 kHz to 1,800 kHz.
These stations include:
– Non-directional beacons (200 kHz to 415 kHz)
– Standard AM broadcast stations (540 kHz to 1,600 kHz).
Non-directional beacons (NDB) are identified by a CW signal
modulated with a 1,020-Hz tone that transmits a three-letter
identification code. Occasionally, NDBs will interrupt the CW
transmission with a voice transmission to provide weather
information and flight advisories.
When an NDB is used in conjunction with instrument landing
system markers, the beacon is referred to as a compass locator.

Compass locators are identified by a continuously transmitted CW two-
letter identification code.
Standard AM broadcast stations are identified by voice transmission of
the station call letters.
The concept of ADF navigation is based on the ability of the airborne
system:
To measure the direction of the arrival of the received signal
Provide a relative bearing indication with respect to the centerline of the
aircraft.
Using the bearing information displayed on the ADF indicator, the pilot
can determine the aircraft's position or can fly directly to the NDB
or AM broadcast station.
To determine the aircraft's position, the pilot simply:
– Plots the headings of two different stations on a navigation
chart
– Triangulates the aircraft location at the point where the two
lines intersect.
Automatic Direction Finder

Principles of ADF Navigation
Radio direction finders were developed in the early 1930's as
the first radio navigation device to be used for airborne
applications.
The early devices used an indicator with a left/right needle that
would center when the aircraft was pointed toward the
station.
The radio direction finder has developed into an automatic
system that continuously displays the direction to the station
by means of a pointer on the ADF indicator.
A means is usually provided to manually or automatically
rotate the compass card on the ADF indicator to the
aircraft's magnetic heading:
The pointer indicates the direction to the station,
The pointer indicates the magnetic heading the aircraft must take
to fly towards the station.

If the compass card is driven by a synchro, which receives
heading information from the compass system:
– The instrument is known as a Radio Magnetic
Indicator (RMI).
All ADF systems employ the directional characteristics of
a loop antenna to find the direction of the NDB or AM
broadcast station and non directional sense antenna to
determine where the station is.
The directional pattern of the loop antenna is:
– If positioned so that the ends of the loop are in
alignment with the incidence of the radio wave, the
received RF signal will be maximum.
If the loop is rotated 90o from this position, the signal will
fade out and this is known as the "null" position.

• A non-directional sense antenna is used to
determine which of the two 180o apart null
positions is the correct bearing to the station.
• Early ADF systems used a rotating loop antenna
and a long-wire sense antenna.
• Modern ADF systems use a goniometer which
eliminates the requirement for the loop to rotate.
ADF systems with non-rotating loops antennas
are packaged in a compact module together with
the sense antenna and RF amplifier to afford less
drag and greater reliability.

ADF indication for Aircraft Position

ADF Antenna Theory
The operation of an ADF system is based on the directional
characteristics of the loop antenna to determine the direction of the
incoming RF signal and a sense antenna which determine from
where the signal is coming.
The loop antenna consists of a continuously wound coil.
When the magnetic lines of force from an incoming RF wave cut across
the coil, a voltage is induced in the antenna. Because of the transit
time of the wave, the voltage induced at the leading edge of the loop
(relative to the direction of the incoming signal) will lead the
voltage induced at the trailing edge. The algebraic sum of the
induced voltages will result in maximum voltage when the plane of
the loop is aligned to the incoming RF wave.
As the loop is turned 90o to the direction of the RF wave equal and
opposite:
– Voltages are induced in the sides of the loop which cancel
each other to result in a zero voltage output.

• The point of rotation where the resultant output is zero
is known as the null position of the antenna. At the null
position, a fairly accurate indication of the station
direction can be determined.
• ADF loop antennas are automatically rotated to the null
position by means of a servomotor. The mechanical
position of the shaft of the servo used to rotate the loop
will reveal the bearing to the station.
• The shaft is mechanically coupled to synchro which
mechanically coupled to the ADF pointer to provide
bearing information.
ADF Antenna Theory

The bidirectional figure-8 pattern of a loop antenna
causes it to null in two positions that are 180o apart.
– This condition can result in wrong ADF pointer
indication since the pilot would not know whether the
aircraft was pointed toward the station or away from
it.
ADF Antenna Theory

This problem is eliminated by the use of an omni-directional,
open-wire sense antenna:
– to provide an additional input signal which is 90o out-of-
phase with the signal received from the loop antenna.
The phase of the loop output will always differ by 90o from
that of the sense antenna,
– A 90o phase shift is added to the loop voltage to cause this
voltage to vary with respect to the constant sense antenna
voltage as the loop changes direction.
By combining the loop and sense antenna voltages, a cardioids
directional pattern results with only one null position.
ADF Antenna Theory

