Automatic Direction Finder
Introduction:-
The eaily aviators used visual aids to guide them
along their route, these visual aids would have
included rivers, roads, iail tracks, coastlines etc
This type of navigation is not possible in low
visibility and so magnetic compasses weie
introduced Magnetic compasses are somewhat
unreliable in the short teim, i e during turning
manoeuvies Directional gyroscopes are reliable
in the short term, but duff ovei longer time
penods A combined magnetic compass stabilised
by a duectional gyroscope (referied to as a gyromagnetic
compass) can oveicome these
deficiencies The gyro-magnetic compass,
together with an airspeed indicator, allowed the
crew to navigate by dead reckoning, i e
estimating their position by extrapolating from a
known position and then keeping note of the
direction and distance travelled Instrumentation
eriois inevitably lead to deviations between the
aircraft’s actual and calculated positions, these
direction finder
deviations accumulate over time. Crews therefore
need to be able to confirm and update their
position by means of a fixed ground-based
reference.
The early airborne navigation systems using
ground-based navigation aids consisted of a
fixed-loop antenna in the aircraft tuned to an
amplitude modulated (AM) commercial radio
broadcast station. Pilots would know the location
of the radio station (indeed, it would invariably
have been located close to or even in the town!
city that the crew wanted to fly to). The fixedloop
antenna was aligned with the longitudinal
axis of the aircraft, with the pilot turning the
aircraft until he received the minimum signal
strength (null reading). By maintaining a null
reading, the pilot could be sure that he was flying
towards the station. This constant turning was
inefficient in terms of fuel consumption and
caused inherent navigation problems in keeping
note of the aircraft’s position during these
manoeuvres! The effects of crosswind
complicated this process since the aircraft’s
heading is not aligned with its track. The introduction of an ‘automatic’ direction
finder (ADF) system addresses this problem. A
loop antenna that the pilot could rotate by hand
solves some ofthese problems; however, this still
requires close attention from the crew. Later
developments ofthe equipment used an electrical
motor to rotate the loop antenna, The received
signal strength is a function of the angular
position of the loop with respect to the aircraft
heading and bearing to the station. If a plot is made of loop angle and
signal strength, the result is a sine wave . The null point is easier to
determine than the maximum signal strength, frequency range 540—1620 lcHz) became an
established method of travelling across country.
With the growth of air travel, dedicated radio
navigation aids were installed along popular air
transport routes. These radio stations, known as
non-directional beacons (NDB), gradually
supplemented the commercial radio stations and a
network of NDBs sprang up in the nations
developing their aeronautical infrastructure.
ADF equipment:-
The rotating loop antenna was eventually
replaced with a fixed antenna consisting of two
loops combined into a single item; one aligned
with the centreline of the fuselage, the other at
right angles This
orthogonal antenna is still referred to as the ‘loop’
antenna. Measuring the signal strength from each
ofthe loops, and deriving an angular position in a
dedicated ADF receiver, determines the direction
to the selected beacon (or commercial radio
station). The loop antenna resolves the directional
signal; however, this can have two possible
solutions 180 degrees apart. A second ‘sense’
antenna is therefore required to detect non
directional radio waves from the beacon; this signal is combined with the directional signals
from the ioop antenna to produce a single
directional solution. The polar diagram for a loop
and sense antenna is shown in Figure 9.4; when
the two patterns are combined, it forms a
cardioid. Most commercial transport aircraft are
fitted with two independent ADF systems
typically identified as left and right systems; the
antenna locations for a typical transport aircraft.
ADF receivers are located in the avionic
equipment bay. The signal received at the
antenna is coupled to the receiver in three ways:
• The sense signal
o A loop signal proportional in amplitude to
the cosine of the relative angle of the
aircraft centreline and received signal
• A loop signal proportional in amplitude to
the sine ofthe relative angle ofthe aircraft
centreline and received signal.
Figure 9.4 Polar diagram for the ADF loop
and sense antennas
The sense antenna signal is processed in the
receiver via a superhet receiver (see page 46)
which allows weak signals to be identified,
together with discrimination of adjacent
frequencies. The output ofthe superhet receiver is
then integrated into the aircraft’s audio system.
Loop antenna signals are summed with the sense
antenna signal; this forms a phase-modulated
(PM) carrier signal. The superhet intermediate
frequency (IF) is coupled with the PM signal into
a coherent demodulator stage that senses the presence of a sense antenna signal from the IF
stage. The PM component of the signal is
rebovered from the voltage controlled oscillator
(VCO) phase lock circuit (see page 53). This
recovered signal contains the bearing information
received by the antenna and is compared to a
reference modulation control signal.
