(TACAN) TACTICAL AIR NAVIGATION
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INTRODUCTION •
TACAN stands for “Tactical Air Navigation”
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Works between U.H.F. band 962 - 1,214 Mhz
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Gives continuous information about range and bearing from a beacon Airborne equipment consists of an interrogating transmitter and a receiver
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INTRODUCTION •
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Provides distance info, from distance measuring eqpt (DME), & Brg (azimuth) info Info is usually provided by two meters –
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Distance in nautical miles from the surface beacon Direction of flight, in degrees-of-brg, to surface beacon
By using the TACAN equipment installed in the aircraft and TACAN ground equipment installed aboard a particular surface ship or shore station, bearing to and distance from that location can be obtained: – –
To fly directly to that particular location, or Use the bearing and distance from a specific beacon to fix geographic location
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TACAN Aircraft Indication
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TACAN PRINCIPLE •
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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 Airborne eqpt generates timed interrogation pulse pairs that 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 beacon5
Distance Measuring Round-Trip Travel Time 6
TRANSPONDER RX GAIN •
If receiver gain is increased, more pulse-pairs will exceed pre-set amplitude level and trigger circuit will fire more frequently: the converse applies when the receiver gain is decreased. Therefore number of pulse-pairs transmitted by the beacon varies directly with the gain and is continuously and automatically adjusted to produce 2,700 pulse pairs per sec. These pulse pairs are of constant amplitude and shape, but have a random recurrence frequency 7
TACAN PULSE PAIRS •
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TACAN transponders use twin-pulse decoders to pass only those pulse pairs having 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 8
TACAN Pulse Train
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TRANSPONDER DUTY-CYCLE •
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TACAN transponder replies to aircraft interrogations at 30 pulse pairs-per-second, per airborne eqpt, to supply necessary distance data. The total pulse output of the transmitter constantly varies, according to the number of interrogating aircraft. In addition, random noise may trigger the transmitter Receiver uses automatic gain & squitter (noise generated output) controls to maintain a constant pulse output to the transmitter. If few interrogations are being received, the gain and squitter of the receiver increase and add noisegenerated pulses to the pulse train. If more interrogating aircraft come into range, gain and squitter decrease and reduce number of noise10 generated pulses
Transponder Output Pulse Train
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If the beacon is interrogated by an airborne equipment, a 12 µsec pulse pair with an amplitude well above the pre-set level will appear at the receiver output as at X in the figure. The trigger circuit therefore will fire and a reply will be transmitted to the aircraft amidst the pulse pairs produced irregularly by the noise from the receiver
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DME TO 100 A/C •
Since the beacon transmits a constant number of pulses per second, the greater the number of interrogation pulses received, the fewer are the noise pulses used by the trigger circuit. This is because the gain will be reduced until the noise pulses with small amplitudes fail to satisfy the requirements of the trigger circuit. As the tracking rate of the interrogating pulses is 24 to 30 per sec, the trigger circuit will be saturated by the interrogations from about 100 aircraft. The receiver gain will be reduced until replies are generated only by strongest 100 interrogations13
ACCURACY •
DME accuracy varies from 3 % of total distance (6 NM at 200 miles out) up to being within 0.1 NM at all distances for the higher end receivers
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BEARING MEASUREMENT •
The timing of the transmitted pulses supplies the actual distance information to the aircraft. TACAN beacon-transponder modulates the strength of the pulse to convey bearing info by producing a specific directional-radiating pattern rotated around a vertical axis. This signal, when properly referenced, indicates the aircraft’s direction from the TACAN facility. This signal and distance data give a two-piece fix (distance and direction) for determining specific aircraft location 15
BEARING MEASUREMENT •
The aircraft TACAN equipment obtains bearing information by comparing the 15-Hz modulated signal with a 15-Hz reference burst signal it receives from the ground facility. The phase relationship between the 15-Hz modulated signal and the 15-Hz reference burst signal depends on the location of the aircraft in the cardioid pattern. The 15-Hz reference burst signals are transmitted when the maximum signal of the cardioid pattern aims due East. This group of 12 pulse pairs is commonly referred to as the North or main reference burst. An additional phase comparison of 135 Hz further reduces the 16 error and improves accuracy.