Loop and Sense antenna pattern
combine to form Cardioid's

A typical ADF system consists of:
– A loop antenna
– Sense antenna
– Receiver
– Control head
– Bearing indicator
The function of the ADF control head is to select the desired
frequency and mode of operation.
These modes include:
– Normal ADF operation using both the loop and sense antennas
– Loop-only mode to manually position the loop antenna to its null
position
– Sense-only mode for radio reception without direction finding.
Other functions include a beat frequency oscillator switch to
produce a 1,020-Hz tone to modulate a CW signal so it is
audible.
ADF Circuit

RF signals induced into the coil windings of the loop antenna are fed to
the loop amplifier contained within the receiver.
From here, the amplified loop signal is shifted 90o and fed to a
balanced modulator which is used to derive the variable-phase signal
from the loop antenna.
A fixed-frequency reference signal from the oscillator is introduced
into the balanced modulator to modulate the carrier signal received
from the loop antenna.
As a result, the carrier signal is replaced with two sideband frequencies,
the upper and lower sidebands.
– The upper sidebands are derived from the sum of the carrier
frequency and the reference frequency.
– The lower sidebands are derived from the difference of the carrier
frequency and the reference frequency. These sideband products are
added to the fixed-phase carrier signal received from the sense
antenna.
ADF Circuit

The resultant signal is detected and amplified in the super
heterodyne receiver. The modulation product from one of
the sidebands is separated from the audio to be used as the
loop signal. The station is to the right of the aircraft if the
loop signal will be in-phase with the reference signal The
station is to the left of the aircraft if the loop signal will be
out-of-phase with the reference signal.
The loop signal is sent to the phase detector, which outputs the
loop drive voltage.
The loop drive voltage positions the loop antenna to its null
position.
– The loop signal will be zero.
The loop antenna is driven by a two phase induction motor:
– One winding is coupled to the reference voltage
– The other winding is coupled to the signal voltage
from the phase detector, to position the loop antenna.
ADF Circuit

The rotatable loop antennas have since been replaced with
stationary loop antennas. The fixed loop antenna consists of
two coils positioned 90o to each other.
– Each coil is connected to one of two goniometer windings
which are also 90o apart.
The goniometer resides in the ADF receiver and has a rotating
winding that positions itself in relation to the induced
voltages in the loop antenna.
In recent years:
– goniometers have since been replaced with solid-state
circuitry
– ADF pointers have been replaced with digital readouts,
thus eliminating all moving parts and increasing reliability.
ADF Circuit

Distance Measuring Equipment
DME (Distance Measuring Equipment) has been
standardized by the ICAO as a radio aid for short and
medium-distance navigation.
It is a secondary type of radar, which allows several
aircraft to simultaneously measure their distance from a
ground reference (DME transponder).
The distance is determined by measuring the propagation
and delay of a RF pulse, which is emitted by the aircraft
transmitter and returned at a different frequency by the
ground station after reception.
The DME 415/435 ground equipment, is constructed by
THALES Air Systems Division - Milan - Italy.

DME provides to aircrafts:
- Straight-line distance to the DME ground
station.
- Aircraft ground-speed.
- Time to DME ground station. (If the aircraft is
flying straight to the DME ground station)
Distance Measuring Equipment

DME Coverage
The DME coverage is limited by the line of
sight, if there isn’t line of sight between the
emitter and the receiver there will not be
communication link.
From 0 to 65 NM radius and above 65 NM.

DME Association
Can be used stand-alone or Master.
It’s always used in association with: TACAN and
VORTAC.
It’s usually used in association with: VOR or
ILS.

Basic Principle of DME
When a signal is sent by the aircraft on board DME
(interrogator), the on board DME starts counting
the time until it gets a reply from the ground
station.
The resulting time depends of the DISTANCE, the
propagation speed and the signal reflections.
The DME ground station transponder generates
replies (artificial echoes) and sends it back to the
aircrafts (“Reverse” Secondary RADAR
principle).

The time interval between interrogation emission
and reply reception provides the aircraft with
the real distance information from the ground
station.
This information may be read by the pilot or the
navigator directly on the airborne indicator.

The Ground station is identified by a Morse (3 or 4
letters) coded tone modulated at 1350 Hz.
DME frequency rang is UHF : 960 MHz to 1215
MHz.
DME have 252 Channels which are separated by 1
MHz .
126 X channel and 126 Y channel.
The ground station signal frequency answer is
always: [interrogator signal frequency] ± 63MHz

The Interrogator sends pulse pairs with a fixed time
separation between the 1st and the 2nd pulse: 12μs
(Channel code X) or 36μs (Channel code Y)
– The time separation between pulse pair is randomly
generated by the on board DME transmitter.
– The pulse time is ~3,5μs

The ground transponder is able to answer up to
about 200 interrogators at a time.
Search Mode : On board DME will send 150
pp/s until it finds the ground station.
Track Mode : When On board DME connected
with the ground station so it sends 24 pp/s.
pp/s= pair pulse / seconds

The error is near to zero when the aircraft is far from the
ground station and it increases when the aircraft is near
from the ground station in the range of 0,5NM.
DME performance is not affected by the weather
conditions.
Control by a Personal Computer (PC) at beacon site,
which can be duplicated at remote site.
Used with below software inside computer:
– WINDOWS SUPERVISOR
– WINDOWS ADRACS SUPERVISOR
– EQUIPMENT MANAGER

DME - Antenna
The suggested antenna for the DME
415-435 DME equipment is the
omnidirectional DME antenna.
This antenna is provided with two
obstruction lights which may be
turned on and off during the day
by an automatic night switch.
The antenna for the DME has
vertical polarization 9-dB gain.