Receivers based on analogue technology send
bearing data to the flight deck displays using
synchro systems. Digital receivers transmit
bearing data to the displays using a data bus
system, typically ARINC 429. The ADF receiver
is often incorporated into a multi-mode receiver
along with other radio navigation systems.
Control pane:-
Aircraft with analogue (electromechanical)
avionics have a dedicated ADF control panel,
located on the centre pedestal, see Figure 9.6(a).
An alternative panel shown in Figure 9.6(b)
enables the crew to select a range of functions
including: frequency selectors/displays and the
beat frequency oscillator (BFO). This function is
used when they want to create an audio frequency
for carrier wave transmissions through their audio
panel.
NDB carrier waves that are not modulated with
an audio component use the beat frequency oscillator (BFO) circuit in the ADF receiver. To
produce an audio output, the receiver heterodynes
(beats) the carrier wave signal with a separate
signal derived from an oscillator in the receiver.
Some ADF panels have an ADF/ANT switch
where ‘ADF’ selects normal operation, i.e.
combined sense and loop antennas; and ‘ANT’
selects the sense antenna by itself so that the crew
can confirm that a station is broadcasting, i.e.
without seeking a null. General aviation products
combine the control panel and receiver into a
single item, A changeover
switch is used to select the active and standby
frequencies.
AOF bearing display:-
The output from the ADF receiver is transmitted
to a display that provides the pilot with both
magnetic heading and direction to the tuned
NDB, this can either be a dedicated ADF
instrument as shown in Figure 9.7(a), or a radio
magnetic indicator (RMI),In the RMI, two bearing pointers (coloured red
and green) are associated with the two ADF
systems and allow the crew to tune into two
different NDBs at the same time. RMIs can have
a dual purpose; pilots use a switch on the RMI to
select either ADF and/or VHF omnidirectional
range (VOR) bearings (see Chapter 10 for the
latter). Referring to Figure 9.7(c), some aircraft
have a bearing source indicator (located adjacent
to the RMI) that confirms ADF or VOR selection.
The evolution of digital electronics together
with integration of other systems has led to the
introduction of the flight management system
(FMS: see Chapter 19) control display unit
(CDU) which is used to manage the ADF system.
Aircraft fitted with electronic flight instrument
systems (EFIS) have green NDB icons displayed
on the electronic horizontal situation indicator
(EHSI) .
Atmospheric conditions: the height and
depth of the ionosphere will vary depending
on solar activity. The sky waves (see Figure
9.8) will be affected accordingly since their
associated skip distances will vary due to
refraction in the ionosphere. This is
particularly noticeable at sunrise and sunset.
• Physical aspects of terrain: mountains and
valleys will reflect the radio waves causing
multipath reception.
• Coastal refraction: low-frequency waves that
are propagated across the surface of the earth
as ground waves will exhibit different
characteristics when travelling over land
versus water. This is due to the attenuation of
the ground wave being different over land and
water. The direction of a radio wave across
land will change (see Figure 9,9) when it
reaches the coast and then travels over water.
This effect depends on the angle between the
radio wave and the coast.
• Quadrantal error (QE): many parts of the
aircraft structure, e.g. the ftselage and wings,
are closely matched in physical size to the
wavelength of the ADF radio transmissions.
Radiated energy is absorbed in the airframe
and re-radiated causing interference; this
depends on the relative angle between the
direction of travel, the physical aspects of the
aircraft and location of the ADF transmitter.
Corrections can be made for QE in the
receiver.
• Interference: this can arise from electrical
storms, other radio transmissions, static build
up/discharges and other electrical equipment
on the aircraft.
The accuracy of an ADF navigation system is in
the order of ±5 degrees for locator beacons and
±10 degrees for en route beacons. Any of the
above conditions will lead to errors in the bearing
information displayed on the RMI. If these
conditions occur in combination then the
navigation errors will be significant. Pilots cannot
use ADF for precision navigation due to these
limitations.
The increased need for more accuracy and
reliability of navigation systems led to a new
generation of en route radio navigation aids; this
is covered in the nextchapter. In the meantime,
ADF transmitters remain installed throughout the
world and the system is used as a secondary radio
navigation aid. The equipment remains installed
on modem aircraft, albeit integrated with other
radio navigation systems.