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Electronics Technician Volume 05-Navigation Systems | Next
Figure 2-2.—Distance measuring round-trip travel time |
TACTICAL AIR
INTRODUCTION
Before we begin discussing TACAN, you need to
recall the definition of the polar-coordinate system.
The polar-coordinate system is a geometric system
used to locate points on a plane. In electronics, it is
usually used for plotting antenna directional patterns.
TACAN is a polar-coordinate type radio air-
navigation system that provides an aircrew with
distance information, from distance measuring
e q u i p m e n t (DME), and bearing (azimuth)
information. This information, as shown in figure 2-
1, is usually provided by two meters. One meter
indicates, in nautical miles, the distance of the aircraft
from the surface beacon. The other meter indicates
the direction of flight, in degrees-of-bearing, to the
geographic location of the surface beacon. By using
the TACAN equipment installed in the aircraft and
TACAN ground equipment installed aboard a
particular surface ship or shore station, a pilot can
obtain bearing to and distance from that location. He
or she can then either:
(1) fly directly to that particular location, or
CHAPTER 2
NAVIGATION (TACAN)
Figure 2-1.—TACAN aircraft indication.
(2) use the bearing and distance from a specific
beacon to fix his or her geographic location.
TACAN PRINCIPLES
The distance measuring concept used in TACAN
equipment is
an outgrowth of radar-ranging
techniques. Radar-ranging determines distance by
measuring the round-trip travel time of pulsed rf
energy.
The return signal (echo) of the radiated
energy depends on the natural reflection of the radio
waves.
However, TACAN beacon-transponders
generate artificial replies instead of depending on
natural reflection.
Now look at figure 2-2. The airborne equipment
generates timed interrogation pulse pairs that the
surface TACAN system receives and decodes. After
a 50-µsec delay, the transponder responds with a
reply. The airborne DME then converts the round-
trip time to distance from the TACAN facility. The
frequency and identification code provide the
geographic location of the transmitting beacon.
TACAN PULSE PAIRS
TACAN transponders use twin-pulse decoders to
pass only those pulse pairs with the proper spacing.
The purpose of this twin-pulse technique is to
increase the average power radiated and to reduce the
possibility of false signal interference.
After the receiver decodes an interrogation, the
encoder generates the necessary pulse pair required
for the transponder’s reply. A TACAN pulse pair
generated by airborne or ground equipment is shown
in figure 2-3.
CONSTANT TRANSPONDER
DUTY-CYCLE
In principle,
reply to aircraft
2-1
the TACAN transponder need only
interrogations at 30 pulse pairs-per-
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