VOR
VOR stands for VHF Omni-directional Range.
A VOR is a radio beacon that transmits a signal
that represents the 360º of the compass.

VOR Ground Station
Above is a VORTAC Station. VORTAC stands for VOR +
TACAN. TACAN Stands for Tactical Air Navigation and it
includes DME.
DME is Distance Measuring Equipment, and it’ll tell you how far
you are from the station. The “counterpoise” is the base of the
station and provides grounding of the station.

VOR Service Coverage
VOR broadcasts
from 108.000-
117.950 MHZ.
It’s operational
service volume
is up to 130 NM
from the station
(upper right).

Types of VOR Stations
There are 3 types of VOR
Stations.

VOR Phases
The VOR signal is comprised of
a Reference Phase and a
Variable Phase.
The Reference Phase is
broadcast in all directions.
The Variable Phase is a rotating
beam.
The difference of phase between
the Reference Phase and the
Variable Phase is used by the
VOR receiver in the airplane to
calculate the bearing from the
station.

VOR Transmitting Signals
The Reference and Variable Phase
signals cannot be mixed during
transmission.
To keep them apart, the Reference
Phase is placed on a “subcarrier”.
At resting frequency of 9960 HZ,
the subcarrier is shifted up and
down in frequency by the
Reference Phase 30x/sec. (FM).
The subcarrier increases in
frequency going positive and
decreases frequency going
negative.
N is indicated at max positive
shown by the left arrow and is at
10,440 HZ.
S is indicated at max negative
shown by the right arrow and is at
9480 HZ.

VOR Signals
The carrier rises and falls in strength
(AM) of VOR signal appears at the
receiver.
The signal is at max strength when
the rotating beam is pointed directly
at the airplane shown by the blue
arrow.
The red arrow shows the highest
frequency of the subcarrier which
occurs at N.
The receiver compares the 2 signals
by measuring the phase of each
signal then calculates the difference
as a magnetic course from the VOR.

VOR Indicator
The VOR course is selected by turning the OBS
(Omni Bearing Selector) knob to the desired course.
The Course Deviation Indicator (CDI) displays
steering commands.

Horizontal Situation Indicator (HSI)
The compass card is slaved to the compass system and shows the
aircraft’s heading automatically.
The course is selected by a knob located elsewhere and is
displayed by digitally in the upper right corner and by the course
needle, here it is 20°.

Radio Magnetic Indicator (RMI)
The RMI displays both VOR and ADF
(automatic direction finder) information. The
compass card is slaved to the compass system.

LOng RAnge Navigation
Loran-C is a hyperbolic radio navigation.
The systems operate on the principle that the
difference in the time of arrival of signals from
two or more stations, observed at a point in the
coverage area, is a measure of the difference in
distance from the point of observation to each of
the stations.
Loran employs time difference measurements of
signals received from at least three fixed
transmitting stations. The stations are grouped to
form a 'chain' of which one stations is labelled the
master (designated M) and the others are called
secondary stations (designated W, X, Y, or Z).

For a given master-secondary pair of stations, a
constant difference in the time of arrival of
signals defines a hyperbolic Line Of Position
(LOP).
Second master-secondary pair results in a second
LOP.
The position fix is achieved by observing the
intersections of the two LOPs on specially
latticed Loran-C charts.
LOng RAnge Navigation

• Master / Slave The master transmits a set of 8 plus 1
pulses. The pulses are received at the aircraft and at W,
X, Y and Z.
• When the aircraft receives the first master pulse, it
starts a timing clock. When the secondary stations
receive the first master pulse, they wait for a short time
known as a coding delay and then each transmits a
similar set of 8 pulses.
• The ship receives the pulses from W, X, Y and Z and
times the interval between receiving the master pulse
and receiving each of the four secondary pulses.
Master / Slave

Coding Delay
The coding delay is such that the aircraft will
always receive the master station pulse first, then
W pulse, then X pulse then Y pulse and finally Z
pulse.
The coding delay also is such that the pulses do not
overlap as they are received.
After a short interval of between one twentieth to
one tenth of a second, the master station transmits
another set of pulses and the cycle repeats.

Time Difference
The position of the aircraft determines the time
differences. If we know the time differences,
we know the aircraft’s position.

