1.2 Aircraft
An aircraft is a machine that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. It counters the force of gravity by using either static
lift or by using the dynamic
lift of an airfoil
or in a few cases the downward
thrust from jet
engines.
In another word aircraft is weight carrying structure that can travel through
the air, supported either by its own buoyancy or by the dynamic action of air
against its surfaces. The human activity that surrounds aircraft is called aviation. Crewed aircraft are flown by an onboard pilot, but unmanned aerial vehicles may be remotely controlled or self-controlled by onboard computers. Aircraft may be
classified by different criteria, such as lift type, propulsion, usage, and
others.
1.3 Airport
An airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an
airport. An airport consists of at least one surface such as a runway for a plane to take off and land, a helipad, or water for takeoffs and landings, and often includes buildings such as control
towers,
hangars and terminal buildings. Larger airports may have fixed base operator services, seaplane docks and ramps, air traffic control, passenger facilities such as restaurants and lounges, and emergency
services. A military airport is known as an airbase or air
station. A water airport is a
water aerodrome (an area of open water used regularly by seaplanes or amphibious aircraft for landing and taking off), usually with passenger
facilities on adjacent land, which acts as an airport.
1.4
Airspace
Airspace means the portion of the atmosphere controlled by a country
above its territory, including its territorial waters or, more generally, any specific three-dimensional portion of
the atmosphere. Airspace may be further subdivided into a variety of areas and
zones, including those where there are either restrictions on flying activities
or complete prohibition of flying activities. By international law, the notion
of a country's sovereign airspace corresponds with the maritime definition of territorial waters as being 12 nautical miles (22.2 km) out from a nation's
coastline.
1.5
Air traffic Controller
Air
traffic control (ATC) involves communication with aircraft to help
maintain separation that
is they ensure that aircraft are sufficiently far enough apart horizontally or
vertically for no risk of collision. Controllers may co-ordinate position
reports provided by pilots, or in high traffic areas (such as the United States)
they may use radar to
see aircraft positions.
ATC is especially important for aircraft flying under Instrument flight rules (IFR), where they may be in weather conditions that do not
allow the pilots to see other aircraft. However, in very high-traffic areas,
especially near major airports, aircraft flying under Visual flight rules (VFR) are also required to follow instructions from ATC.
There
are generally four different types of ATC:
·
Center controllers, who control aircraft
en route between airports
·
Control towers (including tower, ground
control, clearance delivery, and other services), which control aircraft within
a small distance (typically 10–15 km horizontal, and 1,000 m vertical) of
an airport.
·
Oceanic controllers, who control
aircraft over international waters between continents, generally without radar
service.
·
Terminal controllers, who control
aircraft in a wider area (typically 50–80 km) around busy airports.
1.5 Air Traffic
Management
ATM is the regulation of air traffic in order to avoid
exceeding airport or air traffic control capacity in handling traffic, and to ensure that available
capacity is used efficiently. Because only one aircraft can land or depart from
a runway at the same time, and because aircraft must be separated by certain
time to avoid collisions, every airport has a finite capacity; it can only
safely handle so many aircraft per hour. This capacity depends on many factors,
such as the number of runways available, layout of taxi tracks, availability of air traffic
control, and current or anticipated weather. Especially the weather can cause
large variations in capacity because strong winds may limit the number of
runways available, and poor visibility may necessitate increases in separation
between aircraft. Air traffic control can also be limiting, there are only so
many aircraft an air traffic control unit can safely handle. Staff shortages,
radar maintenance or equipment faults can lower the capacity of a unit. This
can affect both airport air traffic control as well as en-route air traffic control center.
2.
AERONAUTICAL COMMUNICATION
2.1. Aviation Band
Aviation Band or Airband or Aircraft band is the name for a group of frequencies in the VHF radio
spectrum allocated to radio communication in civil aviation, sometimes also referred to as VHF, or phonetically as "Victor". Different
sections of the band are used forradionavigational
aids and air traffic
control.
