Introduction
This chapter provides the basic radio principles applicable to
navigation equipment, as well as an operational knowledge
of how to use these systems in instrument flight. This
information provides the framework for all instrument
procedures, including standard instrument departure
procedures (SIDS), departure procedures (DPs), holding
patterns, and approaches, because each of these maneuvers
consists mainly of accurate attitude instrument flying and
accurate tracking using navigation systems.
Navigation
Systems
Chapter 9
Not refracted
Ionosphere
Figure 7-1. Ground, space, and sky wave propagation.
Ground wave
Space wave
Sky wave
Figure 9-1. Ground, space, and sky wave propogation.
Basic Radio Principles
A radio wave is an electromagnetic (EM) wave with
frequency characteristics that make it useful. The wave
travels long distances through space (in or out of the
atmosphere) without losing too much strength. An antenna
is used to convert electric current into a radio wave so it can
travel through space to the receiving antenna, which converts
it back into an electric current for use by a receiver.
How Radio Waves Propagate
All matter has a varying degree of conductivity or resistance
to radio waves. The Earth itself acts as the greatest resistor
to radio waves. Radiated energy that travels near the ground
induces a voltage in the ground that subtracts energy from the
wave, decreasing the strength of the wave as the distance from
the antenna becomes greater. Trees, buildings, and mineral
deposits affect the strength to varying degrees. Radiated
energy in the upper atmosphere is likewise affected as the
energy of radiation is absorbed by molecules of air, water,
and dust. The characteristics of radio wave propagation vary
according to the signal frequency and the design, use, and
limitations of the equipment.
Ground Wave
A ground wave travels across the surface of the Earth. You
can best imagine a ground wave’s path as being in a tunnel
or alley bounded by the surface of the Earth and by the
ionosphere, which keeps the ground wave from going out
into space. Generally, the lower the frequency, the farther
the signal travels.
Ground waves are usable for navigation purposes because
they travel reliably and predictably along the same route
day after day and are not influenced by too many outside
factors. The ground wave frequency range is generally from
the lowest frequencies in the radio range (perhaps as low as
100 Hz) up to approximately 1,000 kHz (1 MHz). Although
there is a ground wave component to frequencies above this,
up to 30 MHz, the ground wave at these higher frequencies
loses strength over very short distances.
Sky Wave
The sky wave, at frequencies of 1 to 30 MHz, is good for
long distances because these frequencies are refracted or
“bent” by the ionosphere, causing the signal to be sent back
to Earth from high in the sky and received great distances
away. [Figure 9-1] Used by high frequency (HF) radios in
aircraft, messages can be sent across oceans using only 50
to 100 watts of power. Frequencies that produce a sky wave
are not used for navigation because the pathway of the signal
from transmitter to receiver is highly variable. The wave is
“bounced” off of the ionosphere, which is always changing
due to the varying amount of the sun’s radiation reaching it
(night/day and seasonal variations, sunspot activity, etc.). The
sky wave is, therefore, unreliable for navigation purposes.
For aeronautical communication purposes, the sky wave
(HF) is about 80 to 90 percent reliable. HF is being gradually
replaced by more reliable satellite communication.
Space Wave
When able to pass through the ionosphere, radio waves
of 15 MHz and above (all the way up to many GHz), are
considered space waves. Most navigation systems operate
with signals propagating as space waves. Frequencies above
100 MHz have nearly no ground or sky wave components.
They are space waves, but (except for global positioning
system (GPS)) the navigation signal is used before it reaches
the ionosphere so the effect of the ionosphere, which can
cause some propagation errors, is minimal. GPS errors
caused by passage through the ionosphere are significant
and are corrected for by the GPS receiver system.
Space waves have another characteristic of concern to users.
Space waves reflect off hard objects and may be blocked if
the object is between the transmitter and the receiver. Site
and terrain error, as well as propeller/rotor modulation error
in very high omnidirectional range (VOR) systems, is caused
by this bounce. Instrument landing system (ILS) course
distortion is also the result of this phenomenon, which led
to the need for establishment of ILS critical areas.