Ninth Pulse
It enables the Loran receiver to identify the
master station.
It is used to transmit warnings if any station is
not transmitting correctly. The warnings
trigger alarms in the Loran receiver.

Time Difference Measurement
Pulse matching
Cycle matching

Group Repetition Interval
Each chain sends its pulses at a specified Group
Repetition Interval (GRI).
There are several different intervals.
Each is a few hundreds of microseconds less
than 50,000, 60,000, 80,000, 90,000 or
100,000 μ seconds.
Examples;49900 μ sec known as Station
499059300 μ sec known as Station 5930

Time Difference Measurement
Uses the third cycle of the received pulse because;
The start of the received pulse may be too weak to
be heard.
The master and secondary signals may not be
received at the same strength.
It is possible to accurately identify the time when
the third cycle ends and time this point.
This part of the pulse arrives at the ship before there
can be any sky wave interference.

Accuracy
The accuracy of measuring the timing delays
(0.1 μ sec).
The angle between the Loran lines of position
(LOP).
The position of the aircraft in the Loran coverage
area, that is whether the position is near the
base line or the base line extension.

Additional Secondary Factor (ASF)
The Latitude/Longitude computation in many
receivers is based upon a pure seawater
propagation path.
Over land distances signals travels at a slower
speed.
For those receivers that accommodate the correction
it is called an Additional Secondary Factor (ASF)
correction, and this is applied automatically when
the receiver computes the latitude and longitude.

eLoran
Enhanced Loran, or eLoran, is independent of GPS but
fully compatible in its positioning and timing
information, and its failure modes are very different.
eLoran is based on the existing low frequency Loran-C
infrastructures that exist today in the United States,
Europe, and Far East, and in fact throughout much of
the northern hemisphere.
It is an internationally recognized positioning and timing
service, the latest evolution of the low frequency long-
range navigation (Loran-C) radio navigation system.

eLoran
Perhaps the most exciting changes from Loran to eLoran are
the new operating concepts.
All transmitters are timed directly to UTC, so that a user
may use all eLoran signals in view and may combine
them with GNSS signals for robust position and time
solutions.
Each transmitter includes a messaging channel; this is an in-
band signaling channel that allows the eLoran signal to
also carry information to improve the user's solution.
Very much like GPS this messaging channel provides
transmitter identification, time of transmission,
differential corrections, and authentication and integrity
signals.

DECCA
Hyperbolic radio navigation system.
Determines the position of an aircraft using radio
signals and fixed navigational beacons.
Uses low frequencies from 70 to 129 kHz.
First deployed- by Royal Navy during World
War II -to predict accurate landing. After the
war it was extensively developed around the
UK and later used in many areas around the
world.

DECCA'S PROGRESS
Decca's was primarily used in ship navigation in
coastal waters.
offered much better accuracy than the LORAN
system.
Decca was replaced, along with Loran and other
similar systems, by the GPS during the 1990s.
The Decca system in Europe was shut down in
the spring of 2000, and the last worldwide
chain, in Japan, in 2001.

PRINCIPLE OF OPERATION-WORKING
• The Decca Navigator System consisted a
number of land-based radio beacons in chains.
Each chain hada master station andthree slave
stations - Red, Green and Purple.
• PLACED at :the slaves positioned at the -
vertices of an equilateral triangle with the
master at the centre. The baseline length - the
master-slave distance - 60–120 nautical miles
(110–220 km).

WORKING STEPS
• Each station transmitted a continuous wave signal.
comparing the phase difference of the signals- resulted
in a set of hyperbolic lines of position - pattern.
• Three Slaves - three patterns.
• The patterns on- nautical charts -set of hyperbolic lines
in the appropriate colour.
• Receivers identified which hyperbola they were onAnd
a position could be plotted at the intersection of the
hyperbola from different patterns - by using the pair
with the angle of cut closest to orthogonal as possible.

Issues faced
If two stations transmit at the
same phase-locked frequency
then - the difference in phase
between the two signals is
constant along a hyperbolic
path.
But if two stations transmit on
the same frequency -
impossible for the receiver to
separate them- so nominal
frequency of 1f was alloted.

• It was phase comparison at this common frequency
that resulted in the hyperbolic lines of position.
Hence Decca receivers multiplied the signals received
from the Master and each Slave by different values to
arrive at a common frequency (least common
multiple, LCM) for each Master/Slave pair, as
follows:
Issues faced

LANES AND ZONES
The interval between two adjacent hyperbolas on -
the signals are in phase was called - lane.
But Early Decca receivers were fitted with three
rotating Decometers - indicated the phase
difference for each pattern.
Each Decometer drove a second indicator - counted
the number of lanes traverse deach 360 degrees of
phase difference - one lane traversed.
In this way, assuming the point of departure was
known, a more or less distinct location could be
identified.

