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Tactical Air Navigation

Introduction:

  • Tactical Air Navigation (TACAN) was developed to provide precise geographical fixing of the aircraft's position for the military where the Distance Measuring Equipment (DME) system is deemed unsuitable
  • Like a VOR, provides 360 radials coming from the station
  • VOR and TACAN systems collocated are called VORTACs
  • An additional advantage is that TACAN ground equipment is compact and relatively easy to transport
  • IDs every 35 seconds
  • The OFF position will disconnect the unit from the aircraft power supply
  • STBY will receive magnetic bearing information only from ground TACAN navigation facilities
  • In the T/R position, the TACAN receives magnetic bearing and distance information
  • Cannot be used to transmit and receive voice
  • Will identify itself with a Morse code identifier about every 37.5/32 seconds
  • Transmitted one time for each 3 or 4 times that a VOR signal is transmitted when identifying a VORTAC

History:

  • For reasons peculiar to military or naval operations (unusual siting conditions, the pitching and rolling of a naval vessel, etc.) the civil VOR/Distance Measuring Equipment (DME) system of air navigation was considered unsuitable for military or naval use
  • A new navigational system, TACAN, was therefore developed by the military and naval forces to more readily lend itself to military and naval requirements
  • As a result, the FAA has integrated TACAN facilities with the civil VOR/DME program
  • Although the theoretical, or technical principles of operation of TACAN equipment are quite different from those of VOR/DME facilities, the end result, as far as the navigating pilot is concerned, is the same
  • These integrated facilities are called VORTACs

Components:

  • TACAN ground equipment consists of either a fixed or mobile transmitting unit
  • The airborne unit in conjunction with the ground unit reduces the transmitted signal to a visual presentation of both azimuth and distance information
  • TACAN is a pulse system and operates in the Ultrahigh Frequency (UHF) band of frequencies
  • Its use requires TACAN airborne equipment and does not operate through conventional VOR equipment

Passage:

  1. DME decreases to minimum
  2. Needle rotates 180°
  3. CDI oscillates from side to side
  4. TO/FORM indicator switches from TO to FROM

NOTE:
Instrument indications will fluctuate when close to the station; this does not necessarily mean your aircraft is off course. Do not chase the needle when close to the station. Limit heading corrections for drift when close to avoid overshooting

Frequencies:

  • TACAN operates in the UHF (1000 MHz) band with 126 two-way channels in the operational mode (X or Y) for 252 total
  • Air-to-ground DME frequencies are in the 1025 to 1150 MHz range
  • Ground-to-air frequencies are in the 962 to 1213 MHz range

Ground Equipment:

  • Consists of a rotating type antenna transmitting bearing and a receiver-transmitter (transponder) for transmitting distance information
  • Ground stations are usually dual transmitter equipped
  • One operational and the other standby
  • Sometimes TACAN reception might be suspected of being in error or bearing/distance unlock conditions may be encountered in flight

Distance Measuring Equipment (DME):

  • The aircraft sends an interrogation to the ground station and the station sends a reply back
  • The elapsed time between transmission and reception is the distance in NM
  • The pulses require about 12 microseconds round trip travel time per NM of distance
  • Replies are random to ensure other aircraft using the same station get a unique signal
  • If the signals are interrupted, a memory circuit maintains the last distance indication for 10 seconds
  • After 10 seconds, a search function begins to lock back onto the station, taking up to 22 seconds
  • Furnishes reliable, line of sight, slant range information at distances up to 199 NM with an accuracy of 2 mile or 3% of the distance, whichever is greater

Limitations:

  • Cannot transmit voice
  • DME equipment is an integral part in providing slant range up to 399.9 NM with an accuracy of 2 miles or 3% of the distance, whichever is greater
  • Cone of confusion:
    • Exists where TACAN azimuth information is not available
    • The "cone" varies from 60° to 110° wide
    • TACAN DME and ID signal will be received and station passage will be noted by minimum DME
    • Although narrow at low altitudes, this "cone" expands to about 18 NM across at 30,000'
    • Because of the size of TACAN cone of confusion, holding on a TACAN will always be established using DME
  • Standard Service Volume applies (T, L, H stations):
    • SSV defines the interference free reception limits of unrestricted NAVAIDs, which are usable for random/unpublished route navigation
    • Reception may be possible beyond the ranges, however interference from other stations on the same frequencies may cause 40° off bearing lock-on
  • 40° off bearing TACAN pointer lock-on error:
    • Locking on to multiples of 40° from the desired radial due to inherent operating functions of the older crystal controlled receivers
    • Solid-state electronics in the T-45C should preclude this problem from occurring
    • Re-channeling the receiver and tuning back to the original station may remedy the error
  • Failure to lock on:
    • Misalignment of equipment
    • Worn aircraft control box
    • Re-channeling the receiver and tuning from the opposite direction back to the original station may remedy the error
  • Co-channel interference:
    • Receiving signals (DME, azimuth, identifier) from more than one TACAN station due to the relationship between aircraft's high altitude and station locations

Conclusion:

  • Pilots should be aware of the possibility of momentary erroneous indications on cockpit displays when the primary signal generator for a ground-based navigational transmitter is inoperative
    • Pilots should disregard any navigation indication, regardless of its apparent validity, if the particular transmitter was identified by NOTAM or otherwise as unusable or inoperative

References: