[MilCom] 254.2 Again!?

Tue Feb 13 14:47:05 EST 2007

Dave,

One has to keep in mind here we're not talking about a lot of aircraft on 
each frequency. This simulcasting we're referring to here only happens 
during the off peak hours (evenings and nights) when traffic is low. One 
controller is handling multiple sectors. He only has one set of ears, and 
can only listen to one voice at a time. Therefore, the fact that the other 
frequencies are "tied up" by the simulcast is probably intentional so that 
aircraft on those frequencies will wait their turn to call the controller. 

Also, as a side note, UHF is very seldom used up here at all in civilian 
airspace. The Canadian military seem to always use VHF while in civilian 
airspace. The only real use of UHF nowadays is at military air bases, in 
military controlled airspace, and for air to air. From my location, if I 
hear an aircraft on UHF talking to civilian ATC, it's usually an American 
military aircraft.

I have inserted additional comments in your email text below.

Regards,

Rob
Regina, Saskatchewan, Canada

"Dave Holford" <holford at cogeco.ca> wrote on 02/12/2007 03:26:58 PM:

> As a retired air traffic controller I find this discussion intriguing, 
and 
> strange.
> 
> I cannot believe that aircraft working Regina ARR/DEP on120.1 would 
tolerate 
> hearing traffic working Saskatoon ARR/DEP on 119.9 without filing 
> complaints.

Remember, it's the same controller. If all the aircraft were on one 
frequency in one area, they would hear each other anyway. In this case, it 
is a Winnipeg based controller handling radar approaches into Regina and 
Saskatoon. During off peak hours, one controller handles both Saskatoon 
and Regina sectors.

> Large numbers of complaints to both their companies and 
> NavCanada; even Transport Canada. I might even expect these complaints 
to 
> show up in CADORS (Canadian Aviation Daily Occurence Reports which 
contain 
> all sorts of picayune problems).
> 
> A controller may have several radio frequencies each with their own 
control 
> module. But only one transmit switch. If two or more frequencies are 
> selected for simultaneous use then all of them will be transmitting at 
the 
> same time. How the system can possibly be configured to allow the 
> transmission of signals being received on the other frequency would 
require 
> some highly unlikely rapid reconfigurations. 

Not at all. They are using some sort of configurable audio matrix 
switching. Received audio from, lets say for an example, a receiver on 
124.3 would be routed and heard by the controller, and it will also be 
routed by the switch matrix to all other selected VHF and UHF transmitters 
being looked after by that controller at that time. The only place the 
audio won't be routed to would be the 124.3 transmitter of course. When 
the controller transmits, he or she will transmit on all selected VHF and 
UHF transmitters. 

This does mean anybody listening on 124.3, or any other of the selected 
VHF and UHF frequencies would hear both sides of the conversations. 

> If the received VHF signals are 
> to be transmitted on UHF two immediate problems arise -
> 1. It will be impossible to hear any aircraft transmissions on UHF while 
an 
> aircraft on VHF is transmitting - I cannot imagine anyone being prepared 
to 
> tolerate such a circumstance. 

I think this is intentional to make sure the pilot on UHF doesn't "step 
on" a transmission on VHF. As I said, the controller's brain can really 
only hear one conversation at a time, so someone has to wait their turn. 
But, yes, you're right, it is basically first come first served on the 
audio, and someone on UHF will not be heard by the controller if someone 
on VHF transmits first. 

> This also would mean that any transmissions 
> from aircraft on the frequency which is being used to simulcast would 
not be 
> recorded - ATC communications are half duplex so you can't listen and 
talk 
> at the same time - this would mean that there would be no guaranteed 
record 
> of aircraft transmissions; the lawyers would have a field day with such 
a 
> situation.

Yes, you're right. If a pilot on UHF made a call while someone on VHF was 
already transmitting (assuming the VHF pilot transmitted first, like I 
said before first come first served), then the recorder would miss the 
call, and so would the controller. But then, that's no different than the 
UHF pilot trying to make a call when the controller is transmitting. If 
the UHF pilot "steps on" either the controller or another pilot on either 
VHF or UHF, then yes the call won't be recorded or heard. 

This simulcasting is set up to prevent "stepping over" other 
transmissions, and to ensure all calls on all frequencies are heard by the 
controller, and recorded. No system in place can guarantee recording a 
call when someone steps on someone else.

