A fighter pilot coming home in bad weather wants to know two things: which direction is base, and how far away is it. For more than seventy years, the answer in military cockpits has come from a system with a slightly clumsy name: TACAN, short for Tactical Air Navigation.

Dial in a channel, and a needle swings round to point at the station while a counter reads out the distance in nautical miles. Simple to use, clever underneath, and still flying in everything from training jets to aircraft carriers in an age of GPS.

Bearing and distance in one box

Civilian aircraft have long navigated with two separate systems: a VOR, which gives a bearing to a ground station, and DME, distance measuring equipment, which gives the distance. TACAN does both from a single, compact station. That matters for the military, because a TACAN beacon can be fitted to a ship, a truck or a small field site, where a VOR with its large antenna array would never fit.

According to the system's history, development began at ITT's laboratories in 1945 under Henri Busignies. The US Air Force and Navy settled on a common airborne set, the AN/ARN-21, around 1950 and 1951. The basic principle has not changed since.

Bogey Dope's plain-language introduction to TACAN, from a pilot's point of view.

How the distance works

The distance part is the easier half to understand. The aircraft sends out pairs of radio pulses. The ground station receives them, waits a fixed delay, and sends a reply. The airborne set measures how long the round trip took and, knowing the speed of light, converts it into distance. Because the signal travels in a straight line from the aircraft to the station, the result is slant range, so an aircraft flying directly over a station at 30,000 feet will still show about five nautical miles.

This part of TACAN uses exactly the same method as civilian DME, which is why the two are compatible. A civilian aircraft can get a distance from a military TACAN, even though it cannot use its bearing information.

How the bearing works

The bearing is where TACAN gets clever. The ground beacon's antenna produces a radiation pattern shaped roughly like a heart, a cardioid, and rotates it 15 times per second. Sitting in one spot, an aircraft sees the signal strength rise and fall 15 times a second. The station also sends a reference pulse group at a fixed point in every rotation. By measuring the time between that reference and the peak it sees, the aircraft can work out its bearing from the station.

That alone gives an accuracy of a few degrees. To sharpen it, the antenna adds a second, finer pattern with nine lobes, producing a 135 Hz ripple on top of the 15 Hz one, with its own reference pulses. The coarse signal tells the receiver which of the nine sectors it is in, and the fine signal pins down the exact bearing. In operational use, TACAN bearing accuracy is about plus or minus one degree. Older stations spun the antenna mechanically; modern ones rotate the pattern electronically.

Ships, tankers and air-to-air

TACAN's compactness makes it ideal for the places military aircraft actually go home to. Aircraft carriers and other ships carry TACAN beacons, so a naval pilot coming back from a mission can find a moving runway far out to sea. Portable TACAN sets can be deployed to forward airstrips.

TACAN also has an air-to-air mode. Two aircraft tuned to channels 63 apart can measure the distance between them, which is useful for joining up with a tanker or a wingman in poor visibility. It gives range only, not bearing, but in thick cloud a closing distance is a very welcome number.

Still useful in the GPS age

Satellite navigation has taken over much of TACAN's day-to-day work, but it has not replaced it. GPS can be jammed or spoofed, as airlines flying near conflict zones have learned the hard way, and military forces want a navigation aid they control themselves. Modern systems blend TACAN with inertial and satellite navigation so that if one source goes bad, the others keep the pilot honest.

So in an era of glass cockpits and helmet displays, the humble TACAN channel is still a routine part of military flying. Dial it in, check the needle and the miles, and you know the way home.

Pilotopedia explains what TACAN is and how it is used.

The idea is the same as in the 1950s: put a beacon where the aircraft need to return, and let them home in on it. Today it can even come in a case carried by soldiers.

Thales shows a modern man-portable TACAN for military forces.

Sources: Wikipedia (Tactical air navigation system), US Federal Aviation Administration, Thales, Bogey Dope, Pilotopedia