Zone Groups
Zone width Like the lanes were grouped into
zones: with 18 green, 24 red or 30 purple lanes
in each zone.
This meant that on the baseline- the zone width
was the same for all three patterns of a given
chain.

MULTIPULSE
Multipulse automatic technique- of lane and zone identification.
METHODOLOGY:
The nominally continuous wave transmissions -divided into a 20
second cycle. With each station in turn simultaneously transmitting
all 4 Decca frequencies (5f, 6f, 8f and 9f) - phase-coherent
relationship for seconds each cycle.This transmission, known as
Multipulse.
Allowed the receiver to extract the 1f frequency -so to identify which
lane the receiver was in.As well as transmitting the Decca
frequencies of - 5f, 6f, 8f and 9f, an 8.2f signal- known as Orange -
was also transmitted.The beat frequency between the 8.0f (Red) and
8.2f (Orange) signals allowed a 0.2f signal to be derived -
corresponds to 5 zones.Accuracy was maintained deeply here.

RANGE AND ACCURACY
During daylight ranges are around 400 nautical miles (740 km)
could be Obtained. Reducing at night to 200 to 250 nautical
miles (460 km).
The accuracy depended on:
Width of the lanes.
Angle of cut of the hyperbolic lines of position
Instrumental errors
Propagation errors (for example, Skywave)
By day these errors could range from a few meters .At night,
skywave errors were greater and on receivers- without
multipulse capabilities -jumps zones.

OMEGA
The first truly global radio navigation system for
aircraft.
It enabled an aircraft to determine their position
by receiving very low frequency (VLF) radio
signals transmitted by a network of fixed
terrestrial radio beacons, using a receiver unit.
It became operational around 1971 and was shut
down in 1997.

OMEGA - History
John Alvin Pierce, the
"Father of Omega,"
first proposed the use
of continuous wave
modulation of VLF
signals for navigation
purposes in the
1940's.

After experimenting with
various frequencies,
he settled on a phase
stable, 10 kHz
transmission in the
1950's. Thinking this
frequency was the far end
of the radio
spectrum Pierce dubbed
the transmission
"Omega," for the last
letter of the Greek
alphabet.
OMEGA - History

There were eight Omega transmitting stations,
located in Norway, Liberia, Hawaii, La Réunion,
Argentina, Australia, USA, and Japan.
The very-low-frequency signals from the
transmitters were detected by aircraft's navigation
receiver, and slight differences (phase differences)
between the signals indicated the position of the
receiver.
The system was accurate to within 4 km/2.5 mi
during the day and 7 km/4 mi at night.
OMEGA

Omega was a Cold War inspired long range
navigation system which expanded the
principles of Decca and Loran. It used
synchronized, ultra-low frequency radio to
create a globally intersecting grid of Lines of
Position (LOP's) which could penetrate
underwater.
OMEGA

OMEGA - Operation
Signal Characteristics
Omega utilized CW (continuous wave) phase
comparison of signal transmission from pairs
of stations. The stations transmitted time-
shared signals on four frequencies, in the
following order: 10.2 kHz, 11.33 kHz, 13.6
kHz, and 11.05 kHz.

Each Omega station transmitted a very low
frequency signal which consisted of a pattern of
three/four tones unique to the station that was
repeated every ten seconds.
If an Omega receiver picked up signals from three
stations, it would compute a vessel's location by
phase comparison. This means that the receiver
determined what direction each signal from was
coming from; the vessel was at the point where
the bearing to Station A intersected the bearings to
Stations B and C.
OMEGA - Operation

OMEGA- Accuracy
• Omega was very accurate for its time. In the late
1960s, when Omega began operation, navigation
was generally the result of a comparison of a dead
reckoning position (the computed position of the
vessel) with the results of "shooting a star" with a
preset sextant. Navigators had to compute the
difference between the position preset from the
dead reckoning position and the position obtained
by observation. This method was accurate, with
errors of not more than 1 nautical mile, but
required about 20 minutes to take three "star
shots" and do the math for each.

By comparison, the Omega signals penetrate not
only water but also sea ice to at least 15
meters, making the very risky business of
surfacing completely unnecessary.
The very low frequency (VLF)
transmissions of Omega would be almost
completely unaffected.
OMEGA- Advantages

OMEGA - Termination
The Omega Navigation System website, operated by the U.S.
Naval Observatory, says it all: "As of September 30, 1997,
0300 UT, the OMEGA Navigation System terminated. All
eight OMEGA stations, NORWAY (A), LIBERIA (B),
HAWAII (C), NORTH DAKOTA (D), LA REUNION (E)
ARGENTINA (F) AUSTRALIA (G) AND JAPAN (H))
around the world have permanently ceased to operate...
OMEGA, the first world wide radio navigation system,
operated for over twenty-six years. Users must no longer
depend on OMEGA broadcasts for navigation of any kind."

