TACAN System (AN/ARN-84)

The TACAN system indicates a slant range accurate to within 0.1 NM and a bearing of 0.5° to any surface station selected. Slant range to airborne stations is provided with an air-to-air (A/A) mode. The operating range is approximately 300 NM, if line of sight is given.

The system offers 126 operating channels in each of 2 modes. Receiving frequencies for surface-to-air operation are 962 to 1024 MHz and 1151 to 1213 MHz, for air-to-air operations, the frequencies are from 1025 to 1150 MHz. The TACAN uses two antennas that automatically switch in a 6-second interval until a threshold signal is received. Note that the TACAN can take up to 2 minutes to warm up when turned on for the first time after a cold start.

TACAN Modes

The system is capable of receiving valid signals from a ground station simultaneously with 99 other aircraft in either REC or T/R mode.

In the A/A mode, the system is capable of transponding with each of five cooperating aircraft, indicating slant range information to each, but the system will interrogate and lock on to only one at a time.

Both pilot and RIO share Identical TACAN control panels on the left consoles. Individual TACAN CMD buttons on both the pilot and RIO left consoles provide transfer of TACAN control between pilot and RIO. Control of TACAN is indicated by a flip-flop indicator in each cockpit showing PLT (pilot) or RIO. Either crewman may adjust the audio level of the identification signal. For TACAN panel description see TACAN Control Panel.

TACAN Displays

Bearing and distance to a TACAN station are displayed on the BDHI, the HSD, and the multiple display indicator. Deviation to the TACAN station is displayed on the HUD and VDI (VDIG) and the HSD and multiple display indicator.

The MDIG displays TACAN bearing marker, deviation ticks, range-to-TACAN station, and course. The HUD and VDI display provide a TACAN deviation bar, which is coded, on the HUD: solid line - TO station, dashed line - FROM station and on the VDI: bright bar - TO station, dark bar - FROM station.

TACAN information is also displayed on both the pilot’s and RIO’s identical BDHI. The bearing and distance functions of the BDHI come alive when the TACAN mode select switch is set to T/R. In the REC and T/R modes, magnetic bearings are displayed by the No.2 (large) needle, which unlocks and enters a search mode (spins) whenever bearing information is unreliable.

Range information received in T/R or A/A mode is displayed in nautical miles on the distance counter. An OFF flag covers the counter window if the range information is unreliable or not available. TACAN information is also displayed on the pilot HSD, HUD, and VDI and on the RIO multiple display indicator in other navigation modes.

TACAN Operation

If after approximately 2 minutes warm up time the range and bearing indications continue to search when a reliable station is selected, check circuit breakers should or select another station. The system has a memory feature so that tracking will not be interrupted by momentary disruption of received signals.

A range signal that is lost and has been previously tracked for at least 10 seconds, will be sustained by memory for 9 to 12 seconds. A bearing signal that has been tracked for at least 15 seconds will be retained for 3 to 8 seconds after signal loss. This allows for automatic antenna switching without a loss of TACAN displays.

During the minimum warmup time, failure indications and erroneous readouts should be disregarded and self-test results may be inconclusive.

TACAN BIT

The TACAN system has a built-in test that continuously monitors the TACAN functionality and provides an interruptive self-test. To start a 22-second interruptive self-test, use the momentary button (BIT switch) and monitor the GO (green) and NO-GO (amber) status lights.

Note: A BIT performed on TACAN stations within 2 NM can give an invalid indication. If a TCN acronym or NO-GO response is observed while tuned to a local station, along with normal TACAN azimuth and range, the acronym and/or the NO-GO should be disregarded.

The normal BIT sequence is as follows:

  1. Set MODE switch to T/R, allow 2 minutes for warmup.
  2. Press and hold BIT button.
  3. Both GO and NO-GO lights illuminate (light test).
  4. BDHI range OFF flag appears.
  5. BDHI bearing needle rotates counterclockwise.
  6. Release button; both lights go out (self-test starts).
  7. After 5 to 6 seconds, BDHI and HSD range reads 2 NM, BDHI and HSD bearing reads 4° (identify TACAN station).
  8. After 22 seconds, if good, green GO light illuminates, if bad, amber NO-GO light illuminates.