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2. NAVIGATION
- Navigation
-
Introduction
- Position Reference System
- Loxodrom and Great Circles
- Directions
- Variation
- Speed
- Speed for the
pilot
- Indicated Air
Speed (IAS)
- True Air Speed
(TAS)
- Ground Speed
(GS)
- Mach (M)
- Wind
- Time
- Navigational aids
- Non Directional Beacon (NDB)
- Very high freq. Omni-directional
Radio range (VOR)
- Distance
Measuring Equipment (DME)
- Intersection /
Fix
- Global
Positioning System (GPS)
- Inerital
Navigation System (INS)
- ATS Routes
- Flight
Management System (FMS)
- Navigational Aids Limitations
- Further reading
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2.1 Introduction
To be able to fly from point A to point B it is
important for the flight crew to know where they are and
where they’re headed.
Before radar, air traffic control
was dependent on pilot position reports via radio. Today,
most of the time, we have radar, which makes it possible
to closely track the position of aircraft.
Good knowledge about navigation and navigation aids is
important for air traffic controllers and therefore we
will start with a basic review of this area.
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2.2 Position Reference System [C]
Positions on the earth are often given as coordinates
in a coordinate system consisting of two parts, latitude
and longitude.
Latitude is the coordinate giving the position in
north-south direction and longitude is the coordinate
giving the position in west-east direction. The earth is
divided into 360 parallels of longitude or meridians.
The reference or zero-meridian is located at a longitude
equal to the position of Greenwich in the UK.
The longitude is expressed as the number of longitudinal
degrees east (E) or west (W) of the reference meridian.
In north-south direction the earth is divided into 180
parallels of latitude. The reference parallel of
latitude is the equator. The latitude is expressed as
the number of latitudinal degrees north (N) or south (S)
of the equator.
To be able to define positions more accurately the
smaller units minutes and seconds have been introduced.
These units are based on the 1/60-system, which means
one degree of latitude or longitude is equal to 60
minutes and one minute is equal to 60 seconds.
- Position coordinates: N59°02′12″ W032°39′55″
N590212 is the latitude and means that the position is
59 degrees, 2 minutes and 12 seconds north of the equator.
W0323955 is the longitude and means that the position is
32 degrees, 39 minutes and 55 seconds west of the zero
meridian.
One minute of latitude is equal to the distance of one
nautical mile (nm), which is equal to 1852 meters.
One minute of longitude is not equal to the same distance
everywhere on earth because the circumference of the earth
is varying with latitude.
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2.3 Loxodrom and Great Circles [C+]
 A Great circle is a circle on the
surface of a sphere that has the same circumference as the
sphere, and devides the sphere into two equal hemispheres.
It is the largest circle that can be drawn on a given
sphere.
The Equator is one example of a great circle as are all
meridians.
In navigation, a loxodrome (or rhumb line) is a line
crossing all meridians at the same angle, i.e. a path of
constant bearing.
If you follow a given compass-bearing on Earth, (having
taken into account magnetic deviation) you will be
following a rhumb line, which spirals from one pole to the
other, with the exception of 90 and 270 degrees, lines of
constant latitude, e.g. the equator.
Near the poles, they are close to being logarithmic
spirals, so they wind round each pole an infinite number
of times but reach the pole in a finite distance.
The pole-to-pole length of a rhumb line is (assuming a
perfect sphere) the length of the meridian divided by the
cosine of the bearing away from true north.
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2.4 Directions [S]
 All
directions in aviation are expressed with the 360-degree
system.
The horizon is divided into 360 equal parts and one
revolution equals 360 degrees.
A direction of due north means a direction of 360, due
east direction 090 and so on.
In aviation there are two (2) basic definitions of
directions. Heading and track.
Heading is defined as the direction where the aircraft
nose points (the longitudinal axis
of the aircraft).
When adding the effect of wind the direction of the path
of the aircraft over the ground will be slightly different
then the aircraft heading and that is called track.
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2.5 Variation [C]
All directions will be given relative to the North Pole.
