Magnetic Compass
The simplest of aeronautic
navigation instruments that is most often used for basic orientation is the magnetic
compass.
Operation
The principle of the compass’s magnetized needle aligning itself to the
field lines of the
earth magnetic field
allows the pilot to determine the heading of the aircraft in relation
to magnetic north. The readings will usually show N, S, E and W with the
intervals between marked for each 30º. Further sub-divisions between
the 30º marks are shown for every 5º.
Figure: magnetic compass with deviation card
There are a number of magnetic compass designs used in aircraft. One
that is present in most aircraft is the floating magnet type. Here the
magnetic needle is integrated into a floating disk that carries the
markings of the compass rose on its circumference. A lubber line
indicates the current magnetic heading of the aircraft. Note, that when
the lubber line indicates North the part of the disk that points North
is actually on the opposite side of the disk. Consequently, the
markings appear backwards – East is on the
West side and vice versa. This could be confusing at first.
Another design that avoids this confusion is the vertical
card magnetic compass. Here the nose of a symbolic aircraft points to
the magnetic heading on a compass rose.
The earth
magnetic field
has a horizontal and a vertical component. The compass needle will
react to both the horizontal and the vertical force. The horizontal
component is used for navigation, as it is responsible for the magnetic
needle pointing to magnetic north. However, the effect that the
vertical component has on the compass performance should also be
understood, as it affects the accuracy of the compass reading
significantly. One could imagine the magnetic needle pointing directly
down when at the magnetic pole. This effect of pulling the magnetic
needle down is called the "magnet dip". Make sure you read the section
on turning and acceleration errors on the
Navigation Basics Page
to understand the effects that the magnetic dip has on compass indication.
Coverage
In principle the magnetic field is present everywhere
on earth. The direction and strength is, however, influenced by a
number of geographic factors. As a result the difference in direction
between the North Magnetic Pole and the North Geographic Pole (the
variation) sometime shows anomalies. This is reflected in bent
variation lines. As the magnetic compass is ultimately used to determine
the direction to Geographic North (in a process that takes into
account the variation) these anomalies are of no practical importance as
they are included in the variation. In other words, it is enough to
know the variation in the area to find the correct direction to
Geographic North. Make sure you read the section on variation on the
Navigation Basics Page
.
It should also be noted that the increasing downward component
of the earth magnetic field the closer one gets to the magnetic poles,
makes the compass less useful at latitudes higher than 60° North or
South.
Accuracy and Errors
An aeroplane is made up of metal, rotating parts of an engine,
electric equipment,
etc., all of which can generate their own magnetic field. Naturally,
these fields affect the compass reading deflecting it from accurately
indicating Magnetic North.
An engineer who has checked the compass in any
particular aeroplane might have tried to minimise the deflection by
placing little magnets around the compass. The remaining deflection
after such corrective action is called Deviation. Each aeroplane
displays a small placard, known as the Deviation Card, which shows the
pilot the corrections to be made to the compass reading to obtain the
magnetic direction.
Obviously the deviation card can only take into
account the influences that were present when the engineer calibrated
the compass. Any magnetic influences introduced after the calibration
procedure can still affect the compass reading. Therefore, one should
keep in mind that materials with their own magnetic field placed in the
vicinity of the compass can have a significant effect. In particular
large metal items or electronic devices, such as headphones or
calculators, can cause large and unpredictable errors.
There are other errors that affect the compass
reading when the aeroplane accelerates or turns (to do with magnet dip
as mentioned above). Make sure you read the applicable paragraphs on
the
Navigation Basics Page
to understand these effects.
The Automatic Direction Finder (ADF)
Operation
The simplest radio navigation aid used in aviation
is a ground-based transmitter which transmits radio signals in all
directions without differentiation: the Non-Directional Beacon (NDB).
The counterpart instrument fitted in the aeroplane is called the
Automatic Direction Finder (ADF) and consists of two arials, a receiver
and an indicator. The indicators needle simply points to the selected
NDB ground station. In that respect it works similar to the magnetic
compass with its needle pointing to magnetic North. Therefore, the
ADF/NDB system is sometimes called the Radio Compass.
There are a number of indicator types – the
fixed-card indicator, the moving-card indicator and the Radio Magnetic
Indicator (RMI). In the fixed-card indicator the card has a fixed
compass rose where north is always in the top position. This type of
indicator displays the relative bearing to the NDB station. The
moving-card indicator and the RMI both have a rotating azimuth card –
the former needs to be adjusted maually, where the latter rotates
automatically, controlled by a gyro compass to represent the aircraft
heading. The RMI also has two needles which can indicate the navigation
information from the ADF or VOR. The head of ADF needle of the RMI
points to the magnetic bearing to the selected NDB station, i.e. the
magnetic heading plus the relative bearing equals the magnetic bearing.
Figure: ADF with fixed card indicator
Figure: RMI
NDB stations transmit on a given frequency in the low-frequency or
medium-frequency LF/MF bands (that is between 200 and 1750 kHz). Each
NDB or Locator is identifiable by a two- or three-lettered Morse code
identification signal which is transmitted along with its normal signal,
the so called IDENT.
The Pilot must identify an NDB before using it for
navigation and, if using it for some length of time, periodically
re-identify it. The lack of an IDENT may indicate that the NDB is out
of service, even though it may still be transmitting (for instance for
maintenance or test purposes). If an incorrect IDENT is heard, then it
must also not be used.
Most NDBs can be identified by selecting AUDIO on
the ADF and listening to the Morse code signal. The correct code can be
found for instance on the ICAO 1:500000 series aeronautical chart or in
the Aeronautical Information Publication (AIP).
There are a number of different IDENT characteristics depending on the type of transmission.
Most NDBs can be identified with the ADF mode
selector in the ADF position. However, there are also NDBs that can only
be identified, if the pilot has selected BFO (Beat Frequency
Oscillator). In this setting a tone is imposed onto the NDB carrier wave
and makes it audible. Such NDBs are shown on charts without inverted
commas, for example MP at Cherbourg in France.
Some NDBs carry voice transmissions, such as the
Automatic Terminal Information Service (ATIS) at some aerodromes. It is
also possible, such as in a situation where the communications radio
(VHF COM) has failed, for ATC to send voice messages to the Pilot on the
NDB frequency and for him to receive them on the ADF if AUDIO is
selected.
Figure: NDB on the chart
Coverage
NDB/ADF is used extensively throughout the world for navigation.
The range of an NDB depends upon:
- the power of the transmitter (10—200 Watt);
- the frequency of transmission; and
- atmospheric
conditions during transmission — signals from an NDB can be
distorted or deflected by electrical storms, as well as during the
periods of sunrise and sunset.
A relatively strong NDB with a range of 100 nm or
more would be required for long-range en route navigation where no other
aids are available. Some NDBs may even have a range of 400 nm when
used for long distance overwater tracking, for instance in the Pacific
area. In more densely populated areas, however, where routes are
relatively short and there are many navigation facilities, most NDBs
have only a short range.
For manoeuvring in the vicinity of aerodromes, only
lower powered NDBs are required. NDBs used for approaches are referred
to as Locators. If a Locator is co-located with an Outer Marker that
serves to fix a position as an aircraft proceeds down an Instrument
Landing System (ILS) approach, then it will be depicted on the ILS
Approach Chart as LOM (Locator Outer Marker).
The range of each Non-Directional Beacon (NDB) or
Locator (L) may be found in AlP COM 2, and within this promulgated range
the NDB should provide bearings accurate to within +/—5º. The
promulgated range also provides guidance as to when attention should be
shifted to the next aid.
Accuracy and Errors
An ideal NDB signal received at an aeroplane may be
accurate to +/-2º. However, there are a number of factors that may
reduce this accuracy to a considerable degree. These include the
following effects:
-
The Thunderstorm Effect causes the ADF needle to be deflected towards a
nearby electrical storm (Cumulonimbus cloud) and away from the
selected NDB.
-
In the Night Effect a fading signal and a wandering ADF needle (most
pronounced at dawn and dusk) is observed when strong skywaves from the
NDB returning to earth from the ionosphere cause interference with the
surface waves from the NDB.
- Interference is possible from other NDBs transmitting on similar frequencies.
- The Mountain Effect is caused by reflections of the NDB signals from mountains.
- The Coastal Effect is caused by the NDB signal bending slightly towards the coastline when crossing it at an angle.