The VHF airband uses
the frequencies between 108 and 137 MHz The lowest 10 MHz of the band,
from 108–117.95 MHz, is split into 200 narrow-band channels of 50 kHz.
These are reserved for navigational aids such as VOR beacons, and
precision approach systems such as ILS localizers.
As of 2012, most
countries divide the upper 19 MHz into 760 channels for amplitude modulation voice transmissions, on frequencies
from 118–136.975 MHz, in steps of 25 kHz. In Europe, it is becoming common
to further divide those channels into three (8.33 kHz channel spacing),
potentially permitting 2,280 channels. Some channels between 123.100 and
135.950 are available in the US to other users such as government agencies,
commercial company advisory, search and
rescue, military aircraft, glider and ballooning air-to-ground,
flight test and national aviation authority use. A typical transmission range
of an aircraft flying at cruise altitude (35,000 ft), is about 200 miles in
good weather conditions.
Aeronautical voice
communication is also conducted in other frequency bands, including satellite
voice on Inmarsat and high
frequency voice in the North
Atlantic and remote areas. Military aircraft also use a dedicated UHF-AM band
from 220.0–399.95 MHz for air-to-air and air-to-ground, including air
traffic control communication. This band has a designated emergency and guard
channel of 243.0 MHz.
Some types of navaids, such as Non-directional beacons and Distance
Measuring Equipment, do not operate on these frequencies; in the case of NDBs
the Low frequency and Medium
frequency bands are used between
190–415 kHz and 510–535 kHz. The ILS glide path operates in the UHF frequency
range of 329.3–335.0 MHz, and DME also uses UHF from 962–1150 MHz
3.
AERONAUTICAL NAVIGATION
3.1 INDRODUCTION
3.1.1.
Introduction to Navigation System
Finding the way from one place to another is called
NAVIGATION. Moving of an aircraft from one point to another is the most
important part for any kind of mission. Plotting on the paper or on the map a
course towards a specific area of the earth , in the past, used to be a task
assigned to a specialized member of the aircraft's crew such a navigator. Such
a task was quite complicated and not always accurate. Since it depended on the
observation, using simple maps and geometrical instruments for calculations.
Today, aerial navigation has become an art which nears to perfection. Both
external Navaids (Navigational Aids) and on-board systems help navigate any
aircraft over thousands of miles with such accuracy that could only be imagined
a few decades ago.
3.1.2.
Piloting
Piloting is the
most common method of air navigation. This method, the pilot keeps on course by
following a series of landmarks on the ground. Usually before take-off, pilot
will making pre-flight planning, the pilot will draw a line on the aeronautical
map to indicate the desired course. Pilot will notes various landmarks, such as
highways, railroad tracks, rivers, bridges. As the pilot flies over each of
landmark, pilot will checks it off on the chart or map. If the plane does not
pass directly over the landmark, the pilot will know that he has to correct the
course.
3.1.3.
Dead Reckoning
Dead reckoning is the primary navigation method used in the
early days of flying. It is the method on which Lindberg relied on his first
trans-Atlantic flight. A pilot used this method when flying over large bodies
of water, forest, deserts. It demands more skill and experience than pilot age
does. It is based on time, distance, and direction only. The pilot must know
the distance from one point to the next, the magnetic heading to be flown.
Pilot works on the pre-flight plan chart, pilot plan a route in advance. Pilot
calculates the time to know exactly to reach the destination while flying at
constant speed. In the air, the pilot uses compass to keep the plane heading in
the right direction. Dead reckoning is not always a successful method of
navigation because of changing wind direction. It is the fundamental of VFR
flight
3.1.4.
Radio Navigation
Radio navigation
is used by almost all pilots. Pilots can find out from an aeronautical chart
what radio station they should tune to in a particular area. They can then tune
their radio navigation equipment to a signal from this station. A needle on the
navigation equipment tells the pilot where they are flying to or from station,
on course or not.