21 15
HDG
Figure 7-2. ADF indicator instrument and receiver.
OFF
VOL
RST SET
ET FLT
FRQ
BFO
ADF
A D F
F R Q
ADF KR 87 TSD
Figure 9-2. ADF indicator instrument and receiver.
Generally, space waves are “line of sight” receivable, but
those of lower frequencies “bend” somewhat over the
horizon. The VOR signal at 108 to 118 MHz is a lower
frequency than distance measuring equipment (DME) at 962
to 1213 MHz. Therefore, when an aircraft is flown “over the
horizon” from a VOR/DME station, the DME is normally
the first to stop functioning.
Disturbances to Radio Wave Reception
Static distorts the radio wave and interferes with normal
reception of communications and navigation signals. Low-
frequency airborne equipment, such as automatic direction
finder (ADF) and LORAN (LOng RAnge Navigation)
,are particularly subject to static disturbance. Using very
high frequency (VHF) and ultra-high frequency (UHF)
frequencies avoids many of the discharge noise effects.
Static noise heard on navigation or communication radio
frequencies may be a warning of interference with navigation
instrument displays. Some of the problems caused by
precipitation static (P-static) are:
• Complete loss of VHF communications.
• Erroneous magnetic compass readings.
• Aircraft flying with one wing low while using
the autopilot.
• High-pitched squeal on audio.
• Motorboat sound on audio.
• Loss of all avionics.
• Inoperative very-low frequency (VLF) navigation
system.
• Erratic instrument readouts.
• Weak transmissions and poor radio reception.
• St. Elmo’s Fire.
Traditional Navigation Systems
Nondirectional Radio Beacon (NDB)
The nondirectional radio beacon (NDB) is a ground-based
radio transmitter that transmits radio energy in all directions.
The ADF, when used with an NDB, determines the bearing
from the aircraft to the transmitting station. The indicator
may be mounted in a separate instrument in the aircraft
panel. [Figure 9-2] The ADF needle points to the NDB
ground station to determine the relative bearing (RB) to the
transmitting station. It is the number of degrees measured
clockwise between the aircraft’s heading and the direction
from which the bearing is taken. The aircraft’s magnetic
heading (MH) is the direction the aircraft is pointed with
respect to magnetic north. The magnetic bearing (MB) is the
direction to or from a radio transmitting station measured
relative to magnetic north.
NDB Components
The ground equipment, the NDB, transmits in the frequency
range of 190 to 535 kHz. Most ADFs also tune the AM
broadcast band frequencies above the NDB band (550 to
1650 kHz). However, these frequencies are not approved
for navigation because stations do not continuously identify
themselves, and they are much more susceptible to sky wave
propagation especially from dusk to dawn. NDB stations
are capable of voice transmission and are often used for
transmitting the Automated Weather Observing System
(AWOS). The aircraft must be in operational range of the
NDB. Coverage depends on the strength of the transmitting
station. Before relying on ADF indications, identify the
station by listening to the Morse code identifier. NDB stations
are usually two letters or an alpha-numeric combination.
ADF Components
The airborne equipment includes two antennas: a receiver
and the indicator instrument. The “sense” antenna (non-
directional) receives signals with nearly equal efficiency
from all directions. The “loop” antenna receives signals
better from two directions (bidirectional). When the loop
and sense antenna inputs are processed together in the ADF
radio, the result is the ability to receive a radio signal well in
all directions but one, thus resolving all directional ambiguity.
The indicator instrument can be one of four kinds: fixed-
card ADF, rotatable compass-card ADF, or radio magnetic
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Figure 7-3. Relative bearing (RB) on a fixed-card indicator. Figure 9-3. Relative bearing (RB) on a fixed-card indicator. Note
that the card always indicates 360° or north. In this case, the RB
to the station is 135° to the right. If the aircraft were on a magnetic
heading of 360°, then the magnetic bearing (MB) would also be 135°.