VOR
Operation
The Very High Frequency Omni-Directional Radio
Range, commonly abbreviated to VOR, VHF Omni Range, or Omni, is a radio
navigation aid operating in the frequency band 108.0 MHz to 117.95
MHz. This is a lower frequency band than that used for VHF
Communications, but significantly higher than that used for NDB/ADF.
There is relatively little interference from atmospheric noise in this
band, so VOR allows high quality line-of-sight reception.
A VOR ground station transmits two VHF radio signals:
- The Reference Phase is omni-directional (i.e. uniform in all directions).
-
The Variable Phase rotates uniformly, with its phase varying at a
constant rate throughout the 360º, being in-phase with the reference
signal on Magnetic North.
The aerial of the VOR airborne receiver then picks up
the signals and measures the phase difference between them. With this
difference depending upon the bearing of the aeroplane from the ground
station, the VOR can determine the magnetic bearing of the aeroplane
from the VOR ground station.
The Pilot should positively identify the VOR by the
Morse code IDENT signal transmitted every 10 seconds or so. Some VORs
may also carry voice transmissions with a relevant Automatic Terminal
Information Service (ATIS).
The airborne component of VOR consists of the
antenna, the receiver and the cockpit display. A separate VHF-NAV radio
is required for navigation purposes, but is usually combined with the
VHF-COM in a NAV/COM set.
There are various types of VOR cockpit display,
however, they are all similar in operation. The VOR cockpit display is
also referred to as the Omni Bearing Indicator, or OBI. It displays the
Omni Bearing selected by the Pilot on the Course Card using the Omni
Bearing Selector (OBS), a small knob which is geared to the card.
Figure: OBI
The straight line magnetic bearings extending outwards from the
ground station are called radials. Radials are identified by a number
starting from 1° east of magnetic north (i.e. 001) clockwise through
360°. If the aeroplane is on the selected radial or its reciprocal, then
the VOR Needle in the OBI, known as the Course Deviation Indicator or
CDI, will be centred. The outer edge of the center circle represents 2°
off-course, each dot represents further 2° out. Full deflection of the
CDI indicates that the aircraft is more than 10° off the selected
radial.
With the aeroplane being on the radial, the TO/FROM flag
indicates whether the selected track would take the aeroplane to or from
the VOR ground station. There is also some means of indicating
insufficient signal strength – usually in form of an OFF flag.
The OBI is only to be used for navigation if:
- the red OFF warning flag is hidden from view;
- the correct Morse Code IDENT is heard.
Figure: VOR on chart
Most aeronautical charts show the position, frequency and Morse Code
identification (IDENT) of each VOR ground station. Information on a
particular VOR can be found in the AIP COM. Changes to this information
will be referred to in Notices to Airmen (NOTAMs). The pilot should
check these during flight preparation.
A VOR ground station may be represented in various ways on a
chart. Since Magnetic North is the reference direction for VOR radials,
a Magnetic North arrow-head usually emanates from the VOR symbol, with
a compass rose heavily marked each 3 degrees and the radials shown in
10º intervals on the rose. This generally adequate for in-flight
estimation of track to an accuracy of +/-2º, however, when flight
planning, it is advisable to be more accurate.
At the Flight Planning stage, the pilot should use a
protractor or plotter for precise measurement of track, although in
some cases this may not be necessary because some much-used tracks are
published on Radio Navigation Charts (RNCs) in degrees Magnetic. If the
pilot measures the track in degrees True (0ºT), then Magnetic Variation
needs to be applied to invert to degrees Magnetic ("Variation West,
Magnetic Best").
Distance Measuring Equipment (DME) is a navigation
aid that is often combined with VOR and works in the UHF band. Such
combination is then called VOR/DME. DME is also an integral part of
VORTAC – a combination of VOR and the military tactical air navigation
(TACAN).
Not all airplanes are DME equipped. DME measures the
direct distance between the aircraft and the VOR station, the so called
slant range distance. Therefore, it depends on the altitude of the
airplane. Most DME receivers are also capable of displaying the ground
speed and time-to-station. However, these work accurately only when
navigating along a radial to or from station.
Coverage
The approximate maximum range of a VHF signal is given by the formula:
VHF Range in nm = the square root of (1.5 x altitude in feet)
Example: At 7,000 ft AMSL, approximate VHF range = square root of (1.5 x 7,000) = square root of (10,000) = 100 nm.
