Aeronautical Information Manual:
1-1-5. Tactical Air Navigation (TACAN)
a. For reasons peculiar to military or naval operations (unusual siting conditions, the pitching and rolling of a naval vessel, etc.) the civil VOR/Distance Measuring Equipment (DME) system of air navigation was considered unsuitable for military or naval use. A new navigational system, TACAN, was therefore developed by the military and naval forces to more readily lend itself to military and naval requirements. As a result, the FAA has integrated TACAN facilities with the civil VOR/DME program. Although the theoretical, or technical principles of operation of TACAN equipment are quite different from those of VOR/DME facilities, the end result, as far as the navigating pilot is concerned, is the same. These integrated facilities are called VORTACs.
b. TACAN ground equipment consists of either a fixed or mobile transmitting unit. The airborne unit in conjunction with the ground unit reduces the transmitted signal to a visual presentation of both azimuth and distance information. TACAN is a pulse system and operates in the Ultrahigh Frequency (UHF) band of frequencies. Its use requires TACAN airborne equipment and does not operate through conventional VOR equipment.
I have never flown in the military, so I can only take at what the FAA puts out. VOR's work on two phases, a reference and a variable phase. Reference is a pulse in all directions while variable changes throughout different radials (360 of them.) One way of thinking of it is that at the 360 radial, the reference and variable are identical, like two sine curves that are the same (starting from the origin.) At the 180 radial, the variable phase is roughly inverted. The wave is shifted so the null points are an equal distance from each other. Another way of thinking is that the variable sine curve is flipped across the X axis.
Now DME’s work with an interrogation signal sent from DME equipment in the aircraft and the station sends a reply. The equipment measures the time it takes and calculates the distance.
TACAN’s, in my opinion from the articles I read above, are the fancier versions of NDB/ADF’s. The NDB sends out a pulse and the ADF in the plane is able to indicate the relative bearing to the station. I believe the TACAN emits a pulse and the 9 different receivers measure the signal to determine the direction and DME in the same way above. It appears that the main different between a NDB and TACAN is the LF/MF to the UHF.
So, as a CFI with an opinion and a guess, I believe TACAN’s are on ships and not VOR’s because the shifting of the ship would screw with a VOR indication on the CDI on the instrument. Navigating in close proximity to the ship could have the CDI dancing all over the place. For a TACAN, the shifting of the boat (I think) would be less detrimental due to the fact that radials are not shifting all over the sky.
As for instrument flying and IAP only being over the station, that is incorrect. With distance information, a cross radial, GPS or radar with the assistance of ATC, an IAP for a VOR type approach can be off the station.
Does this make any sense to the crustier individuals out there?