24 21
6 3
S W
E N
HDG
Figure 7-4. Relative bearing (RB) on a movable-card indicator. Figure 9-4. Relative bearing (RB) on a movable-card indicator. By
placing the aircraft’s magnetic heading (MH) of 045 ° under the
top index, the RB of 135° to the right is also the magnetic bearing
(no wind conditions), which takes you to the transmitting station.
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ADF needle
VOR needle
Figure 7-5. Radio magnetic indicator (RMI). Figure 9-5. Radio magnetic indicator (RMI). Because the aircraft’s
magnetic heading (MH) is automatically changed, the relative
bearing (RB), in this case 095 °, indicates the magnetic bearing
(095°) to the station (no wind conditions) and the MH that takes
you there.
indicator (RMI) with either one needle or dual needle. Fixed-
card ADF (also known as the relative bearing indicator (RBI))
always indicates zero at the top of the instrument, with the
needle indicating the RB to the station. Figure 9-3 indicates
an RB of 135°; if the MH is 045°, the MB to the station is
180°. (MH + RB = MB to the station.)
The movable-card ADF allows the pilot to rotate the aircraft’s
present heading to the top of the instrument so that the head
of the needle indicates MB to the station and the tail indicates
MB from the station. Figure 9-4 indicates a heading of 045°,
MB to the station of 180°, and MB from the station of 360°.
The RMI differs from the movable-card ADF in that it
automatically rotates the azimuth card (remotely controlled
by a gyrocompass) to represent aircraft heading. The RMI
has two needles, which can be used to indicate navigation
information from either the ADF or the VOR receiver. When
a needle is being driven by the ADF, the head of the needle
indicates the MB TO the station tuned on the ADF receiver.
The tail of the needle is the bearing FROM the station. When
a needle of the RMI is driven by a VOR receiver, the needle
indicates where the aircraft is radially with respect to the
VOR station. The needle points the bearing TO the station
as read on the azimuth card. The tail of the needle points to
the radial of the VOR the aircraft is currently on or crossing.
Figure 9-5 indicates a heading of 360°, the MB to the station
is 005°, and the MB from the station is 185°.
Function of ADF
The ADF can be used to plot your position, track inbound
and outbound, and intercept a bearing. These procedures
are used to execute holding patterns and nonprecision
instrument approaches.
Orientation
The ADF needle points TO the station, regardless of aircraft
heading or position. The RB indicated is thus the angular
relationship between the aircraft heading and the station,
measured clockwise from the nose of the aircraft. Think of
the nose/tail and left/right needle indications, visualizing the
ADF dial in terms of the longitudinal axis of the aircraft.
When the needle points to 0°, the nose of the aircraft points
directly to the station; with the pointer on 210°, the station
is 30° to the left of the tail; with the pointer on 090°, the
station is off the right wingtip. The RB alone does not indicate
aircraft position. The RB must be related to aircraft heading
in order to determine direction to or from the station.
Station Passage
When you are near the station, slight deviations from
the desired track result in large deflections of the needle.
Therefore, it is important to establish the correct drift
correction angle as soon as possible. Make small heading
corrections (not over 5°) as soon as the needle shows a
deviation from course, until it begins to rotate steadily toward
a wingtip position or shows erratic left/right oscillations. You
are abeam a station when the needle points 90° off your track.
Hold your last corrected heading constant and time station
passage when the needle shows either wingtip position or
settles at or near the 180° position. The time interval from
the first indications of station proximity to positive station
passage varies with altitude—a few seconds at low levels to
3 minutes at high altitude.
