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Old 8th Mar 2004, 20:11   #1 (permalink)
 
Join Date: Jan 2003
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Why do the military use TACANs, why not VOR/DME?

Why do the military use TACANs and not VOR/DME instead?
My understanding of a TACAN is that it is like a VOR but is UHF rather than VHF (except the DME part which is UHF on both). Does a TACAN have a greater range? Is that why They are used?

Whit the exception of the DME part, do any civilian aircraft have TACAN equipment installed?
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Old 9th Mar 2004, 00:58   #2 (permalink)
 
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Greater bearing accuracy.

Under military control - (they can put them where they want them, without having to co-ordinate with civilian authorities).

No need to de-conflict with VHF VOR frequencies - the military need total control in war or limited war situations.

You can put them on a ship (VOR needs a level mown field and a permanent installation).

You can deploy them tactically. A team of 6 soldiers can put one into a forward battle area unprepared field.

Don't have to put out NOTAMs every time you start one up or switch one off. No need to tell civilians at all, if you don't want to.

Can switch them on and off at times that favour your own side, but deny them to the enemy.

The enemy won't have TACAN receivers, but they will have VOR receivers.

And lots of others. The military see a bigger picture than just compatibility of kit.



TACAN does not have a greater range - it's line of sight, just like VOR.


I don't know of any civilian aircraft with TACAN fitted. I would be surprised if there were any, other than those on military contracts, but I may be wrong.
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Old 9th Mar 2004, 01:29   #3 (permalink)
 
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To add to Oxford Blue's comments, the higher frequency of TACANs means that the equipment can be smaller and more easily deployable in the field. Some photos give an idea of the difference in size.

My understanding of the history is that the Victor airway system in the US was designed after a compromise: the military wanted TACANs for the reasons stated, the civil authorities wanted VORs because of the lower cost of airbrone equipment. The compromise was the VORTAC, a colocated VOR and TACAN facility of which all parties can use the DME element.

By contrast, in the UK, because of the location of military airfields outside the airway structure, the civil and military went their separate ways. I don't think we have any VORTACs in the UK, though the TACAN route chart in the AIP suggests that SAM, MAY, DVR and LND are part of the network, so may also have TACAN equipment installed.
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Old 9th Mar 2004, 08:51   #4 (permalink)
 
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Hi there,

I seem to remember that the DME/TACAN frequencies were paired to the VHF VOR frequencies, and there was a table in the AERAD/JEPP giving the links. As I recall, the TACANS were marked with a small cartwheel symbol on the Jepp charts, with a channel number. If one checked the list it would give a corresponding VHF NAV freuency. When this was tuned on the VOR receiver it automatically set the appropriate UHF frequency on the DME side of things and up would come the range side of the TACAN (no bearing info).

We used to use it a lot in the Frozen North of Europe!
Cheers
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Old 12th Mar 2004, 19:06   #5 (permalink)
 
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Some BAe 146 aircraft were fitted with TACAN. The first were those operated by Aspen Airways (now Air Wisconsin / United Express) for approaches into Aspen airport (late 1980s).

The Queen’s Flight aircraft also had TACAN, but of course they were 'military aircraft'.
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Old 12th Mar 2004, 23:26   #6 (permalink)

 
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Stornoway VORTAC? I could be out by a decade here...
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Old 13th Mar 2004, 01:13   #7 (permalink)
 
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When I were a lad there were a number of co-located TACAN and VORs - WAL/WAZ, OTR/OTZ, STN/STZ. The TACANs (which also supplied the DME function) had the 'Z' on the end to show that they were not exactly co-located. To have the same ident they both have to be within ??? metres.

As time went by and military route-flying aircraft were fitted with VOR receivers the TACANs were removed and DMEs were put in with the VORs. In the UK now TACAN is mainly an airfield approach and orientation aid. However there is still a TACAN 'airways' structure (info cribbed from another thread):

Details of the TACAN route structure can be found at:

http://www.ais.org.uk/aes/pubs/aip/pdf/enr/26030502.PDF

you need to register to get to the site but it's free.


TACAN is more accurate because, instead of having a single rotating reflector producing an AM cardioid (or is it a limacon?), TACAN has 9 extra rotating reflectors. Rather than using an omnidirectional FM signal, TACAN uses timing pulses: a 'Master Reference Burst' and others that tie in with the small reflectors. The only problem this can lead to is a 40° bearing error which can be a bit embarrassing.

Last edited by Stan Evil; 13th Mar 2004 at 01:28.
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Old 13th Mar 2004, 04:54   #8 (permalink)
 
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Wasn't there also an esoteric way of working out the right DME freq to get the range from a TACAN. I think you add 53 to the low channels and 63 to the higher ones to get the last 3 digits of the freq to dial up. Eg, for channel 70, add 53 to get 123 and thus dial up 112.3 to get the range.

Unless you know better...........

Cheers,
mcdhu
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