Direction finding (DF), or radio direction finding (RDF), is the
measurement of the direction from which a received signal was
transmitted. This can refer to radio or other forms of wireless
communication, including radar signals detection and monitoring
(ELINT/ESM). By combining the direction information from two or more
suitably spaced receivers (or a single mobile receiver), the source of
a transmission may be located via triangulation.
finding is used in the navigation of ships and aircraft, to locate
emergency transmitters for search and rescue, for tracking wildlife,
and to locate illegal or interfering transmitters. RDF was important
in combating German threats during both the
World War II
World War II Battle of
Britain and the long running Battle of the Atlantic. In the former,
the Air Ministry also used RDF to locate its own fighter groups and
vector them to detected German raids.
RDF systems can be used with any radio source, although very long
wavelengths (low frequencies) require very large antennas, and are
generally used only on ground-based systems. These wavelengths are
nevertheless used for marine radio navigation as they can travel very
long distances "over the horizon", which is valuable for ships when
the line-of-sight may be only a few tens of kilometres. For aerial
use, where the horizon may extend to hundreds of kilometres, higher
frequencies can be used, allowing the use of much smaller antennas. An
automatic direction finder, which could be tuned to radio beacons
called non-directional beacons or commercial
AM radio broadcasters,
was until recently, a feature of most aircraft, but is now being
phased out 
For the military, RDF is a key tool of signals intelligence. The
ability to locate the position of an enemy transmitter has been
invaluable since World War I, and played a key role in World War II's
Battle of the Atlantic. It is estimated that the UK's advanced
"huff-duff" systems were directly or indirectly responsible for 24% of
all U-Boats sunk during the war. Modern systems often used phased
array antennas to allow rapid beamforming for highly accurate results,
and are part of a larger electronic warfare suite.
Radio direction finders have evolved, following the development of new
electronics. Early systems used mechanically rotated antennas that
compared signal strengths, and several electronic versions of the same
concept followed. Modern systems use the comparison of phase or
doppler techniques which are generally simpler to automate. Early
British radar sets were referred to as RDF, which is often stated was
a deception. In fact, the
Chain Home systems used large RDF receivers
to determine directions. Later radar systems generally used a single
antenna for broadcast and reception, and determined direction from the
direction the antenna was facing.
2 Single channel DF or single site location (SSL)
2.1 Pseudo-doppler DF technique
2.2 Watson-Watt /
Adcock antenna array
2.3 Correlative interferometer
3.2 Location of illegal, secret or hostile transmitters - SIGINT
3.3 Emergency aid
3.4 Avalanche rescue
4 See also
Direction finding requires an antenna that is directional (more
sensitive in certain directions than in others). Many antenna designs
exhibit this property. For example, a
Yagi antenna has quite
pronounced directionality, so the source of a transmission can be
determined simply by pointing it in the direction where the maximum
signal level is obtained. However, to establish direction to great
accuracy requires more sophisticated technique.
The crossed-loops DF antenna atop the mast of a tug boat
A simple form of directional antenna is the loop aerial. This consists
of an open loop of wire on an insulating former, or a metal ring that
forms the antenna elements itself, where the diameter of the loop is a
tenth of a wavelength or smaller at the target frequency. Such an
antenna will be least sensitive to signals that are normal to its face
and most responsive to those meeting edge-on. This is caused by the
phase output of the transmitting beacon. The phase changing phase
causes a difference between the voltages induced on either side of the
loop at any instant. Turning the loop face on will not induce any
current flow. Simply turning the antenna to obtain minimum signal will
establish two possible directions from which the signal could be
emanating. The NULL is used, as small angular deflections of the loop
aerial near its null positions produce larger changes in current than
similar angular changes near the loops max positions. For this reason,
a null position of the loop aerial is used.
To resolve the two direction possibilities, a sense antenna is used,
the sense aerial has no directional properties but has the same
sensitivity as the loop aerial. By adding the steady signal from the
sense aerial to the alternating signal from the loop signal as it
rotates, there is now only one position as the loop rotates 360° at
which there is zero current. This acts as a phase ref point, allowing
the correct null point to be identified, thus removing the 180°
ambiguity. A dipole antenna exhibits similar properties, and is the
basis for the Yagi antenna, which is familiar as the common
VHF or UHF
television aerial. For much higher frequencies still, parabolic
antennas can be used, which are highly directional, focusing received
signals from a very narrow angle to a receiving element at the centre.