3.2.6 Advantages of NDB
Although
there are now several more accurate navigational systems available on other
radio frequency bands, the NDB is still used in every country in the world, and
will continue to do so for many more years to come. The reasons are obvious
which can be outlined as follows:
#
Very simple air-borne and ground equipment
#
Inexpensive to install and maintain
#
Omni-directional information
# Any number of aircraft can use the same radio
beacon
#
Responsibility of accuracy mainly depends on airborne receiver
#
Multi-purpose uses
3.2.7 Limitation of NDB
1.
Night effect
Radio
waves take two paths to the radio compass receiver. The first and normal path
is along the earth’s surface. If only these waves were received, the compass
would point directly to the NDB. The second path is via one or more wave
refracting layers above the earth (the ionosphere) returning to earth to mix
with directs waves. Complete changes in the nature of the waves take place on
this path and produce errors in direction.
The
ratio of the intensity of indirect to direct waves in the total received signal
determines the liability of error of the radio compass. As the strength of the
indirect waves is far greater at night, errors then are more common and of
greater magnitude: this is called ‘night effect’. Often this effect is more
pronounced within an hour of sunrise or sunset, when the changes in the state
of ionisation of the upper atmosphere are particularly violent.
The
night time range of an NDB is only dependable over distances where the ground
wave transmission predominates, which is approximately 60 miles over land and
100 miles over sea under reasonable propagating conditions. As the distance
increases the ratio of indirect to direct waves will increase and bearing
indications will become erratic. Treat with caution NDB reception beyond these
ranges.
3.2.10.2
Radiation resistance:
The
base radiation resistance is another important characteristic. It is a
characteristic, which has a direct relationship to the radiated power and
consequently to effective range of the NDB. Because the NDB antennas are
electrically very short (less than 30°),
the current distribution along the antenna is linear and radiation resistance
may be calculated to reasonably close approximation by the formula:
R
= q2
/328 , where q is the electric
length of the antenna in degree. l = 360°
With
the above formula it is evident that by increasing the length of the antenna its
radiation resistance increases, and hence the efficiency increases. See
following table.
Antenna
Height (ft)
|
Radiation
resistance in Ohms
|
200KHz
|
300KHz
|
450KHz
|
50
75
100
150
200
250
|
0.041
0.092
0.163
0.367
0.657
1.020
|
0.092
0.207
0.367
0.825
1.467
2.295
|
0.206
0.466
0.825
1.857
3.300
5.164
|
3.2.10.3
Antenna Q
:
Antenna
system Q is the ratio of the reactance of the antenna capacitance to the
antenna total system resistance. It is always preferable to keep the Q as low
as possible to reduce losses in the antenna system.
Since Q = Xc/R ,
Q
can be reduced by increasing capacitance of the aerial. I.e. by addition of top
loading or by increasing the height. NDB transmitters are usually required to
have a bandwidth of at least 2X1020 Hz.
1020 Hz being the max ident frequency.
Bandwidth
= f/Q
Therefore,
at 300 KHz Q = 300,000/2040 = 147
Which
means a Q of 147 at 300 KHz NDB station will insure that the ident modulation
will be radiated without any distortion. If bandwidth of the antenna is low ( Q
is high) then instead of 1020 Hz ident modulation of 400Hz should be used.
3.2.10.4
Expected range
NDB
antenna should be designed in such a way that it should radiate reliable signal
up to the required coverage area. ICAO has specified that in the coverage areas
the field strength should not be less than 70mV
per meter. Between the latitudes 30°N
and 30°S field strength
of 120mV may be
required.
Conclusion
: To increase the efficiency and to improve the performance of an NDB antenna
its capacitance should be as high as possible and should be more than 500pF at
the lower frequencies. This can be achieved either by increasing the height of
the antenna or by providing additional top loading.
3.2.11 Transmitting Equipment
The
NDB transmitter is relatively very simple equipment. The RF carrier is
amplitude modulated either by 400Hz or by 1020 Hz tone, which is coded with two
to three letters station identification in Morse Code. A simplified block diagram
is shown in Fig. below:
Antenna
Monitor
equipment monitors the performance of the radiating signal. Radiation is done
in A0/A2 mode. Depending upon the use an NDB could be classifies as one of the
following:
High
Power: usable range
extends up to 400 NM. Radio beacons of this type are considered as en-route or homing radio navigational aids. The transmitter output is normally
100W to 5KW.
