[MilCom] TACAN Channel numbers vs frequency

[dan wanchic]

To make a long story short and as Larry has already indicated
- the frequencies for channels 92X and 92Y

The aircraft interrogator freq is 1116 MHz.
The X channel ground station freq is 1179 MHz.
The Y channel ground station freq is 1053 MHz.

I'm not familiar with any milcoms using that freq range other than for 
TACAN/DME
purposes and would be interested in reading anything that anyone has to 
offer.

For those who are interested.

TACAN is what is known as a Rho-Theta system. It provides distance
Rho data and Azimuth Theta data to the aircraft.

It is a pulse modulation system. The aircraft transmits two pulses,
what we call a pulse pair.

In an X channel system the pulses are 3.5 microseconds wide and they
are spaced 12 microseconds apart. Most civilian systems use X channel
spacing.

In a Y channel system the pulses are 3.5 microseconds wide and they
are spaced 36 microseconds apart.

Regardless of the type of channel, the ground station receives the 
interrogation
from the aircraft. It waits exactly 50 microseconds and then it sends a 
pulse pair reply.

In an X channel system the ground station pulse spacing is 12 microseconods 
and a
Y channel it is 30 microseconds. A typical ground station transmits about 
3500 watts.
Pure DME systems usually run 1000 watts if located with a VOR or 100 watts 
if
associated with an ILS.

The aircraft measures the time it takes to receive its reply from the ground 
station and
uses this time to calculate its slant range distance from the ground 
station. This part of
the system is also known as DME or distance measuring equipment in civilian 
aircraft.

The azimuth portion of the system is much more complicated and would be 
difficult to
capture in a brief message. Most TACAN systems use a rotating antenna which 
is essentially
a three element yagi that rotates at 900 rpm. There are a few phased array 
antennas but
most are motor driven. In addition to replies, the ground station also sends 
a series of pulses
know as the North reference burst or NRB's and Auxillary reference bursts or 
ARB's that are triggered by the antenna rotation. And it transmits a 3 
letter morse ID.

The aircraft uses these pulse streams to measure where it is relative to the 
station. This is
the Theta part of the system. There is no civilian equivalent to this 
component and it does
not operate on the same prinicple as a VOR.

Both military and civilain aircraft use the DME part of the system.

TACAN channels are numbered 1X through 126X and 1Y through 126Y.

Channel 1X the aircraft interrogator transmits on 1025 MHz and the ground 
station
replies on 962 MHz.

Channel 1Y the aircraft interrogator transmits on 1025 MHz and the ground 
station
replies on 1088 Mhz.

There is a 63 MHz difference between the transmit and receive freqs for 
TACAN/DME.
So all of these systems conveniently have 63 MHz IF's.

The frequencies assigned to the channels increases by 1 MHz for increases 
for the
aircraft interrogator 1 MHz for each channel number above channel 1. It's 
necessary
to subtract 1 from the channel number. As an example channel 57 is 1081 MHz 
-
add 56 to 1025 MHz = 1081. the aircraft interrogation freq is the same for 
both
X and Y channels.

The ground station frequency plan is more complicated.

As a side note - spent a lovely day yesterday working on an ILS at the 
Duluth airport.
Plenty of F-16's buzzing back and forth. They were using the Zipgun ident 
yesterday.
Those boys are somewhat loud when they fly directly over you and they are 
about
100 feet off the deck!

Dan

If this address bounces
-- wa8vzq at frontiernet.net--
-- Apple Valley, MN --

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