Also, remember this simulcasting only occurs when traffic is light, and 
individual controllers are handling more than one sector. If there are a 
lot of aircraft and a lot of calls, the liklihood of "stepping on" 
increases. But then, if that happens, it's likely more controllers would 
be used, and you wouldn't have a single controller looking after so many 
aircraft on different frequencies. 

> 2. When the controller needs to transmit on UHF the system mst be 
> reconfigured to feed audio from the controller rather than the VHF 
receiver 
> to the UHF transmitter.
> 
> Basically, if transmissions from one aircraft are simulcast on another 
> frequency then that second frequency becomes essentially unavailable for 

> communications from aircraft - the transmitter is in use anytime the 
> controller or another aircraft uses any of the simulcast frequencies.

As I said before, this seems to be intentional to prevent pilots from 
"stepping on" other transmissions from other pilots, which wouldn't do 
anyone any good because the controller only has one set of ears.

> 
> As one who has used the system in several military and civil airports 
over 
> the last half century, been a listener to the frequencies since the days 
of 
> crystallized channel aircraft radios, and taught the technology of 
modern 
> ATC communications systems I find this thread very interesting and will 
be 
> awaiting enlightenment on what is really happening.
> 
> While technically such a situation is not difficult to implement, if it 
was 
> actually to occur I would anticipate there would immediately be UCRs 
> (Unsatisfactory Condition Reports) flooding into the system support 
centre.

I doubt anyone has complained about this. If they have, the complaints 
have fallen on deaf ears. NavCanada has been doing this (at least in the 
Regina area) for several years now. I can't remember exactly when it 
started, but I'm guessing at least 5 years. 

> 
> The only time I have heard anything approaching these reports has been a 

> result of third order intermod at the receiver.

It's not intermod. :-)

> 
> Dave Holford.
> 
> 
> ----- Original Message ----- 
> From: <rob.berezowski at sasktel.sk.ca>
> To: <milcom at mailman.qth.net>
> Sent: Monday, February 12, 2007 3:11 PM
> Subject: [MilCom] 254.2 Again!?
> 
> 
> > Yes, I hear this all the time here in Regina. It's part of 
configuration
> > ofthe telecom equipment being used by NavCanada. I hear simulcasting 
all
> > the time. For example, when Winnipeg Centre is simulcasting on UHF, 
both
> > the controller audio and the audio from aircraft on the matching VHF
> > frequency will be broadcast on UHF. This seems to be normal procedure 
to
> > simulcast the aircraft transmissions.
> >
> > I've also heard a lot of VHF to VHF simulcasts as well. A good example 
is
> > when the Winnipeg-based controller handling approaches into Regina is
> > looking after Saskatoon approaches as well. On the VHF frequency in 
Regina
> > (120.1), I can hear the controller, local aircraft, as well as 
simulcasts
> > of the aircraft approaching Saskatoon (they are on 119.9, but well out 
of
> > direct reception range of my scanner).
> >
> > So Jody your report doesn't surprise me based on what I normally hear 
in
> > Western Canada.
> >
> > Rob Berezowski
> > Regina, Saskatchewan, Canada
> >
> > ------------------------------
> >
> > Message: 8
> > Date: Sat, 10 Feb 2007 01:33:52 +0000
> > From: vampire11 at comcast.net
> > Subject: [MilCom] 254.2 Again!?
> > To: milcom at mailman.qth.net
> > Message-ID:
> >
> > <021020070133.28727.
> 45CD21000001BB9A000070372207003201CECE0A9D079F030E90 at comcast.net>
> >
> >
> > Ok just when I thought 254.2 was explained and the mystery was solved,
> > This happened. I currently have the that same Calgary Alberta approach
> > controller on 254.2 right now, BUT I am hearin the aircraft answer him
> > back. The VHF side is gettin in the sat now. I hear the civilian 
aircraft
> > on VHF answering the controller.  I have never heard of a airport
> > rebroadcasting all of the VHF side on UHF before. The only thing that 
is
> > usally similucasted is the controller on UHF. Any ideas anyone? I got 
this
> > all recorded as always.
> >
> > Jody
> > North Georgia
> >
> >
> > ------------------------------
> >
> 

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