Low
Power : usable range extends from 10 NM to 25 NM. Radio
beacons of this type are called locators
and are normally used for approach or holding purposes. The transmitter output
power is kept below 100W.
3.2.12 Monitoring and Calibration
Normally
the NDB beacon has two transmitters and two monitors, i.e. dual equipment
system. Monitor analyzes the radiated signal and checks the following:
#
Gives alarm if the transmitted carrier power is reduced more than 3dB. i.e. 50%
#
Gives alarm if the identification signal is removed or continuous by any
reason.
#
Gives alarm if the monitor itself becomes faulty.
3.3.4 VOR Transmission
Techniques
A
9960 Hz
9960 Hz generator produces the sub-carrier
frequency, which is the basis for the REF signal. The 9960 Hz is fed to a 30Hz
frequency modulator in order to produce the frequency deviation of ± 480 Hz. This again is amplitude
modulated with 30% on the VHF transmitter, normally 50 to 200 Watts output. The
REF signal is fed to an omnidirectional antenna, which normally is a loop
antenna, called Alford loop antenna.
The REF signal is also in parallel fed to a modulation eliminator, which
removes the modulation, and the signal output from this block is then a clean
continuous wave (CW) signal with the same carrier frequency. This signal is fed
to the horizontally polarized dipole antenna to obtain the figure of eight
pattern.Since the frequency of these two signals are the same, they will
combine together to form a cardioid. The goniometer rotates the figure of eight
at 1800 rpm, which will also cause the cardioid to rotate at the same rate.
A
cardioid has maximum and minimum radiation pattern. While rotating, when the
maximum pattern is towards the receiver it will receive maximum signal and for
minimum pattern the signal received will be minimum. Therefore, if the cardioid
is made to rotate at 1800 times per minute (30 times per second), the receiver
will get the signal as 30 Hz AM. The following Fig. 2.2.6 explains the rotation
of the cardioid pattern and the resulting AM signals received by the airborne
VOR receivers at north, east, south and west directions. Form these figures it
is evident that the variable phase of the amplitude modulation (space
modulation) is dependent of azimuth degree by degree.
VOR
is a phase comparison system. This means
simply that the
phase of one signal
is compared with
the phase of
another. However, a
problem arises in that
this type of
comparison is possible
only between signals
whose frequencies are identical, and
one also needs
to be able
to identify the
source of each
signal. Some-times this
identification can be
accomplished by time
multiplexing the signals
and storing the phase
information in the
receiver circuitry for
later comparison. But in
the VOR system both
signals are transmitted
simultaneously, and there
needs to be a
way to prevent
the two from
producing a resultant
as they pass
through space. This is
accomplished by transmitting
one signal as
amplitude modulation and
the other as frequency
modulation. Detection in
the receiver produces
two separate audio signals
of exactly the
same frequency but
with measurable phase
difference.
The
two signals being compared are both 30 Hz signals. One
is transmitted in a
manner so as to
produce a circular
radiation pattern. Thus
all aircraft at
the same instant
in time, and
at the same
distance from the
transmitter, receive the same
exact phase of
this 30 Hz
signal. Consequently, this signal
is called the reference signal. The other
signal has a
radiation pattern shaped
like a cardioid, and
this pattern is
caused to rotate
about the station
30 times per
second. The airborne
equipment which receives
this cardioid signal detects
a signal strength
which depends on
which part of
the cardioid it is
receiving at any
particular instant of
time. By the
rotation of this
pattern amplitude modulation is
created, and the
received signal strength
from the variable
signal undergoes a cyclic
change which is
repeated 30 times
per second.
Since both
signals occur at the same
30 Hz rate
there is a
repetitive synchronization between the
two, and all
that is needed
is an index
point so that
they may be compared
for phase difference.
