Major system characteristics
||Rotating beam beacon navigation
||962 - 1214 Mc/s
"Tacan" stands for
"Tactical Air Navigation" and is a system which, working in the u.h.f. band,
between 962 and 1,214 Mc/s, gives to a pilot continuous information as to his
range and bearing from a beacon. The airborne equipment consists of an
interrogating transmitter and a receiver which includes suitable demodulating
circuits to enable the information contained in the beacon's response to be
extracted. The ground equipment consists of a beacon provided with a rotating
aerial system. In the absence of interrogating signals the beacon transmits a
series of random pulses together with groups of marker or reference pulses which
are locked to the aerial rotation. Bearing information can be obtained without
interrogation since the beacon is continuously transmitting.
In addition to transmitting a random series of
pulses the beacon also periodically transmits a signal by which it identifies
itself. To determine range the airborne transmitter radiates a series of pairs
of pulses. These are received at the beacon and, after a fixed delay, are
re-transmitted in place of the particular random pulses that the beacon would
have transmitted in the absence of interrogation. The time delay between the
emission of any interrogating pulse and the receipt of the reply is measured by
one of the standard radar techniques described in Chapter 10. To avoid mistakes
arising from multiple interrogation the airborne transmitter emits its pairs of
pulses at random intervals. The receiver only recognises an exactly similar set
of reply pulses all delayed by the same amount.
The aerial polar diagram of the beacon is of the
shape shown in Fig. 3.13. This is produced by placing parasitic radiators,
comprising a single radiator and a ring of nine, in the positions shown in Fig.
By rotating these parasitic elements around the
radiator the polar diagram is caused to rotate in space.
The parasitic elements spin round the aerial at
900 r.p.m. so that the received signal is amplitude modulated at 15 c/s, caused
by the rotation of the single radiator, and at 135 c/s caused by the rotation of
the group of nine radiators. A marker signal is transmitted every time the main
lobe of the polar diagram passes due magnetic East. Somewhat misleadingly this
is known as the "N " or north marker. Further marker signals are transmitted
after every 40o of rotation of the aerial pattern. This system of
markers provides reference signals at 15 and 135 c/s for phase comparison with
the signals received in the aircraft. As the aircraft flies round the beacon the
time of receipt of the marker signals with respect to the phase of the amplitude
modulation of the received signal varies as shown in Fig. 3.15.
The airborne receiver has five detectors. The
first is an envelope detector whose output consists of a composite 15 and 135
c/s signal due to the rotation of the aerial pattern. The second and third
detectors are used to isolate the 15 and 135 c/s reference signals respectively.
The fourth detector is used to feed the range measurement circuits and the fifth
to extract the beacon identification signal.
The signal from
the envelope detector is passed through filters to separate the 15 c/s and 135
c/s modulations. A phase comparison circuit is used to measure the phase
difference between the 15 c/s modulation and the 15 c/s reference signal from
detector number two. This circuit enables the bearing of the beacon to be
measured to within ±20o A second phase comparison circuit compares
the phase of the 135 c/s modulation with that of the 135 c/s reference signal
from detector number three. This gives the bearing of the beacon to an accuracy
of about ±1o
There is no limit to the number of aircraft
which can simultaneously obtain bearing information from a Tacan beacon but no
more than 100 aircraft at a time can obtain distance information. The major
claim of the Tacan system is that by employing an antenna pattern comprising a
main lobe and "ninth harmonic" lobes a nine-fold increase in accuracy is
obtained. Whether such an accuracy can be obtained in varied conditions of
siting has yet to be proved.