More sophisticated techniques such as phased arrays are generally used
for highly accurate direction finding systems called goniometers such
as are used in signals intelligence (SIGINT). A helicopter based DF
system was designed by
ESL Incorporated for the U.S. Government as
early as 1972.
Radio direction finder
The RDF antenna on this B-17F is located in the prominent teardrop
housing under the nose.
Single channel DF or single site location (SSL)
Single-channel DF uses a multi-antenna array with a single channel
radio receiver. This approach to DF obviously offers some advantages
and drawbacks. Since it only uses one receiver, mobility and lower
power consumption are obvious benefits but without the ability to look
at each antenna simultaneously (which would be the case if one were to
use multiple receivers, also known as N-channel DF) more complex
operations need to occur at the antenna in order to present the signal
to the receiver.
The two main categories that a single channel DF algorithm falls into
are amplitude comparison and phase comparison. Some algorithms can be
hybrids of the two.
Pseudo-doppler DF technique
The pseudo-doppler technique is a phase based DF method that produces
a bearing estimate on the received signal by measuring the doppler
shift induced on the signal by sampling around the elements of a
circular array. The original method used a single antenna that
physically moved in a circle but the modern approach uses a
multi-antenna circular array with each antenna sampled in succession.
Adcock antenna array
Main article: Adcock antenna
The Watson-Watt technique uses two
Adcock antenna pairs to perform an
amplitude comparison on the incoming signal. An
Adcock antenna pair is
a pair of monopole or dipole antennas that takes the vector difference
of the received signal at each antenna so that there is only one
output from the pair of antennas. Two of these pairs are co-located
but perpendicularly oriented to produce what can be referred to as the
N-S (North-South) and E-W (East-West) signals that will then be passed
to the receiver. In the receiver, the bearing angle can then be
computed by taking the arctangent of the ratio of the N-S to E-W
The basic principle of the correlative interferometer consists in
comparing the measured phase differences with the phase differences
obtained for a DF antenna system of known configuration at a known
wave angle (reference data set). The comparison is made for different
azimuth values of the reference data set, the bearing is obtained from
the data for which the correlation coefficient is at a maximum. If the
direction finding antenna elements have a directional antenna pattern,
then the amplitude may be included in the comparison.
A portable, battery operated GT-302 Accumatic automatic direction
finder for marine use
Radio direction finding,
Radio direction finder, or RDF was once the
primary aviation navigational aid. (Range and Direction Finding was
the abbreviation used to describe the predecessor to Radar.)
Beacons were used to mark "airways" intersections and to define
departure and approach procedures. Since the signal transmitted
contains no information about bearing or distance, these beacons are
referred to as non-directional beacons, or NDB in the aviation world.
Starting in the 1950s, these beacons were generally replaced by the
VOR system, in which the bearing to the navigational aid is measured
from the signal itself; therefore no specialized antenna with moving
parts is required. Due to relatively low purchase, maintenance and
calibration cost, NDB's are still used to mark locations of smaller
aerodromes and important helicopter landing sites.
Further information: Non-directional beacon
Similar beacons located in coastal areas are also used for maritime
radio navigation, as almost every ship is (was) equipped with a
direction finder (Appleyard 1988). Very few maritime radio navigation
beacons remain active today (2008) as ships have abandoned navigation
via RDF in favor of GPS navigation.
In the United Kingdom a radio direction finding service is available
on 121.5 MHz and 243.0 MHz to aircraft pilots who are in
distress or are experiencing difficulties. The service is based on a
number of radio DF units located at civil and military airports and
certain HM Coastguard stations. These stations can obtain a "fix"
of the aircraft and transmit it by radio to the pilot.
