What is Tactical Air Navigation?
Tactical Air Navigation in general can be described as the military version of the VOR/DME system. It operates in the Fx band 960-1215 MHz. The bearing unit of TACAN is more accurate than a standard VOR since it makes use of a two-frequency principle, with 15 Hz and 135 Hz components, and because UHF transmissions are less prone to signal bending than VHF.
The distance measurement component of Tactical Air Navigation operates with the same specifications as civil DMEs. Therefore to reduce the number of required stations, stations are frequently co-located with VOR facilities. These co-located stations are known as VORTACs. This is a station composed of a VOR for civil bearing information and a Tactical Air Navigation for military bearing information and military/civil distance measuring information. The TACAN transponder performs the function of a DME without the need for a separate, co-located DME. Because the rotation of the antenna creates a large portion of the azimuth (bearing) signal, if the antenna fails, the azimuth component is no longer available and the Tactical Air Navigation downgrades to a DME only mode.
The range function of both Tactical Air Navigation and DME beacons is identical in operation, and is based on the transponder principle, but the bearing information of a Tactical Air Navigation beacon is in the form of a continuous transmission.
Bearing Measurement by TACAN
The pulses produced initially by the beacon, whatever their origin, are of constant shape and amplitude. However, in order to convey bearing information they are amplitude modulated by the aerial system. As the beacon transmits 2,700 pulse pairs per sec, the modulation is carried as accurately as if a CW bearer was used.
The beacon’s aerial system comprises a central vertical aerial around which two concentric fibre glass cylinders rotate at 15 revolutions per second. Embodied in the surface of the inner and outer cylinders respectively, are one vertical reflector and nine equally-spaced directors
Use of Tactical Air Navigation
The maximum range of Tactical Air Navigation 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.
AIR-TO-AIR Tactical Air Navigation
Air-to-air TACAN enables suitably equipped aircraft to measure the range between each other, up to a maximum of 195 nm but the 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. However, in order 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 this would mean that one or more of the aircraft would receive replies to less than 90% of its transmissions and lock would be lost. In addition, the responder’s gain is set by the amplitude of the strongest signal seen and if there is a large difference between the signal strength received from different aircraft it may be the case that the weaker signals are not seen. It is thus impossible to state precisely the maximum number of aircraft that can lock-on simultaneously.
Tactical Air Navigation Offset Computer
The addition of a offset computer to an airborne installation enables bearings and distances to be obtained from a point offset from the beacon.
The range and bearing of the offset point relative to the beacon are manually set on the computer control. The computer then converts the range and bearing into electrical signals and transmits them to the navigation display where they are added to the direct Tactical Air Navigation signals. The combination of signals causes the display to indicate the range and bearing of the offset point instead of the beacon.
Theoretically a Tactical Air Navigation should provide a 9-fold increase in accuracy compared to a VOR but operational use has shown only an approximate 3-fold increase.
Accuracy of the 135 Hz azimuth component is ±1° or ±63 m at 3.47 km. Accuracy of the DME portion is 926 m (±0.5 nm) or 3 percent of slant range distance, whichever is greater.
Tactical Air Navigation stations can provide distance up to 390 nautical miles.
Modern Tactical Air Navigation are much more accurate. The requirement now is to have portable Tactical Air Navigation that is IFR certifiable, both Station and Portable systems. The latest modern version of Tactical Air Navigation has been tested to an average error of 0.00 in both range and azimuth, and could be a feasible back-up to future Air Traffic Control Systems and may even be integrated into systems for a seamless back up.
Past Tactical Air Navigation have relied on high output power (up to 10,000 Watts) to ensure good signal in space to overcome nulls present in antenna design and to provide their required 200 mile range. With the advancement of technology, antenna design has improved with higher gain antennas, much shallower nulls, and lighter construction. Now it’s feasible to have a 200 nm range with a 400 Watt Tactical Air Navigation/DME Transmitter, making the Tactical Air Navigation package much smaller, more portable and more reliable (Power = Heat which shortens the life of electronics).
TACAN is getting smaller: Full Tactical Air Navigation coverage can now be provided in a system that can be carried on a single trailer weighing less than 4000 lbs, and set up by two people in less than an hour. Transceivers can now be as small as lunch boxes (with full coverage and range) and the antennas can be reduced from 800 pounds to less than 100 pounds.
Because the azimuth and range units are combined in one system it provides for simpler installation. Less space is required than a VOR because a VOR requires a large counterpoise and a fairly complex phased antenna system. A Tactical Air Navigation system theoretically might be placed on a building, a large truck, an airplane, or a ship, and be operational in a short period of time. An airborne Tactical Air Navigation receiver can be used in air-to-air mode which allows two cooperating aircraft to find their relative bearings and distance.
For military usage a primary drawback is lack of the ability to control emissions (EMCON) and stealth. Naval Tactical Air Navigation operations are designed so an aircraft can find the ship and land. There is no encryption involved, an enemy can simply use the range and bearing provided to attack a ship equipped with a Tactical Air Navigation. Some Tactical Air Navigation have the ability to employ a “Demand Only” mode wherein they will only transmit when interrogated by an aircraft on-channel. It is likely that Tactical Air Navigation will be replaced with a differential GPS system similar to the Local Area Augmentation System called JPALS. The Joint Precision Approach and Landing System has a low probability of intercept to prevent enemy detection and an aircraft carrier version can be used for autoland operations.
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- Department of Transportation and Department of Defense (March 25, 2002).
- Department of Transportation and Department of Defense (March 25, 2002).
- FIS Book on Avionics