Instrument Landing System (ILS)
Before Avionics, Landing and departure followed
Visual Meteorological Conditions (VMC) when
the weather conditions are good. When weather
worsens, it becomes IMC for instrument
Meteorological Conditions. Pilots then fly IFR
under instrument flight rules. Two crucial
parameters under IMC are Visibility and Ceiling.
Visibility is the horizontal distance one can see
and recognize objects whereas Ceiling is the
height of the bottom of the clouds.

A precision landing system is required for visibility less than
2600 feet and ceiling less than 200 feet.
A precision landing system must provide horizontal and vertical
guidance along with the approach path to the desired runway.
ADF provides directional info towards NDB located at airport.
VOR and NDB provides non – precision landing guidance.
A precision of an approach is provided by Ceiling and Visibility
and these are reported by weather observers and broadcast to
pilots.
At large airports, visibility is measured by a transmissometer,
which sends a light beam in runway and measures how much
is lost over a short distance. This results in and Runway Visual
Range.

The beginning of an instrument approach is initial approach
fix (IAF) and final approach fix (FAF) is where the
aircraft takes the same heading as the runway and the
remainder of the approach is a straight line to the runway.
Today instrument landings end with a visual touchdown
where the pilot must see the runway to land. If the pilot
does not make visual contact with the runway, he must do
a go-around, called as misses approach point.
Some aircraft equipped with autoland facility which enables
a touchdown and rollout in completely.

ILS is a ground-based instrument
approach system that provides precision
guidance to an aircraft approaching and
landing on a runway, using a combination of
radio signals and, in many cases, high-intensity
lighting arrays to enable a safe landing
during instrument meteorological conditions
(IMC), such as low ceilings or reduced
visibility due to fog, rain, or blowing snow.

ILS Components
ILS Consists of the following:
LOCALIZER
GLIDE PATH/SLOPE
MARKER BEACON
APPROACH LIGHTING SYSTEM

LOCALIZER
One of the main components of the ILS system is
the localizer which handles the guidance in the
horizontal plane. The localizer is a VHF radio
transmitter and antenna system using the same
general range as VOR transmitters (between MHz
and MHz). Localizer frequencies, however, are
only on odd-tenths, with 50 kHz spacing between
each frequency. The transmitter and antenna are
on the centerline at the opposite end of the
runway from the approach threshold.

The localizer, or VHF course marker, emits two
directional radiation patterns. One comprises of a
bearing amplitude-modulated wave with a
harmonic signal frequency of 150 Hz and the
other one with the same bearing amplitude-
modulated wave with a harmonic signal
frequency of 90 Hz. These two directional
radiation patterns do intersect and thus create a
course plane, or a horizontal axis of approach,
which basically represents an elongation of the
runway’s axis.
LOCALIZER

The signal of the localizer launches the vertical indicator
called the track bar (TB). Provided that the final
approach does occur from south to north, an aircraft
flying westward from the runway’s axis is situated in an
area modulated at 90 Hz, therefore the track bar is
deflected to the right side.
LOCALIZER

On the contrary, if the plane’s positioned east from the
runway’s axis, the 150 Hz modulated signal causes the track
bar to lean out to the right side. In the area of intersection,
both signals affect the track bar, which causes to a certain
extent a deflection in the direction of the stronger signal.
Thus if an aircraft flies roughly in the axis of approach
leaned out partially to the right, the track bar is going to
deflect a bit to the left.
LOCALIZER

GLIDE SLOPE/PATH
The glide slope, or angle of the
descent plane provides the
vertical guidance for the pilot
during an approach. It’s
created by a ground UHF
transmitter containing
an antenna system operating
in the range of 329 MHz, with
a channel separation of 50
kHz.

GLIDE SLOPE/PATH
Like the signal of the localizer, so does the signal of
the glide slope consist of two intersected radiation
patterns, modulated at 90 and 150 Hz. However
unlike the localizer, these signals are arranged on
top of each other and emitted along the path of
approach

Marker Beacon
The outer marker is located 3,5-6 NM ( km)
from the runway’s threshold. Its beam
intersects the glide slope’s ray at an altitude of
approximately 1400 ft ( m) above the runway.
It also roughly marks the point at which an
aircraft enters the glide slope under normal
circumstances, and represents the beginning of
the final part of the landing approach.

The middle marker is used to mark the point of
transition from an approach by instruments to a
visual one. It’s located about 0.5 to 0.8 NM (926
to 1482m) from the runway’s threshold. When
flying over it, the aircraft is at an altitude of 200
to 250 ft (60.96-76.2 m) above it. The audio
signal is made up of two dashes or six dots per
second. The frequency of the identification tone is
1300 Hz. Passing over the middle marker is
visually indicated by a bulb of an amber (yellow)
colour.
Marker Beacon

The inner marker emits an AM wave with a
modulated frequency of 3000 Hz. The
identification signal has a pattern of series of
dots, in frequency of six dots per second. The
beacon is located 60m in front of the runway’s
threshold.
Marker Beacon

APPROACH LIGHTING SYSTEM
It assists the pilot in transitioning from
instrument to visual flight, and to align the
aircraft visually with the runway centerline.
Pilot observation of the approach lighting
system at the Decision Altitude allows the pilot
to continue descending towards the runway,
even if the runway or runway lights cannot be
seen, since the ALS counts as runway end
environment.