The magnetic North Pole is not located on the same
position as the true North Pole and this results in two
possible references of directions. If the direction is
given with reference to magnetic north it is expressed in
degrees magnetic, and if the direction is given with
reference to true north it is expressed in degrees true.
The heading indicator in the aircraft will show the
direction relative to magnetic north and consequently all
headings assigned to aircraft should be in degrees
magnetic.
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2.6 Speed [S]
In aviation speed is normally measured in knots, which
are defined as nm/hour.
100 knots equals 185 km/h.
There are different ways to measure speed.
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2.6.1 Speed for the pilot [C]
The conventional airspeed indicator depends on the
effect of air being forced into a small tube mounted on
the outside of the aircraft (pitot tube). This airspeed
indication is affected by the density of the air, which
changes with altitude and ambient air temperature. As
the aircraft climbs, the air becomes less dense, and the
airspeed indicator shows a lower speed than the aircraft
is actually moving through the air.
There are other ways of measuring the speed, that aren’t
affected by the temperature and air-pressure, such as
GPS.
Pilots rely on indicated airspeed to control the
aircraft, because it is a true representation of how the
aircraft will behave in respect to stall speeds and
other known aircraft characteristics.
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2.6.2 Indicated Air Speed (IAS) [S+]
IAS is the speed that the pilot can read in the cockpit
and speed restrictions issued by air traffic control is
normally given in IAS in the lower airspace (see Mach
below).
IAS is based on measurements of how many particles of
air that hit the aircraft in a given period of time. The
faster the aircraft flies and the higher the density of
the air is, the more particles will hit the aircraft.
An aircraft keeping the same groundspeed (see below)
will get a lower IAS when the aircraft climbs.
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2.6.3 True Air Speed (TAS) [S+]
True airspeed is the speed at which the aircraft is
moving through the air. It has no relation to the wind.
In some aircraft, a true airspeed indicator is provided
along with the conventional "indicated airspeed" gauge.
Most pilots estimate their true airspeed prior to a
flight, and calculate the actual true airspeed during
the flight. This data is needed for flight planning
purposes. It is used along with wind data to arrive at,
you guessed it, ground speed.
A thumb of rule, which can be used to obtain TAS is to
increase IAS by 2% for every 1000’ of increase in
altitude.
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2.6.4 Ground Speed (GS) [S]
While true airspeed is the speed at which an aircraft
moves through the air irrespective of the wind, ground
speed is the speed the aircraft is moving over the
ground.
If an aircraft is flying at a TAS of 250 knots
with a 30 knot tailwind the GS will be 280 knots.
That
said, just remember that the speed you see on your radar
is ground speed, and the speed the pilot normally sees
is indicated airspeed. Your speed will usually show a
higher value than the pilot's, depending on his
altitude, unless he has a strong headwind.
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2.6.5 Mach (M) [S+]
In the upper airspace Mach is normally used to express
speeds.
Mach is a quotient of the local speed of sound and Mach
1.0 is equal to the speed of sound.
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2.7 Wind [S]
 The
wind direction is given in degrees just like it is given
for aircraft direction.
The direction of wind is always given as the direction
from where the wind comes.
In a weather report for an airport (METAR) the wind
direction and strength is given as for example 18003KT,
where 180 is the direction (south) and 03 is the
strength in knots.
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2.8 Time [S+]
When giving time in aviation, it is important to
define what time you refer to since an aircraft often
flies trough many time zones. Hence time is always given
in UTC (Universal Time Co-ordinated) in aviation.
The big difference between UTC and other time format is
that UTC doesn’t change with DST (Daylight Saving Time
or summertime). Adding the letter ”Z”, which is
pronounced ”Zulu”, marks times given in UTC.
WET (Western European Time) = UTC during winter
CET (Central European Time) = UTC+1 hour during winter
WET = UTC+1 hour during DST
CET = UTC+2 hours during DST
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2.8.1 Date and reading time [S+]
Dates can also be added in the format 211020Z, which
means time 10:20 UTC on the 21:st.
