Now even handheld aviation radios have GPS, VOR and ILS! I wonder why modern LCD screens with radios and navigation doesn't have TACAN? Is it coded, not useful, not certified or not used anymore? I don't think there is a problem to implement the electronics nowadays.
Civilian aircraft can use part of TACAN, the Distance Measuring Equipment (DME) signal is the same as civilian DME. As for the directional component one big reason is that the military don't want you to.
Even if the military didn't mind there would likely be no demand as the costs of using TACANs outweigh the benefits. TACAN is essentially a more accurate version of VOR, and although the complexity and weight of the system has reduced the antenna is still apparently 100 pounds (45kg). For light aircraft that's a fair chunk of useful weight taken off. Cost-wise a VOR setup will set you back somewhere around \$6000 just for the equipment, call it closer to $8000 installed (roughly). A TACAN is a more sophisticated system which would cost a great deal more, say finger in the air of \$20,000, maybe more. TACAN antennas would need to be certified for every single airplane model, and that's going to add to the costs.
So for a light aircraft I'm going to lose usable weight to an antenna that is also going to throw off my weight and balance, and that's going to cost me at least \$20k when all the equipment, installation and recertification costs are factored in. That's big cash when I already have VOR, ILS, DME, NDB and 3+ satellite navigation systems already available. Oh, and I can also call up ATC most places and say 'help!' if it all goes wrong. There's just no benefit. My phone can tell me where I am if every system on the airplane fails as it has a built in GPS!
For commercial large airplanes it's the same situation. In addition to all the above most will have redundant inertial navigation systems, so even less need for it.
Let's clarify some areas
TACAN provides both distance and bearing information. The system is interrogated and the reply is pulse coded, meaning information is delivered by the TACAN ground beacon upon request from the airborne interrogator, and not permanently. This is valid both for distance and bearing.
TACAN pulses are identical to the pulse of the civil DME system.
TACAN may deliver bearing information permanently by sending dummy reply pulses (at a constant rate of 2,700 pairs of pulses per second, adding squitter pulses if necessary to fill the blanks when not interrogated).
TACAN is often collocated with a VOR, so that civil aviation can use the DME, and military can use the VOR. The resulting equipment is named VORTAC. The TACAN can be down for maintenance or tactical imperatives.
- The DME system is a civil system at the origin, and the DME function of a VORTAC is likely managed (calibrated, maintained, powered down...) under a joint agreement between civil and military authorities.
TACAN uses upper UHF band while VOR uses VHF. Back in 50s and 60s, UHF components where expensive for limited performance:
Less powerful than VHF components for emitter amplifiers.
UHF components had a lot more noise and a lower SNR than VHF, preventing affordable high performance receivers.
Today the advantage is for UHF, with equipment being more compact and allowing better performance.
Why two systems?
The divergence between VOR-DME and TACAN results from some competition between civil and NATO authorities at the time the TACAN was designed:
The civilian authorities couldn't wait for the TACAN to be operational for use in new airways.
It seems NATO agreed to use the VOR-DME while at the same time agreeing at international level to use a different system without making this decision public. See Flying Magazine, June 1955.
A TACAN ground beacon is compact, it can be easily installed on ships, high altitude sites, and temporary landing sites. VOR-DME cannot do that due to the large radius of the antenna phased array (at least today with Doppler VOR).
TACAN compact station:
AS-4502/T instantaneous All-Band TACAN ground beacon, from ONUR
VORTAC regular size:
German VORTAC TGO near Aichtal, Source
On the image above, the low white antennas are the VOR circular phased array and the white cylinder at the top is the TACAN radome with the DME aerial at the top).
Airborne equipments are similar for VOR and TACAN, except an aircraft with both VOR and DME needs two set of antennas and two radio equipment, one for VHF, one for UHF:
TCN-550 complete airborne equipment to use a TACAN ground station, from Rockwell Collins
Differences between VOR-DME and TACAN
A TACAN ground beacon can be used for precision approach lateral guidance, due to the higher precision of the bearing information. A well known use was for STS Orbiter landing (3 TACAN onboard). Precision landing is not possible with VOR lateral guidance, an ILS must be used.
VOR can be used to transmit voice messages when a communication failure occurs with an aircraft. ATC will try to transmit messages using the VOR audio modulation (used to transmit the VOR ID). TACAN has no audio capability, the system works with DME pulses, including for bearing information.
A TACAN ground station is a transponder, and as such limits the number of interrogators, due to jamming occurring after some threshold, which may be around 200 interrogators.
Clearly both systems could be used by civilians and military, with some limitations for one user or the other:
Compactness: Military would be prevented to use VOR on small platforms like ships. Probably a no-go for military.
Control: TACAN needs to be under the control of military authorities, meaning maintenance and shutdown not managed by civilians. Likely a big problem for civil authorities.
Audio feature of VOR not available to civilians as an emergency channel.
User number limit of TACAN could be a problem for civil navigation.
Civil authorities not using TACAN
- Early competition with military and military delaying agreement.
- Control over ground stations.
- Unlimited number of users for VOR (still limited for DME).
- Easier maintenance.
- Voice capability.