[MilCom] 254.2 Again!?

[Dave Holford]

This is getting lengthy, however here goes.

Obviously you have never had much contact with controllers. One of the most
basic requirements is being able to listen to several voices at one time.
The other is "the picture", a mental image of where all your aircraft are
and will be in relationship to each other over the next period of time -
"loosing the picture is the stuff of nightmares" I have seen it twice and
the look on their faces is something I would not care to see again. (When I
was checking out my monitor used to amuse himself by starting up
conversations about my family, or hobbies, or sports whenever I was trying
to follow multiple control conversations - then give me a hard time because
I couldn't keep up).

Not only do they have multiple frequencies, they have multiple hotlines -
just talk and the other controller hears you - but they also have
interphones, intercoms, dial-up lines and, of course, direct communications
with the folks next to them,

I shall never forget the sense of utter confusion the first time I walked
into a tower and heard all the conversations going on at once - speakers
everywhere, even on the ceiling. That and the tray of ulcer medication are
the two things that I still remember from my first exposure.



As to your insertions:

One controller working several sectors - been there done that, standard
procedure. Never heard of any one attempting to deny other aircraft the use
of a frequency by transmitting messages from aircraft in different airspace
which would be of no interest, and could even confuse the situation. The
controller controls transmissions - that is the basis of the system. If
aircraft A needs to know about aircraft B who is on a different frequency
then it is the controllers responsibility to provide that information in a
form that is useful, not just a rebroadcast - again this is standard
procedure (traffic is a B737 1 o'clock 6 miles, opposite direction at
FL370).

That configurable audio matrix has existed for at least 30 years, there are
two of them in each unit, and would never permit a transmission on a
controllers frequency unless the controller has activated his talk switch.

Once again, listening to multiple conversations is part of a controller's
job - you pick out the important bits and if you don't think there are
potentially important transmissions which might come up on one frequency
while you are listening to another you ain't been there. If I had not heard
the "Air Canada XXX emergency descent" while I was listening to a clearance
readback on another frequency the board of enquiry would probably have taken
action which involved me having a much higher pitched voice, plus maybe loss
of my license.

If I had missed the "high speed unidentified 15north southbound" on the
co-ordinators hotline while I was delivering a clearance life might have
been even more interesting; and my heart rate gone even higher.

And, the real kicker "departure XXX airborne off 07 {the ident would
immediately identify the specific operator) when the active runway was 25,
while I was listening to another conversation could have made CNN if I
missed it. There was a commercial carrier on final for 25.

It is not first come first served, it is like a computer interrupt system,
highest priority first - others get added to the queue.

You would be surprised how many "stepped on calls" can be recovered from a
tape by a board of inquiry.

Sorry to be so long winded, but there are valid reasons why it is the way it
is.

Dave

----- Original Message ----- 
From: <rob.berezowski at sasktel.sk.ca>
To: <milcom at mailman.qth.net>
Cc: "Dave Holford" <holford at cogeco.ca>
Sent: Tuesday, February 13, 2007 2:47 PM
Subject: Re: [MilCom] 254.2 Again!?


> 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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