In radio
communication time is often given with only the number
of minutes, for example time 14:54 is expressed as “time
54”. Time 16:00 is expressed as “on the hour” and 11:30
can be expressed either “time 30” or “on the hour and a
half”.
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2.9 Navigational aids [S]
Navigation aids are used by flight crew to navigate
between different positions on the earth and may consist
of transmitters on the ground, receivers in aircraft and
most recently also satellites.
The crew navigates between different navigation aids and
points called VOR, NDB and intersections. With a joint
name these are called waypoints.
2.9.1 Non Directional Beacon – NDB [S+]
NDB is a radio beacon transmitting non-directional radio
signals.
The pilot can determine the direction or “bearing” to
the beacon with an instrument called ADF indicator.
NDBs have a maximum range of about 50-100 miles,
depending on equipment. NDBs with three letters have a
longer range then NDBs with two letters.
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2.9.1.1 More on NDB [C+]
The NDB operates between 200 and 1750 kHz, but in Europe
most frequencies are between 255 and 455.
The ADF receiver can be used when line-of-sight
transmission becomes unreliable, or when there is no VOR
equipment on the ground or in the aircraft.
It is used as a means of identifying positions, receiving
low and medium frequency voice communications, homing,
tracking, and for navigation on instrument approach
procedures.
The low/medium frequency navigation stations used by ADF
include Non-Directional Beacons, ILS radio beacon
locators, and commercial broadcast stations.
The ILS radio beacon is a beacon which is placed at the
same position as the outer marker of an ILS system (or
replaces the OM).
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2.9.2 Very high freq. Omni-directional Radio range – VOR [S+]
A VOR is transmitting directional radio signals and is
more accurate then a NDB.
With the navigation instruments the pilot can intercept
and fly with a specified direction towards or from the
beacon. These directions are called radials. If you fly
due south from a VOR, you fly on radial 180. If you are
due south of the VOR and point the aircraft towards the
VOR (north) you are still on radial 180, but flying on
heading 360.
If you fly towards the VOR it is common to say you are
flying on a radial inbound it, and if you fly away from it
you are flying on a radial outbound it.
The maximum range of a VOR is about 100-200 miles.
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2.9.2.1 More on VOR [C+]
VHF Omni-directional Range (VOR) operates between 108 and
117.950 MHz.
The range of a VHF-signal can be calculated using the
formula [1.25 x √ (height of the transmitter in feet) +
1.25 x √ (height of the receiver in NM)]. The signal is
weakened by several factors such as terrain and different
weather-phenomena.
The VOR works on a "light-tower" principle. Imagine that
it has a rotating light bundle, and a steady 360 degree
light. The light bundle swings around one time per minute
(1 RPM = 6 degrees/sec). If the light bundle hits the
magnetic North, the steady light flashes once. Take a
stopwatch and start timing when the steady light flashes,
stop timing when the light bundle hits you.
In practice, the steady 360 degree light is actually a
steady 30 Hz signal on the VOR, and is modulated (pasted)
on the VOR frequency. The rotating bundle is also a 30 Hz
signal. The receiver will compare both signals and
determine the difference in phase, and in this way the
position relative to the VOR is determined.
A VOR usually also includes a Morse Ident, and sometimes
there is also a VOICE channel (ATIS) pasted on the VOR
frequency.
A VOR is often accompanied by a DME.
The accuracy of a VOR is about 2 degrees.
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2.9.3 Distance Measuring Equipment – DME [S+]
 DME
is another type of beacon with which the range between the
aircraft and the transmitter can be measured and presented
to the pilot.
A DME does not make it possible to see the direction from
the aircraft to the DME station, only the range.
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2.9.3.1
More on DME [C+]
Distance Measuring Equipment (DME) operates between 962
and 1213 MHz.
The measuring is initiated from the aircraft. The
interrogator sends out two pulses which are received by
the DME station. After a 50 microsecond delay, the ground
station sends a pulse back, which is 63 MHz higher or
lower than the original frequency. The further the DME
station is away, the longer the pulse needs to travel. By
timing the time difference between sending and receiving
the signal (minus the 50 microsecond delay) you can
determine the distance.
