Tactical air navigation system

Typical US Air Force TACAN site using a dB Systems Model 900E TACAN Antenna

A tactical air navigation system, commonly referred to by the acronym TACAN, is a navigation system initially designed for naval aircraft to acquire moving landing platforms (i.e., ships) and later expanded for use by other military aircraft. It provides the user with bearing and distance (slant-range or hypotenuse) to a ground or ship-borne station. It is, from an end-user perspective, a more accurate version of the VOR/DME system that provides bearing and range information for civil aviation. The DME portion of the TACAN system is available for civil use; at VORTAC facilities where a VOR is combined with a TACAN, civil aircraft can receive VOR/DME readings. Aircraft equipped with TACAN avionics can use this system for enroute navigation as well as non-precision approaches to landing fields. However, a TACAN-only equipped aircraft cannot receive bearing information from a VOR-only station.

History

TACAN symbol on aeronautical charts

In 1945, development of the system commenced with ITT Inc.'s Federal Communications Laboratory under Henri G. Busignies. A 1000 MHz system using a polar coordinate system for direction and distance was identified as project goals. In February 1946, the Wright Air Development Center produced a study stating a 1000 MHz polar coordinate system using omnidirectional radio range and distance measuring equipment was optimal for short range navigation. In April, ITT was awarded a contract to produce the AN/APN-34 airborne interrogator and the AN/GPN-4 ground beacon. In August, a prototype 3000 MHz bearing, and a 1000 MHz distance measuring, system was demonstrated. In June 1947, ITT was awarded a contract by the Rome Laboratory to develop the L. A. deRosa and L. Himmel 1000 MHz omnidirectional radio range with 0.5 degree bearing accuracy. That autumn, S. H. Dodington proposed rotating a reflector around the distance measuring beacon, so as to also provide bearing. In June 1948, the navy's Bureau of Ships, led by the "Father of the Modern Tacan" J. Loeb, became involved in development when it awarded ITT a contract to replace its YE/YG beacon with an airborne AN/ARN-16 and shipboard AN/URN-1. In 1949, the air force started taking delivery of the AN/APN-34, while the navy attached one to its AN/ARN-16. By the end of 1950, both the air force and the navy, agreed on a common combined bearing and distance system. The specifications for this AN/ARN-21 included 126 channels, accuracy of 0.75 degrees, and a range up to 200 nautical miles. In 1951, ITT delivered the 50 channel AN/ARN-21 and AN/URN-3 systems. In September 1952, the 126 channel versions were demonstrated.

Hoffman Laboratories Div. of the Hoffman Electronics Corp.–Military Products Division provided services in the 1950s.

Operation

The 1000 MHz omnidirectional range bearing and distance measuring tactical air navigation system includes transponders and beacons. The air to ground distance function consists of 126 two-way channels spaced 1 MHz apart between 1025 and 1150 MHz. The ground to air bearing and distance functions consist of 63 channels between 962 and 1024 MHz, and another 63 channels between 1151 and 1213 MHz. Pulse coding is used to increase the average power, and multiplexing the bearing function onto the distance channel.

Ranging

The distance function is based on radar ranging, but instead of using reflections displayed on a scope, timing circuits convert the timing delay between interrogation and reply into the associated distance on a meter. The airborne transmitter sends out narrow, widely spaced interrogation pulses, 2 per send when tracking, and 150 per second when searching. The rate varies in an irregular fashion for each aircraft, so as to prevent interference. A searching process on the aircraft determines which reply pulse time delay corresponds to its interrogation rate. The timing circuits include a memory function that maintains the distance indication for up to 10 seconds without a reply, before initiating a new search. A ground based beacon sends out a constant 2700 pulses per second independent of the number of interrogating aircraft, and a Morse code identifier every 75 seconds.

Bearing

Bearing information is derived from amplitude modulation (AM) of the responding station's pulse-pair signals, the AM signal being generated via physical rotation of a station's directional antenna or electronic steering of the same signal using an antenna array. Two AM signals are generated: a fundamental AM signal at 15 Hz, and an auxiliary AM signal (implemented using fixed signal reflectors in rotating-antenna installations) at 135 Hz, the ninth harmonic of the fundamental signal. These correspond to a "coarse" and "fine" bearing signal, the latter improving the accuracy of the former. The time is compared between the point of peak positive signal strength with a reference train or "burst" of pulse-pairs of specific repetition rate and duration, timed to transmit at a specific point in the signal's sweep; these replace all other pulse types when transmitted. The civilian VOR system differs from TACAN in utilizing a single continuous-wave 30 Hz modulation signal, using the phase difference between a fixed-phase and variable phase (rotating) component to derive bearing info.

Squitter function

TACAN stations transmit pulse-pairs at a composite rate of 3600 pairs/second: 900 of which are bearing reference bursts, and the other 2700 being composed of ranging and identification pulses. When insufficient interrogation pulses from aircraft are present, the station will use a squitter circuit to inject additional randomized pulse-pairs to maintain the desired pulse rate. This ensures that sufficient signal is present to support demodulating bearing signals.

Identification

TACAN stations are identified by Morse code. The transmitting station periodically replaces the randomized ranging pulse-pairs with regularly spaced pairs that de-modulate to a 1350 Hz tone, keying a three-letter identification code at approximately 6-7 wpm every 40 seconds. Ranging and squitter pulses are permitted during the gaps between dots and dashes. There is no capability for voice transmission in a TACAN-only system.

