MIL Aircraft TACAN proximity alert?

[Edelweiss]

I had this idea some time ago and not sure if it's already been done. I suppose most MIL a/c will have their TACAN running when not operating covertly, and we're probably interested in those which do not show up on a tracker. Has anybody tried intercepting their TACAN downlink signals successfully? I tried tuning into it with an SDR but get quite a lot of local interference probably from all things computer so I'm not sure if I caught something. The transponder signal should come in reasonably strong with good range I would have thought, so far no luck even with those who claimed doing a TACAN approach. The pulses are of course very short and that might be the issue here. Should be some irregular chirps? I'm quite certain I was on the right channel. Need to play more but if you have any contributions please post them here!

Mike

 

[MUTNAV]

All aircraft for a specific TACAN interrogates the ground station on the same frequency...

A specific aircraft sends out pulse pairs to interrogate the TACAN at irregular intervals, only when there is enough coincidence of replies to the interrogations are they considered valid for a particular aircraft. This keeps the chance of mistaking replies meant for other aircraft as replies to themselves.,,

Basically trying to pick up interrogations of the ground station would be possible but, hard to do..

Thanks
Joel

 

[AB5ID]

I'm not sure if TACAN interrogations are any different than DME interrogations? Both use the same radio channels and had the same pulse characteristics. If you were able to decode the pulse pairs you might not be able to differentiate between a military aircraft and a civilian aircraft. The ground station will reply to the interrogations but also will include a North burst and auxiliary burst for azimuth used by military aircraft.

 

[MUTNAV]

I'm not sure if TACAN interrogations are any different than DME interrogations? Both use the same radio channels and had the same pulse characteristics. If you were able to decode the pulse pairs you might not be able to differentiate between a military aircraft and a civilian aircraft. The ground station will reply to the interrogations but also will include a North burst and auxiliary burst for azimuth used by military aircraft.

They are on the same frequencies, but once the replies are identified by the aircrafts receiver, they become less frequent.

I don't see how to effectivly use the interrogations as a reasonable proximity sensor ( you would have to be able to differentiate the difference between a 1 Ghz signal from 5 miles away at 3,000 feet and one from 20 miles away, keeping in mind that the aircraft twenty miles away may have a much better line of sight to your antenna.

Maybe you could use a surplus TACAN receiver that has the right bandwidth and response, of course you would still have the problem of the interrogations coming in irregularly.

I don't know if this helps. Probably not.

Thanks
Joel

 

[AB5ID]

My only point was you wouldn't no who (military or civilian) or where the interrogations were coming from.

 

[MUTNAV]

My only point was you wouldn't no who (military or civilian) or where the interrogations were coming from.

Agreed.

I was assuming they wanted only some sort of proximity alerting. but yes, we would have no idea where they were coming from without a df antenna, or how far without elevation / distance information.

Thanks
Joel

 

[Edelweiss]

1st step is to pick something up. Much easier to get a rough direction from the higher frequencies. Any kind of alert is better than none. Can then check the radios...
Let me know if you can hear something!

 

[dlwtrunked]

All aircraft for a specific TACAN interrogates the ground station on the same frequency...

A specific aircraft sends out pulse pairs to interrogate the TACAN at irregular intervals, only when there is enough coincidence of replies to the interrogations are they considered valid for a particular aircraft. This keeps the chance of mistaking replies meant for other aircraft as replies to themselves.,,

Basically trying to pick up interrogations of the ground station would be possible but, hard to do..

Thanks
Joel

I do not understand you lass sentence as it is very easy to do. Identifying the sender is hard.

 

[MUTNAV]

I do not understand you lass sentence as it is very easy to do. Identifying the sender is hard.

True enough...
The big problem I see, to the point of making the exploration of any alternatives worthwhile, would be telling the difference between an interrogation from an aircraft on final, versus one 40 miles out.

To make a long story short though, you basically need an interrogation receiver, home made or taken from a DME.

Thanks Joel

 

[majoco]

Civilian DME and TACAN channels are 1X to 16Y and 60X to 79Y with corresponding frequencies ranging from 1025MHz (1X) to 1040MHz (16Y) with channel spacing of 1MHz for the interrogation frequencies. The channel 60X interrogates on 1084MHz and 126Y interrogates on 1150MHz. The "Y" channel ground station reply is always 63MHz above the interrogation frequency with a 12uS pulse spacing, the "X" channels are 63MHz below and a 36uS spacing.

Each of the military channels is paired with a VHF navigation band frequency from 108MHz right up to 117.95MHz and so the 108MHz nav frequency is paired with the 17X channel right up to the 117.95MHz channel is paired with 126Y. All other parameters are same as the civilians.

I doubt if there's many civilian aircraft equipped with TACAN and I also doubt that there are any TACAN's in the civilian channel plan - except for our calibration aircraft back in the 70's and 80's which was fitted with a military unit. Nice Fokker F27 fitted with all sorts of goodies, even Omega receivers and sat nav as soon as it was operational. Long range tanks allowed us to fly all around the South Pacific islands at the tax payers expense!

 

[AB5ID]

The "Y" channel ground station reply is always 63MHz above the interrogation frequency with a 12uS pulse spacing, the "X" channels are 63MHz below and a 36uS spacing.

The Y Channel replies from a ground station are 30 microsecond pulse spacing.

The X channel replies from a ground station are 12 microsecond pulse spacing.

 

[Edelweiss]

AFAIK the interrogation frequency is not a fixed one, it changes with the beacon which makes total sense otherwise more beacons would reply. MIL-STD-291 gives you most info and also a channel table.