This is the direct distance from the aircraft to the
ground station, and not the distance over the ground. If
an aircraft fly at 10 km overhead a VOR/DME, the DME will
read 5.5 nm!
A lot of VOR frequencies are coupled with a DME frequency.
Every VOR frequency has a fixed DME frequency.
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2.9.4 Intersection / Fix [S+]
An intersection or fix is not a ground-based navigation
aid, but only a position on the surface of the earth
defined with the position reference system.
As an intersection is not a transmitter it cannot be flown
to with conventional navigation instruments. To fly to an
intersection the aircraft has to be RNAV (area navigation)
equipped.
RNAV means that the aircraft will calculate its position
and the direction to the next waypoint by means of
different ground based navigation aids (VOR/DME) as well
as by positions from GPS and INS/IRS.
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2.9.5 Global Positioning System – GPS [S+]
GPS is a satellite positioning system developed by the
United States Department of Defense (DOD) for use on land,
sea and in the air.
It will likely be the major component of the ICAO -
designated GNSS - Global Navigation Satellite System.
The full GPS constellation has 24 operational satellites
to provide continuous, highly accurate three-dimensional
position information globally.
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2.9.5.1 More on GPS [C+]
GPS is operating in 1,900 NM orbits, each satellite
continuously transmits signals on 1227.6 and 1575.42 MHz.
The GPS receiver automatically selects the signals from
four or more satellites to calculate a three-dimensional
position, velocity and time.
Using the un-encrypted coarse acquisition navigational
signal (C/A code) which will be available to all civil
users, system accuracy will be at least 100 meters
horizontally and 140 metres vertically, 95% of the time.
Unlike ground based navigation systems, GPS provides
global coverage with virtually no signal inaccuracies
associated with propagation in the earth's atmosphere.
Signal masking can occur with mountainous terrain,
man-made structures and with poor antenna location on the
aircraft.
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2.9.6 Inerital Navigation System (INS) [C+]
Inertial Navigation Systems (INS) are completely
self-contained and independent of ground based navigation
aids. After being supplied with initial position
information, it is capable of updating with accurate
displays of position, attitude, and heading. It can
calculate the track and distance between two points,
display cross error, provide ETAs, ground speed and wind
information. It can also provide guidance and steering
information for the pilot instruments.
The system consists of the inertial platform, interior
accelerometers and a computer. The platform, which senses
the movement of the aircraft over the ground, contains two
gyroscopes. These maintain their orientation in space
while the accelerometers sense all direction changes and
rate of movement. The information from the accelerometers
and gyroscopes is sent to the computer, which corrects the
track to allow for such factors as the rotation of the
earth, the drift of the aircraft, speed, and rate of turn.
The aircraft's attitude instruments may also be linked to
the inertial platform.
The accuracy of the INS is dependent on the accuracy of
the initial position information programmed into the
system. Therefore, system alignment before flight is very
important. Accuracy is very high initially following
alignment, and decays with time at the rate of about 1-2
NM per hour. Position updates can be accomplished in
flight using ground based references with manual input or
by automatic update using multiple DME or VOR inputs.
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2.9.7 ATS Routes [S+]
ATS routes are pre-determined routes connecting waypoints
to each other, which the aircraft will follow.
ATS routes are named by a letter followed by two or three
numbers. If the route is used in the upper airspace it is
also given the prefix “Upper”. For example UN872 (“Upper
November 872”).
Some ATS routes are for aircrafts flying in one direction
only.
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2.9.7.1 More on ATS Routes [C+]
ICAO states that: “The designation of specific ATS routes
within the network should be made so that the majority of
recurring flight operations can identify them in flight
plans with the least number of designators.”
For routes forming part of the basic ATS route network
- A, B, G, R – routes which form part of the regional
networks of ATS routes and are not area navigation
routes.
- L, M, N, P – area navigation routes (RNAV) which
form part of the regional networks of ATS routes.
For routes not forming part of the basic ATS route
network
- H, J, V, W – for routes which are not area
navigation routes.