Errors and accuracy
Reception may be affected by the terrain surrounding
the ground station, the height of the VOR beacon, the altitude of the
aeroplane and its distance from the station.
Since the VOR uses VHF signals, it is accurate as
long as the aircraft is in "line of sight" of the transmitter. The only
problem may be that if the aircraft is at a sufficiently high altitude,
then there is the possibility that two VORs on the same frequency could
cause interference.
To counteract this, the AIP publishes the range and
altitude at which one can expect to receive a reliable signal for each
VOR, the so called "Designated Operational Coverage" (DOC) figures.
When a VOR is operating normally, the radials are transmitted to an accuracy of at least +/- 2º.
Ground radar
Operation
Primary radar involves the transmission of energy in
the form of very short pulses, to determine the range and bearing of an
object. Any object in the path of the pulses, will reflect and scatter
the energy. Some of the reflected energy will reach the receiver
allowing the calculation of range and bearing to be made.
The strength of the reflected echoes depends upon the following factors:
- Power of transmitter.
- Range of the reflected object.
- Shape, material and altitude of the reflected object.
- Size of the object in relation to the wavelength.
The calculation of range and bearing of the
reflecting object from the transmitting source does not require any
co-operation from the target.
Most radar screens are simply Cathode Ray Tubes (CRT)
that resemble circular television screens. Using the same principle as
television, a beam of electrons is directed onto the fluorescent
coating of the CRT to provide a radar picture. Radar controllers
generally have circular displays showing the position of the radar
antenna in the centre, with range marks to aid in estimating distance.
The radar screen is also known as a Plan Position Indicator (PPI).
The actual radar dish may be located away from the
position of the radar controller, possibly on a nearby hill or tower. As
the radar antenna rotates slowly, the small electron beam in the
controller's CRT also rotates, leaving a faint line or trace on the
screen in a direction aligned with the direction of the antenna at that
moment. Any radar return signal appears as a blip at the appropriate
spot on the screen.
An indication of North on the screen allows the
controller to estimate the direction of the target, and the range marks
assist in estimating its distance. The blip of the target remains
visible for some seconds after the small trace line has moved on, and
will still be visible (but fading) as its next blip occurs in the
following revolution. This fading trail of blips allows the Controller
to determine the motion of the target in terms of direction and speed.
In areas of high traffic density, the radar
responsibility may be divided between various controllers, each with
their own screen and radio communications frequency, and will go under
such names as:
- Approach Control; and
- Zone Control.
Other markings besides the range circles may be
superimposed upon the screen as a video map to indicate the location of
nearby controlled airspace, aerodromes, radio navigation aids such as
VORs and NDBs, restricted areas, etc.
Coverage
All radar systems operate at VHF or above, therefore
range is limited to line of sight as with VOR, the maximum range may be
limited by the following:
- height of the radar head,
- height of the target,
- intervening high ground between the transmitter and receiver.
Errors and accuracy
The main factors that affect the reliability and accuracy of primary radar are:
-
characteristics of the reflecting object – The strength of the echo
depends on the shape, size material and altitude of the reflecting
object.
-
clutter from precipitation and high ground – weather returns can make
it difficult to monitor the echo from the target aircraft.
- transmission power – the greater the transmission power, the greater the range.
- blind spots (e.g. valleys)
In an attempt to remove some of the "clutter" from
the radar screen, an electronic sifting device called a Moving Target
Indicator (MTI) will only show return signals from a target that is in
motion (i.e. not clouds or high terrain).
Secondary surveillance radar (SSR)
Secondary Surveillance Radar removes most of the
limitations of Primary Radar simply by adding energy to the return pulse
from the aeroplane, using a device carried on board the aeroplane
known as a Transponder.
Operation
The SSR ground equipment consists of:
- an interrogator that provides a coded signal asking a transponder to respond;
-
a highly directional rotating radar antenna that transmits the coded
interrogation signal, then receives any responding signals, and passes
them back to the interrogator; and
- a decoder, which accepts the signals from the interrogator, decodes them and displays them on the radar screen.
The SSR airborne equipment consists of a transponder carried in the individual aeroplane.
The originating signal transmitted from the ground
station triggers an automatic response from the aeroplane's transponder.
It transmits a strong answering coded signal which is then received at
the ground station. This response signal is much, much stronger than
the simple reflected signal used in primary radar. Even a very weak
signal received in the aircraft will trigger a strong response from the
transponder.
