The airborne equipment also contains a multichannel transmitter-receiver (transceiver). Bearing information is automatically obtained with the correct channel selected. Distance is determined by measuring the elapsed time between transmission of interrogating pulses of the airborne set and reception of corresponding reply pulses from the ground stations. This sequence is initiated by the aircraft transmitter and requires about 12 microseconds per NM round trip. Since the DME gives a readout of slant range rather than ground range, a correction has to be applied to the reading.
DME is designed to provide reliable information to a maximum distance of 199 NM, dependent on aircraft equipment and LOS. Accuracy is plus or minus one-half NM or 3 percent of the distance, whichever is greater.
Since a large number of aircraft could be interrogating the same station, the aircraft TACAN must sort out the pulses that are replies to its own signal. Interrogation pulses are transmitted on an irregular and random basis by the airborne set which then searches for replies synchronized to its own interrogations. If the signals are interrupted, a memory circuit maintains the last distance indications on the range indicator for approximately 10 seconds to prevent the search operation from recurring. This process starts automatically when a new station is tuned or when there is a major interruption of signals. Depending upon the actual distance from the station, the searching process may require up to 22 seconds. The maximum number of aircraft that can be accommodated by one station at any one time is 100. With the development of the X and Y bands, this number can be doubled.
Bearing and/or Distance Unlock
Since TACAN bearing and DME are subject to LOS restrictions, this information could be lost any time signals are blocked. Temporary obstructions can occur in flight any time any part of the aircraft gets between the ground and aircraft antenna. Other aircraft, terrain, and buildings are external causes for unlock. Any time the signal is obstructed for more than 10 seconds for DME and 2 seconds for azimuth, the unlock conditions are indicated by a rotating bearing needle and a tumbling DME readout.
Azimuth Cone of Confusion
TACAN antennas transmit radio energy in circular patterns out from the transmitter. However, waves are not transmitted directly above the station. Therefore, as the aircraft approaches a TACAN station, signals are lost. This is indicated by a rotating TACAN bearing needle in the RMI. The azimuth cone can be up to 100° or more in width or approximately 15 NM wide at 40,000 feet. Thus, one may enter the cone of confusion at approximately 7.5 DME at this altitude. Approaching the station, usable TACAN information is lost before the cone is reached as aircraft memory circuits maintain the last information.
Range Indicator Fluctuations
Slight oscillations up to approximately one forth NM are normal for range indicator operation due to the pulses generated by the transmit or receive function. When a usable signal is lost, the memory circuit maintains the indicated range for about 10 seconds, after which unlock occurs unless usable signals are regained.
Forty Degree Azimuth Error Lock-on
The construction of the TACAN ground antenna is such that it transmits a series of nine signal lobes (eight auxiliary and one main reference pulse) 40° apart. With the airborne receiver working correctly, these pulses lock on the airborne equipment with the main reference at 090°. With a weak signal, the main reference pulse may slide over or miss the 090° slot and lock on at one of the auxiliary positions. When this occurs, azimuth indications are 40° or a multiple of 40° in error. Forty degrees azimuth lock-on error does not cause a course warning flag to appear on the indicator. Rechanneling the airborne receiver may give the set another chance to lock on properly.
Co-channel interference occurs when an aircraft is in a position to receive TACAN signals from more than one ground station on the same frequency. This normally occurs only at high altitudes when distance separation between like frequencies is inadequate. DME, azimuth, or identification from either station may be received. This is not a malfunction of either airborne or ground equipment, but a result of position.
Tuning and Controls
The basic controls of most TACAN systems are shown in Figure 5-11. The proper channel is tuned by rotating channel selector knobs (1) to any of 126 channels. Knob (2), the internal test mode, validates working condition of TACAN. The channel mode selector (4) allows X or Y band to be selected. These controls are presented in the channel indicator (3). A volume control (5) adjusts the audio level of the station identifier signal. The TACAN test button permits the user to perform a system self-test. The function selector (6) has four settings:
- OFF—Removes power to the set.
- REC—Energizes the receiver to obtain bearing information.
- T/R—Energizes both receiver and transmitter to obtain both bearing and distance information.
- A/A—Transmits and receives interrogations and replies to measure range to another A/A TACAN-equipped aircraft. Bearing information is not provided on this set.
In order to provide both military and civilian pilots the capability of positioning from the same radio NAVAIDs, a combination of VOR and TACAN station was developed. Each facility offers three services. VOR azimuth signals are transmitted on the published VOR frequency. TACAN azimuth and DME signals are broadcast on the published UHF channel.
Fix-to-Fix Navigation (Using RMI and BDHI)
Flying from one radial and DME to another is basic to many departures and approaches. A heading to the desired point may be derived quickly through the use of an RMI, providing a radial and a separate readout of DME. The same procedures apply for a BDHI. The following technique and example are provided in order to demonstrate how to compute a heading. [Figure 5-12]
Present position = 180°/60
Desired position = 090°/30
Present heading = 000°
- Tune, identify, and monitor correct VOR and TACAN.
- Turn the aircraft in the general direction of the desired fix by turning to a heading approximately halfway between the head of the bearing pointer (000°) and the radial on which the desired fix is located (090°). In this case, turn to 045°. [Figure 5-12A]
- Visualize the aircraft position and the desired fix on the RMI as follows:
- The center of the RMI is considered to be the VOR or TACAN, and the compass rose simulates the radials around the station.
- The fix with the greater range (180°/60) is established at the outer edge of the compass card.
- The fix with the lesser range (090°/30) is established at a point that is proportional to the distance represented by the outer edge of the compass card.
- Determine the heading to the desired fix by connecting the present position to the desired fix with an imaginary line on the RMI. [Figure 5-12D to C] Establish another imaginary line parallel to the line labeled B to C through the center of the RMI. This line indicates the no-wind heading to the desired fix (030°).
- Turn to 030° and apply any drift correction. With 5° right drift, turn to 025°. Cross-check position continually and correct as necessary.
Fix-to-Fix Navigation (Using the MB-4 Computer)
A fix-to-fix can also be computed on the wind face side of an MB-4 computer. First, give the pilot a general heading toward the fix. (NOTE: Work in bearings; however, all work must be done in either bearings or radials to compute the solution.) For the following example, radials are used. The fix to navigate to is the 280° radial at 30 DME. Set up a graphic depiction on the wind face side of the computer with present position (350° radial at 050 DME) and the desired fix (280°/030). Use the following steps:
- Place present position (350°/050) on the wind face side using the square grid at the bottom of the MB-4. Align 350° on the compass rose under the true index. Mark the point by counting down 50 NM from the TAS grommet and mark with a ×. Use the scale set up on the square grid or set up an applicable scale. The scale used must remain constant throughout the problem. [Figure 5-13]
- Place the fix radial and DME (280°/030) on the computer the same as in step one. [Figure 5-14] Mark as a fix symbol (Δ).
- Determine the no wind heading by rotating the compass rose so that the present position (×) is directly above the fix (Δ). Use the square grid at the bottom to help with alignment. [Figure 5-15] Turn the aircraft to MC under the true index (206° for this example) and kill the drift. (NOTE: Place present position (×) on the 0 NM horizontal baseline then, using a NM increment scale, count down to the fix position (Δ) to determine how far you are from the fix (48 NM in this example).
- Repeat the procedure as necessary to keep all progress updated.