- Q, T, Y, Z – for area navigation (RNAV) routes.
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2.9.8 Flight Management System – FMS [C]
Flight management system (FMS) is the term used to
describe an integrated system that uses navigation,
atmospheric and fuel flow data from several sensors to
provide a centralized control system for flight planning,
and flight and fuel management.
 The
system processes navigation data to calculate and update a
best computed position based on the known system accuracy
and reliability of the input sensors.
This system may also be referred to as a multi-sensor
RNAV.
FMS controls differ widely between aircraft types and
manufacturers, but the Typical FMS Control Unit figure, to
the right, gives a typical arrangement.
The heart of any FMS is the navigation computer unit. It
contains the micro processor and navigation data base. A
typical base contains a regional or worldwide library of
navaids, waypoints, airports and airways.
FMS sensor input is supplied from external DME, VOR, air
data computer (ADC) and fuel flow sensors. Depending on
the capabilities of the navigation sensors, most flight
management systems are approved for en route IFR in most
classes of RNAV airspace.
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2.10 Navigational Aids Limitations [C+]
Navigational aids can be classified as almost anything,
visual or otherwise, as long as it provides an aircraft
with positional data.
For our purposes in the VACC, the limitations we are
concerned with is the useful range of VOR's, VORTAC's,
VOR/DME, and NDB's. VOR's without DME are becoming a
rarity, and most navaids, except for NDB's, have distance
information available.
Navaids are classified by their useful altitude and
distance. This takes into consideration signal strength,
their protection from navaids on the same frequency, and
other factors. The classes of navaids are depicted on a
low altitude chart. The symbology shows the type of
navaid, and it can be assumed to be classified as at least
"L"-class unless the notation (T) for TVOR appears in the
communications box next to the navaid, where name,
frequency, and identifier appears. Look at a low altitude
chart and try to find a TVOR. The navaids on high altitude
charts can be assumed to be "H"-class unless a (L) or (T)
appears in the communications box.
The table below shows the useful range of navaids at
various altitudes. Naturally, you do not need to commit
this to memory, or even keep it for reference, but be
aware of the limitations of navigational aids. You are not
required to reference the following tables when assigning
routes and altitudes, because you are always monitoring
the flights on radar. However, the tables may explain why
aircraft do not always receive the signal when you expect
they should. It may also prevent you from embarrassment
when clearing a high altitude aircraft over a TVOR, and
wondering why the pilot is too lame to navigate.
VOR/VORTAC/TACAN
| Class |
Altitude |
Distance |
| T |
12,000 and below |
25 |
| L |
Below 18,000 |
40 |
| H |
Below 14,500 |
40 |
| H |
14,500 – 17,999 |
100 |
| H |
18,000 – FL450 |
130 |
| H |
Above FL450 |
100 |
LF/MF Radio Beacon (NDB)
| Class |
Power (watts) |
Distance |
| CL |
Under 25 |
15 |
| MH |
Under 50 |
25 |
| H |
50 – 1,999 |
50 |
| HH |
2,000 or more |
75 |
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2.11 Further reading
If you with to learn more on the topics covered above we
list a number of external links which we believe could be
of interest to members in general
Time and date:
http://www.timeanddate.com/
GPS - Positioning system:
http://www.colorado.edu/geography/gcraft/notes/gps/gps.html
http://www.gps.oma.be/
Speed:
http://www.womanpilot.com/past%20issue%20pages/2000%20issues/jan%20feb%202000/airspeed.htm
https://ewhdbks.mugu.navy.mil/mach-as.htm
Calculate between IAS, CAS, TAS:
http://www.flightplan.za.net/trueAirspeed.php
NDB:
http://en.wikipedia.org/wiki/Non-directional_beacon
VOR:
http://en.wikipedia.org/wiki/VHF_omnidirectional_range
DME:
http://en.wikipedia.org/wiki/Distance_Measuring_Equipment
Naviagtional Aids:
http://www.centennialofflight.gov/essay/Government_Role/navigation/POL13.htm1Back
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