The secondary responding pulse from the aeroplane's
transponder can carry coding which will allow the controller to
distinguish the aeroplane from all others on the radar screen.
Depending upon the code selected in the transponder by the pilot, it can
also carry additional information such as:
- the identity of the aeroplane;
- its altitude;
- any abnormal situation such as radio failure, distress, emergency, etc.
A significant advantage is that SSR is not degraded to the same extent as primary radar by weather or ground clutter:
- it presents targets of the same size and intensity to the controller,
- it allows the controller to select specific displays, and
- the system has minimal blind spots.
Modes and codes
Typical transponder modes, selected by the Function Selector Knob, include:
- OFF: off
-
STANDBY: warmed up, and ready for immediate use. This is the normal
position until ready for take-off, when one would select ALT or ON (if
transponder is to be used in flight).
- ON: transmits the selected code in Mode A (aircraft identification mode) at the normal power level.
-
LO SENS: low sensitivity, transmits the selected code the same as in
the ON position but at a lower power level. This may be requested by
the radar controller to avoid over-strong blips on his screen from
aircraft close to the interrogating antenna. After landing, a pilot
would normally switch to STANDBY or OFF for the same reason.
-
ALT: altitude, which may be used if the altitude reporting capability
(known as mode C) is installed in your aircraft. This is a special
"encoding" altimeter which feeds your altitude to the transponder for
transmission on to the ATC radar screen. (If not installed, the
transponder still transmits in Mode A, i.e. aircraft identification
without altitude reporting).
-
TST: tests that the transponder is operating correctly and if so,
illuminates the reply monitor light. It causes the transponder to
generate a self-interrogating signal to provide a check of its
operation.
Knobs are provided to select the appropriate code for
the transponder, the selected code being prominently displayed in
digital form.
Figure: transponder
An important procedure to follow when selecting and altering codes is
to avoid passing through vital codes (such as 7700 for emergencies,
7600 for radio failure) when the transponder is switched ON. This can
be avoided by selecting STANDBY whilst the code is being changed.
The Reply-Monitor Light will flash to indicate that the
transponder is replying to an interrogation pulse from a ground station.
It will glow steadily when:
-
The pilot presses the TEST button or moves the function switch to the
TEST position (depending upon the design of your particular transponder)
to indicate correct functioning; or
- transmitting an IDENT pulse.
When the IDENT button is pressed by the pilot upon
request from the radar controller to SQUAWK IDENT, a special pulse is
transmitted with the transponder's reply to the interrogating ground
station. This causes a special symbol to appear for a few seconds on the
radar screen around the return from the aircraft's transponder, thus
allowing positive identification by the radar controller.
The term Squawk, which is often used in radio
communication, is confined to transponder usage, and the instruction
following squawk is usually quite clear, for instance: "Squawk Ident";
"Squawk Code 4000"; "Squawk Mayday" (7700), etc.
Very High Frequency Direction Finding (VHF D/F or VDF)
Operation
One of the benefits of modern navigation and radio
systems is that a pilot has the option of asking a suitably equipped
ground station for his relative bearing to or from that particular
ground station.
This means that the pilot can determine his
position (by using another VDF readout or NDB station) and heading more
accurately should he become disorientated.
Some aerodromes are equipped with radio aerials
which can sense the direction of VHF-COM signals (i.e. normal voice
signals) received from an aeroplane.
This information is
presented to the air traffic controller (usually the approach
controller) as a radial line on a Cathode Ray Tube similar to a radar
screen or, with the most modern VDF equipment, as a very accurate
digital readout of bearing.
The controller can then advise the pilot of his
bearing relative to the aerodrome. This is known as Very High Frequency
Direction Finding, and is often abbreviated to VHF D/F or VDF.
An advantage of VDF is that no specific airborne
equipment is required other than a VHF-COM, i.e. a normal VHF
communications radio.
A typical VDF air/ground exchange would be a pilot
requesting ATC to provide his QDM (magnetic bearing to the ground
station), followed by the controller advising it. By steering the QDM,
the pilot is able to home to the ground station, i.e. head towards it.
Consequently, ground stations that are equipped to provide VDF are
designated by the term Homer.