TACAN Modulation Envelope
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X & Y TACAN AND DME BEACONS •
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The ground equipment is more correctly known as an X beacon (TACAN or DME). In order to increase the number of channels from 126 to 252 within the same freq allocation, modified beacons known as Y beacons are employed A Y beacon receiver operates on the same freq as an X beacon receiver of the same channel number, but the transmitter freqs are different. Where an X beacon transmitter is 63 MHz lower than the receiver freq (Ch Nos 1 to 63), the Y beacon transmitter is 63 MHz higher; and vice versa for (Ch Nos 64 to 126). DME beacons are distinguished by the final digit on the VHF setting freq ie a 5 for a Y beacon, and a 0 for an X beacon, eg 114.15, Y, and 114.20, X. 18
USE OF TACAN AND DME Navigational Use The maximum range of TACAN and DME primarily depends on the aircraft height since radio waves in the 1,000 MHz band are quasi-optical in character. A TACAN ground beacon has a 70° cone above it in which the bearing function is inoperative. It should be remembered that the distance shown on the airborne equipment is slant range, not ground range, from the beacon. When TACAN bearing information is used it must be remembered that the arrowhead of the needle points towards the beacon. Therefore when plotting a position from the beacon, the tail end of the needle gives the required magnetic bearing. •
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LIMITATIONS Interference To overcome possible mutual interference, the operational requirement for each beacon for coverage in range and altitude has been specified. Each ground installation has been protected against interference up to the stated range and altitude. Outside the protected range and altitude, interference may occur and navigational information may be unreliable. Accuracy Brg indications ±0.5° Distance ±0.1 nm or ±1% of distance, whichever is greater. •
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AIR-TO-AIR TACAN •
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Enables suitably equipped a/c to measure the range between each other, up to a maximum of 195 nm; bearing information is not available. The changeover between air-to-ground and air-to-air operation is controlled by a selector switch on the airborne equipment. The maximum number of interrogating aircraft to which one responder can reply simultaneously is theoretically 33. To remain locked-on an aircraft must receive replies to 90% of its transmissions. If 33 were locked-on and a further interrogation was attempted, the responder would reply to this interrogation, but lock would be lost for one or more of the aircraft.
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TACAN DATA
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TACAN CHANNELS –
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There are 126 Tx and Rx channels
CHANNEL FREQUENCY –
Total channels
126 for Tx and 126 for Rx
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Range
962 1213 Mc
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Transmitting Channel
1025
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Receiver channel
63 between 962 1024 and 63 between 1151 1213 Mc
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1150 Mc 1 Mc spacing –
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CONE OF SILENCE –
AT 30,000 Feet
17 NM
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AT 50,000 Feet
29 NM
T h e d i s t a n c e i n d i c a t o r r e ad s t h e s l a n t r a n g e .
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TACAN OPERATING RANGE
Channel Low Band 1 63
Airborne Rx
Airborne Rx
Ground Tx
Ground Tx
962
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1024 MHz
1025 1087 MHz
1213 MHz
1088 1155 MHz
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High Band 64 126
1151
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DIFFERENCE BETWEEN TACAN AND DME CAPABILITY
TACAN
DME
Bearing
Yes
No
Slant range
Yes
Yes
Range rate
No
Yes
Channel capacity
126
100
Off set capability
Yes
No
Numerical
By Veh Freq
Channel selection
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AN – ARN 118 (V) •
Power source
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Total channel
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Max range
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FREQUENCY ALLOCATION –
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28 volt 115 volt 126 X 126 Y 390 NM 200 NM
ESS DC ESS AC
CP upper
(In case of surface beacon) (In case of airborne beacon)
Low band 962 1024 MC 100 KC High band 1151 1213 100 KC Intermediate Freq 63 70 KC Airborne Tx Freq 1025 - 1150 MC 100 KC –
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COMPONENTS –
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Control panel flight control pedestal R/T ID 310 range indicator (B Model) ID 307 bearing indicator (B Model) 161 B Couplers (2) (E Model) –
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