ILS Categories
Two Types
Precision ( All elements of ILS)
Non-precision (NDB, VOR and DME)
Precision Approaches
CAT I
CAT II
CAT III
CAT III A
CAT III B
CAT III C

ILS Limitations
It only has 40 channels. It only can serve one runway,
causing congestion in bad weather.
It is subject to interference by powerful FM broadcasts. It
can be blocked by terrain.
The azimuth and glide slope beams are fixed and narrow. As
a result, aircraft have to be sequenced and adequately
separated which causes landing delays.
There are no special procedures available for slower aircraft,
helicopters, and Short Take Off and Landing (STOL)
aircraft.
ILS cannot be sited in hilly areas and it requires large
expanses of flat, cleared land to minimize interference with
the localizer and glide slope beams.

Microwave Landing System
The Microwave Landing System (MLS) was
designed to replace ILS with an advanced
precision approach system that would overcome
the disadvantages of ILS and also provide greater
flexibility to its users.
It is a precision approach and landing system that
provides position information and various ground
to air data. The position information is provided
in a wide coverage sector and is determined by an
azimuth angle measurement, an elevation
measurement and a range measurement.

Advantages of MLS
It has 200 channels and can handle curved and stepped approaches &
be used to land aircraft on aircraft carriers.
The glide slope is selectable, which can handle steeper approaches used
for helicopters.
It is not subject to interference from FM radio stations. It is not subject
to blockage from terrain.
The azimuth coverage is ± 40° of the runway on- course line and glide
slopes from .9° to 20° can be selected. The usable range is 20 nm
from the MLS site
It operates in the SHF band, MHZ. This enables it to be sited in hilly
areas without having to level the site.
Course deviation errors (bending) of the localizer and glide path
caused by aircraft, vehicles and buildings are no longer a problem
because the MLS scanning beam can be interrupted and therefore
avoids the reflections. Because of its increased azimuth and
elevation coverage aircraft can choose their own approaches. This
will increase runway utilization and be beneficial to helicopters.

MLS Azimuth Beam
A narrow scanning beam from the
MLS sweeps back and forth
beyond either side of the runway.
A new receiver was made to
receive MLS, called a Multi-
Mode receiver and can handle
ILS, MLS and GPS.
An arriving aircraft picks up the
sweeps called “TO” and “FRO”.
A time difference between the TO
and FRO beams is used to
compute where the runway
centerline is.
A curved approach can be
computed if the aircraft is
equipped with a Flight
Management System (FMS).

Azimuth Transmitter
One of the 2 major components
of an MLS system is the
azimuth transmitter.
The azimuth signal is similar to
the localizer signal in ILS.
The azimuth signal sweeps a
wide area beyond the left and
right sides of the runway
allowing for many inbound
courses.
The station is located about
400 feet beyond the end of the
runway as seen by an arriving
aircraft.

MLS Elevation Beam
Just like with the azimuth
beam, the elevation beam scans
To and Fro.
However, the beam goes up
and down instead of side to
side.
The aircraft again uses the time
difference to compute
glidepath.
Steeper glidepaths can be
computed for helicopter use.

Elevation Beam Transmitter
The 2nd major component of the
MLS , Elevation Beam Transmitter
is similar to the Glide slope signal in
ILS. The Elevation Beam sweeps a
wide area, allowing for steeper
approaches.
The Elevation Beam Transmitter is
located about 400 feet off the
approach end of the runway.
Co-located with the Elevation Beam
Transmitter is the P-DME
transmitter.
P-DME (Precision DME) is 10x
more accurate than conventional
DME.
P-DME provides range to
touchdown information.

MLS Time Reference
MLS signals arriving at the
airplane produce 2 peaks as the
beam sweeps back and forth
over the receiver antenna. The
airborne equipment computes
the time difference between the
peaks to determine the
centerline (AZ) or glide path
(EL). This equipment tells the
difference between the 2
signals by a short identifier
known as a “preamble”.
The AZ signal sweeps at 13.5
scans a second and the EL
signal scans at 40.5 scans a
second. The EL signal has a
higher frequency due the
necessity of a more accurate
signal for glidepath.

PRINCIPLE OF OPERATION
MLS employs the principle of Time Division
Multiplexing (TDM) whereby only one frequency is
used on a channel but the transmissions from the
various angle and data ground equipments are
synchronized to assure interference free operations on
the common radio frequency.
Time referenced scanning beam (TRSB) is utilized in
azimuth and elevation as follows: the aircraft computes
its azimuth position in relation to the runway centre-
line by measuring the time interval in microseconds
between the reception the ‘to’ and ‘fro’ scanning beams.

