[MilCom] 254.2 Again!?

Tue Feb 13 14:57:59 EST 2007

The uplink for the satellite in question is on or near 294.9 MHz. Calgary 
Approach uses 294.9 MHz. The frequencies I was quoting in my email were 
just examples of local ATC frequencies.

Jody reported in her email she was hearing aircraft transmissions coming 
over the satellite, but what's happening is that VHF aircraft 
transmissions from aircraft approaching Calgary are being simulcast on 
294.9 MHz, and then being downlinked by the satellite.

Jody in North Georgia can monitor the bird, but I  believe the satellite 
is in an elliptical orbit, and she can only hear the satellite when it is 
above the horizon for her. So the signal comes and goes. The same would 
apply at your location in Boston. 

Rob
Regina, Saskatchewan, Canada

"Tom Greenwood" <TheGreenwoods1 at Verizon.net> 
02/12/2007 09:17 PM

To
"Dave Holford" <holford at cogeco.ca>, <milcom at mailman.qth.net>, 
<rob.berezowski at sasktel.sk.ca>
cc

Subject
RE: [MilCom] 254.2 Again!?

Maybe I have missed something here but I do not recall anybody indicating
what the uplink frequency is for this bird nor indicating that they may 
have
heard something on that frequency.  I am just barely hearing changes in 
the
noise level on 254.2 with audio being present but at a very poor SNR here 
in
Boston.  But my setup is not optimized for it either.

Any ideas?

73's

Tom

-----Original Message-----
From: milcom-bounces at mailman.qth.net
[mailto:milcom-bounces at mailman.qth.net]On Behalf Of Dave Holford
Sent: Monday, February 12, 2007 16:27
To: milcom at mailman.qth.net; rob.berezowski at sasktel.sk.ca
Subject: Re: [MilCom] 254.2 Again!?

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. 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. 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. 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.
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 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.

The only time I have heard anything approaching these reports has been a
result of third order intermod at the receiver.

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 com
cast.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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