This index point
is established on the reference
signal, and the
phase of the
variable signal is initially adjusted
so that the phase
difference between the
two is 0o
at magnetic north.
The rotation of
the variable signal
from north azimuth
creates a phase
difference of one
electrical degree for each
rotational degree; therefore,
an observer can
determine his geographical azimuth by
simply measuring the
phase difference between
the phase of
the reference signal and
the phase of
the variable signal. A
receiver anywhere within
the coverage area
of a facility
will receive two
30 Hz signals-one
from the reference
field and one
from the variable
field cardioid that
is passing by. The
phase of the
signal from the
variable field will
lag that of
the reference field by
the exact number
of degrees the
receiver bears from
magnetic north.
3.3.6. VOR Errors
1) Multi-path errors: The major bearing
errors in the VOR system are caused by multi-path reception. Signals reaching
the aircraft receiver may include those that arrive after reflections from
natural or man-made objects as well as those arriving by a direct path. The
multi-path signals will add and subtract as the phases of direct and reflected
signals vary while the aircraft flies along the course.
2) Ground station errors: results from
misphasing of the 30 Hz reference and variable phases, misalignment of the
north and other calibration errors at the VOR station. The major ground station
error is due to spurious vertical polarization generated by the antennas
resulting in undesirable vertically polarized 30 Hz azimuth dependent
component. This spurious 30Hz component will not be in phase with the actual 30
Hz horizontally polarized variable phase. The aircraft antenna, although
horizontally polarized, will pick up some of this vertically polarized signal
when the aircraft will tilt. These factors may cause an additional error to the
tune of ±1°.
3) Aircraft receiver error: It is a function of
the cost and age of the aircraft receiver. The older generation aviation
receivers tend to have errors, which in new equipment have been essentially
eliminated. The modern aircraft receivers have performance better than ±2°.
4) Pilotage or flight technical error: It
is a function of many parameters, which are all difficult to measure. Studies
of flight technical errors show the error to be higher when the aircraft makes
a turn than on a straight-line route.
Instrument
Landing System
The Instrument
Landing System, abbreviated as ILS, is a system of electronic equipment, which
assists the landing aircraft to make straight in approach by using cockpit
indications at any non-visual meteorological conditions. ILS is a standard aid,
adopted by the members of ICAO since its development in 1940's. There are
hundreds of ILS's in operation at all modern airports throughout the world. It
is still considered to be the most reliable, most utilized and most implemented
precision approach system in the world.
The system
comprises of a Localizer, a Glide Slope, and two to three Marker
Beacons. The landing path is determined by the intersections of two
planes, as shown in Fig.1-A, and could be explained as follows:
Ø A vertical plane containing the runway centerline,
is defined by a VHF Transmitter, called Localizer (LLZ).
Ø A horizontal plane of 2º-4º vertical angle
containing the runway centerline, is defined by an UHF transmitter, called
Glide Slope (GS).
Ø Vertically
radiated VHF Markers (IM,MM & OM) transmitters provide fixed distance
information
Ø
(Radiation pattern of an ILS)
All these
stations form a system that provides an electronic passage, exactly at an
approach angle that is required for a safe landing.
The ILS helps to
bring the aircraft safely down to a pre-defined height, called the Decision Height, from where the
pilot has to make his own decision whether to land or to make a missed
approach. The missed approach is an aviation terminology for unsuccessful
landing. In this case, the aircraft has to make a turn and try to land once
again. In category- IIIC, visibility is not needed and a blind landing can be
made using electronic equipment. Therefore, based on decision height and the visibility
of the runway, three categories of ILS are defined by the International Civil
Aviation Organization (ICAO) which is tabulated below.
Table
-T1
ILS CATEGORIES
|
DECISION HEIGHT (M)
|
VISIBILITY (M)
|
CAT - I
CAT - II
CAT - IIIA
CAT - IIIB
CAT - IIIC
|
60
30
0
0
0
|
800
400
200
50
0
|
Since the pilots
fully rely on ILS guidance for landing, the signals radiated by an ILS should
be very accurate and authentic. ICAO Annex-10 specifies the necessary technical
tolerances that have to be maintained for the above three categories of ILS's.
