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What the heck is TACAN, anyway?

Those of us who haven't served in the armed forces in some aviation capacity probably see words like "VORTAC" and wonder where the "TAC" comes from now and again. VORTAC is a term used freely by controllers, pilots, and instructors without knowing where the latter half of the word originates.

To be fair, many of you are former military airman and could spot a TACAN in a Where's Waldo? book with little effort. Spotting a TACAN isn't difficult when you know what to look for. Cylindrical and typically white, TACAN antennas stand up in the middle of the VOR antenna, which is much larger and cone-shaped.

However, even if GA pilots did know what to look for, that and a token get us on the NYC subway. We wouldn't know what one does or how a pilot would use it.

Nuts and Bolts

TACAN stands for "Tactical Air Navigation." It originated in military aviation in the 1950s, where it has been traditionally used by western militaries for primary electronic navigation. We in the civilian world would see it as a UHF-equivalent of the VOR/DME.

Operationally, azimuth (bearing) from a TACAN is about twice as accurate as VOR, though the technology could do even better. Unlike VOR, which must be stationary to operate properly, TACAN can be mobile. In addition to its ground stations--either stand-alone TACANs or VORTACs collocated with civilian NAVAIDs--the military routinely places TACAN units on tanker aircraft and ships so aircraft can home in on an airborne fueling rendezvous or a floating runway.

That's pretty convenient, especially if you're tired after a long game of tag at 40,000 feet.

Good Ol' DME

TACAN transmits distance information using interrogation and response via pulse modulation to create pseudorange, which is read by the onboard receiver. Sound familiar? This is DME--literally. The UHF component of the VOR/DME is TACAN technology at work. It could be the TACAN itself (if from a VORTAC) or the same technology, but a separate installation such as a VOR/DME or ILS/DME. Military aircraft, therefore, receive their distance information in the same way, and often from the same piece of equipment, as do civilian aircraft.

The main difference between civil and military radio navigation is the transmission and receipt of azimuth information. Civilian VOR uses a stationary antenna to emit dual signals that differ only by their phase as you fly around the station. The frequency and amplitude of both signals are the same. Your receiver knows you are on the 020-degree radial when it picks up two signals that are 20 degrees out of phase.

TACAN uses the same concept, but TACAN transmits azimuth information using a combination of pulse and amplitude modulation. TACAN has one antenna that is stationary orbited by a second antenna that moves. The inner antenna emits a pulse called the reference burst once per revolution of the outer antenna.

Assisted by a radio-frequency (RF) energy reflector, the rotating element creates a 15-Hz, amplitude-modulated waveform that is seen differently depending on where you are in relation to the TACAN. Further reflectors increase the accuracy by creating a secondary, 135-Hz waveform.

These together create the variable signal. This concept of determining bearing is identical to that used in a VOR--identifying differences between a reference and variable signal to determine bearing--but the details are different.

TACAN will be replaced someday. The forerunner currently appears to be the Joint Precision Approach and Landing System (JPALS). Much like our soon-to-be-released (or not) Local Area Augmentation System, JPALS uses differential GPS for navigation. TACAN can also be intercepted and honed by enemy aircraft without much effort. JPALS cannot.
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Title Annotation:Tactical Air Navigation
Author:McNamee, Jeff
Geographic Code:1USA
Date:Sep 1, 2006
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