Whereas no special equipment is required in the
aeroplane for VDF other than a VHF-COM radio, it does require a special
installation at the ground station. Two typical designs for VDF aerials
at aerodromes are the H-type aerial (a double-H dipole aerial in
technical terms), or the Doppler-type VDF aerial.
Bearings that a pilot may request from a VDF operator are:
- QDM: magnetic bearing TO the station;
- QDR: magnetic bearing FROM the station (i.e. the reciprocal of the QDM);
- QTE: true bearing from the station.
QDR, the magnetic bearing from the station, is
useful for orientation (where am I?). QDR is similar information to a
VOR radial.
QTE, the true bearing from the station, is useful if
the pilot wants to plot a position line from the VDF ground station to
the aeroplane on a map (against True North).
QDM is the most commonly requested bearing. It is the
heading to steer direct to the VDF station provided no crosswind
exists. In a crosswind, however, a Wind Correction Angle (WCA)
into-wind must be used to counteract the drift if a reasonably straight
track is to be achieved, rather than a curved homing.
At typical light aircraft speeds, it is reasonable
for the pilot to request a QDM each half-minute or so to check tracking,
and to modify heading if necessary.
Accuracy and Errors
The main problem associated with VDF is that of
wind drift. If the pilot is flying in unknown wind conditions, any
bearing he is given by the ground station assumes that there is no
wind, and so no allowance is made to the bearing.
As successive bearings are obtained, the pilot
should be able to see that he is drifting left or right of the desired
track and therefore make the appropriate adjustments to his heading.
The quality of the bearings obtained by VDF is classified by the VDF ground operator to the pilot as:
- Class A: accurate to within +/-2°
- Class B: accurate to within +/-5°
- Class C: accurate to within +/-10°
- Class D: CAP 46 also lists some Class D VDF Stations with an accuracy poorer than even +/-10°.
Most modern equipment is generally accurate to +/-1°, although accuracy may be decreased by:
- VDF site errors such as reflection from nearby uneven ground, buildings, aircraft or vehicles; and
-
VHF propagation errors caused by irregular propagation over differing
terrain, especially if the aeroplane is at long range from the VDF
ground station.
GPS
Global Positioning System (GPS) is a satellite-based
radio navigation and time dissemination system that is used by many
people to accurately determine their position at any point on earth. It
has been developed by the U.S. Department of Defense.
The global GPS system consists of three segments:
-
The space segment consists of a constellation of 26 satellites orbiting
the earth. Each satellite transmits a unique code and navigation
information in the UHF band. Therefore, it is not affected by weather
interferences, but subject to line of sight limitations.
-
The control segment consists of a master control station and a number
of monitoring stations and ground antennas on earth. They fulfil the
function of monitoring the satellites and communicating updates and
corrections to the satellites.
-
The user segment is associated with the receiving end of the system.
Knowing the exact time and position of signal transmission and the time
of travel, an accurate determination of the receiver position is
possible. A three-dimensional fix can be achieved with receiving
signals from 4 satellites.
Operation
There are a wide variety of receivers on the market,
which differ in functionality, operating procedures and in their
suitability to be used for aeronautical navigation. Knowing how to
operate the receiver is vital before relying on it for navigation. GPS
receivers contain extensive databases which should be current and
checked to be correct and suitable before used in navigation. After
switching the unit on, initialisation will proceed, which can be
extensive – in particular when the unit has not been used for some time
or has been relocated over long distances while switched off. Allow
time for this initialisation process.
Figure: GPS receiver
Coverage
The system will operate anywhere on earth as long as the
appropriate number of satellites are in line of sight (i.e. above the
horizon of the receiver).
Accuracy and Errors
GPS course deviation is linear, i.e. the tracking
sensitivity is independent of the position of the receiver. Most low
cost GPS units will provide a reading that is accurate up to 100m.
Higher cost units can be accurate to within 1m. The US Government limits
the accuracy attainable with a GPS unit to the person or organisation
using it e.g. military users are able to achieve more precise
positioning than members of the public (including General Aviation
Pilots).
GPS receivers can be very complex devices – so most
errors probably occur due to incorrect operation or interpretation.
Pilots should be careful to observe placards, selector switch
positions, and annunciator indication when using GPS. Receivers should
only be utilised for its approved purpose.
GPS NOTAMs will announce satellite outages where necessary.