In many
occasions, where the geographical conditions do not permit installation of
Marker beacons at predefined distances, Distance Measuring Equipment (DME) is
co-located with ILS. The DME can be co-located with an ILS in the following
three ways
- DME with Glide Slope
- DME with Localizer
- DME installed independently.
Thus, while
landing on ILS, a pilot determines his position from the runway end by DME. In
some locations, an NDB is installed on the centerline of the runway in stead of
a Marker. Such an NDB is then called a compass
locator.
Coverage of an ILS
Localizer: The
horizontal localizer coverage sector is extended from the center of the
localizer antenna array to the distances of:
5.3.4 Comparison
with Primary Radar
Primary Radar depends on the reflection of radar signal and
operates on one frequency. As a consequence reflections from ground, buildings,
precipitations, etc., will be fed to the display along with aircraft echoes.
This can cause the display to be cluttered with unwanted signals. Also, the
power output of a primary radar must be sufficiently high to allow for the
two-way path and for the losses on reflection. Nevertheless, primary radar is
self-contained and does not require equipment to be installed in the aircraft.
Secondary radar is independent of aircraft echoeing area. In
fact, because the airborne equipment has its own transmitter, much less power
is needed than in primary radar for the same operational range. Also, as the
airborne responses of the secondary radar is on a different frequency to the
ground transmission, ground clutter and other unwanted echoes are not accepted
for presentation. With suitable coding, secondary radar can provide aircraft
indentification without aircraft maneuvres and thus with much less use of other
communication system. The coding system may also be used to send printed
information from aircraft to air-traffic controllers on ground. Unfortunately,
the coding system relies on pulses being either “present” or “not present” and
therefore weak signals cannot be tolerated as they may generate false
information. Secondary radar also suffers from the disadvantage that it
requires each aircraft to carry a transponder.
5.3.5 The Coding System
The method used in SSR for communicating information consists
of the transmission and reception of pulses. The ground interrogator first
transmits a pair of pulses with definite width and spacing to the aircraft.
Depending upon various modes used in radar for various purposes, the pulse
spacing could vary from each other. In secondary radar it is necessary to
design the system to reduce the chances of transponder operation as a result of
receiving spurious pulses. For this reason, two pulses with a known spacing are
transmitted from the interrogator. The ground based interrogation pulses are
produced differently depending upon “mode of operation” of the secondary radar.
0.8 ± 0.1ms
Mode-A
8 ± 0.2ms
Mode-B
17 ± 0.2ms
Mode-C
21 ± 0.2ms
Mode-D
25 ± 0.2ms
(Fig:
Various modes of SR and their pulse spacing)
After receiving the interrogation pair of pulses from the
ground interrogator, the aircraft transponder transmits back a reply to ground.
In the process of replying, the aircraft transponder does not reply to any
other signals at least for the duration of the reply pulses train. It is known
as receiver dead time of radar transponder. This dead time does not last more
than 125 microseconds after transmission of the last reply pulses. The aircraft
transponder recognizes the interrogation mode by the time spacing between two
pulses. The transponder reply, as illustrated below, consists of a train of
pulses each of which is 0.45 ms wide
and 1ms
apart.
0.45ms
F1 6 or 12 information
pulses F2
Ident
1.45ms
20.3ms 4.35ms
(Fig. 5.14 Reply Pulse
Train from transponder)
The train of pulses is built up as follows:
Two framing pulses 20.3ms apart (F1 and F2). The first frame pulse is always the
first pulse in the train. The information pulses lie between two frame pulses
and composed of either 6 or 12 pulses. The information signal is formed in
binary codes, which means either presence or absence of pulses. If six pulse positions are used then the number of
codes available is 26 = 64. For 12 positions the number of codes
available is 212 = 4096. The identity pulse is transmitted only when
the switch is activated in the aircraft at the request of the air trafic
controller. The identity pulses are then transmitted automatically for a period
of twenty seconds.