Operating modes

There are two basic channel configurations available: X (the original implementation) and Y (added in the 1960s to expand available channels and reduce mutual interference between closely-spaced stations). These configurations differ in pulse-pair width, fixed receiver response delay, and polarity of frequency offset from the interrogation channel. TACAN interrogators can operate in four modes: receive (for bearing/identification only), transmit/receive (for bearing, range, and ID), and air-to-air versions of the previous two.

The typical TACAN onboard user panel has control switches for setting the channel (corresponding to the desired station's assigned frequency), the operation mode for either transmit/receive (T/R, to get both bearing and range) or receive only (REC, to get bearing but not range). Capability was later upgraded to include an air-to-air mode (A/A), where two airborne users can get relative slant-range information depending on specific installations, though an air-to-air mode allows distance to be established between transmitters/receivers.

TACAN operating specifications, derived from MIL-STD-291C

Channel/operating mode	Interrogator frequency (MHz)/channel	Response frequency offset (±63 MHz)	Interrogator pulse-pair width (μs)	Response delay spacing (μs)	Response pulse-pair width (μs)	Main/auxiliary reference burst length (pulse pairs)	Main/auxiliary reference burst pulse-pair spacing (μs)	Main/auxiliary reference burst synchronization point
X channels, air-to-ground	1025-1150 (1-126)	negative (1-63) positive (64-127)	12	50	12	12/6	12/24	Positive peak of AM signal pointed due east when main burst triggered; auxiliary burst synchronized to same event, but suppressed during main burst transmission
Y channels, air-to-ground		positive (1-63) negative (64-127)	36	74	30	13/13	30/15 (both single pulse)
X channels, air-to-air			12	62	single pulse	Same as air-to-ground, if supported
Y channels, air-to-air			24	74

Performance and accuracy

A DVORTAC installation in Germany; the TACAN antenna is elevated above the center DVOR-antenna on the counterpoise.

When initially deployed, TACAN was intended to provide a bearing accuracy of ±0.22°, based on the main bearing signal's own accuracy of ±2° and the corrections applied by the ninth-harmonic auxiliary bearing signal. Theoretically a TACAN should provide a 9-fold increase in accuracy compared to a VOR, but operational use has shown only an approximate 3-fold increase.

Operational accuracy of the 135 Hz azimuth component is ±1° or ±63 m at 3.75 km.

Manufacturers of TACAN sets mention the ability to track stations out to 400NM, though these systems will cap their instrumented range signals at approximately 200NM. Per official FAA service volume information, reliable TACAN/DME reception can be guaranteed out to 130NM below 45,000 feet above the surface for a high-altitude certified unit.

On the first Space Shuttle flight, Capcom Joseph P. Allen reported up to the crew that their TACANs had locked onto the Channel 111X signals at St. Petersburg, FL at a range of 250 miles.

Benefits

A shipboard TACAN antenna on USS Raleigh (LPD-1) with a lightning rod extending above it

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 TACAN 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 TACAN receiver can be used in air-to-air mode to provide the approximate distance between two coordinating aircraft by selecting channels with 63 channels of separation (e.g., aircraft #1 sets channel 29 into its TACAN and aircraft #2 sets channel 92 into its TACAN.). It does not provide relative bearing.

Drawbacks

For military usage a primary drawback is lack of the ability to control emissions (EMCON) and stealth. Naval TACAN operations are designed so an aircraft can find the ship and land. Since there is no encryption, an enemy can use the range and bearing provided to attack a ship equipped with a TACAN. Some TACANs have the ability to employ a "Demand Only" mode: only transmitting when interrogated by an aircraft on-channel. It is likely that TACAN 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.

Some systems used in the United States modulate the transmitted signal by using a 900 RPM rotating antenna. This antenna is fairly large and must rotate 24 hours a day, possibly causing reliability issues. Modern systems have antennas that use electronic rotation (instead of mechanical rotation), hence no moving parts.

Integration with modern navigation systems

Although TACAN was originally developed as a stand-alone military navigation aid, it is increasingly integrated with civil navigation systems in mixed-use airspace. Many modern TACAN installations are co-located with VOR/DME facilities (VORTAC), allowing both military and civilian aircraft to use the same site for en route navigation. This interoperability supports performance-based navigation (PBN) route structures and allows legacy TACAN stations to provide backup capability for GNSS-based navigation. In some military operations, TACAN is also used in conjunction with inertial and satellite navigation to provide multi-sensor navigation redundancy and improved accuracy.

See also

• Battle of Lima Site 85 (SAC TACAN captured March 1968)
• Distance measuring equipment
• Electronics technician
• Global Positioning System
• VHF omnidirectional range
• Wide Area Augmentation System

External links

• Tactical Air Navigation - 1955 USN training film on TACAN, including information on the AN/URN-3 ground-based station
• dB Systems, Inc. - Manufacturer of mechanically scanned, electronically scanned, shipboard, man-portable, and tactical TACAN Antennas - Complete TACAN Antenna profile with datasheets and photos
• Rantec Microwave Systems - Manufacturer of non-rotating TACAN antennas - Complete with antenna internal photos and specs
• Moog Navigation and Surveillance Systems Archived 2014-11-29 at the Wayback Machine - Fixed site, shipboard, mobile and man-portable TACAN systems
• Leonardo Air Traffic Management - Fixed site TACAN systems