The beam starts the ‘to’ sweep at one extremity of
its total scan and travels at a uniform speed to the
other extremity. It then starts its ‘fro’ scan back to
its start position. The time interval between the
reception of the ‘to’ and ‘fro’ pulses is
proportional to the angular position of the aircraft
in relation to the runway on- course line. The pilot
can choose to fly the runway on-course line or an
approach path which he selects as a pre-
determined number of degrees ± the runway
direction.
PRINCIPLE OF OPERATION

Glide Slope Location
Another beam scans up and down at a uniform
speed within its elevation limits. The aircraft’s
position in relation to its selected glide slope
angle is thus calculated in the same manner by
measuring the time difference between the
reception of the pulses from the up and down
sweep. The transmissions from the two beams and
the transmissions from the other components of
the MLS system are transmitted at different
intervals i.e. it uses ‘ time multiplexing’.

Other components of the system
• Flare: Although the standard has been developed to
provide for flare elevation, this function is not intended
for future implementation.
• Back azimuth: Gives overshoot and departure guidance
± 20° of runway direction up to 15° in elevation.
• DME Range along the MLS: This course is provided
not by markers but by a DME.
• Transmission of auxiliary data consists of:
• station identification
• system condition
• runway condition
• weather information

Inertial Navigation Systems
Inertia ---
The property of bodies to maintain constant translational and rotational velocity, unle
ss disturbed by forces or torques, respectively (Newton’s first law of motion)An
Inertial reference frame
A coordinate frame in which Newton’s laws of motion are valid. Inertial reference fr
ames are neither rotating nor accelerating.
Inertial sensors
Measure rotation rate and acceleration, both of which are vector‐ valued variables.
Gyroscopes
Sensors for measuring rotation: 1. Rate gyroscopes measure rotation rate 2.
Integrating gyroscopes measure rotation angle.
Accelerometers
sensors for measuring acceleration.
Input axis
of an inertial sensor defines which vector component it measures. Multi‐axis sensors
measure more than one component.
Inertial measurement unit (IMU) contains a cluster of sensors:
Accelerometers (three or more, but usually three) and
Gyroscopes (three or more, but usually three).
These sensors are rigidly mounted to a common base to maintain the same relative o
rientation.

Basic Principle of Inertial Navigation
• Given the ability to measure the acceleration of vehicle
it would be possible to calculate the change in velocity
and position by performing successive mathematical
integrations of the acceleration with respect to time.
• In order to navigate with respect to our inertial
reference frame, it is necessary to keep track of the dire
ction in which the accelerometers are pointing.
• Rotational motion of the body with respect to inertial
reference frame may be sensed using gyroscopic
sensors that are used to determine the orientation of the
accelerometers at all times. Given this information it is
possible to resolve the accelerations into the reference
frame before the integration process takes place.

What does an INS consist of?
An inertial navigation
uses gyroscopes and accelerometers to maintain an
estimate of the position, velocity, and attitude rates of the vehicle in
or on which the INS is carried, which could be a land vehicle, aircra
ft, spacecraft, missile, surface ship, or submarine.
An INS consists of the following:
– An IMU
– Instrument support electronics
–Navigation computers (one or more) calculate the
gravitational acceleration (not measured by
accelerometers) and doubly integrate the net
acceleration to maintain an estimate of the position of
the host vehicle.

Stabilized Platform and
Strapdown Technologies
There are many different designs of INS with different performance ch
aracteristics, but they fall generally into two categories:
–Gimbaled or stabilized platform techniques, and
– Strapdown
The original applications of INS technology used stable platform techni
ques. In such systems, the inertial sensors are mounted on a stable
platform and mechanically isolated from the rotational motion of t
he vehicle. Platform systems are still in use, particularly for those a
pplications requiring very accurate estimates of navigation data, su
ch as ships and submarines.
Modern systems have removed most of the mechanical complexity of
platform systems by having the sensors attached rigidly, or “strapp
ed down”, to the body of the host vehicle. The potential benefits of
this approach are lower cost, reduced size, and greater reliability c
ompared with equivalent platform systems. The major disadvantag
e is a substantial increase in computing complexity.

A gimbal is a rigid with rotation bearings for
isolating the inside of the frame from external
rotations about the bearing axes. At least three
gimbals are required to isolate a subsystem from host vehicle
rotations about three axes, typically labeled as
roll, pitch, and yaw axes.
The gimbals in an INS are mounted inside one another. Gimbals
and torque servos are used to null out the rotation of stable
platform on which the inertial sensors are mounted.
Gimbaled Inertial Platform

Strapdown Inertial Navigation System
Block diagram

Strapdown Inertial Navigation System

Relation to guidance and control

AE8751 - UnitIV.pdf

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