5.4 Monopusle SSR and
Mode-S
5.4.1 Monopulse SSR
The new Mode S system was
intended to operate with just a single reply from an aircraft, a system known
as monopulse. The accompanying diagram shows a conventional main or
"sum" beam of an SSR antenna to which has been added a
"difference" beam. To produce the sum beam the signal is distributed
horizontally across the antenna aperture. This feed system is divided into two
equal halves and the two parts summed again to produce the original sum beam.
However the two halves are also subtracted to produce a difference output. A
signal arriving exactly normal, or boresight, to the antenna will produce a
maximum output in the sum beam but a zero signal in the difference beam. Away
from boresight the signal in the sum beam will be less but there will be a
non-zero signal in the difference beam. The angle of arrival of the signal can
be determined by measuring the ratio of the signals between the sum and
difference beams. The ambiguity about boresight can be resolved as there is a
180° phase change in the difference signal either side of boresight.
7.2 Radio direction finder (Radio compass)
7.2.1 Operation
Radio Direction Finding works by comparing the signal
strength of a directional antenna pointing in
different directions. At first, this system was used by land and marine-based
radio operators, using a simple rotatable loop antenna linked to a degree
indicator. This system was later adopted for both ships and aircraft, and was
widely used in the 1930s and 1940s. On pre-World War II aircraft,
RDF antennas are easy to identify as the circular loops mounted above or below
the fuselage. Later loop antenna designs were enclosed in an aerodynamic,
teardrop-shaped fairing. In ships and small boats, RDF receivers first employed
large metal loop antennas, similar to aircraft, but usually mounted atop a
portable battery-powered receiver.
Fig: radio compass
In use, the RDF
operator would first tune the receiver to the correct frequency, then manually
turn the loop, either listening or watching an S meter to
determine the direction of the null (the direction at which a
given signal is weakest) of a long wave (LW)
or medium wave (AM) broadcast beacon or station
(listening for the null is easier than listening for a peak signal, and
normally produces a more accurate result). This null was symmetrical, and thus
identified both the correct degree heading marked on the radio's compass rose
as well as its 180-degree opposite. While this information provided a baseline
from the station to the ship or aircraft, the navigator still needed to know
beforehand if he was to the east or west of the station in order to avoid
plotting a course 180-degrees in the wrong direction. By taking bearings to two
or more broadcast stations and plotting the intersecting bearings, the
navigator could locate the relative position of his ship or aircraft.
Later, RDF sets
were equipped with rotatable ferrite loopstick antennas,
which made the sets more portable and less bulky. Some were later partially
automated by means of a motorized antenna (ADF). A key breakthrough was the
introduction of a secondary vertical whip or 'sense' antenna that
substantiated the correct bearing and allowed the navigator to avoid plotting a
bearings 180 degrees opposite the actual heading. After World War II, there
many small and large firms making direction finding equipment for mariners,
including Apelco,
Aqua Guide, Bendix, Gladding (and its marine
division, Pearce-Simpson), Ray Jefferson, Raytheon,
and Sperry. By the 1960s, many of these
radios were actually made by Japanese electronics manufacturers, such as Panasonic, Fuji Onkyo,
and Koden Electronics
Co., Ltd. In aircraft equipment, Bendix and Sperry-Rand were two of the
larger manufacturers of RDF radios and navigation instruments.
7.6.1SSR Transponder Modes
Mode A interrogations are sent to
request the specified aircraft identification code. Mode C is used to request
altitude reporting together with identification. The coded reply signal is
composed of a series of pulses. In Mode A operation, the number of pulses
generated in a reply signal is determined by setting the four octal (0 to 7)
digit code switches on the Transponder to the assigned identification code.
This allowes for 4'096 different identification codes. Certain transponder
codes are reserved for special applications to activate an alert on the
controller's console:
7500 indicates
that a hijack is in process
- 7600 reports a
communication radio failure
- 7700 indicates an
emergency condition
Replies to Mode C interrogations
generate pulses that correspond to the aircraft's altitude. The received
altitude information is then displayed directly on the controller's console.