Location of illegal, secret or hostile transmitters - SIGINT
High frequency direction finding
High frequency direction finding and SIGINT
British Post Office RDF lorry from 1927 for finding unlicensed amateur
radio transmitters. It was also used to find regenerative receivers
which radiated interfering signals due to feedback, a big problem at
World War II
World War II considerable effort was expended on identifying secret
transmitters in the United Kingdom (UK) by direction finding. The work
was undertaken by the
Radio Security Service (
RSS also MI8). Initially
three U Adcock HF DF stations were set up in 1939 by the General Post
Office. With the declaration of war,
RSS developed this into a
larger network. One of the problems with providing coverage of an area
the size of the UK was installing sufficient DF stations to cover the
entire area to receive skywave signals reflected back from the ionised
layers in the upper atmosphere. Even with the expanded network, some
areas were not adequately covered and for this reason up to 1700
voluntary interceptors (radio amateurs) were recruited to detect
illicit transmissions by ground wave. In addition to the fixed
RSS ran a fleet of mobile DF vehicles around the UK. If a
transmitter was identified by the fixed DF stations or voluntary
interceptors, the mobile units were sent to the area to home in on the
source. The mobile units were HF Adcock systems.
By 1941 only a couple of illicit transmitters had been identified in
the UK; these were German agents that had been "turned" and were
MI5 control. Many illicit transmissions had been
logged emanating from German agents in occupied and neutral countries
in Europe. The traffic became a valuable source of intelligence, so
the control of
RSS was subsequently passed to MI6 who were responsible
for secret intelligence originating from outside the UK. The direction
finding and interception operation increased in volume and importance
The HF Adcock stations consisted of four 10m vertical antennas
surrounding a small wooden operators hut containing a receiver and a
radio-goniometer which was adjusted to obtain the bearing. MF stations
were also used which used four guyed 30m lattice tower antennas. In
RSS began experimenting with Spaced Loop direction finders,
developed by the Marconi company and the UK National Physical
Laboratories. These consisted of two parallel loops 1 to 2m square on
the ends of a rotatable 3 to 8m beam. The angle of the beam was
combined with results from a radiogoniometer to provide a bearing. The
bearing obtained was considerably sharper than that obtained with the
U Adcock system, but there were ambiguities which prevented the
installation of 7 proposed S.L DF systems. The operator of an SL
system was in a metal underground tank below the antennas. Seven
underground tanks were installed, but only two SL systems were
installed at Wymondham, Norfolk and Weaverthorp in Yorkshire. Problems
were encountered resulting in the remaining five underground tanks
being fitted with Adcock systems. The rotating SL antenna was turned
by hand which meant successive measurements were a lot slower than
turning the dial of a goniometer.
Another experimental spaced loop station was built near Aberdeen in
1942 for the Air Ministry with a semi-underground concrete bunker.
This, too, was abandoned because of operating difficulties. By 1944 a
mobile version of the spaced loop had been developed and was used by
RSS in France following the D-Day invasion of Normandy.
The US military used a shore based version of the spaced loop DF in
World War II
World War II called "DAB". The loops were placed at the ends of a
beam, all of which was located inside a wooden hut with the
electronics in a large cabinet with cathode ray tube display at the
centre of the beam and everything being supported on a central axis.
The beam was rotated manually by the operator.
Royal Navy introduced a variation on the shore based HF DF
stations in 1944 to track U-boats in the North Atlantic. They built
groups of five DF stations, so that bearings from individual stations
in the group could be combined and a mean taken. Four such groups were
built in Britain at Ford End, Essex, Goonhavern, Cornwall, Anstruther
and Bowermadden in the Scottish Highlands. Groups were also built in
Iceland, Nova Scotia and Jamaica. The anticipated improvements were
not realised but later statistical work improved the system and the
Ford End groups continued to be used during the Cold
Royal Navy also deployed direction finding equipment on ships
tasked to anti-submarine warfare in order to try to locate German
submarines, e.g. Captain class frigates were fitted with a medium
frequency direction finding antenna (MF/DF) (the antenna was fitted in
front of the bridge) and high frequency direction finding (HF/DF,
"Huffduff") Type FH 4 antenna (the antenna was fitted on top of the
A comprehensive reference on
World War II
World War II wireless direction finding
was written by Roland Keen, who was head of the engineering department
RSS at Hanslope Park. The DF systems mentioned here are described
in detail in his 1947 book Wireless Direction Finding.
