Tacan

[Yorks.ppl]

What is a TACAN and how, if possible is one used?

I have searched the forums with no joy, and can find no reference to them in any of my books but loads are listed in pooleys.

I hope this isnt a dumb question:O

[Blinkz]

TACANs are a military beacon that can be used by civil aircraft too. As far as I know (having not used one) that they just provide DME information to us.

On looking around pprune it looks like they can be used as more then DME, but I'm not sure if there use is similair to a NDB or VOR.

[BEagle]

A TACAN (tactical air navigation) beacon is similar to, but much more accurate than a VOR/DME beacon. Civil a/c are unable to obtain bearing information from TACANs due to the UHF frequencies used in the 950-1150 MHz band. However, the DME element can be used by civil DME receivers.

Beware that the DME frequency radiated from the Brize Norton TACAN (Ch 56X) is the same as that which should be frequency-paired with the ILS frequency 111.9. But, unlike I-DME information, it will give you range to the TACAN beacon, NOT to touchdown. This may not be made clear in the relevant ILS approach plates......

[FlyingForFun]

As others have said, you can use the DME information from a TACAN, but not the VOR information.

However (someone please correct me if I'm wrong) there are other beacons known as VORTACs, which are a TACAN co-located with an ordinary VOR. So military aircraft can use them as a TACAN, and civilian aircraft can use them as if they were a co-located VOR/DME.

FFF
--------------

[Engineer]

TACAN has 126 X and 126 Y channels. Normally only X channels
are used, unless crowded. TACAN frequencies are tied to VOR frequencies. (Note: there are more TACAN frequencies than VOR frequencies, some are blanked around the ATCRBS Beacon frequencies, and others are for expansion and military use).

Frequency pairings as follows

Channel VOR
-------------------
17X 108.00 MHz
17Y 108.05 MHz
18X 108.10 MHz
18Y 108.15 MHz
19X 108.20 MHz
19Y 108.25 MHz
and at 50Khz spacing
upto
58X 112.10 MHz
58Y 112.15 MHz
59X 112.20 MHz
59Y 112.25 MHz
. . . (Unused to protect Beacon)
70X 112.30 MHz
70Y 112.35 MHz (Unused to protect Beacon)

126X 117.90
126Y 117.95 (Last VOR pairing)

29Y and 92Y Favorites for Military Air Refueling (Air-Air)
Check the heavens if active. All Air-Air pairs are 63 Y apart.

[Aussie Andy]

TACAN is simply the MIL equivalent of VOR/DME - it was originally supposed to have better accuracy (not sure how true this is in practice with modern receivers and ranges though).

I believe the real reason the MIL wanted a system different to VOR is that it is more "tactical" in that it can be deployed more easily at short notice, in theatre, on un-even surfaces, ships at sea or even on airborne platforms. So TACAN receivers abound in MIL aircraft, and hence MIL airfields typically have TACANs for NAV and/or APP purposes.

VORTAC is simply when an VOR and TACAN are co-located - common in the US.

Andy :ok:

[IO540]

Incidentally is there somewhere a comprehensive reference for the channel numbers versus VHF frequencies?

I've found some partial tables but they weren't clear as to how it worked.

I know the DME receiver has a lookup table in it so when you select a VHF frequency you actually get the paired UHF DME frequency.

The U.S. ONC charts have only channel numbers on them.

[Aussie Andy]

I believe it's in the AIP somewhere...?

[Dan Winterland]

TACAN bearing system works in a similar way to a VOR by sending out multiple signals from which the receiver can determine the radial by measuring the phase difference in the signals. However, the TACAN's phase difference is repated every 40 degrees thus making it (in theory) nine times more accurate. This is acheived by a rotating drum and reflectors and is the size of a largish dutbin, thus is much slmaller and portable compared with a VOR. It can also be installed on a ship.

[IO540]

How old is this information?