Homing
The ADF may be used to “home” in on a station. Homing
is flying the aircraft on any heading required to keep the
needle pointing directly to the 0° RB position. To home in
on a station, tune the station, identify the Morse code signal,
and then turn the aircraft to bring the ADF azimuth needle to
the 0° RB position. Turns should be made using the heading
indicator. When the turn is complete, check the ADF needle
and make small corrections as necessary.
Figure 9-6 illustrates homing starting from an initial MH of
050° and an RB of 310°, indicating a 50° left turn is needed
to produce an RB of zero. Turn left, rolling out at 50° minus
50° equals 360°. Small heading corrections are then made
to zero the ADF needle.
If there is no wind, the aircraft homes to the station on a direct
track over the ground. With a crosswind, the aircraft follows
a circuitous path to the station on the downwind side of the
direct track to the station.
Tracking
Tracking uses a heading that maintains the desired track
to or from the station regardless of crosswind conditions.
Interpretation of the heading indicator and needle is done to
maintain a constant MB to or from the station.
To track inbound, turn to the heading that produces a zero
RB. Maintain this heading until off-course drift is indicated
by displacement of the needle, which occurs if there is a
crosswind (needle moving left = wind from the left; needle
moving right = wind from the right). A rapid rate of bearing
change with a constant heading indicates either a strong
crosswind or close proximity to the station or both. When
there is a definite (2° to 5°) change in needle reading, turn in
the direction of needle deflection to intercept the initial MB.
The angle of interception must be greater than the number
of degrees of drift, otherwise the aircraft slowly drifts due to
the wind pushing the aircraft. If repeated often enough, the
track to the station appears circular and the distance greatly
increased as compared to a straight track. The intercept angle
depends on the rate of drift, the aircraft speed, and station
proximity. Initially, it is standard to double the RB when
turning toward your course.
For example, if your heading equals your course and the
needle points 10° left, turn 20° left, twice the initial RB.
[Figure 9-7] This is your intercept angle to capture the
RB. Hold this heading until the needle is deflected 20° in
the opposite direction. That is, the deflection of the needle
equals the interception angle (in this case 20°). The track has
been intercepted, and the aircraft remains on track as long
as the RB remains the same number of degrees as the wind
correction angle (WCA), the angle between the desired track
and the heading of the aircraft necessary to keep the aircraft
tracking over the desired track. Lead the interception to avoid
overshooting the track. Turn 10° toward the inbound course.
You are now inbound with a 10° left correction angle.
NOTE: In Figure 9-7, for the aircraft closest to the station,
the WCA is 10° left and the RB is 10° right. If those values
do not change, the aircraft tracks directly to the station. If you
observe off-course deflection in the original direction, turn
again to the original interception heading. When the desired
course has been re-intercepted, turn 5° toward the inbound
course, proceeding inbound with a 15° drift correction. If the
initial 10° drift correction is excessive, as shown by needle
deflection away from the wind, turn to parallel the desired
course and let the wind drift you back on course. When the
needle is again zeroed, turn into the wind with a reduced
drift correction angle.
WIND
ADF
HDG 050°
050°
24 21
W S
24 2I
ADF
HDG 360°
360°
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24 2I I5
ADF
HDG 330°
330°
3W
2I
I5 I2
ADF
HDG 320°
320°
15 12
2I
I5 I2
ADF
HDG 310°
310°
15 12
2I
I5 I2
Station
OBS
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NAV
GS
Instrument view is from
the pilot’s perspective,
and the movable card is
reset after each turn
Figure 9-6. ADF homing with a crosswind.
Figure 9-7. ADF tracking inbound.
OBS
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NAV
GS
Instrument view is from
the pilot’s perspective,
and the movable card is
reset after each turn
ADF
HDG 005°
005°
ADF
HDG 360°
360°
ADF
HDG 340°
340°
ADF
HDG 340°
340°
ADF
HDG 350°
350°
°
24 2I I5
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24 2I I5
2I I5 I2
2I I5 I2
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2I I5
WCA = 10° LEFT
RB = 10° RIGHT
Station
WIND
Operational Errors of ADF
Some of the common pilot-induced errors associated with
ADF navigation are listed below to help you avoid making
the same mistakes. The errors are:
1. Failure to keep the heading indicator set so that it
agrees with the corrected magnetic compass reading.