At the end of
World War II
World War II a number of
RSS DF stations continued to
operate into the Cold War under the control of GCHQ the British SIGINT
Most direction finding effort within the UK now (2009) is directed
towards locating unauthorised "pirate" FM broadcast radio
transmissions. A network of remotely operated
VHF direction finders
are used mainly located around the major cities. The transmissions
from mobile telephone handsets are also located by a form of direction
finding using the comparative signal strength at the surrounding local
"cell" receivers. This technique is often offered as evidence in UK
criminal prosecutions and, almost certainly, for
Radio direction finder
There are many forms of radio transmitters designed to transmit as a
beacon in the event of an emergency, which are widely deployed on
civil aircraft. Modern emergency beacons transmit a unique
identification signal that can aid in finding the exact location of
Avalanche transceivers operate on a standard 457 kHz, and are
designed to help locate people and equipment buried by avalanches.
Since the power of the beacon is so low the directionality of the
radio signal is dominated by small scale field effects and can be
quite complicated to locate.
Location of radio-tagged animals by triangulation is a widely applied
research technique for studying the movement of animals. The technique
was first used in the early 1960s, when the technology used in radio
transmitters and batteries made them small enough to attach to wild
animals, and is now widely deployed for a variety of wildlife studies.
Most tracking of wild animals that have been affixed with radio
transmitter equipment is done by a field researcher using a handheld
radio direction finding device. When the researcher wants to locate a
particular animal, the location of the animal can be triangulated by
determining the direction to the transmitter from several locations.
Phased arrays and other advanced antenna techniques are utilized to
track launches of rocket systems and their resulting trajectories.
These systems can be used for defensive purposes and also to gain
intelligence on operation of missiles belonging to other nations.
These same techniques are used for detection and tracking of
Main article: Amateur
Radio Direction Finding
Events hosted by groups and organizations that involve the use of
radio direction finding skills to locate transmitters at unknown
locations have been popular since the end of World War II. Many of
these events were first promoted in order to practice the use of radio
direction finding techniques for disaster response and civil defense
purposes, or to practice locating the source of radio frequency
interference. The most popular form of the sport, worldwide, is known
Radio Direction Finding or by its international
abbreviation ARDF. Another form of the activity, known as "transmitter
hunting", "mobile T-hunting" or "fox hunting" takes place in a larger
geographic area, such as the metropolitan area of a large city, and
most participants travel in motor vehicles while attempting to locate
one or more radio transmitters with radio direction finding
Modern dead drop techniques
Radio direction finder
AN/FLR-9, a cold war US Air Force HF direction finding system.
AN/FRD-10, a cold war US Navy HF direction finding system.
^ "Next Gen Implementation Plan 2013" (PDF). Archived from the
original (PDF) on 2013-10-23.
^ a b "
Radio Direction Finding) - The Eyes of Fighter
^ Smith, D.J. (2005). Air Band
Radio Handbook (8th Edition). Sutton
Publishing. pp. 104–105. ISBN 0-7509-3783-1.
^ Elliott (1972), p. 264
^ Keen, R (1947). Wireless Direction Finding (4th ed.). London, UK:
^ deRosa, L.A. (1979). "Direction Finding". In J.A. Biyd; D.B. Harris;
D.D. King; H.W. Welch, Jr. Electronic Countermeasures. Los Altos, CA:
Peninsula Publishing. ISBN 0-932146-00-7.
^ *J. Hereford & B. Edgerly (2000). "457 kHz Electromagnetism and
the Future of Avalanche Transceivers" (PDF). International Snow
Science Workshop (ISSW 2000). Archived from the original (– Scholar
search) on July 22, 2011.
^ Titterington, B.; Williams, D.; Dean, D. (2007).
- The ARDF Handbook.
Radio Society of Great Britain.
Elliott, Peter (1972). "The Lend-Lease Captains". Warship
International Naval Research Organization
International Naval Research Organization (3):
Appleyard, S.F.; Linford, R.S.; Yarwood, P.J. (1988). Marine
Electronic Navigation (2nd Edition). Routledge & Kegan Paul.
pp. 68–69. ISBN 0-7102-1271-2.
Radio Direction Finding Applications Literature (RDF Products)
Doppler Systems Application Notes (Doppler Systems)
Why You Can't Track Your Stolen GPS
Time (magazine) April 28, 2008
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