Editing Tactical air navigation system (section)

==Operation==
TACAN in general can be described as the military version of the VOR/DME system, though despite providing similar information as its civilian counterpart, its method of operation is significantly different. It operates in the [[UHF]] [[frequency]] band 962-1213 [[MHz]], utilizing a pulse-pair transponder system not dissimilar to that of [[secondary surveillance radar|secondary surveillance RADAR]]. Interrogating aircraft transmit in the 1024-1150 MHz band, split into 1 MHz channels numbered 1-126; the responding station (ground, ship, or another aircraft) is 63 MHz (63 channels) above or below the originating frequency, depending on the channel and mode of operation selected. Spacing between pulses in an individual pulse-pair is also determined by TACAN operating mode.

===Ranging===
Range information is functionally identical to the method provided by civilian DME: pairs of 3.5 microsecond (μs) pulses (measured edge-to-edge at 50% modulation strength) from an aircraft are repeated by the station being interrogated, using the round-trip time to calculate slant-range distance. Randomized spacing between interrogation pulse-pairs allows the interrogator to separate its own signal from that of other aircraft, enabling multiple users to access the ranging function without mutual interference. A fixed-round trip delay time (dependent on system mode) is added to each pulse-pair when being retransmitted by its station. The interrogator will generate up to 150 pulse-pairs per second when first acquiring a station in range in "search" mode, then drop down to ≈30 per second when acquired in "track" mode. Memory circuits in the ranging function enable a track to be quickly reestablished when ranging pulses are temporarily suppressed by other TACAN functions (see below).

===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,<ref>{{cite web | url=http://www.patentstorm.us/patents/5128873/description.html | author=Rockwell International | date=July 7, 1992 | title=Aircraft rendezvous using low data rate two-way TACAN information | url-status=dead | archive-url=https://web.archive.org/web/20110612222405/http://www.patentstorm.us/patents/5128873/description.html | archive-date=June 12, 2011 }}</ref> though an air-to-air mode allows distance to be established between transmitters/receivers.

{| class="wikitable"
|+ TACAN operating specifications, derived from MIL-STD-291C<ref>{{Cite web |title=MIL-STD-291 C TACTICAL AIR NAVIGATION SIGNAL |url=http://everyspec.com/MIL-STD/MIL-STD-0100-0299/MIL-STD-291C_11561/ |access-date=2024-03-18 |website=everyspec.com}}</ref>
! 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
| rowspan=4 | 1025-1150 (1-126) || negative (1-63)<br>positive (64-127)<br /> || 12 || 50 || 12 || 12/6 || 12/24 || rowspan=2 | 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
| rowspan=3 | positive (1-63)<br>negative (64-127)<br /> || 36 || 74 || 30 || 13/13 || 30/15 <br>(both single pulse)<br />
|-
!X channels, air-to-air
| 12 || 62 || rowspan=2 | single pulse || rowspan=2 colspan=3 | Same as air-to-ground, if supported 
|-
!Y channels, air-to-air
| 24 || 74
|}