Every VOR I've seen here in the UK is an obvious phased array. One has about 30 aerials in a circle some 15-20m dia and one feeds signals to all of these, phase shifted so as to create an interference pattern which forms a beam (or two; I don't know the details but it's probably two or four).

Phased array radars work on the same principle.

VORs with rotating aerials still AFAIK exist somewhere in the USA.

A TACAN, whether constructed as a phased array or as the old rotating aerial, working as it is on a frequency several times greater than a VOR, would be correspondingly smaller in diameter and that has got to be the #1 advantage. One could put it on the back of a truck.

Both a VOR and a TACAN emit a continuously varying radial so there is no difference in resolution - the res is infinite.

UHF will be a bit less affected by terrain than VHF but I can't see that is relevant since e.g. a localiser is VHF.

[BEagle]

TACAN transmissions take place on 252 frequency channels:

126 channels in X mode, called "X channels", numbered 1X to 126X

126 channels in Y mode, called "Y channels", numbered 1Y to 126Y (0.05 MHz offset from the "X channels".

Channel separation is 1 MHz.

Each channel consists of a beacon transmit frequency between 962 MHz and 1213 MHz and a beacon receive frequency, between 1025 MHz and 1150 MHz.

TACAN bearing informaton is more accurate than VOR bearing information; this is a function of the transmitted characteristic, not the receiver selectivity.

[Dan Winterland]

Just dug out my old CFS notes.

The TACAN transmits 2700 pulse pairs a second from a central vertical aerial which is surrounded by 2 concentric cylinders rotaing at 15 revolutions per second. The inner cylinder has one reflector, the outer cylinder, 9 equally spaced reflectors. The inner cylinder produces a signal at 15hz. As the single reflector passes east, the aerial transmits a burst of 12 nanosecond pulsed pairs - this is known as the Main Reference Burst (MRB). The receiver computes a rough radial by comparing the phase difference between the 15hz and MRB signals.

For further accuracy, the outer drum with the 9 reflectors produces a 135 hz (9 x 15) signal on top of the 15hz signal. An additional 12 nanosecond Auxillery Reference Burst (ARB) also produced as the main signal passes east allows the receiver to calculate a more accurate radial. The phase difference between the 15hz and 135hz signals determines which of the nine 40 degree sectors the aircraft is in and is therefore 9 times more accurate than a VOR (in theory).

It is a very different animal to the VOR and I believe a lot of installations still use the rotaing drums, although electri=onic version must exist as one aircraft I used to fly had the ability to transmit TACAN azimuth information as well as just the air to air DME facility. The one at Brize is still a rotaing drum by the look of it. It was designed to be portable - TACANS would have been relocated at regular intervals during any conflict.

The DME element of TACAN uses the same transmissions as the radial signals - therefore the frequencies used are the same.

Most modern VORs are Doppler VORs. These are the ones with a series of white balls around the centre aerial. These transmit a phased pulsed signal which to the receiver looks just like the physically produced rotaing signal of old.

FYI, phased array radars work on a slightly different principle. The radar head is stationary, the beam is moved by adjusting its' frequency. 360 degree coverage is achieved by having more than one aerial which are used in turn (as seen on the AEGIS radar on Ticonderoga class ships).

I knew there was a good reason for keeping those notes!

[Engineer]

BEagle

Are you sure about the 1MHz spacing ?

[BEagle]

I believe that it's 1 Mhz between adjacent channels of the same family (X or Y) and that the Y channels are offset between 2 adjacent X channels.

But I shall wait for TACAN Dan's confirmation once he's checked his Boys' Book of TACAN Facts and Figures!

[Dan Winterland]

Errr.............!

[Circuit Basher]

There is a Table of TACAN Frequencies (http://microvoltradio.com/tacan.htm) here, which confirms the 1MHz spacing. I nearly popped up to comment, as (based on VOR freqs) it seemed like 25 kHz would be the spacing. Luckily I kept my powder dry and checked before I posted!! :D