Initiating an ADF approach without verifying that the
heading indicator agrees with the corrected compass
indicator reading may cause the pilot to believe that
he is on course but still impact the terrain (CFIT).
2. Improper tuning and station identification. Many pilots
have made the mistake of homing or tracking to the
wrong station.
3. Positively identifying any malfunctions of the RMI
slaving system or ignoring the warning flag.
4. Dependence on homing rather than proper tracking.
This commonly results from sole reliance on the
ADF indications rather than correlating them with
heading indications.
5. Poor orientation due to failure to follow proper steps
in orientation and tracking.
6. Careless interception angles, very likely to happen if
you rush the initial orientation procedure.
7. Overshooting and undershooting predetermined
MBs, often due to forgetting the course interception
angles used.
8. Failure to maintain selected headings. Any heading
change is accompanied by an ADF needle change.
The instruments must be read in combination before
any interpretation is made.
9. Failure to understand the limitations of the ADF and
the factors that affect its use.
10. Overcontrolling track corrections close to the station
(chasing the ADF needle) due to failure to understand
or recognize station approach.
Very High Frequency Omnidirectional Range
(VOR)
VOR is the primary navigational aid (NAVAID) used by civil
aviation in the National Airspace System (NAS). The VOR
ground station is oriented to magnetic north and transmits
azimuth information to the aircraft, providing 360 courses
TO or FROM the VOR station. When DME is installed with
the VOR, it is referred to as a VOR/DME and provides both
azimuth and distance information. When military tactical air
navigation (TACAN) equipment is installed with the VOR,
it is known as a VORTAC and provides both azimuth and
distance information.
To track outbound, the same principles apply: needle moving
left = wind from the left, needle moving right = wind from the
right. Wind correction is made toward the needle deflection.
The only exception is while the turn to establish the WCA is
being made, the direction of the azimuth needle deflections is
reversed. When tracking inbound, needle deflection decreases
while turning to establish the WCA, and needle deflection
increases when tracking outbound. Note the example of
course interception and outbound tracking in Figure 9-8.
Intercepting Bearings
ADF orientation and tracking procedures may be applied to
intercept a specified inbound or outbound MB. To intercept
an inbound bearing of 355°, the following steps may be used.
[Figure 9-9]
1. Determine your position in relation to the station by
paralleling the desired inbound bearing. In this case,
turn to a heading of 355°. Note that the station is to
the right front of the aircraft.
2. Determine the number of degrees of needle deflection
from the nose of the aircraft. In this case, the needle’s
RB from the aircraft’s nose is 40° to the right. A rule
of thumb for interception is to double this RB amount
as an interception angle (80°).
3. Turn the aircraft toward the desired MB the number of
degrees determined for the interception angle, which
as indicated (in two above) is twice the initial RB (40°)
or, in this case, 80°. Therefore, the right turn is 80°
from the initial MB of 355° or a turn to 075° magnetic
(355° + 80° + 075°).
4. Maintain this interception heading of 075° until the
needle is deflected the same number of degrees “left”
from the zero position as the angle of interception 080°
(minus any lead appropriate for the rate at which the
bearing is changing).
5. Turn left 80° and the RB (in a no wind condition and
with proper compensation for the rate of the ADF
needle movement) should be 0° or directly off the
nose. Additionally, the MB should be 355° indicating
proper interception of the desired course.
NOTE: The rate of an ADF needle movement, or any bearing
pointer for that matter, is faster as aircraft position becomes
closer to the station or waypoint (WP).
Interception of an outbound MB can be accomplished by the
same procedures as for the inbound intercept, except that
it is necessary to substitute the 180° position for the zero
position on the needle.