Altitude information is not selected by the code switches of the transponder,
but is obtained directly from an encoding altimeter.
7.7
Flight Data Recorder (FDR)
Flight recorders
comprise two systems, a flight data recorder (FDR) and a cockpit voice recorder
(CVR).
Sometimes, both FDR and CVR functions are combined into a single unit (ICAO
Definition: Combination recorders). Combination recorders need to meet
the flight recorder equipage requirements as specifically indicated in ICAO
Annex 6 - Operation of Aircraft.
7.7.1 ICAO Requirements
According to the
provisions in ICAO Annex 6 - Operation of Aircraft, Vol 1 and Vol. III, a Type
I FDR shall record a number of parameters the parameters required to determine
accurately the aeroplane flight path, speed, attitude, engine power,
configuration and operation. Types II and IIA FDRs shall record the parameters
required to determine accurately the aeroplane flight path, speed, attitude,
engine power and configuration of lift and drag devices.
7.7.2 Objectives
The recorder is
installed in the most crash survivable part of the aircraft, usually the tail
section. The data collected in the FDR system can help investigators determine
whether an accident was caused by pilot error, by an external event (such as
windshear), or by an airplane system problem. Furthermore, these data have
contributed to airplane system design improvements and the ability to predict
potential difficulties as airplanes age. An example of the latter is using FDR
data to monitor the condition of a high-hours engine. Evaluating the data could
be useful in making a decision to replace the engine before a failure occurs.
7.7.3 Principles of Operation
The FDR onboard the
aircraft records many different operating conditions of the flight. By
regulation, newly manufactured aircraft must monitor at least eighty-eight
important parameters such as time, altitude, airspeed, heading, and aircraft
attitude. In addition, some FDRs can record the status of more than 1,000 other
in-flight characteristics that can aid in the investigation. The items
monitored can be anything from flap position to auto-pilot mode or even smoke
alarms. It is required by regulations that on an annual basis an FDR
verification check (readout) is performed in order to verify that all mandatory
parameters are recorded.
§ Magnetic
Tape -
The introduction of the CVR in the late 1960s and DFDRs in the early 1970s made
magnetic tape the recording medium of choice until the introduction of
solid-state flight recorders in the late 1980s. There were a variety of tapes
and tape transports used by the various recorder manufacturers. The most widely
used tapes were mylar, kapton, and metallic. The tape transports were even more
varied, using designs such as coplaner reel to reel, coaxial reel-to-reel,
endless loop reel packs and endless loop random storage. Tape CVRs record four
channels of audio for 30 minutes, and the DFDR records 25 hours of data. CVRs
and FDRs record over the oldest data with the newest data in an endless
loop-recording recording pattern.
§ Digital
Recording -
Most DFDRs require a flight data acquisition unit (FDAU) to provide an
interface between the various sensors and the DFDR. The FDAU converts analog
signals from the sensors to digital signals that are then multiplexed into a
serial data stream suitable for recording by the DFDR. Industry standards
dictated the format of the data stream, which for the vast majority of
tape-based DFDRs is 64 12-bit data words per second. The recording capacity of
the tape DFDR is limited by the length of tape that can be crash-protected and
the data frame format. The capacity of the tape DFDRs was adequate for the
first generation of wide-body transports, but was quickly exceeded when
aircraft like the Boeing 767 and Airbus A320 with
digital avionics were introduced.
§ Solid
State Technology -
The introduction of solid-state flight recorders in the late 1980s marked the
most significant advance in evolution of flight recorder technology. The use of
solid-state memory devices in flight recorders has expanded recording capacity,
enhanced crash/fire survivability, and improved recorder reliability. It is now
possible to have 2-hour audio CVRs and DFDRs that can record up to 256 12-bit
data words per second, or 4 times the capacity of magnetic tape DFDRs.
Devices which carry out electrical operations by using moving parts are known as electromechanical.
